December 12, 2017

Collets for the Atlas 12x36 lathe

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:

MT3 collets

The spindle of the Atlas lathe has a number 3 Morse Taper machined inside of it. It comes with an adapter sleeve to allow it to accept a number 2 Morse Taper (things like a drill chuck).

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.

MT2 holder and ER collets

I ran into this set on the "Banggood" site (China). Interesting for $100 with a holder. This fits into a MT2 taper. The Atlas has an MT3 taper in the spindle, so if I can find the MT3 to MT2 sleeve somewhere, these will work fine.

3C collets

3C collets are hollow and do allow stock to pass through the spindle. The are available in diameters up to 1/2 inch. They require a hollow drawbar. There are 3C collet holders that fit a 3 Morse Taper, but they preclude passing stock through the spindle. So, to use 3C collets with the atlas lathe, you need both a drawbar and whatever tapered collet closer is appropriate.

5C collets are essentially bigger versions of the 3C. I am not clear whether they can be used nicely with the Atlas lathe.

ER collets

These are the size and shape of a biscuit and require a collet holder. Many people machine their own collet holder, which will then run very true (being machined on and for the specific machine on which they will be used). They are specified in metric dimensions, but a give size can be used for diameters plus or minus 1/64 of an inch, so a set will cover all possible dimensions, english or metric. There are two series that seem to be in common use:

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

WW stands for Webster Whitcomb. A google search on "ww lathe" will yield a lot.

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.


Have any comments? Questions? Drop me a line!

Tom's home page / tom@mmto.org