A type, M type and UM type ER nuts: which one fits
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An ER collet chuck holds the tool with a collet and a nut. The ER nuts and chucks we sell come in three types: A, M and UM. These letters are trade names, and other makers may use different ones, so check the shape and thread of your own nut before ordering.
The three types
- A type: a hex nut. You tighten it with an ordinary spanner.
- M type: a mini nut. It has a smaller outside diameter, a smaller and finer thread than the standard nut, and notches. It is tightened with an M type spanner. Use it where space is tight, for example when the nut must clear a part or a fixture.
- UM type: a round nut, tightened with a UM type (multihook) spanner or a C-hook spanner. It is the type we list for ER32 and ER40.
What we list
| ER size | Nut types in our listings |
|---|---|
| ER11, ER16, ER20, ER25 | A type and M type |
| ER32, ER40 | UM type |
Nut, chuck and spanner must match
The nut, the chuck thread and the spanner must be the same type and the same ER size. An M type nut has a different thread from an A type nut of the same ER size, so it will not screw onto an A type chuck. Each type also needs its own spanner.
Before you order, look at your chuck. A hex nut is usually A type. A small round nut with notches is usually M type. A larger round nut with slots for a hook spanner is usually UM type. If your chuck is from another maker, measure the thread as well.
Which should I buy?
- Replacing a nut on an existing chuck: copy the type and ER size from the old nut. Check the spanner you own as well.
- Tight clearance around the tool: choose an M type chuck and nut, if the ER size you need comes in M type.
- ER32 or ER40 chucks from our range: these take the UM type nut and a matching UM spanner.
- No special spanner on hand: A type needs only an ordinary spanner.
Our BT40 ER11, ER16 and ER20 collet chuck listings state A type and M type options, and our ER32 and ER40 cylindrical shank chucks are UM type.
If you are not sure, send us the chuck size and a photo of the nut and we will check it with you.