This abrasive has a paper or cloth backing to which the abrasive is applied directly (in the form of a powder or "grain"). This “graininess” is one of the main characteristics of the product, which is also known under other “names”: sandpaper, sandpaper. The size of the grains determines what type of work can be done with this sandpaper, and this is indicated by the marking.
You should know that artificial abrasives (electrocorundum, carborundum, etc.) are used in production. The entire "family" of sandpaper is classified according to Russian and foreign standards. Therefore, further, for simplicity, all indicators will be indicated in accordance with Russian GOST. It complies with the foreign standard FEPA, or ISO 6344, as generally accepted in the world. However, some countries also use their own (Canada, USA, China, Japan). Sandpaper is produced either in separate sheets (sheet) or rolls.
Appointment of sandpaper depending on the marking
The letter "P" in GOST denotes graininess and is characterized by numbers from 12 to 2500. The larger the number in the designation of "sandpaper", the smaller it is (smaller grain size). However, in some former Soviet republics, GOST USSR is also used to designate graininess. For example, 20 - N. This is if the size is measured in tens of microns. If just in microns, then the designation will be as follows - M20. So designate one of the smallest sandpaper, commonly called "null". Let's bring brief classification sandpaper.
- For very rough work P22, P24, P36 80-N, 63-N, 50-N
- For rough work P40, P46, P60 40-N, 32-N, 25-N
- For primary grinding P80, P90, P100, P120 20-H, 16-H, 12-H, 10-H
- For final grinding P150, P180 8-H, 6-H
For fine-grained types of sandpaper, there is a classification.
- Sanding hardwood P240, P280 5-H, M63
- Polishing, grinding
- before painting P400, P600 M28, M40; 2-H, 3-H
- ceramic grinding,
- plastic, metal P1000 M20, 1-N
- Polishing P1200, P1500, M14, M10, M7, M5
- R2000, R2500 N-0, N-00, N-01
There are other designations on the back of sandpaper that can be used to determine what its basis is, manufacturing technology, type of abrasive material, etc. Here are some examples:
- if there is no separate letter, then this paper is rolled. The leaf has an index "L";
- "1" - designed for grinding soft materials;
- "2" - for grinding metals;
- the letters L1, L2 and M denote wet-strength paper;
- the letters P warn that the paper is afraid of dampness.
There are a number of other designations, but they are of interest only to a specialist, and they do not play a special role for the practical use of sandpaper.
Sandpaper is the most common material for grinding various surfaces. There are many types and brands. Without understanding them, you run the risk of damaging the material being sanded or the abrasive itself - this happened to me once at the dawn of a foggy youth.
What is sandpaper?
Sandpaper is a flexible abrasive. It is also called sandpaper, sandpaper or just sandpaper. It is made of a paper or fabric base and a layer of abrasive grains glued to it.
The sandpaper is intended for processing of surfaces from concrete, a tree, a brick, metal, plastic, glass and other surfaces. With its help you can:
- remove old coatings (for example, varnish) and their residues;
- prepare the base for priming and painting;
- remove chips and burrs from cuts of different materials;
- leveling, grinding, polishing surfaces.
Abrasives for the production of sandpaper
Natural emery is a mixture of magnetite and corundum. Now it is practically not used in the production of abrasives.
- Carborundum (silicon carbide) and electrocorundum (aluminum oxide) - most often used for the production of sandpaper. Get them artificially.
- Borazon (elbor), synthetic diamond and pomegranate are used less frequently.
Electrocorundum
Aluminum oxide is the most common abrasive. These are hard crystals that have sharp edges at the break:
- Electrocorundum is synthesized by reducing charge melting in an arc furnace. The raw material consists of iron filings, bauxite agglomerate and low-ash carbonaceous materials.
- Aluminum oxide has excellent cutting ability. It can withstand high pressure.
- Often alloying additives are added to the charge during melting. They improve the characteristics of electrocorundum. So, chromium oxide increases the strength and abrasive qualities of the material. Aluminum oxide can be identified by its ruby tone.
Carborundum
- Silicon carbide is obtained by sintering graphite and silica in an Acheson electric furnace. These are crystals of various shapes, the edges of which are very sharp.
- Carborundum is harder than electrocorundum. But his fragility is higher.
- The pressure during grinding breaks the crystals. This creates new cutting edges. This feature of carborundum keeps the sandpaper working for a long time and prevents clogging of the abrasive layer.
Sanding paper with carborundum abrasive is best suited for processing plastic and glass. It can also be used for metal.
Elbor and artificial diamond
Diamond has the highest hardness of any known substance.. Elbor slightly loses to him in hardness, 3 times superior to carborundum and 4 times electrocorundum. But elbor surpasses diamond in temperature resistance.
The disadvantage of these abrasives is their high price.. Therefore, they are rarely used for the manufacture of emery skins.
Pomegranate
Garnet is a relatively soft mineral.. Its hardness on the Mohs scale is 6.4-7.5 units. Therefore, garnet abrasive is used to process soft substrates and materials. In most cases, it is wood.
Lack of pomegranate skin- fast wear.
Dignity- the abrasive layer has the same grain size. Therefore, this sandpaper grinds more smoothly than materials with other abrasives.
Sandpaper classification
The most important characteristic of a sanding pad - grain. The purpose of sandpaper can be different:
- polishing,
- grinding,
- rough preliminary stripping.
Based on it, the grain diameter can vary from 3 microns - for fine polishing, up to 1 mm - for the roughest work.
The grit size of the sandpaper is regulated by the international standard ISO No. 6344 (FEPA). Its analogue in the Russian Federation is GOST R No. 52381/2005, adopted in 2005.
According to these documents, the grain size of the material is indicated by the letter P and a number from 2500 to 12. The higher the number, the smaller the diameter of the grains. The number itself indicates the number of wires per sieve per inch.
In the CIS countries, the Soviet GOST No. 3647/80 is still used:
- According to it, the number indicates the minimum grain diameter in microns. It is followed by the letter N.
- On a fine skin, the letter M is first put, followed by a number indicating the size of the grains in microns.
Some of the countries have different standards:
- In Canada and the USA - American National Standards Institute (ANSI);
- In China, GB-2478;
- In Japan - Japanese Industrial Standard (JIS).
Grades of sanding paper by grain size
Sandpaper is marked with two alphanumeric groups. They are printed with ink on the reverse side of a roll or sheet. The first of them speaks of the graininess of the material. Types of sandpaper on this basis are presented in the tables.
coarse paper
Coarse-grained skin is intended for rough work.
| Coarse-grained emery cloth | |||
| purpose | Marking according to ISO No. 6344 | Grain diameter in micrometers | |
| The roughest job | R-22 | 80-N | 1000-800 |
| R-24 | 63-N | 800-630 | |
| R-36 | 50-N | 630-500 | |
| rough work | R-40 | 40-N | 500-400 |
| R-46 | 32-N | 400-315 | |
| R-60 | 25-N | 315-250 | |
| Primary grinding | R-80 | 20-N | 250-200 |
| R-90 | 16-N | 200-160 | |
| R-100 | 12-N | 160-125 | |
| R-120 | 10-N | 125-100 | |
| Final sanding of softwoods, old paintwork for painting | R-150 | 8-N | 100-80 |
| R-180 | 6-N | 80-63 | |
Fine-grained skin
Finer sandpaper is for fine work.
| Fine-grained emery cloth | |||
| purpose | Marking according to ISO No. 6344 | Marking according to GOST No. 3647/80 | Grain diameter in micrometers |
| Final sanding of hardwoods, sanding between coats | R-240 5 | M-63 and 5-N | 63-50 |
| R-280 | M-50 and 4-H | 50-40 | |
| Finish polishing, sanding between paint coats, wet sanding | P-400 | M-40 and 3-N | 40-28 |
| P-600 | M-28 and 2-N | 28-20 | |
| Sanding plastic, metal and ceramics, wet sanding | P-1000 | M-20 and 1-H | 20-14 |
| Finest polishing and sanding | P-1200 | M-14 | 14-10 |
| P-1500 | M-10 and 0-H | 10-7 | |
| P-2000 | M-7 and 01-N | 7-5 | |
| R-2500 | M-5 and 00-N | 5-3 | |
Skin marking according to the state standard
The second marking indicates all the data about the sandpaper. It can be printed with ink or placed on the product label. I will give an example: L2E600 × 40P125A25-L1MA GOST No. 13344/79. I will decipher:
- The first letter L means that the sandpaper is sheet. Roll material is not marked in any way.
- The number 2 indicates the type of paper. In our case, it is designed for grinding on metal. The number 1 would mean that the skin is used for processing materials with low hardness.
- The letter E indicates that the abrasive was applied by the electrostatic method. The letter M would mean a mechanical way.
- 600×40 are the dimensions of the sheet, its width and length in millimeters. For a rolled skin, its width is indicated in millimeters, and the length in meters.
- The alphanumeric group L1 indicates the basis of the material. In our case, it is moisture-resistant paper.
- Marking P2 is a paper base 0-200.
- Moisture-resistant paper is designated M, L1 and L2.
- Paper not resistant to water is marked P1 and P11.
- The twill warp is designated S2G, S1G, S1, U2G, U1G, U2 and U1.
- Semi-double fabric is marked with the letter P.
- The alphanumeric group 25A indicates the type and brand of abrasive. In our case, it is white electrocorundum. It can also be referred to as 24A.
- Marking 15A indicates normal electrocorundum.
- 45A and 43A - mono-corundum.
- 55C, 54C and 53C - black carborundum.
- 62C, 63C and 62C - green silicon carbide.
- 81Kr - flint.
- 71St indicates glass abrasive.
- The number 25 indicates the diameter of the grains of the main fraction of the abrasive in micrometers.
Fine-grained skin is marked in this part with an alphanumeric group from M3 to M63.
- -H indicates the content of the main fraction of the abrasive. In our case, it is 55 percent.
- The letter B will indicate 60%.
- Letter D - by 41%.
- The letter H - by 45%.
- The next letter indicates what composition the abrasive material is glued to. In my example, this is M. So, hide glue was used.
- The letter C will indicate a synthetic composition.
- K means a combined mixture of skin and synthetic glue.
- YAN-15 indicates amber lacquer.
- SFK stands for phenol-formaldehyde resin.
- The last letter indicates the wear resistance class of the skin in relation to the presence of defects. In my example, this is class A - less than 0.5% of the defective surface of the material.
- The letter B means less than 2%.
- The letter B indicates less than 3% of defects on paper.
- GOST No. 13344/79 indicates the standard according to which the material is made. A moisture-resistant sandpaper is produced on it. The material not resistant to water is produced in accordance with GOST No. 6456/82.
Classification by abrasive distribution method
- Semi-open and open filling. With this method of distribution, the abrasive covers from 60 to 40% of the base. Such a filling excludes the filling of gaps between the grains with grinding waste. Lumps on the skin are not formed.
Sandpaper with open and semi-open coating is optimal for sanding soft and loose materials. For example, resinous woods, putty surfaces.
- Solid (closed) backfill. With this method, the abrasive is applied to the entire base. This sandpaper is optimal for grinding hard materials. For example, hardwoods, metals.
Sandpaper base materials
The type of sandpaper depends on the material of its base.
paper base
Paper for abrasive material should be as strong as possible. So it will withstand mechanical stress longer. Paper is classified based on density in grams per square meter. This classification has been adopted according to ISO No. 6344. Marking is carried out by letters.
Paper can be ordinary and moisture resistant. Its water resistance can also be increased by the adhesive used for the abrasive.
Pros:
- low cost;
- when grinding, it does not lengthen;
- the surface of the paper makes it possible to apply the smallest grains of abrasive on it.
Minuses:
- low strength;
- low wear resistance.
Fabric base
Polyester or cotton are most often used as the basis for sandpaper. They are impregnated with polyester resins. Impregnation gives the material moisture resistance and increases its strength.
Tear strength and elasticity- these are the main characteristics of fabric bases. According to these parameters, they are divided into four classes:
- Group J fabrics are used for finishing sanding profiles and edges.
- Class X - used for heavy and dirty work.
- Fabrics of groups Y and W - are used if maximum strength of the abrasive material is needed. For example, during industrial grinding of cladding panels.
When choosing a fabric-based skin, prefer its most rigid appearance - such as the upcoming grinding and the shape of the surface to be sanded will allow. The durability of the abrasive material depends on the rigidity of the base.
Grinding discs are most often made on a fiber basis.
The fiber base is intended for the production of grinding wheels. It is produced by treating cellulose with zinc chloride. The result is a dense and hard material.
Lack of fiber- It is not moisture resistant, as it absorbs water strongly.
Production of sandpaper
In the production of sandpaper, two methods of applying abrasive material are used.
Methods for applying abrasive grains
- electrostatic method. The negatively charged abrasive material is attracted in an electrostatic field to the adhesive layer deposited on the base. Under the influence of the field, the particles are pressed into the binder. The grains are placed vertically and look down with the tip.
Advantage of the method- the abrasive layer created by electrostatic means is more aggressive. It makes it possible to grind off more material (compared to a mechanically created abrasive layer) with the same physical effort.
- mechanical way. Abrasive particles under the action of gravity fall on the adhesive layer applied to the base. They are arranged in random order.
The disadvantage of the method- the abrasive layer created mechanically is less aggressive.
Adhesives
Synthetic and natural adhesives of different brands are used for the production of sanding paper. The type of binder affects the strength and use of sandpaper.
The main tasks of glue- hold the abrasive on the base and remove heat from it during grinding. The fixation strength of the grains in the adhesive layer should be greater than the strength of the particles themselves.
The type of adhesive depends on the elasticity and rigidity of the sandpaper and, to some extent, moisture resistance. Manufacturers often add special additives to the binder. They give the material certain qualities. For example, anti-loading, antistatic properties.
- Epoxy, urea-formaldehyde and phenol-formaldehyde resins, as well as amber varnish- the most common types of synthetic adhesives.
- Skin glue- the most commonly used natural composition. The skin produced with it is not waterproof. Therefore, it is not suitable for wet work.
Features of the use of material
When sanding surfaces, follow the universal rule: you need to start processing with coarser sandpaper. Gradually change it to a skin with a thinner abrasive layer.
- Very coarse and coarse abrasive used for pre-treatment of wood, removal of the old paint layer and corrosion centers from metal.
- Very thin and thin skin It is used for various stages of fine grinding and polishing of surfaces.
- For plastic best suited emery material coated with carborundum.
- For wet processing you need to use a waterproof type of sandpaper with an abrasive layer P-600 / P-400. Sanding in an aqueous environment makes it possible to achieve greater surface smoothness and prevents dust formation.
For manual processing of volumetric surfaces of complex and curvilinear shape, it is most convenient to fix the sandpaper on a thick piece of soft rubber.
Small instruction:
- To work with sanding paper was more comfortable, wrap it around the bar. It can be of any suitable material - foam, plastic or wood;
- Pre-glue or nail a piece of porous rubber or felt to the bar.
With such a simple device, the quality of your work will increase. You will be able to control the pressure when sanding, and productivity will increase.
Machine application
Sandpaper serves as a consumable for such devices:
- Surface grinding (vibration grinding) machine. On its working part, rectangular sheets of skin are fixed with Velcro or in clamps. The device may have an outlet for a dust collector.
- delta grinder. Its triangular shape resembles the Greek letter Δ (delta). The device makes it possible to grind surfaces in hard-to-reach areas. On it, sheets of sandpaper in the form of a triangle are fixed with Velcro. The device is equipped with a dust collector outlet.
- Orbital (eccentric) sander. On it the skin in the form of a circle is fixed with Velcro. The machine has a dust collector outlet.
- Angle grinder (grinder) and an electric drill with an additional nozzle. The emery wheels on the support plate are fixed with Velcro. Or have a hole in the center for the clamping washer. This tool does not have a dust collector outlet.
- Universal vibrating cutter. On its support platform, triangular sheets of sanding paper are fixed with Velcro. The machine does not have an outlet for the dust collector.
- Belt sander. Cuts of the rolled skin are fixed on the roller and the drive shaft of the device. The device does not have an outlet for the dust collector.
Conclusion
Sandpaper is the best material for sanding. To make surface treatment efficient and productive, choose the most suitable skin for your case - I talked about the types and features of sandpaper.
The video in this article will continue to introduce you to the topic. If you have any questions, ask them in the comments.
Grinding wheels are characterized by a geometric shape (type), type of abrasive material, its grain size, bond type, hardness, etc. And when choosing a grinding wheel, such characteristics as hardness or structure may be more significant than the type of abrasive.
Complete marking of grinding wheels contains:
- circle type;
- its dimensions;
- type of abrasive material;
- grain number;
- degree of hardness;
- structure (ratio between abrasive, bond and pores in the tool body);
- type of ligament;
- maximum speed;
- accuracy class;
- imbalance class.
The marking of circles, made in accordance with various editions of GOSTs, has some differences regarding the designations of grain size, hardness, grade of abrasive and bond. Manufacturers label their wheels differently, using old or new designations and omitting some features. Below are examples of deciphering the designations of grinding wheels.
3 - hardness: K - medium soft;
4 - structure: 6 - medium;
6 - unbalance class: 2
1 - abrasive material: 25A - white electrocorundum;
2 - grain size (old marking): 60 (according to GOST it should be 63) - 800-630 microns;
3 - hardness: K-L - depending on the circumstances, it can be K or L - medium soft;
4 - bundle: V - ceramic.
1 - abrasive material: 25A - white electrocorundum;
2 - grain size (old marking): 25 - 315-250 microns;
3 - hardness (old marking): CM2 - medium soft;
4 - structure: 6 - medium;
5 - bundle (old marking): K - ceramic;
6 - accuracy class: B
7 - unbalance class: 3
1 - abrasive material: 25A - white electrocorundum;
2 - grain size: F46 - average size 370 microns;
3 - hardness: L - medium soft;
4 - structure: 6 - medium;
5 - bundle: V - ceramic;
6 - circumferential speed: 35 m/s;
7 - accuracy class: B
8 - unbalance class: 3
1 - abrasive material: 14A - normal electrocorundum;
2 - grain size: F36-F30 - extended range including F36 (average size 525 microns) and F30 (average size 625 microns);
3 - hardness: Q-U - depending on the circumstances, it can be medium hard, hard, very hard;
4 - bundle: BF - bakelite with the presence of reinforcing elements;
5 - unbalance class: 1
The choice of brand of a grinding wheel should be done taking into account all its characteristics.
Types of grinding wheels and their size
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The following types of grinding wheels are produced (the designations are given in brackets according to the old GOST 2424-75):
- 1 (PP) - straight profile;
- 2 (K) - ring;
- 3 (3P) - conical;
- 4 (2P) - double-sided conical;
- 5 (PV) - with one-sided undercut;
- 6 (ChTs) - cup cylindrical;
- 7 (PVD) - with two grooves;
- 9 - with bilateral undercut;
- 10 (PVDS) - with a bilateral undercut and a hub;
- 11 (ChK) - cup conical;
- 12 (T) - poppet;
- 13 - poppet;
- 14 (1T) - poppet;
- 20 - with one-sided conical groove;
- 21 - with a double-sided conical groove;
- 22 - with a conical groove on one side and cylindrical on the other;
- 23 (PVK) - with conical and cylindrical grooves on one side;
- 24 - with a conical and cylindrical undercut on one side and a cylindrical undercut on the other;
- 25 - with conical and cylindrical grooves on one side and conical on the other;
- 26 (PVDK) - with conical and cylindrical grooves on both sides;
- 27 - with a recessed center and reinforcing elements;
- 28 - with a recessed center;
- 35 - straight profile, working end face;
- 36 (PN) - with pressed fasteners;
- 37 - annular with pressed fasteners;
- 38 - with one-sided hub;
- 39 - with a double-sided hub.
All types are described in GOST 2424-83.
In addition to the profile shape, the circles are characterized by the size DxTxH, where D is the outer diameter, T is the height, H is the diameter of the hole.
Types of diamond and elbor wheels are regulated by GOST 24747-90. Marking the shape of CBN and diamond wheels consists of 3 or 4 characters that carry information about the shape of the section of the body, the shape of the section of the CBN or diamond-bearing layer, the location of the latter on the wheel, and the design features of the body (if any).
The designation of the grinding wheel with the shape of the body 6, the shape of the diamond or elbor containing layer A, with the location of the diamond or elbor containing layer 2, with the design features of the body C.
All types are described in GOST 24747-90.
The type and dimensions of the wheel are selected based on the type and configuration of the surfaces to be ground, as well as the characteristics of the equipment or tool used.
The choice of circle diameter usually depends on the number of revolutions of the spindle on the selected machine and on the ability to provide the optimum circumferential speed. Specific wear will be the smallest with the largest circle size in diameter. Smaller wheels have fewer grains on the working surface, each grain has to remove more material, and therefore they wear out faster. When working with circles of small diameters, uneven wear is often observed.
When choosing a diamond wheel, it is desirable to pay attention to the width of the diamond layer. When working "on the pass" it should be relatively large. When grinding using the "plunge" method, the width of the diamond coating should be commensurate with the width of the surface to be treated. Otherwise, ledges may appear on the surface of the circle.
Abrasives
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The most commonly used abrasive materials for grinding wheels are: electrocorundum, silicon carbide, CBN, diamond.
Electrocorundum available in the following grades: white - 22A, 23A, 24A, 25A(the higher the number, the higher the quality); normal - 12A, 13A, 14A, 15A, 16A; chromium - 32A, 33A, 34A; titanic - 37A; zirconium - 38A and others.
Silicon carbide. Two varieties of silicon carbide are produced: black - 52C, 53C, 54C, 55C and green- 62C, 63C, 64C, differing from each other in some mechanical properties and color. Green carbide is more brittle than black carbide.
Diamond It is widely used for making diamond grinding wheels used for honing and sharpening carbide tools, machining hard alloy parts, optical glass, ceramics, etc. It is also used for dressing grinding wheels made of other abrasive materials. When heated in air to 800°C, the diamond begins to burn.
Elbor(CBN, CBN, borazone, cubonite) is a cubic modification of boron nitride. Having the same hardness as diamond, it significantly exceeds the latter in heat resistance.
Abrasive materials are characterized by hardness, grain size, abrasive ability, strength, thermal and wear resistance. High hardness is the main distinguishing feature of abrasive materials. Below are comparative characteristics of microhardness and heat resistance of the main abrasive materials.
| materials | Microhardness, kgf / mm 2 |
| Diamond | 8000-10600 |
| Elbor (cubic boron nitride, KNB) | 8000-10000 |
| Boron carbide | 4000-4800 |
| silicon carbide green | 2840-3300 |
| Silicon carbide black | 2840-3300 |
| monocorundum | 2100-2600 |
| Aluminum oxide white | 2200-2600 |
| Titanium electrocorundum | 2400 |
| Chromium electrocorundum | 2240-2400 |
| Aluminum oxide normal | 2000-2600 |
| Corundum | 2000-2600 |
| Quartz | 1000-1100 |
| Titanium carbide | 2850-3200 |
| Wolfram carbide | 1700-3500 |
| Hard alloy T15K6, VK8 | 1200-3000 |
| Mineral ceramics TsM332 | 1200-2900 |
| High speed steel hardened P18 | 1300-1800 |
| Steel tool carbon sealed U12 | 1030 |
| Sealed carbon steel St.4 | 560 |
The choice of one or another abrasive material is largely determined by the characteristics of the material being processed.
| Abrasive | Application |
| Aluminum oxide normal | It has high heat resistance, good adhesion to the binder, mechanical strength of the grains and significant viscosity required to perform the operation with variable loads. Machining materials with high tear resistance (steel, ductile iron, iron, brass, bronze). |
| Aluminum oxide white | in terms of physical and chemical composition more uniform, has a higher hardness and sharp edges, has better self-sharpening and provides a lower surface roughness compared to normal electrocorundum. Processing of the same materials as normal electrocorundum. Provides less heat generation, higher surface finish and less wear. Grinding of high-speed and alloyed tool steels. Processing of thin-walled parts and tools, when the removal of heat generated during grinding is difficult (dies, gear teeth, threaded tools, thin knives and blades, steel cutters, drills, woodworking knives, etc.); parts (flat, internal and profile grinding) with a large contact area between the wheel and the machined surface, accompanied by abundant heat generation; when finishing grinding, honing and superfinishing. |
| Silicon carbide | It differs from electrocorundum in increased hardness, abrasive ability and brittleness (grains have the form of thin plates, as a result of which their fragility increases during operation; in addition, they are worse retained by the ligament in the tool). Green silicon carbide differs from black silicon carbide in increased hardness, abrasive ability and brittleness. Machining of materials with low tear resistance, high hardness and brittleness (carbide alloys, cast iron, granite, porcelain, silicon, glass, ceramics), as well as very tough materials (heat-resistant steels and alloys, copper, aluminium, rubber). |
| Elbor | It has the highest hardness and abrasive ability after diamond; has high heat resistance and increased brittleness; inert to iron Grinding and finishing of hard-to-cut steels and alloys; fine grinding, sharpening and finishing of high-speed steel tools; fine and final grinding of high-precision workpieces made of heat-resistant, corrosion-resistant and high-alloy structural steels; fine and final grinding of machine guides, lead screws, the processing of which is difficult with conventional abrasive tools due to large thermal deformations. |
| Diamond | It has high wear resistance and low heat resistance; reactive to iron; has increased brittleness and reduced strength, which contributes to self-sharpening; synthetic diamond of each subsequent brand (from AC2 to AC50) differs from the previous one in higher strength and less brittleness. Grinding and finishing of brittle and highly hard materials and alloys (hard alloys, cast irons, ceramics, glass, silicon); fine grinding, sharpening and finishing of hard-alloy cutting tools. |
Diamond wheels are able to process material of any hardness. However, it must be borne in mind that diamond is very brittle and does not withstand shock loads well. That's why diamond circles it is advisable to use for the final processing of carbide tools, when you need to remove a small layer of material, and there is no shock load on the grain. In addition, diamond has a relatively low heat resistance, so it is desirable to use it with a coolant.
Grain
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
Abrasive grit is a characteristic of grinding wheels that determines the cleanliness of the resulting surface. A grain is either intergrowths of crystals, or a separate crystal, or its fragments. Like all solids, it is characterized by three dimensions (length, width and thickness), but for simplicity, they operate with one - width. Many parameters depend on the size of the grain - the amount of metal removed in one pass, the purity of processing, the productivity of grinding, the wear of the wheel, etc.
According to GOST 3647-80, in the designation of the grit size of grinding wheels, the grain size is indicated in units equal to 10 microns (20 = 200 microns), for micropowders - in microns with the addition of the letter M.
In the new GOST R 52381-2005, which basically corresponds to the international FEPA standard, the grit size of grinding powders is indicated by the letter F with a number. The higher the number, the finer the grain and vice versa.
Diamond and elbor wheels have their own grain size designations. Their granularity is indicated by a fraction, the numerator value of which corresponds to the size of the side of the upper sieve in microns, and the denominator - the lower sieve.
The table below shows the grit ratios of grinding wheels according to old and current standards.
| Designation according to GOST 3647-80 | Designation according to GOST 9206-80 (diamond powders) | Size, microns | FEPA | |
| Designation for abrasive materials, excluding materials with a flexible backing | Average size, microns | |||
| F4 | 4890 | |||
| F5 | 4125 | |||
| F6 | 3460 | |||
| F7 | 2900 | |||
| 200 | 2500/2000 | 2500-2000 | F 8 | 2460 |
| F10 | 2085 | |||
| 160 | 2000/1600 | 2000-1600 | F 12 | 1765 |
| 125 | 1600/1250 | 1600-1250 | F 14 | 1470 |
| 100 | 1250/1000 | 1250-1000 | F 16 | 1230 |
| F 20 | 1040 | |||
| 80 | 1000/800 | 1000-800 | F22 | 885 |
| 63 | 800/630 | 800-630 | F24 | 745 |
| 50 | 630/500 | 630-500 | F 30 | 625 |
| F 36 | 525 | |||
| 40 | 500/400 | 500-400 | F 40 | 438 |
| 32 | 400/315 | 400-315 | F 46 | 370 |
| 25 | 315/250 | 315-250 | F 54 | 310 |
| F 60 | 260 | |||
| 20 | 250/200 | 250-200 | F 70 | 218 |
| 16 | 200/160 | 200-160 | F 80 | 185 |
| 12 | 160/125 | 160-125 | F 90 | 154 |
| F 100 | 129 | |||
| 10 | 125/100 | 125-100 | F 120 | 109 |
| 8 | 100/80 | 100-80 | F 150 | 82 |
| 6 | 80/63 | 80-63 | F 180 | 69 |
| 5, M63 | 63/50 | 63-50 | F 220 | 58 |
| F 230 | 53 | |||
| 4, M50 | 50/40 | 50-40 | F 240 | 44,5 |
| M40 | 40/28 | 40-28 | F 280 | 36,5 |
| F 320 | 29,2 | |||
| M28 | 28/20 | 28-20 | F 360 | 22,8 |
| M20 | 20/14 | 20-14 | F 400 | 17,3 |
| M14 | 14/10 | 14-10 | F 500 | 12,8 |
| M7 | 10/7 | 10-7 | F 600 | 9,3 |
| M5 | 7/5 | 7-5 | F 800 | 6,5 |
| M3 | 5/3 | 5-3 | F 1000 | 4,5 |
| 3/2 | 3-2 | F 1200 | 3,0 | |
| 2/1 | 2-1 | F 1500 | 2,0 | |
| F 2000 | 1,2 | |||
| 1/0 | 1 and | |||
| 1/0,5 | 1-0,5 | |||
| 0,5/0,1 | 0,5-0,1 | |||
| 0,5/0 | 0.5 and | |||
| 0,3/0 | 0.3 and | |||
| 0,1/0 | 0.1 and | |||
The choice of the grain size of the wheel should be determined by a number of factors - the type of material being processed, the required surface roughness, the amount of allowance to be removed, etc.
The smaller the grain size, the cleaner the surface to be treated. However, this does not mean that in all cases preference should be given to a smaller grain size. It is necessary to choose the grain size that is optimal for a particular processing. Fine grain gives a higher surface finish, but at the same time can lead to burning of the processed material, clogging of the wheel. When using fine grit, grinding performance is reduced. In the general case, it is advisable to choose the largest grain size, provided that the required surface cleanliness is ensured.
If it is necessary to reduce the surface roughness, the grain size must be reduced. Large allowances and increased productivity require increased grit.
In general, the harder the workpiece material and the lower its viscosity, the higher the grain size of the wheel can be.
| Grit numbers according to GOST 3647-80 | Grit numbers according to GOST R 52381-2005 | Purpose |
| 125; 100; 80 | F14; F16; F20; F22 | Editing of grinding wheels; manual peeling operations, cleaning of blanks, forgings, welds, castings and rolled products. |
| 63; 50 | F24; F30; F36 | Preliminary round external, internal, centerless and flat grinding with surface roughness of the 5-7th purity class; finishing of metals and non-metal materials. |
| 40; 32 | F40; F46 | Preliminary and final grinding of parts with surface roughness of the 7th-9th cleanliness class; sharpening of cutting tools. |
| 25; 20; 16 | F54; F60; F70; F80 | Fine grinding of parts, sharpening of cutting tools, preliminary diamond grinding, grinding of shaped surfaces. |
| 12; 10 | F90; F100; F120 | Fine grinding of diamonds, sharpening of cutting tools, finishing grinding of parts. |
| 8; 6; 5; 4 | F150; F180; F220; F230; F240 | Finishing of cutting tools, thread grinding with fine thread pitch, finishing grinding of parts made of hard alloys, metals, glass and other non-metallic materials, fine honing. |
| M40-M5 | F280; F320; F360; F400; F500; F600; F800 | Final finishing of parts with an accuracy of 3-5 microns or less, roughness of the 10th-14th grade of cleanliness, superfinishing, final honing. |
Grinding wheel hardness
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The hardness of the grinding wheel should not be confused with the hardness of the abrasive material. These are different concepts. The hardness of the grinding wheel characterizes the ability of the bond to keep abrasive grains from being pulled out under the influence of the material being processed. It depends on many factors - the quality of the bond, the type and shape of the abrasive, the manufacturing technology of the circle.
The hardness of the wheel is closely related to self-sharpening - the ability of the abrasive wheel to restore its cutting ability due to the destruction or removal of blunt grains. Wheels in the process of work are intensively self-sharpening due to the splitting of cutting grains and their partial chipping from the bond. This ensures that new grains enter the work, thereby preventing the appearance of burns and cracks in the processed material. The lower the hardness of the wheel, the higher the self-sharpening. By hardness circles are divided into 8 groups.
| Name | Designation according to GOST 19202-80 | Designation according to GOST R 52587-2006 |
| Quite soft | VM1, VM2 | F, G |
| Soft | M1, M2, M3 | H, I, J |
| Medium soft | CM1, CM2 | K,L |
| Average | C1, C2 | M, N |
| Medium hard | ST1, ST2, ST3 | O, P, Q |
| Solid | T1, T2 | R, S |
| Quite hard | WT | T, U |
| Extremely hard | Thu | V, W, X, Y, Z |
The choice of hardness of the grinding wheel depends on the type of grinding, the accuracy and shape of the parts to be ground, the physical and mechanical properties of the material being processed, the type of tool and equipment. In practice, in most cases, medium-hard wheels are used, which have a combination of relatively high productivity and sufficient durability.
A slight deviation of the characteristics of the circles from the optimal one leads either to burns and cracks of the surface being sharpened, when the hardness of the circle is higher than required, or to intensive wear of the circle and distortion of the geometric shape of the tool being sharpened, when the hardness of the circle is insufficient. Particularly precisely in terms of hardness, wheels for sharpening tools with carbide inserts should be selected.
Here are some guidelines that may be helpful when choosing grinding wheels for hardness. When sharpening tools with carbide cutters, the wheel must have a high self-sharpening capability. Therefore, when sharpening them, circles of low degrees of hardness are used - H, I, J (soft), less often K. The more tungsten or titanium carbides in the hard alloy, the softer the grinding wheel should be.
When it is required to maintain high accuracy of shape and size, preference is given to those types of grinding wheels that have increased hardness.
With the use of cutting fluids, harder wheels are used when grinding than when grinding without cooling.
Bakelite bonded wheels should have a hardness 1-2 steps higher than ceramic bonded wheels.
To prevent the appearance of burns and cracks, softer circles should be used.
Structure
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The structure of the tool is usually understood as the percentage of the volume of abrasive material per unit volume of the tool. The more abrasive grain per unit volume of the wheel, the denser the structure of the tool. The structure of the abrasive tool affects the amount of free space between the grains.
When sharpening cutting tools, it is desirable to use wheels with a freer space between the grains, as this facilitates the removal of chips from the cutting zone, reduces the possibility of burns and cracks, and facilitates cooling of the tool being sharpened. For sharpening cutting tools, circles are used on a ceramic bond of the 7-8th structure, on a bakelite bond - of the 4-5th structure.
Bundle
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
In the manufacture of grinding wheels, abrasive grains are bonded to the base and to each other with a bond. The most widely used bonds are ceramic, bakelite and volcanic.
ceramic bond It is made from inorganic substances - clay, quartz, feldspar and a number of others by grinding and mixing them in certain proportions. Vitrified bonded grinding wheels are marked with the letter ( V). Old designation - ( To)
The ceramic bond gives the abrasive tool rigidity, heat resistance, shape stability, but at the same time increased brittleness, as a result of which it is undesirable to use wheels with a ceramic bond under shock loading, for example, in rough grinding.
bakelite bond mainly consists of artificial resin - Bakelite. The marking of circles with bakelite has a Latin letter in the designation ( B). Old designation - ( B). In comparison with the ceramic bond, the bakelite bond has greater resilience and elasticity, heats the metal being processed less, but has lower chemical and temperature resistance, and worse edge resistance.
Bakelite bond can be with reinforcing elements ( bf, old designation - BOO), with graphite filler ( B4, old designation - B4).
Vulcanite bond is a vulcanized synthetic rubber. The marking of the abrasive wheel has the letter ( R). Old designation - ( AT).
In most cases, ceramic or bakelite bonded abrasive wheels are used. Both have their own characteristics, which determine their choice for a particular job.
The advantages of a ceramic bond include strong fixation of the grain in the bond, high thermal and wear resistance, good retention of the working edge profile, and chemical resistance. The disadvantages are increased brittleness, reduced bending strength, high heat generation in the cutting zone, and, consequently, a tendency to burn the material being processed.
The advantages of the bakelite bond are elasticity, good self-sharpening of the wheel due to the reduced strength of the grain in the bond, and reduced heat generation. Disadvantages - more intense wear compared to the ceramic bond, reduced edge resistance, low resistance to coolants containing alkalis, low heat resistance (bakelite begins to become brittle and burn out at temperatures above 200 ° C).
Accuracy class
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The accuracy of the dimensions and geometric shape of abrasive tools is determined by three classes AA, BUT and B. For less critical abrasive operations, a tool of the class B. More accurate and high quality is a class tool BUT. To work in automatic lines, on high-precision and multi-circular machines, high-precision tools are used AA. It is distinguished by a higher accuracy of geometric parameters, uniformity of the grain composition, balance of the abrasive mass, and is made from the best grades of grinding materials.
Unbalance class
| 1 | 150x16x32 | 25A | F46 | L | 6 | V | 35 | B | 3 |
The unbalance class of the grinding wheel characterizes the unbalance of the mass of the wheel, which depends on the accuracy of the geometric shape, the uniformity of the mixing of the abrasive mass, the quality of pressing and heat treatment of the tool during its manufacture. Four classes of permissible unbalance of the mass of circles are established ( 1 , 2 , 3 , 4 ). Unbalance classes are not related to the accuracy of balancing wheels assembled with flanges before installing them on a grinding machine.
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Having understood the technology of sandpaper production, you can easily find the right sheet. Conventionally, it is divided into three groups: coarse-grained, medium-grained and close to zero for finishing. There are other subtleties, not taking into account which, you can ruin the material or abrasive.
Choose graininess. From removing old paint to polishing glassware
There is a marking on the back of the sandpaper, but it may differ depending on the manufacturer and year. It is better to buy it by inspecting it personally, rather than trusting inexperienced people or ordering it on the Internet. If this is not possible, rely on a combination of indicators, not numbers. Since the same grain size can be indicated by three different markings: P 800-21.8, 400-23 and J 700-21. We list all options in the table.
Table No. 1. Grain marking
| USSR | Modern marking | Purpose | ||||
| GOST-3647-80 | Size, microns | Educational materials are not flexible | Size, microns | Educational materials on a flexible basis | Size, mk | |
| F4 | 4890 | Rough cleaning of seams or removal of old coating | ||||
| F5 | 4125 | |||||
| F6 | 3460 | |||||
| F7 | 2900 | |||||
| 200 | 2500/2000 | F 8 | 2460 | |||
| F10 | 2085 | |||||
| 160 | 2000/1600 | F12 | 1765 | P12 | 1815 | |
| 125 | 1600/1250 | F 14 | 1470 | |||
| 100 | 1250/1000 | F 16 | 1230 | P16 | 1324 | |
| F 20 | 1040 | P20 | 1000 | |||
| 80 | 1000/800 | F22 | 885 | |||
| 63 | 800/630 | F24 | 745 | P 24 (24) | 764 (708) | Surface leveling |
| 50 | 630/500 | F 30 | 625 | P 30 (30) | 642 (632) | |
| F 36 | 525 | P 36 (36) | 538 (530) | |||
| 40 | 500/400 | F 40 | 438 | P 40 (40) | 425 (425) | |
| 32 | 400/315 | F 46 | 370 | |||
| 25 | 315/250 | F 54 | 310 | 60 | 265 | |
| F 60 | 260 | P60 | 269 | |||
| 20 | 250/200 | F 70 | 218 | P80 | 201 | |
| 16 | 200/160 | F 80 | 185 | P 100 (80) | 162 (190) | |
| 12 | 160/125 | F 90 | 154 | Rough surface sanding to remove scratches | ||
| F 100 | 129 | P120 (120) | 125 (115) | |||
| 10 | 125/100 | F 120 | 109 | P 150 (150) | 100 (92) | |
| 8 | 100/80 | F 150 | 82 | P 180 (180) | 82 (82) | |
| 6 | 80/63 (80-63) | F 180 | 69 | P 220 (220) | 68 (68) | |
| 5 M63 | 63/50 (63-50) | F 220 | 58 | P240 (240) | 58,5 (58,5) | |
| F 230 | 53 | P 280 (J 280) | 52,2 (52) | |||
| 4 M50 | 50/40 (50-40) | F 240 | 44,5 | P 320 (J 320) | 46,2 (46) | |
| P 360 (J 360) | 40,5 (40) | |||||
| M40 | 40/28 (40-28) | F 280 | 36,5 | P 400 (320 or J 400) | 35 (36 or 34) | Removing traces of rough grinding, sharpening metal for the blade |
| F 320 | 29,2 | P 500 (360 J 500) | 30,2 (28) | |||
| M 28 | 28/20 (28-20) | F 360 | 22,8 | P 600 (J600) | 25,8 (24) | |
| P 800 (400 J 700) | 21,8 (23 21) | |||||
| M20 | 20/14 (20-14) | F 400 | 17,3 | P 1000 (500 J 800) | 18,3 (20 18) | |
| P 1200 (600 J 1000) | 15,3 (16 15,5) | |||||
| M14 | 14/10 (14-10) | F 500 | 12,8 | P 1500 (800, J 1200) | 12,6 (12,6, 13) | Finishing grinding of products to perfect condition, processing of the blade after sharpening |
| P 2000 (1000, J 1500) | 10,3 (10,3, 10,5) | |||||
| M7 | 10/7 (10-7) | F 600 | 9,3 | P 2500 | 8,4 | |
| M5 | 7/5 (7-5) | F 800 | 6,5 | 1200 (J2000) | 5,5 (6,7) | |
| J2500 | 5,5 | |||||
| M3 | 5/3 (5-3) | F 1000 | 4,5 | J 3000 | 4 | |
| 3/2 (3-2) | F 1200 | 3 | J4000 | 3 | ||
| 2/1 (2-1) | F 1500 | 2 | J6000 | 2 | ||
| F 2000 | 1,2 | J8000 | 1,2 | |||
| 1/0,5 (1-0,5) | ||||||
| 0,5/0,1 (0,5-0,3) | ||||||
| 0,3/0,1 (0,3-0,1) | ||||||
| 0.1 and< | ||||||
In all situations, choose at least three variations and start with a larger grain, and end with the smallest.
Material processing options:
- glass, plastic and stone use the wet sanding method, so choose sheets with a moisture-resistant backing. If there are chips, start with 3 thousand microns, with light scratches from 1500 microns. Then move on to 1k or 600um and finish with 100 or 30um. To restore gloss, GOI paste is used. On lenses, lenses or screens, more gentle formulations;
- wood and plaster surfaces - it is better to choose smaller fractions so as not to leave deep scratches. You can start from 1 thousand microns and finish from 30 and below;
- iron - polished in different sizes, depending on the tasks. To give the desired shape, they take the coarsest fractions from 4890 microns and finish with zeros. As a rule, 4-5 intermediate options are used. Coarse abrasives for soft metals such as gold and tin should not be used;
- surface painted with water-based paint - finish sanding with sandpaper with a fraction close to zero. Otherwise, it will show all the small scratches.
Be careful not to confuse type and micron. If the marking shows from F4 to F22, this is coarse sandpaper, but its grain size is measured only in microns and is equal to from 4890 to 885 microns. When choosing, it is better to name the grain size, 1 µm = 0.001 mm.
Pros and cons of different bases
Some bases are not suitable for use in a humid environment, others are elastic and curl well, there is a particularly strong base or, conversely, soft on thin paper and inexpensive.
The most common bases:
- paper ones are inexpensive, do not stretch, and come in all sorts of grain sizes, but they are not strong enough. It can be waterproof, but inferior to fabric in these properties;
- fabric - elasticity is their main plus and minus. On the one hand, the base easily takes the form of any product, on the other hand, the abrasive crumbles when strongly stretched. Durable and moisture resistant, but expensive;
- fiber - more often made for discs, but are indispensable for stone processing. It is used for very hard materials;
- combined - layers of fabric and paper are glued together and an abrasive is applied to the base. It has the advantages of paper and fabric, but has a high price.
The base also has its own marking, which can be seen on the article or on the reverse side. The values can be viewed in the table.
Table number 2. Marking the bases
Types and methods of applying abrasive
When choosing, pay attention to the type of application, some of them are more suitable for products with a fragile surface, others for grinding hard material.
Table No. 3. Marking the type of application
| Marking | Application type |
| 1 | open application |
| 3 | Stearate coating |
| 4 | Closed type of application |
Coating according to the method of application:
- by open filling - 60% of the area is covered. Due to the voids, it is suitable for crumbling material, chips and gypsum are poured through them. Such a skin is less likely to clog;
- closed filling - the abrasive closes the canvas by 100%. Choose for metal processing, as when used on a soft surface, it quickly clogs;
- mechanical method - less uniform application due to the use of gravity. Particles fall in different directions;
- with the help of electrostatics - the most "sharp" emery layers are created. The impact on each particle with an electrostatic field turns them over with a point in one direction.
The abrasive is glued to the surface with resins and glue. Some of them have waterproof or antistatic properties.
Each abrasive has its own tasks
A soft abrasive will not help when processing glass or stone, and a very hard one will make deep grooves on plastic or putty. Usually, the sharpest materials are used for rough peeling or shaping, and soft materials are used for leveling and smoothing.
Most often you can find such materials:
- diamond is the strongest and sharpest material, but expensive;
- garnet is harder than alumina, but wears out faster. Most often used for wood;
- quartz - known as "glass skin" due to its frequent use in optics and ceramics;
- aluminum oxide (emery) - with strong friction, it is updated due to chips that form new faces;
- silicon carbide - the sharpest and most affordable, replaces the properties of diamond chips. Use for metal, ceramics and paint stripping;
- electrocorundum - differs from aluminum oxide in greater strength, as it is alloyed with titanium, aluminum or chromium.
If it is impossible to determine the material externally, be guided by the letter marking.
Table No. 4. Abrasive marking
Homemade grinding sheets according to old recipes
The first sandpaper appeared not in 1833, but millennia ago. Coastal dwellers used shark skin or boiled glue on fish scales. Further, they smeared pieces of leather or cloth with it and sprinkled it with sand. Craftsmen living far from the coast extracted glue from the bones and veins of animals or used the resin of trees.
To change the impact on the treated surface, the craftsmen made several versions of the canvas. Some of the sheets had crumbled gemstones, others had sand, and still others had ground shells or plant seeds. To process large stones or objects, metal plates were created, on the hot surface of which an abrasive was applied.
Modern sheets are produced with the same type of marking, adopted throughout the world. Old sandpaper inherited or purchased from the construction market can be very different from accepted standards, so it’s better to focus on appearance, this will help to avoid mistakes.
What sandpaper to choose? It depends on what kind of work you are going to do - primary rough processing, surface leveling, grinding or polishing. To quickly make a choice, see the table:
|
GOST R 52381-2005 (Russia) |
GOST 3647-80 (USSR) |
Grain size (µm) |
Purpose |
| Coarse-grained | |||
|
Roughing |
|||
|
Rough woodwork |
|||
|
Primary grinding |
|||
|
Preparing hardwoods for sanding |
|||
| Fine-grained | |||
|
Final sanding of hardwoods |
|||
|
Polishing of final coatings |
|||
|
Grinding metal, plastics, ceramics |
|||
|
Even finer grinding, polishing |
|||
The main characteristic of "sandpaper" is graininess. The most common grain size standard is FEPA (Europe, India, Turkey, South Africa), also known as ISO 6344. This standard for the most part corresponds to the current Russian standard GOST R 52381-2005 and is marked with the letter P and numbers from 22 to 2500. The larger the number on the marking , the finer the particle size of the abrasive.
At the same time, there are also earlier GOSTs, even those that existed in the USSR, for example, 20-N or M5 / N-00 “null” - the finest sandpaper.
In addition, there are ANSI (CAMI / UAMA) and "0" grade (USA, Canada), JIS (Japan), GB (China) standards.
An equally important parameter of sanding paper (or fabric-based) is the material of the abrasive.
Currently used abrasives:
silicon carbide (carborundum);
ceramics;
pomegranate;
aluminum oxide (electrocorundum);
synthetic diamond.
Electrocorundum is the most widely used abrasive. Sandpaper with this coating is the strongest and most durable. Additional strength and an increase in the abrasive abilities of electrocorundum are given by the addition of chromium oxide in the manufacture of such a “sandpaper”. It is easily distinguished by its ruby color.
Sandpaper with carborundum. Most suitable for processing plastic, glass, fine grinding for metal.
Garnet coated paper. Wears out faster due to the fact that garnet is a relatively soft mineral. Therefore, it is used for grinding softer materials, such as wood. Leaves behind a smoother surface.
Grinding paper is also distinguished by the type of abrasive application (filling).
Open (or semi-open) coating - the abrasive covers 40-60% of the base surface. This type of backfill eliminates the formation of lumps on the abrasive surface. Suitable for processing loose materials: putty surfaces, wood.
Closed coating (or solid) - suitable for grinding hard materials: hard wood, metal.