Here are my notes on collets, with particular emphasis on the Atlas 12x36 lathe.
Collets are ideal for holding small diameters precisely. They are also safer, as there are no chuck jaws to crash into, which is especially important when you are working on small items close to the chuck or collet.
Some links and references:
R8 collets are what I have a set of for my mill. Their "shank" has a diameter of 0.95 inches, which will not fit into the 0.795 inch hole in the spindle of my Atlas lathe.
MT3 collets (also referred to as 3-MT) are "Morse Taper 3" collets, and would be dandy for use with the Atlas lathe. They require a 3/8-16 thread drawbar and would not allow stock to pass through the spindle. They would be fine for tool holding or holding short work. They do not need to be (and should not be) overtightened in order to do their job.
5C collets are essentially bigger versions of the 3C. I am not clear whether they can be used nicely with the Atlas lathe.
Do an Ebay search for ER closing nut with drawings. You can typically buy the closing nut and drawings for about $20, and the nut will include (or should) and eccentric release ring which ensures that the collet will not bind up inside the nut.
If you want to hold really tiny things, ER16 or ER20 collets may be useful.
My friend Alan tells me:
The MT3 and 3C collets are designed to hold their exact sizes (essentially). I think you can hold any size if you have collets spaced by 1/64 inch. In other words, the collet can squeeze down about .015 inches from nominal size.
On the other hand, the ER40 collets (because they have a double taper) can squeeze down .039 inches (1 mm). So you need fewer collets to cover the whole range. The ER32 collets also squeeze down .039 inches for the larger sizes (.094 inches and larger), but the smaller ones only squeeze down .015.
WW collets are tiny watchmaker collets. I often see them referred to as "8mm collets".
Another style of 8mm collets is the Moseley style, which have a conoidal (curved) head and may be longer (like 40 mm).
An 8mm collet has an 8mm diameter body, the threads are somewhat smaller and differ slightly among manufacturers. They have a 20 degree taper on the end that closes.
The 1907 Hardinge catalog says the single angle is 20 degrees, the body is .3147 inch diameter and the thread is .270 inch diameter with a 63 metric pitch. That equals 40.3 TPI.
The Levin catalog sells a WW tap that is .275-40, which is a generally accepted standard for these collets. Levin says the body is .3147 and the cone angle is 40 degrees.
Some European makers may call the pitch .635 mm, which is exactly 40 TPI. They might call the thread diameter 7 mm, which is .2756 inch.
Different makes and ages of collets have a variety of body and thread diameters, but they are generally close enough to interchange.
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