Cut-off wheel types: Type 1, 41, 27 and 42 compared, with guards and speed limits

01What is a cut-off wheel?

A cut-off wheel is a thin abrasive wheel, bonded with resin and reinforced with fibreglass, that cuts metal, stone or masonry with its rim. It does not grind with its face: Cal/OSHA, HAZET and FLEX all state that cutting wheels cut with the periphery only and must not be used for side grinding.

Cut-off wheels come in two families by machine. Angle grinder wheels have a 7/8 in centre hole in the US, the same bore European makers give as 22.23 mm, or a threaded hub. Larger chop saw wheels fit a 1 in arbor and run on a dedicated abrasive saw. This page deals mainly with grinder wheels, and covers chop saw wheels where the two meet.

Three attributes decide whether a wheel is right: its type number, which sets the shape and therefore the guard; its maximum speed, which must match the grinder; and its thickness and grain, which suit the material. The sanding discs and grinder wheels hub sets cut-off wheels alongside flap discs, grinding wheels and sanding discs.

02What do the type numbers on a cut-off wheel mean?

The type number describes the wheel's shape. Type 1 and Type 41 are flat; Type 27 and Type 42 have a depressed centre; Type 27A is a depressed-centre wheel for large cutting-off machines. The number is printed on the label and decides which guard and flanges the wheel needs.

The overlap of names causes most of the confusion. US catalogues group Type 1 and Type 41 as one flat cutting-wheel shape, and European discs are often simply marked T41 alongside their size, thickness and bore. So the belief that a Type 41 is a different wheel from a Type 1 is wrong in practice: they are the same flat profile under two naming systems. The same applies to Type 27 and Type 42 cutting wheels. Bosch even labels one of its wheels Type 1A with the ISO equivalent of 41 in brackets.

03Type 1 or Type 27: which cut-off wheel shape should you use?

Use a flat Type 1 or Type 41 wheel for the deepest, squarest cut, and a depressed-centre Type 27 or Type 42 wheel when you need to cut flush against a surface. United Abrasives and Saint-Gobain (Norton) say the flat shape gives the maximum depth of cut and square, burr-free cuts.

The depressed centre exists for clearance. United Abrasives explains that the raised hub lets the lock nut sit below the face of the wheel, so the wheel can cut closer to an adjoining surface and you can see the cut better. That makes Type 27 and Type 42 cutting wheels popular for trimming bolts and brackets flush to a wall. The trade-off is a little less cutting depth, because the hub takes up part of the radius.

A Type 27 cutting wheel looks like a Type 27 grinding wheel, and that resemblance leads people to grind with it. It is still a thin cutting-only wheel. Using it to deburr or grind can make it fail, whatever its shape.

04Which guard does each wheel type need?

A flat Type 1 or Type 41 cut-off wheel needs the closed cutting guard, often called the Type 1 guard, which encloses both sides and the rim. HAZET, Milwaukee and FLEX all say the open Type 27 grinding guard supplied with most grinders is not enough for a cutting wheel. On any portable grinder, OSHA and HAZET limit the exposed part of the wheel to 180 degrees.

Depressed-centre grinding wheels, Type 28 saucer wheels and Type 29 flap discs take a Type 27-style guard with an inward-curled lip. Cup wheels need a cup guard with an adjustable skirt. The table below lists each type with its guard and the working angle the makers' labels show.

The sources disagree on one point. United Abrasives says its thin Type 27 and Type 42 cutting wheels can be used on all angle grinders under ANSI B7.1, while FLEX's wheel and guard guide pairs those wheels with the Type 1 cutting guard. Because the cutting guard covers more of the wheel, this page follows FLEX: fit the cutting guard for any cut. Some grinders come with one; Bosch, for example, supplies its GWS 12-125 with a separate clip-on cutting guard. The belief that whatever guard came on the grinder will do for cutting is the most common guard mistake.

05How fast can a cut-off wheel safely spin?

No faster than the maximum speed printed on the wheel, and that figure must be at least the grinder's no-load speed. OSHA, FLEX, HAZET and DeWalt all set the check this way round: the wheel's rating must equal or exceed the spindle speed before the wheel goes on.

Smaller wheels carry higher speed ratings than larger ones. United Abrasives and Saint-Gobain rate 4-1/2 in (115 mm) cut-off wheels at 13,300 RPM, and United Abrasives and Klingspor rate 125 mm wheels at 12,200 RPM. For the next size up, United Abrasives and Milwaukee both give 10,200 RPM. At 7 in the makers differ slightly: United Abrasives and Klingspor give 8,500 RPM, while Milwaukee's thin wheel of that size carries a higher figure, shown with the other makers' numbers in the second table below. That spread is why the wheel's own label is what counts. DeWalt's thicker cutting and notching wheel follows the same pattern as thin wheels: DeWalt rates its 4-1/2 in notching wheel at 13,300 RPM, matching United Abrasives' thin wheels of that size, so the thicker wheel carries the same rating here.

European discs are rated by rim speed rather than revolutions. Klingspor and Bosch's grinder manual give 80 m/s as the maximum peripheral speed for cutting discs on hand-held angle grinders, and the RPM on the label is that speed converted for the disc's diameter; it must not be exceeded. The size table below gives the US and European pairs side by side.

The danger is a large wheel on a small, fast grinder. A wheel that fits the spindle is not therefore safe on it: a large wheel rated for a big, slow grinder would run far beyond its rating on a small, fast one. The table below lists the disc size, spindle thread and no-load speed makers publish for grinders in our tool base, so you can compare them with a wheel's label.

06How thick should a cut-off wheel be?

Thin enough to cut fast, thick enough for the job: 0.045 in is the standard thin metal cut-off wheel for US grinder wheels, and 1.0 mm is the common thin cutting disc for small European grinders.

Thicker wheels exist for different work, not for extra safety. United Abrasives and DeWalt sell wheels of about 0.090-3/32 in for cutting and notching metal, and United Abrasives lists thicker combination wheels that can also deburr and lightly grind; those combination wheels are marked for that use, which ordinary cut-off wheels are not. For stainless, United Abrasives lists 1/16 in contaminant-free wheels. Cal/OSHA sets an upper limit of 3/16 in for the smaller cutting-off wheels used on hand-held grinders, and says the centre hole should be no larger than 1/4 of diameter.

The belief that a thicker wheel is always safer is wrong. Safety comes from the speed rating, the right guard and using the wheel only for cutting. Thickness should be matched to the task.

07Which cut-off wheel suits steel, stainless, aluminium or masonry?

Choose the grain and formulation the wheel maker lists for the material: aluminium oxide, zirconia or ceramic for carbon steel, a contaminant-free (INOX) wheel for stainless, an anti-loading aluminium formula for non-ferrous metals, and silicon carbide or a diamond blade for masonry, concrete and stone.

Aluminium oxide is the most common cutting grain and has the lowest initial cost, according to United Abrasives, DeWalt and Bosch. United Abrasives adds that ceramic wheels last a long time, cut fast and run cooler with less discolouration. For stainless, a contaminant-free wheel keeps iron, chlorine and sulphur out of the cut; United Abrasives sets its limit at 0.1 percent combined. European makers often mark these wheels INOX.

For masonry, United Abrasives uses silicon carbide grain in its abrasive wheels. Diamond blades are the other route and are mounted the same way on a grinder, but HAZET says they should be used only on the materials listed on the blade label. The guide to diamond blades for concrete compares rim styles for block, brick and slab.

The grain list above has no entry for wood, and none of the wheel makers behind this page lists a cut-off wheel for it. A 2022 study from one Argentine hospital, covering 928 patients with angle grinder hand injuries treated in 2016 to 2020, shows the cost of ignoring that: most had been cutting wood or firewood, and 15.5% of those injuries ended in finger amputation.

08Do chop saw wheels fit an angle grinder?

No. Chop saw abrasive wheels have a 1 in arbor and are far larger and slower-rated than grinder wheels, so they belong only on the abrasive saw they are made for. The common large chop saw wheel is about 3/32 in / 3.0 mm thick, and United Abrasives rates its own at 4,400 RPM.

The table below lists United Abrasives' chop saw wheels by diameter. Bosch's 355 mm disc confirms the same arbor size, though Bosch's speed rating was not shown on the page fetched. Very large Type 27A wheels for cutting-off machines are a separate class again, mounted on flat flanges.

09How do you choose and fit a cut-off wheel, step by step?

These steps follow the checks OSHA, FLEX, HAZET and the grinder makers set out for mounting a wheel.

1. Disconnect the grinder. Unplug it or remove the battery before changing anything.
2. Match diameter, hole and speed. Pick a wheel of the grinder's rated size with a matching centre hole or thread, and a printed maximum speed at least the grinder's no-load speed.
3. Check the wheel. Look for cracks and chips, read the expiry date on the metal ring, and discard any wheel that has been dropped. OSHA calls for a ring test before mounting.
4. Fit the right guard. Swap the grinding guard for the closed cutting guard before fitting a Type 1 or Type 41 wheel.
5. Mount on clean flanges. Use clean, flat, undamaged flanges of matching diameter. The wheel should slide on freely; tighten the nut only enough to hold it.
6. Run it before cutting. Stand out of line and run the wheel at operating speed with the guard on for at least 1 minute.
7. Cut straight. Cut in a straight line with the rim, without twisting, bending or jamming the wheel, and never grind with its face.

10Why do cut-off wheels break or wear out early?

Cut-off wheels most often fail through side loading: twisting the wheel in the cut, jamming it, bending it, or using its face to grind or deburr. Cal/OSHA, HAZET and FLEX all prohibit side grinding with a cutting wheel, and United Abrasives says to cut straight without twisting or jamming.

Age and storage also count. Resin-bonded discs carry a use-by date on the metal ring, and Saint-Gobain sets it 3 years after manufacture. Wheels do not last forever on the shelf: DGUV tests found that warm, damp storage can reduce bursting speed within months, while normal dry storage does not cause a safety-critical loss. Store wheels flat, dry and away from heat.

Other failures trace back to setup: a wheel run above its speed rating, a hole reamed out to fit a different spindle, dirty or mismatched flanges, or a wheel that was dropped and cracked. If a new wheel vibrates or wobbles, stop and check the mounting before cutting; the guide to grinder disc vibration covers the checks. To clean the burr after a cut, switch to a flap disc rather than turning the cut-off wheel on its side.

Twisting or jamming a wheel is also how kickback starts, and grinder kickback tends to throw the tool towards the face. A wheel that breaks up throws fragments the same way. US emergency department records show where that ends: of the injuries involving workshop grinders, buffers and polishers in CPSC's NEISS emergency department data for 2024, a category that counts angle and bench grinders together, about 48% were eye injuries, by AccessoryFit's estimate from the public case file, and foreign bodies were a frequent diagnosis. That is the case for a full face shield over safety glasses whenever a cut-off wheel is running.

12What should you read next?

Fit comes before type, so if you are unsure whether a wheel suits your grinder's spindle, the guide to grinder spindle and arbor fit covers bore sizes, threads and US-to-EU equivalents. For blending welds and cleaning up cut edges, flap discs for metal are the accessory makers intend for that work. Finer surface preparation moves to a sander, and the orbital sander disc picks cover that step. To see every accessory listed for one grinder model, use the tool accessory lookup, and the other technique guides explain saw blades, bits and abrasives.

13Questions readers ask