Pull studs (retention knobs): sizes and why yours doesn't fit
A retention knob is the screw-in stud the tool changer grips. It looks like a trivial part and it is one of the most frequently bought-wrong items in the whole toolholding system.
Why the same taper takes different studs
Three independent variables have to match, and none of them are visible from the outside of the holder:
- Thread. BT uses metric throughout. CAT exists in both a metric (DIN 69872) and an inch (ASME B5.50) version, and they are not interchangeable.
- Shoulder angle. Most knobs are offered with a 45°, 60° or 90° seating angle. The angle has to suit the gripper in your machine's changer, not the holder.
- Through-hole or solid. Coolant-through holders need a through-hole knob; a solid knob on a coolant holder blocks the flow entirely.
Match the knob to the machine, not to the holder.
The holder determines the thread. The machine's gripper determines the shoulder angle and the head profile. A BT40 knob that threads in perfectly can still be wrong for your changer, and that is the single most common reason a "correct" knob gets returned.
BT retention knobs — MAS 403
| Model | L (mm) | L1 (mm) | L2 (mm) | Head Ø (mm) | Thread | Shoulder angles offered |
|---|---|---|---|---|---|---|
| BT30 | 43 | 23 | 18 | 12.5 | M12 | 45° / 60° / 90° |
| BT40 | 60 | 35 | 28 | 17 | M16 | 45° / 60° / 90° |
| BT50 | 85 | 45 | 35 | 25 | M24 | 45° / 60° / 90° |
CAT / V-flange retention knobs
| Model | L (mm) | L1 (mm) | L2 (mm) | Head Ø (mm) | Thread |
|---|---|---|---|---|---|
| CAT30 | 44 | 24 | 19 | 13 | M16 |
| CAT40 | 54 | 26 | 20 | 17 | M16 |
| CAT50 | 74 | 34 | 25 | 25 | M24 |
CAT thread: published data conflicts, so check before you buy.
Several supplier tables list CAT40 knobs as
M16, which is the DIN 69872 metric form. The inch form per ASME B5.50 is commonly 5/8"-11 UNC on a #40 machine. Both are real and both are sold. Measure the thread in your holder — or read the machine's manual — rather than trusting a size code alone.
Machine-specific knobs
Some builders use their own geometry even where the taper is standard. Mazak, for example, uses shorter knobs than the generic equivalents:
| Model | L (mm) | L1 (mm) | L2 (mm) | Head Ø (mm) | Thread |
|---|---|---|---|---|---|
| Mazak BT40 | 44.1 | 19.1 | 14.0 | 17 | M16 |
| Mazak CT40 | 41.25 | 16.25 | 11.2 | 17 | M16 |
| Mazak BT50 | 65.2 | 25.2 | 17.7 | 25 | M24 |
A generic BT40 knob is around 60 mm long; the Mazak equivalent is about 44 mm. Thread it in and the gripper won't engage — or worse, it will engage partially and drop the tool.
Buying checklist
- Taper and size of the holder (BT40, CAT50, SK40…)
- Thread measured in the holder — metric or inch
- Shoulder angle required by your machine's gripper
- Through-hole or solid depending on whether you run coolant through the tool
- Machine-specific pattern if the builder deviates from the generic form
Worth knowing: knobs are a wear item. They take the full retention load on every tool change and the seating shoulder degrades. If a tool has ever dropped, replace the knob rather than reusing it — the cost is trivial next to the damage.