Depressed Center Grinding Wheels: Type 27, 41 and 42 Explained

Every bonded disc carries a type number, and it is not a catalogue code — it is the shape, and the shape is the safety instruction. A Type 27 has a raised hub and a dished face, made to grind at an angle. A Type 41 is dead flat and thin, made to cut on its edge and nothing else. Put a Type 41 flat against the work and you are loading a 1 mm disc sideways, which is exactly how discs come apart.

The three type numbers you will actually meet

The distinction that matters on a busy site: Type 27 grinds, Types 41 and 42 cut. The depressed centre on a Type 42 is not there to let it grind — it is there to keep the clamp nut clear of the workpiece when cutting flush, so the disc can sink deeper before the flange touches. It is still a cutting disc, still edge-only.

Size, thickness and max RPM chart

Diameter sets the maximum RPM, because bonded abrasives are designed around a rim speed limit of 80 metres per second. Bigger disc, same rim speed, lower permitted RPM:

The 100 mm size dominates India and Southeast Asia because the compact grinders that take it are what most workshops own; 115 mm rules the Middle East, Africa and Europe. That single fact decides half of a container mix — full machine-and-disc pairing is in the angle grinder disc size guide.

Thickness: what each one is for

• 1.0 mm — thin-wall stainless tube, threaded rod, sheet. Least heat into the work, so the least discolouration on stainless, and the fastest cut. Also the least forgiving of a twisted grinder.
• 1.6 mm — the general workshop cutting disc. Enough body to survive a slight wobble, still fast.
• 2.5–3.2 mm — rebar, angle iron, heavy section. Slower and hotter, but it will not snatch in a deep cut.
• 6.0 mm — grinding, not cutting. This is Type 27 territory: weld caps, edge preparation, removing tack welds.

For stainless, thickness is only half the story — the disc must also be INOX-rated, meaning under 0.1% iron, sulphur and chlorine. Contaminant-free discs exist because free iron embedded in a stainless surface rusts, and a customer who sees rust spots on new stainless blames the fabricator, not the disc. That is the single most useful thing a hardware counter can explain to a fabrication buyer.

From our range

Industrial 4.5" (115mm) Ultra-Thin 1.0mm Cut-Off Wheel for INOX – OEM

115 mm × 1.0 mm INOX cut-off wheel — the thin disc for stainless where discolouration matters.

From our range

Industrial 4.5" (115mm) Depressed Center Grinding Wheel for Metal – OEM

115 mm Type 27 depressed centre wheel for weld dressing and heavy metal removal.

The bore: 22.23 mm and the M14 exception

Almost every disc from 115 mm up has a 22.23 mm bore — that is 7/8", an imperial dimension surviving in a metric world. 100 mm discs use 16 mm. The exception is discs supplied with an M14 threaded flange moulded in, which screw straight onto the spindle without flange nuts; they are used mainly on diamond blades and cup wheels for flush cutting, and they cannot be fitted to a machine with a different spindle thread. Check the spindle before ordering threaded discs in volume — M14 is standard in Europe and the Gulf, but not universal.

Grain: aluminium oxide, zirconia, ceramic

The grain argument on a counter is always cost-per-disc versus cost-per-cut, and it only lands with numbers. A zirconia flap disc that costs 60% more and lasts three times as long is cheaper on the second job. For a workshop grinding stainless daily that is an easy sale; for a homeowner grinding one gate hinge, aluminium oxide is the right answer and pushing ceramic is how you lose a customer.

What to check before the disc goes on

• Max RPM on the label ≥ the grinder’s no-load speed. Not "about the same" — equal or higher. This is the whole reason the number is printed.
• Expiry date. Resin-bonded discs are date-stamped and good for three years from manufacture. Bond strength falls off after that; the disc looks perfect and is not.
• Ring test on larger wheels, and a visual check on all of them — chips at the bore, cracks radiating from the edge, any disc that has been dropped.
• Correct flanges, both of them, and the guard fitted. Most disc-burst injuries are guard-off injuries.

The full pre-use routine, including storage humidity and why discs that live in a damp store fail early, is in the cut-off wheel safety checks.

Buying discs by the container

Bonded abrasives are the highest-turnover line in this trade and the easiest to get wrong on a first order. The mix that works for most distributors in India, Pakistan and the Gulf: 60% cut-off in 1.0 and 1.6 mm, 25% Type 27 grinding at 6 mm, 15% flap discs, weighted to 100 mm or 115 mm depending on which grinders dominate locally. Get the diameter split right and everything sells; get it wrong and half a container sits. Tell us the market and the machine mix and we will build the disc assortment against it.