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 (KB: Mounted Point). 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; KB: Mounted Point).
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; KB: Mounted Point). The host tool is quick: straight die grinders commonly free-run at 18,000 to 25,000 RPM, and pencil grinders go far higher (KB: Die Grinder).
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 (KB: Mounted Point). 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 (KB: Mounted Point). 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 (KB: Mounted Point). A few makers also publish "actual-size" shape charts where the printed silhouette equals the real point (KB: Mounted Point).
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 (KB: Mounted Point), 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 (KB: Die Grinder).
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 (KB: Maximum Operating Speed). Surface speed scales with diameter: SFPM = diameter in inches × RPM × 0.262 (KB: RPM and SFPM Matching). 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 (KB: RPM and SFPM Matching).
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 (KB: Die-Grinder Internal Work). 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 (KB: Mounted Point).
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 (KB: Mounted Point). A long unsupported shank bends under centrifugal force and vibrates; a loose mandrel or excessive overhang is the dominant ejection hazard with these tools (KB: Mounted Point).
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 (PFERD data; KB: Mounted Point):
| 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 (KB: Die Grinder; KB: Die-Grinder Internal Work). For how the standards fit together, see our EN 12413, oSa and ANSI explainer.
The working rules that follow (KB: Die-Grinder Internal Work):
- 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 (KB: Wheel Ring Test And Mounting Safety). 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; KB: Mounted Point). Resin-bonded points would not ring anyway — organic bonds deaden the tone at any size (KB: Wheel Ring Test And Mounting Safety).
What replaces the ring test on a mounted point (KB: 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 (KB: Maximum Operating Speed; KB: 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.
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 (KB: Mounted Point).
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 (KB: Mounted Point).
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 (KB: Mounted Point; KB: Die-Grinder Internal Work).
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; KB: Mounted Point).
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 (KB: Mounted Point).
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 (KB: Die Grinder).