Quick Answer
The letter on a mounted point code is a size-class family, not a shape: W is the broad workhorse group (heads roughly 3/32" to 2" on 1/8", 1/4" or 3/8" shanks), A covers larger heads on 1/4" shanks, and B is the small precision series. The number picks one specific profile, so a W220 or A11 reads the same across the major catalogues. Two catches: the printed max RPM is valid only at the catalogue's reference overhang, usually 1/2" of exposed shank, and the ring test does not apply to mounted points — ANSI B7.1 excludes them; inspect visually and spin-test instead (ANSI B7.1-2010).
This guide also covers mounted flap wheels for die grinders, the shank-mounted coated wheels used for blending and brushed finishes in the same tight spots (see Flap wheels for die grinders).
First, what counts as a mounted point
A mounted point is a small bonded-abrasive head, vitrified or resin bond, permanently fixed to a steel shank and chucked in a die grinder collet for internal grinding, deburring and detail work in geometry a disc cannot reach. Under ANSI B7.1, "mounted wheels" are wheels usually 2" (about 50 mm) in diameter or smaller, permanently mounted to mandrels or small spindles (ANSI B7.1-2010). The host tool is quick: straight die grinders commonly free-run at 18,000 to 25,000 RPM, and pencil grinders go far higher.
Reading the code: the W, A and B series decoded
Mounted-point heads follow a standardised letter-and-number system shared across the major makers, so the same code means the same point brand to brand. The decode chart:
| Series | Mandrel (shank) | Head diameter | Typical max RPM at 1/2" overhang | Typical work | Common codes |
|---|---|---|---|---|---|
| W — the broadest group: cylinders, cones, balls, tapers, trees | 1/8", 1/4", 3/8" | ~3/32"–2" | ~8,250–123,000 | Steel castings, large die work, general grinding | W187, W220, W222, W236 |
| A — larger heads, longer cylinders and points | 1/4" | ~3/4"–1-5/8" | ~16,100–76,500 | Heavy deburring, blending, edge and snag work | A1, A3, A11, A15, A31, A38, A40 |
| B — small precision points | 1/8", 1/4" | ~1/8"–3/4" | up to ~105,000 | Fine detail, fillet and radius blending, tight access | B46, B52, B136 |
RPM ranges are catalogue figures at the reference overhang. The binding number is always the one marked on the specific point in your hand.
The letter is a size class, not a geometry
Inside each family the number maps to one profile and head size. W220, W222 and W236 are all cylinders in the W group, with W187 the small-diameter sibling; A11 is one of the long A-series points for heavier deburring and blending; B52 is a typical small B head for fillet work. A few makers also publish "actual-size" shape charts where the printed silhouette equals the real point.
Within a family, the profile follows the job: cylinders for flat and surface grinding, cones for angled surfaces and chamfers, balls for concave and radius blending, and bullets or trees for tight-access detail (manufacturer data).
Shank sizes and the collet trap
- 1/4" (6 mm) — the dominant shank, used across the W and A groups and most general fabrication points.
- 1/8" (3 mm) — the smaller B and W heads, for fine detail work in small collets.
- 3/8" (10 mm) — only the largest W heads, where head mass needs a heavier shank.
Metric catalogues list the same parts as 3 mm and 6 mm shanks, but the collet must match the actual shank: 1/4" and 6 mm differ by about 0.35 mm, and a forced mismatch grips too little metal and can throw the point.
Grain, bond and grit
Grain is typically aluminum oxide for ferrous work (steel, iron, stainless) and silicon carbide for non-ferrous metals, carbide, ceramics, glass and stone, with black silicon carbide the usual choice on cast iron and brittle materials (manufacturer data).
Vitrified heads are strong, heat-tolerant and hold their form, which makes them the default for precision and cool cutting on steel, cast iron and stainless. Resin heads are more flexible, cut cooler on heat-sensitive alloys and give a smoother finish, but wear faster (manufacturer data). Run coarse grits (about 24 to 46) for aggressive stock removal, then step down to finer points (about 60 to 120 and finer) to finish and blend.
Why smaller heads carry higher RPM
The safety limit on a spinning abrasive is really a peripheral (surface) speed limit, and centrifugal stress rises with the square of speed — double the RPM and the load on the bond quadruples. Surface speed scales with diameter: SFPM = diameter in inches × RPM × 0.262. A small head has a tiny circumference, so it must spin far faster to do the same work. A 1" point at 22,000 RPM sees only about 5,764 SFPM — die grinder speeds that sound alarming are exactly what small points need.
Real catalogue figures show the spread: a common 1" × 2" W222 cylinder is rated 15,900 RPM, while a tiny 8.8 × 8.4 mm point is rated 65,000 RPM. Across the families, maxima run from roughly 8,000–16,000 RPM for the ~2" and 1-5/8" heads up to 100,000+ RPM for the smallest 1/8" heads.
Overhang derating: the printed RPM assumes a short shank
Overhang is the unsupported shank length between the collet face and the abrasive head. Published maximum RPM is normally quoted at a reference overhang, commonly 1/2", and stops applying the moment you pull the point further out. A long unsupported shank bends under centrifugal force and vibrates; a loose mandrel or excessive overhang is the dominant ejection hazard with these tools.
The derating is steep. Manufacturer data shows the same point dropping from about 40,500 RPM at 1/2" overhang to about 30,000 RPM at 1", and a heavier cast-edge point falling from ~15,900 to ~12,370 RPM over the same change (manufacturer data):
| Example point (manufacturer data) | Max RPM at 1/2" overhang | Max RPM at 1" overhang |
|---|---|---|
| Cylindrical point | ~40,500 | ~30,000 |
| Heavier cast-edge point | ~15,900 | ~12,370 |
This is not one maker's habit. Maximum permissible speed for mounted points is defined as a function of shape, shank diameter and unsupported overhang under DIN 69170 / EN 12413 and under ANSI B7.1 alike; the oSa marking requirements likewise tie the ceiling to overhang and the minimum shank length held in the collet. For how the standards fit together, see our EN 12413, oSa and ANSI explainer.
The working rules that follow:
- Bury the shank. Clamp at least 10 mm in the collet and seat it as deep as the job allows.
- Expose only the head you need. If you must reach deeper into a bore, drop the RPM; use a proper shank extension, never a half-pulled-out point.
- Check concentricity before each use. A bent or badly clamped shank shakes, and a shaking point fails.
Why the ring test does NOT apply to mounted points
The ring (tap) test — suspend a vitrified wheel, tap it lightly, listen for a clear tone — is the classic pre-mount crack check for full-size wheels under OSHA 1910.215 and ANSI B7.1. But ANSI B7.1 lists the test as not applicable to mounted wheels, small wheels 4" and under, and plugs and cones: the size, the shapes and the steel mandrel damp the sound, so a cracked point can still sound fine (ANSI B7.1-2010 §6.1.1.2). Resin-bonded points would not ring anyway — organic bonds deaden the tone at any size.
What replaces the ring test on a mounted point:
- Visual inspection for chips and cracks before every mount.
- A one-minute no-load spin test after mounting, standing clear of the plane of rotation — a cracked head usually lets go here, away from the work.
- The spindle-speed check: the tool's free-running speed must not exceed the RPM marked on the point (ANSI B7.1-2010 §6.2).
- Respect the mark on a worn point. The marked RPM, not the worn head diameter, is the safety figure; never remount a part-worn point at a higher speed unless the original peripheral speed is not exceeded (ANSI B7.1-2010 §6.2).
The user-side routine carries the whole load for a reason: under ANSI B7.1, mounted points, like wheels under 4", are exempt from the factory overspeed proof test larger wheels must pass before shipment (ANSI B7.1 Standard). No factory spin margin stands behind a point; your inspection and RPM discipline are the safety system. The same mount-match logic for full-size wheels is in our max RPM safety guide.
Where mounted points fit on the shop wall
Mounted points are the hard-cutting end of the die grinder consumable family. For the full family tree — including when a small flap wheel or a Type R / TR quick-change disc does the job better — see our mounted points and flap wheels guide. For blending in the same tight spots, small flap wheels carry much lower marked speeds than abrasive points, and the backing pads and die grinder accessories must match the shank and collet sizes above. Outfitting a fab bench from scratch? Start with the metal fabrication essentials collection. Whitby Abrasives is a supplier and stockist; the decode chart above is brand-neutral and works on any maker's catalogue.
Flap wheels for die grinders
A mounted flap wheel is a coated abrasive built from abrasive-cloth flaps arranged radially around a shank, with a non-woven variant for surface conditioning. It chucks into the same die grinder collet as a mounted point and finishes complex geometry, such as pipes, tubes, railings and hardware, where a flat disc cannot reach (manufacturer data).
These are shank-mounted wheels for die grinders. The flap discs sold for angle grinders are a separate product: a disc that threads onto the grinder's arbor, commonly 5/8-11 UNC, and runs at roughly 5,000 to 12,000 RPM. Flap wheels also come unmounted (also called "log rolls"), with a central bore that slides onto a bench-grinder or lathe spindle. For die-grinder work the shank type is the one that matters (manufacturer data).
Because the flaps radiate around a cylinder, each one strikes the work at a slightly different angle. That avoids the repeating scratch pattern a flap disc tends to leave and gives a linear, brushed finish, which is why fabricators reach for a flap wheel to match an existing brushed grain across a weld (manufacturer data).
Shank sizes and grit
Die-grinder flap wheels come in two size bands and a wide grit range (manufacturer data):
- Small: 10 to 30 mm diameter on a 3 mm or 6 mm shank.
- Inch range: 3/4" to 3" diameter with 1" or 2" face widths on a 1/4" stem. Standard sizes are 1" × 1", 1-1/2" × 1", 2" × 1" and 3" × 1".
- Grit: coarse is 36 to 60, medium 80 to 240 and fine 320. Non-woven and fine cloth reach about 400 to 800 for polishing. A common ladder is 60 then 240.
The collet rule above applies to flap wheels too: match the collet to the actual shank, 1/4" or 6 mm.
Grain for metal
Zirconia alumina is the metalworking workhorse: it self-sharpens and takes higher heat and pressure. Ceramic alumina covers the most demanding stock removal. Aluminum oxide is the general-purpose grain for soft metals and wood, and the non-woven web handles surface conditioning. On metal that generates heat, choose zirconia or ceramic (manufacturer data).
Speed: read the stamp
Flap wheel max RPM depends on diameter. Small shank wheels are rated highest, up to about 25,000 RPM, and the rating falls as diameter rises to keep peripheral speed inside the safe envelope. Ratings also vary widely by maker and construction, and two wheels of the same diameter can carry different maximum speeds. Run to the rating stamped on the specific wheel. Distributors quote a working range of roughly 60 to 80% of the rated maximum (manufacturer data).
Die-grinder consumables also sit under a low peripheral-speed ceiling. Manufacturers cite 40 m/s (DIN EN 12413) for bonded mounted points and small wheels, well below the 80 m/s of off-hand discs, so a 25 mm point tops out around 30,000 RPM. A larger flap wheel must run slower still, so always go by the RPM marked on the wheel. Fit an oversized accessory to a 25,000 RPM grinder and it is already over-speed (manufacturer data).
Mounted point or flap wheel
| Job in tight geometry | Reach for | Why |
|---|---|---|
| Hard internal grinding, weld roots, casting cleanup | Mounted point | The rigid bonded head removes metal in confined bores and cavities |
| Blending, deburring and brushed-finish matching on contours | Flap wheel | Radial flaps follow curves and leave a linear, repeat-free grain |
| Fine fillet or radius detail in tiny access | Small B-series point or small shank flap wheel | The smallest heads and wheels reach where larger tools cannot |
| Stepping coarse to fine on one surface | Both, in sequence | Rough with coarse grit, finish with a finer point or wheel |
For edge deburring that must not round the edge over, see our guide to convolute, unitized and flap deburring wheels. Sizes and grits in stock are listed in the mounted flap wheel catalogue.
FAQ
What does the letter in a mounted point code like W220 or A11 mean?
The letter is the family: a size class and mandrel diameter, not a geometry. W is the broadest group, A is larger heads on 1/4" shanks, and B is the small precision series. The number picks the exact profile inside the family, and the codes read the same across major brands.
How fast can I run a mounted point?
At or below the RPM marked on that specific point, at the overhang the catalogue assumed — commonly 1/2". Family maxima run from roughly 8,000–16,000 RPM for the biggest heads to 100,000+ RPM for the smallest, because the true limit is peripheral speed.
Why does pulling the shank further out of the collet lower the max RPM?
The unsupported shank bends under centrifugal force and vibrates; the longer the overhang, the lower the speed it can take before it whips or snaps. Manufacturer data shows a point rated ~40,500 RPM at 1/2" overhang falling to ~30,000 RPM at 1". Clamp at least 10 mm of shank in the collet and derate whenever you extend.
Does the ring test work on mounted points?
No. ANSI B7.1 lists mounted wheels as not applicable for the ring test, along with wheels 4" and smaller and plugs and cones; the mandrel and small head damp the tone. Inspect visually for chips and cracks and run a one-minute no-load spin test after mounting instead (ANSI B7.1-2010 §6.1.1.2).
What shank sizes do mounted points come in?
1/4" (6 mm) is the dominant shank across the W and A groups; 1/8" (3 mm) serves the smaller B and W heads; 3/8" (10 mm) appears only on the largest W heads. Metric and imperial pairs are catalogue equivalents, but the collet must match the actual shank.
Can I put a 6 mm shank in a 1/4 inch collet?
No. The two differ by only about 0.35 mm, and a mismatched collet grips on too little metal — the point can be thrown at speed. Swap to the collet that matches the shank exactly.
What is the difference between a mounted point and a mounted flap wheel?
A mounted point is a rigid bonded-abrasive head on a shank, used for hard internal grinding, weld roots and casting cleanup in confined bores. A mounted flap wheel is abrasive-cloth flaps arranged radially around a shank, used for blending, deburring and brushed-finish matching on contours. Both chuck into a die grinder collet.
Is a mounted flap wheel the same as a flap disc?
No. A mounted flap wheel has an integral shank and runs in a die grinder collet, and its radial flaps leave a linear, brushed finish. A flap disc threads onto an angle grinder arbor, commonly 5/8-11 UNC, and runs at roughly 5,000 to 12,000 RPM for heavier weld grinding and surface stock removal.
What grain and grit should I use for a mounted point?
Use aluminum oxide for steel, iron and stainless, and silicon carbide for non-ferrous metals, carbide, ceramics, glass and stone, with black silicon carbide on cast iron. Start with a coarse grit (about 24 to 46) for stock removal, then step to a finer point (about 60 to 120) to finish and blend (manufacturer data).
Which grain should I choose for a flap wheel for metal?
For heat-generating metal stock removal, choose zirconia alumina (self-sharpening) or ceramic alumina. Use aluminum oxide for wood and soft metals. Pick the finest grit that still removes the defect, then step up, for example 60 then 240, to refine the finish (manufacturer data).
What RPM should I run a small flap wheel at?
Run to the rating stamped on the specific wheel, because ratings vary widely by maker. Small shank wheels are rated up to about 25,000 RPM, and distributors recommend a working range of roughly 60 to 80% of the rated maximum (manufacturer data).