MT is Morse Taper. It has been around for a long time and is often found on larger drills. It comes in different sizes from MT0 (small) to MT7 (large). It is good for drills but not so good for milling. As a result when Morse Tapers are used for milling they require a drawbar. This is like a long bolt that pulls the tooling into the spindle to act against milling forces that loosen a standard Morse Taper. The problem with Morse tapers is that they can cause problems when trying to remove them from their socket. There are a number of threads on this forum regarding stuck tapers.
The Bridgeport Company wanted something to replace Morse Tapers so they designed the R8 taper. This still uses a drawbar to pull the tooling into the spindle but is easier to release than Morse tapers.
There are many other tapers available due to limitations with R8 as well such as ISO30.
Morse Taper and R8 finger collets are made with slots in them that allow the collet to close onto a parallel shank of a tool and grip it when the drawbar pulls them into the spindle. The fingers are fixed at one end so the grip inside the collet is not as high as it is at the end where the fingers can move. This means that these collets need to be closely matched to the diameter they have to hold. A 1/2" collet (Ø12.7mm) will not grip a Ø12mm tool as well as a 12mm collet. The result is you may end up with a lot of collets for each size required.
ER collets are split from both ends in alternating positions and have two angles for the holder and the closing nut. This gives a more parallel squeeze and so the ER collets can hold a range of sizes. For instance you can get an ER32 collet that will hold tools from Ø13mm down to Ø12mm. Smaller ER collets such as ER11 have a smaller size range. The size for an ER32 is nominally the outside diameter of part of the collet, not the size of tooling it can hold. ER32 typically holds up Ø20mm for example.
To use an ER32 collet (for example) in a mill with MT or R8 spindles you need an MT to ER32 collet chuck or an R8 to ER collet chuck and ER collets to go into the ER collet chuck. You only need to buy the collet chuck that suits the spindle of the machine you end up with.
You can also buy an ER collet chuck with a parallel shank and fit it into a finger collet that suits your spindle. Here are some typical examples of ER11 on a parallel shank that can be fitted into a larger ER collet or finger collets. An R8 to ER32 collet chuck. An R8 finger collet with a parallel shank ER32 collet chuck.
Hope this helps.
Martin C
![img_20200614_093452.jpg img_20200614_093452.jpg](data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==)
Spelling and finger trouble corrected.
Edited By Martin Connelly on 21/11/2020 09:57:09