Grinding wheels and surface finish

November 3, 2026  ·  Indumor

Grinding wheel: a body of revolution made of hard grains held by a bond, which cuts with the edges of thousands of grains at once and renews itself as dull grains break away. Grinding: removing metal with a wheel, in cuts of hundredths of a millimeter, to bring a part to size and finish. Surface roughness: the footprint every process leaves on a surface, measured in microns or microinches. Those three definitions are the whole chapter.

How to read a wheel marking

The marking follows ISO 525 and the FEPA system and reads left to right: abrasive, grain size, grade, structure and bond. A 46 K 5 V, the gray wheel on the bench grinder, says regular aluminum oxide (A), grain 46 (medium), grade K (medium), structure 5 (medium) and vitrified bond (V). Whatever comes before the abrasive and after the bond is the maker's own code. American wheels follow ANSI B74.13, which uses the same fields in the same order.

The grade is not the hardness of the grain but how firmly the bond holds it. A soft wheel sheds its grains before they dull and keeps cutting; a hard one holds them until they glaze, and then it rubs and burns. Hence the rule that never fails: hard material, soft wheel; soft material, hard wheel.

Catalog bands, as a guide. ANSI B74.13 only distinguishes soft (A–H), medium (I–P) and hard (Q–Z): on that scale K is medium.

Which wheel for which job

The table gives catalog specifications for shop jobs. They are a guide, and the real test is still the spark and the work. A wheel that loads (the material sticks to it) wants a more open structure or a softer grade; one that burns the work wants a softer grade, a coarser grit or more coolant.

The shapes are numbered in ISO 525. A straight wheel (type 1) cuts on its periphery only: grinding on its side is the commonest way to burst one. For face work there are the cups (6 and 11) and the dish (12); for the angle grinder, the depressed-center disc (27) and the cut-off wheels (41 and 42), which are never used on their side.

Peripheral speed and maximum rpm

A wheel bursts from centrifugal force, and that force depends on peripheral speed, not on rpm. That is why the maximum speed is marked in meters per second (or sfpm) and the maximum rpm comes out of the formula with the diameter: as the wheel wears down it may run faster, never the other way round. A common vitrified wheel is marked 35 m/s; resin wheels go to 63, and the fiber-reinforced ones on angle grinders and chop saws to 80 m/s, about 16 000 sfpm.

Color stripe on the wheel (EN 12413 annex; no longer mandatory, still widely used): 50 m/s blue, 63 m/s yellow, 80 m/s red, 100 m/s green, 125 m/s blue and yellow, 140 m/s blue and red, 160 m/s blue and green. The speed on the label rules: an unmarked wheel is not mounted.

Before mounting, the ring test: hang the wheel on a finger through the hole and tap it with the handle of a screwdriver 45° off the vertical. A sound vitrified wheel rings like a bell; a cracked one sounds dead and goes in the bin. Resin and rubber wheels do not give that ring. Then blotters on both sides, matched flanges of at least a third of the diameter, the guard in place, the work rest within 1/8 in (3 mm) and the tongue guard within 1/4 in, as ANSI B7.1 and OSHA ask, and one minute running free with nobody in front. Resin wheels expire three years after manufacture.

Roughness: Ra, Rz and the N grades

A profilometer drags a stylus across a stretch of the surface and draws the profile. A mean line is fitted to it, and Ra is the average distance from that line, taken as an absolute value. Rz is the average, over five equal lengths, of the distance from the highest peak to the deepest valley in each. Rt (or Rmax) is that distance over the whole trace: a single scratch sends it up, moves Rz a little and leaves Ra untouched. That is why drawings call for Rz or Rt on sealing and fatigue surfaces.

There is no exact formula between Ra and Rz because they depend on the shape of the profile. As a guide, Rz is 4 to 7 times Ra: near 4 for turning, near 7 for grinding. The N grades of the old ISO 1302 still turn up on drawings: N6 is Ra 0.8, N7 is 1.6 and each grade doubles the one before. American drawings call for Ra in microinches, 1 µm being 39.4 µin, and the preferred values (32, 63, 125…) match the N grades.

The instrument filters out waviness with the sampling length lr, the cut-off, chosen from the expected Ra; another cut-off gives another number. When a drawing calls for a roughness, the symbol goes as in chapter 25, and its relation to the fit is in chapter 3.

The roughness each process leaves

Every process has its range, and asking for less than the process gives is asking for one more operation. Finish turning reaches Ra 0.8 with a good machine and a fresh edge; below that you grind, and below 0.1 you lap or hone. A laser leaves an edge that gets painted, plasma one that sometimes gets ground and oxy-fuel one that almost always gets machined (laser, plasma or oxy-fuel).

Values as a guide, from the ASME B46.1 chart reproduced in Machinery's Handbook; plasma and sheet laser cutting are shop figures.

In turning the theoretical roughness comes from the feed and the nose radius, and it is what you use to pick both: a 0.2 mm feed with a 0.8 mm radius gives 1.6 µm; with a 0.4 radius, twice that. The formula assumes a profile of perfect arcs; the real part adds chatter and wear.

Cutting fluids

The fluid cools, lubricates, carries the chips away and keeps rust off: neat oil lubricates and hardly cools, a synthetic does the opposite, an emulsion sits in between. Concentration is checked with a hand refractometer: the reading in degrees Brix times the product's factor, which is not 1 for semi-synthetics and synthetics. The pH is checked with strips and kept where the data sheet says, nearly always between 8.8 and 9.5: below that bacteria take over and the Monday-morning smell appears; above it, skin and paint suffer.

Values as a guide, from data sheets: the product's own sheet rules. Concentration = refractometer reading in degrees Brix × the product's factor. SN: not water-miscible; SE: water-miscible (SEM emulsifiable, SES soluble); a trailing W means already mixed.

In grinding the fluid is compulsory to keep the work from burning, and a synthetic or semi-synthetic, as a guide at 3 or 4 %, works best, aimed at the contact zone with plenty of flow: the wheel drags a layer of air that deflects the jet. A burned part (brown or blue marks) has altered hardness under the surface.

Frequently asked questions

What does A 46 K 5 V mean on a grinding wheel?

A is aluminum oxide, 46 the grain size (medium), K the grade (medium), 5 the structure (medium) and V a vitrified bond. It is the general-purpose wheel for unhardened steel on a bench grinder.

How fast can a 200 mm (8 inch) grinding wheel turn?

It depends on the rated speed: 3342 rpm at 35 m/s, 4775 at 50 m/s. A 2850 rpm bench grinder with an 8 inch wheel runs at 29.8 m/s (5900 sfpm), within the rating of any vitrified wheel.

What surface finish does turning give?

Usually Ra 0.8 to 6.3 µm (32 to 250 µin, grades N6 to N9). With a rigid machine, a fresh edge and a small feed it reaches 0.4; for less you grind, hone or lap.

Sources: ISO 525:2020 (bonded abrasive products: designation and shapes), FEPA (marking system and grain sizes), ISO 8486-1 and ISO 8486-2 (grain sizes), EN 12413:2019 (safety of bonded abrasive products), ISO 16089 (safety of stationary grinding machines), ANSI B7.1-2017 (safety code for abrasive wheels), ANSI B74.13 (wheel markings), ISO 21920-1:2021, ISO 4287, ISO 4288 and ISO 1302 (surface texture: parameters, cut-off and N grades), DIN 51385 (classification of cutting fluids), Machinery's Handbook (roughness by process).