A tool holder designation reads left to right as three segments: BT40-ER32-100 means a BT40 spindle interface (7:24 V-flange taper per JIS B 6339, 44.45 mm gauge diameter), an ER32 collet chuck (ISO 15488 collet system, clamping shanks up to 20 mm), and a 100 mm gauge length from the gauge line to the holder nose. Only the first segment is fully standardized — clamping-system codes such as ER, SLN, SF, and APU are industry conventions that vary by manufacturer, so the maker's dimensional drawing, not the code string, is the ordering contract.
Tool holder catalogs compress three buying decisions into one product code, and misreading any segment produces a holder that bolts into the spindle but fails the job — wrong reach, wrong collet series, or coolant that never arrives at the cut. This guide decodes each segment in order and finishes with worked examples and the ordering mistakes that generate most returns. It assumes the taper system itself is already chosen; for that upstream decision, see the BT vs CAT vs HSK comparison, and for choosing between clamping technologies on performance grounds, see the tool holding solutions guide.
The Three-Segment Anatomy of a Holder Designation
Nearly every machine-tool holder code follows the pattern interface — clamping system + capacity — length, with optional suffixes for coolant form and balance. Only the spindle-interface segment of a tool holder designation is governed by a published standard; the clamping-system mnemonic, its capacity number, and the length convention are manufacturer practice. Two catalogs can print the same string with slightly different dimensions behind it, which is why experienced buyers treat the code as a search key and the drawing as the specification.
✦ Standardized by the Interface Standard
- Taper geometry and gauge diameter (7:24 No. 30/40/50; HSK 1:10)
- V-flange and drive-slot dimensions
- Pull stud thread (MAS 403, ANSI/ASME B5.50, DIN 69872)
- HSK form letter and nominal flange diameter
✦ Manufacturer Convention — Verify on the Drawing
- Clamping-family mnemonic (ER, SLN, SF, APU, SDC, FMB)
- What the capacity number means in each family
- Gauge-length measuring convention and offered lengths
- Speed and balance ratings engraved on the body
Segment 1 — The Spindle Interface Code and Size
The first segment names the standard that machined the back end of the holder. BT tapers follow JIS B 6339, CAT tapers follow ANSI/ASME B5.50, SK tapers follow DIN 69871, and HSK hollow tapers follow DIN 69893 — four standards, four mutually incompatible interfaces. JIS B6339 defines the BT V-flange geometry while MAS 403 separately specifies the BT pull stud, so a BT holder only runs when both match the machine; MAS 403 studs also come in 45° and 60° head-angle variants that must match the drawbar gripper. ANSI/ASME B5.50 fills the same role for North American CAT spindles with inch-series pull stud threads (5/8"-11 on CAT40, 1"-8 on CAT50). DIN 69871 is the European SK (Steilkegel) V-flange standard — its holders take DIN 69872 pull studs and, despite sharing the 7:24 taper, are not interchangeable with BT or CAT because flange and stud dimensions differ.
| Code | Standard | Common sizes | What the number means | Pull stud |
|---|---|---|---|---|
| BT | JIS B 6339 | BT30, BT40, BT50 | 7:24 taper series No. (30 = 31.75 mm, 40 = 44.45 mm, 50 = 69.85 mm gauge dia.) | MAS 403 (metric: M12/M16/M24) |
| CAT | ANSI/ASME B5.50 | CAT30, CAT40, CAT45, CAT50 | Same 7:24 taper series No. as BT | Inch thread per B5.50 (CAT40: 5/8"-11) |
| SK | DIN 69871 | SK30, SK40, SK50 | Same 7:24 taper series No. | DIN 69872 |
| HSK | DIN 69893 | HSK-A63, HSK-A100, HSK-E50, HSK-F63 | Nominal flange diameter in mm + form letter (A/B/C/D/E/F) | None — internal clamping |
The size number in BT40, CAT40, and SK40 refers to the same 7:24 taper series: No. 30 = 31.75 mm, No. 40 = 44.45 mm, and No. 50 = 69.85 mm gauge diameter. HSK numbering works differently: the HSK size number states the nominal flange diameter in millimeters, so an HSK-A63 holder has a 63 mm flange over a hollow 1:10 taper of roughly 48 mm at its large end. The form letter (A through F per DIN 69893) encodes drive slots, flange size, and coolant routing. DIN 69893-1 is the part of the series that dimensions Forms A and C, so the HSK-A holders on most machining centers are specified against DIN 69893-1 rather than the umbrella number. Catalogs write the size either as HSK-A63 (standard notation) or HSK63A (product-code notation) — both name the same interface. Which interface to buy is a spindle-speed and stiffness decision covered in the BT vs CAT vs HSK comparison; for nomenclature purposes the rule is simpler: the interface segment must match the machine's spindle exactly, letter and number.
Segment 2 — The Clamping System Code and Capacity Number
The middle segment names the mechanism that grips the cutting tool, and here the standardization stops. Clamping-system mnemonics such as ER, SLN, APU, SF, and FMB are industry conventions that vary by maker — the same side-lock holder is coded SLN in one catalog and SLA in another, and a face mill arbor appears as FMB, FMA, or FMH depending on the brand. Precision collet lines carry maker-specific prefixes too (PNER, PGER, or "UP"-suffixed variants). The decoder below covers the most common families and what the trailing capacity number means in each:
| Code family | Clamping system | Capacity number means | Typical runout (TIR) | Typical maker speed rating |
|---|---|---|---|---|
| ER (ISO 15488 / DIN 6499) | Forward nut-clamped collet chuck | Holder taper-bore dia. in mm (ER32 = 32 mm bore, clamps 2–20 mm) | ≤0.015 mm (d₁ ≤ 10 mm), ≤0.020 mm (10–26 mm), Class 2 | 10,000–15,000 RPM (G6.3); 25,000 RPM (G2.5 precision) |
| SLN / SLA | Side-lock (Weldon) end mill holder, DIN 1835-B flat | Bore dia. in mm for the tool shank (SLN20 = 20 mm) | ~0.01–0.02 mm system | 8,000–15,000 RPM (G6.3) |
| SF / SFC / SFH | Shrink-fit chuck | Nominal bore dia. in mm (SF12 = 12 mm h6 shank) | ≤0.003 mm at 3xD | up to 25,000 RPM (G2.5) |
| APU | Integrated keyless drill chuck | Max drill capacity in mm (APU13 = 1–13 mm) | 0.06 mm (BT), 0.05 mm (HSK) | ~10,000 RPM |
| SDC | Pull-back collet chuck (HSK) | Collet size designation | ≤0.003 mm at 3xD | 20,000+ RPM |
| MTA / MTB | Morse taper socket adapter (DIN 228) | Morse taper No. (MTA4 = MT4); A = tang ejection, B = threaded drawbolt | ~0.015–0.02 mm added per stage | ~8,000 RPM |
| FMB / FMA / FMH | Face mill arbor | Arbor pilot dia. in mm from the ISO 6462 series (22/27/32/40/50) | face runout ≤0.005–0.025 mm | 3,000–6,000 RPM |
The capacity number changes meaning with each clamping family: ER32 names a 32 mm taper-bore collet series clamping up to 20 mm, SLN20 a 20 mm Weldon bore, APU13 a 13 mm drill capacity, MTA4 a Morse taper No. 4 socket, and FMB27 a 27 mm arbor pilot. A few anchors behind those rows: ISO 15488 (equivalent to DIN 6499, the original ER standard) fixes the collet's 8° half-angle taper, sizes ER11–ER40, and the Class 2 runout limits — ≤0.015 mm TIR for shanks ≤10 mm and ≤0.020 mm for 10–26 mm per ISO 15488:2003 Table 4. DIN 1835 Form B defines the Weldon flat that the SLN set screw bears on, which is why side-lock holders resist axial pull-out in heavy roughing. DIN 228 governs the self-holding Morse tapers behind MTA/MTB adapters, where the A/B letter distinguishes tang ejection from threaded-drawbolt retention. ISO 6462 standardizes the face mill arbor pilot series so cutter bodies from different makers interchange on the same FMB arbor.
The speed column deserves its own caution. The same SLN side-lock code is rated 8,000 RPM by one manufacturer and 15,000 RPM by another — speed and balance ratings belong to the maker's build quality, not to the code family. None of the interface or collet standards specify maximum RPM; those numbers come from manufacturer testing. For the performance trade-offs between collet, hydraulic, and shrink-fit clamping (rather than their naming), see the collet chuck vs hydraulic chuck comparison.
Segment 3 — Gauge Length: The Number Most Often Ordered Wrong
The final number is a length in millimeters, and its reference plane depends on the interface. Gauge length on a 7:24 V-flange holder (BT, CAT, SK) is conventionally measured from the gauge line — the plane of the taper's gauge diameter, approximately at the flange face — to the holder nose; on HSK it is measured from the flange contact face. Some catalogs print it as a bare number (BT40-ER32-100), others prefix it (H100 or L100); a few quote overall length instead, which is exactly the ambiguity the dimensional drawing resolves.
Gauge length matters for three reasons. First, reach: the holder must clear the fixture and workpiece, so deep-cavity work needs 120–150 mm projections while standard milling typically uses 60–100 mm. Second, rigidity: for a slender holder body, beam mechanics predicts free-end deflection growing roughly with the cube of projection, so moving from a 70 mm to a 100 mm gauge length can roughly triple nose deflection ((100/70)³ ≈ 2.9) in otherwise similar holders — longer is never free. Third, offsets: tool presetters and CAM tool tables reference the gauge line, so an unplanned length substitution silently shifts every Z offset built on it.
Order the shortest gauge length that clears the job
Pick the projection by measuring the deepest feature plus fixture clearance, then round up to the maker's next standard length — typically offered in steps such as 70/100/150 mm on BT40 lines. Buying one "universal" long holder for everything trades away stiffness on the 90% of jobs that never needed the reach.
Suffix Codes — Coolant Form AD/B and Balance Markings
Two suffix groups commonly trail the main string. The first is the coolant form on V-flange holders: on JIS B 6339 and DIN 69871 holders, Form AD routes coolant centrally through the pull-stud bore, Form B routes it laterally through holes in the flange collar, and holders marked AD/B are convertible between the two. Form A (no through-hole) still appears on arbors and side-lock bodies for dry or external-coolant work. Ordering Form A or B for a machine that delivers coolant through the drawbar typically means no coolant reaches the cut at all — the most expensive way to discover the suffix matters.
The second suffix group is the balance marking engraved on the body, such as "G2.5 25,000 RPM". A balance marking is only meaningful as a grade-plus-speed pair, because ISO 1940-1 defines each G grade as a permissible residual-unbalance level that shrinks as rated speed rises. ISO 1940-1 is used for tool holder balance classification because it ties allowable unbalance to operating speed: G6.3 at 15,000 RPM is a common standard-line rating, while G2.5 at 25,000 RPM marks premium lines, and the same physical holder could pass G2.5 at low speed yet fail it at high speed. Treat an engraved grade without a speed as marketing, not data.
Worked Decodes: Reading Three Real Designations
Worked decode 1 — BT40-ER32-100 (Form AD):
Worked decode 2 — HSK63A-SLN20-90:
Worked decode 3 — CAT50-FMB32-60: a CAT V-flange taper No. 50 per ANSI/ASME B5.50 (69.85 mm gauge diameter, 1"-8 pull stud thread), carrying a face mill arbor with a 32 mm pilot from the ISO 6462 series, projecting 60 mm from the gauge line. The 32 mm pilot suits roughly 80–100 mm cutter bodies, and the short 60 mm projection keeps the heavy interrupted-cut load close to the spindle face. Reading the string in segment order — interface, clamping family, capacity, length — recovers every ordering-critical dimension except the ones only the drawing can confirm.
Common Ordering Mistakes the Code Won't Catch
Three mistakes account for most wrong-holder deliveries, and all three live in the conventional (unstandardized) segments:
- Wrong gauge length. Substituting a 70 mm holder where the process plan assumed 100 mm causes fixture collisions and invalidates preset offsets; substituting longer than needed typically costs stiffness by the cube of the extra projection (Section 04).
- Mismatched nut and collet series. ER, TG, and DA collets look alike but have different taper geometries — a TG collet will not seat in an ER chuck. Within ER, mini-nut (M-suffix) chucks use smaller-diameter nuts than standard chucks of the same ER number, and the nuts are not interchangeable. Order nut, collet, and chuck from the same series designation, and remember the ER collet must snap into the nut's extraction ring before insertion.
- Wrong coolant form (AD vs B). A through-spindle-coolant machine needs Form AD (or AD/B); a flange-fed machine needs Form B. The form letter is easy to drop when copying a code from another shop's setup sheet written for a different machine.
The code is a search key, not a specification
Two manufacturers can sell "BT40-ER32-100" with different nut styles, different balance grades, and even slightly different length conventions. Before a purchase order goes out, confirm the dimensional drawing for: length reference plane, nut series and wrench type, coolant form, balance grade at rated RPM, and (for BT/CAT/SK) the pull stud specification — studs are almost never included.
Quick Ordering Reference by Scenario
| Scenario | Designation to order | Interface / clamping standard | Key spec to verify | Why |
|---|---|---|---|---|
| Job-shop milling, BT40 spindle, mixed 3–20 mm shanks | BT40-ER32-100 AD | JIS B 6339 + ISO 15488 | ≤0.020 mm TIR Class 2; M40 × 1.5 nut | One ER32 collet set covers 2–20 mm shanks with the broadest consumable supply |
| North American VMC, 20 mm Weldon roughing end mills | CAT40-SLN20-90 | ANSI/ASME B5.50 + DIN 1835-B | Set screw on Weldon flat; 5/8"-11 pull stud | Positive screw-on-flat drive resists axial pull-out under heavy roughing loads |
| High-speed finishing above 20,000 RPM, HSK spindle | HSK63A-SF12-90 | DIN 69893 + maker shrink-fit line | ≤0.003 mm TIR at 3xD; G2.5 balance marking | Symmetric one-piece shrink-fit body balances finely and grips 25,000–40,000 N |
| Drilling 1–13 mm on a machining center | BT40-APU13-100 AD | JIS B 6339 (chuck is maker design) | 0.06 mm TIR; keyless grip | Integrated chuck removes arbor stack-up and shortens gauge length 20–40 mm vs chuck-on-arbor |
| Face milling an 80–100 mm cutter, CAT50 spindle | CAT50-FMB32-60 | ANSI/ASME B5.50 + ISO 6462 pilot | Pilot 32 mm; drawbolt face seating | ISO 6462 pilot series gives cutter-body interchange with ±0.005 mm radial repeatability |
| Tang-shank MT4 drills on a BT50 horizontal mill | BT50-MTA4-120 | JIS B 6339 + DIN 228 | MT4 socket; tang slot for drift ejection | Self-holding Morse taper retains tang-end drills and ejects with a standard drift |
Decode left to right: standardized interface, maker-convention clamping code, gauge length in millimeters.
The interface segment (BT/CAT/SK/HSK plus size) is fixed by JIS B 6339, ANSI/ASME B5.50, DIN 69871, or DIN 69893 and must match the spindle exactly. The clamping code (ER, SLN, SF, APU, SDC, MTA, FMB) and its capacity number are manufacturer conventions — ER32 means a 32 mm collet bore, SLN20 a 20 mm Weldon bore, APU13 a 13 mm drill capacity, MTA4 a Morse No. 4. The final number is gauge length: shortest that clears the job wins. Always confirm length reference plane, nut/collet series, coolant form (AD vs B), and balance grade at rated RPM on the maker's drawing before ordering.
What does BT40-ER32-100 mean on a tool holder?
It decodes as a BT40 spindle interface (7:24 V-flange taper per JIS B 6339, 44.45 mm gauge diameter), an ER32 collet chuck per ISO 15488 (32 mm taper bore, clamping shanks from 2 to 20 mm), and a 100 mm gauge length measured from the gauge line to the holder nose. The MAS 403 pull stud is ordered separately.
Is tool holder nomenclature standardized?
Only the spindle-interface segment is. BT follows JIS B 6339, CAT follows ANSI/ASME B5.50, SK follows DIN 69871, and HSK follows DIN 69893. The clamping-system codes (ER, SLN, SF, APU, FMB) and gauge-length conventions are manufacturer practice — the same side-lock holder may be coded SLN or SLA — so verify dimensions on the maker's drawing.
What does the number after the clamping code mean?
It changes meaning by family: ER32 names a 32 mm taper-bore collet series (clamps 2–20 mm shanks per ISO 15488), SLN20 a 20 mm side-lock bore for Weldon shanks, SF12 a 12 mm shrink-fit bore for h6 shanks, APU13 a 1–13 mm drill chuck capacity, MTA4 a Morse taper No. 4 socket, and FMB27 a 27 mm face mill arbor pilot.
What is gauge length on a tool holder and why does it matter?
Gauge length is the projection from the holder's reference plane to its nose — measured from the gauge line on BT/CAT/SK V-flange holders and from the flange face on HSK. It sets reach, Z offsets, and stiffness: deflection grows roughly with the cube of projection, so a 100 mm holder deflects close to 3x more than a 70 mm one under similar load.
What do AD and B mean on a BT or SK tool holder?
They are coolant-form codes on JIS B 6339 and DIN 69871 holders: Form AD feeds coolant centrally through the pull-stud bore, Form B feeds it laterally through holes in the flange collar, and AD/B holders convert between both. Form A has no through-hole. A through-spindle-coolant machine needs AD or AD/B — Form B alone delivers no central coolant.
Sources
- JIS B 6339 — Tool shanks with 7/24 taper for automatic tool change (Japanese Standards Association)
- ANSI/ASME B5.50 — V-Flange Tool Shanks for Machining Centers
- DIN 69893-1 (HSK Form A and C)
- ISO 15488:2003 — Collets with 8° setting angle for tool shanks
- ISO 6462 — Face and shoulder milling cutters with arbor hole (arbor pilot series)
- ISO 1940-1 — Balance quality of rotors
- Manufacturer catalog data: Harlingen Tools (BT, JIS B 6339), SYIC/Sanjet General Catalogue (CAT, ANSI B5.50), Falcon Toolings (HSK, DIN 69893), Gaetano Caporali (ER collet chucks, DIN ISO 15488)
