Collet Capacity Atlas 3996 and 3986 (12" lathe)

[ShagDog]

I was looking over an old Atlas catalog for the 3996 and 3986 lathe, and am having a difficult time with the specifications. The lathes have a 1-1/2-8 spindle thread with a mt3 internal taper, and mt2 taper in spindle nose sleeve. It says that the "Collet Capacity" is 1/2". I thought that an mt3 taper would take up to a 3/4" collet, and that mt3 collets run from 1/8" to 3/4", while mt2 tapers run from 1/8" to 1/2". Is it the "sleeve" that only allows 1/2" collet? What exactly is the "sleeve"?

Can someone please explain what I am missing here.

 

[mmcmdl]

My 3996 took 3A collets with 1/2 " being the largest .

What exactly is the "sleeve"?

It has an OD taper of MT3 and fits in your spindle taper , and has the ID taper to fit the collets . You then an use a drawbar thru the spindle .

I took pictures of all the tooling when I sold the lathe and they are on the site somewhere . They show the spindle nose sleeve , collets and drawbar setup .

 

[ShagDog]

Thanks. I think I might understand now. Just to make sure I understand, the spindle taper is 3mt. The collet holder sleeve fits in the 3mt taper. It is just like a drawbar on any other lathe; except, it has an internal mt2 to fit the 3at collet, right?.

So a 3/4" 3mt collet (not 3at) can be used with a standard drawbar with a 3/8-16 threaded rod connected to the 3/4" 3mt collet, right?

 

[wa5cab]

Yes, you can use 3MT collets with a 3/8"-16 drawbar for holding milling cutters with shank diameters up to 3/4". However, the 3MT collets are only available in 1/16" increments and have nowhere near a 1/16" range. So in practice they are not suitable for holding general work.

For holding tubing and solid round stock, you may use either 3AT or 3C collets with a draw tube and the appropriate 3AT or 3C to 3MT collet closer. Note that the 3AT and 3C closers are very definitely not interchangeable but are close enough in appearance to the same that if you have both, you had best mark them somehow. The primary difference is in the female taper. Both collet families are available with ID's of 1/64" to 1/2" in 1/64" increments. The draw tube's internal threads are actually effectively the same except that the spec is in common fractions for the 3AT and in decimal fractions for the 3C. However, the collet lengths are significantly different so to use one draw tube for both, the length must be adjustable or you need
a spacer with the 3C;s.

You may also be able to buy or make an ER type (usually either ER25 or ER42) collet closer that may have either 1-1'2"-8 female threads or a 3MT arbor and draw bar. The 3MT version has the disadvantage of not being able to pass long stock through the spindle, so should generally be avoided.

One comment that I will add is to NEVER EVER attempt to use either a 3MT collet or a 3MT cutter holder or an ER closer on an MT arbor without using a drawbar. There are a lot of tanged type cutter holders around and a few arbor mounted ER closers but if you use one without a drawbar, it is pretty much guaranteed to come loose and ruin the part that you are milling or turning, probably when it is almost finished.

 

[ShagDog]

Thanks. Still wondering about 1 thing. What exactly Is the mt2 taper in "spindle nose sleeve" referenced in the brochure specs (see below)?

The brochure states: "Taper in spindle nose sleeve..................No. 2 MT".

 

[wa5cab]

It was the short version 3MT to 2MT adapter that was supplied with all Atlas 9", 10" and 12" lathes ever built. Same adapter was supplied with most of the other makes of similar size lathes. Such as South Bend, Clausing and Logan. It has the advantage over what you would get if you went into a machine tool supply house today and asked for a 3MT x 2MT adapter in that a 2MT arbor would be in about the same stick-out position as if the spindle taper was 2MT instead of 3MT.

 

[ShagDog]

Thanks. I now understand.

 

[mickri]

If you are wanting to hold something that has to extend into the spindle bore the only collets that I know of that will allow you to do that are ER collets held in an ER collet chuck. I made one for my ER32 collets.

 

[wa5cab]

No, both the 3AT and the 3C collets, which were available for decades before the ER types came on the scene, allow for any length of work piece that will fit the collets to fit through the hollow spindle. In addition to that, there are or were several 5C collet closers made. And more recently many 5C collet chucks that will fit the Atlas 10" and 12" lathes. All allow work pieces up to 3/4" diameter to extend through the spindle. Not that there is anything inherently wrong with the ER collets, but all of the other types have been available for decades longer than the ER's. And some have been available for nearly a century.