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techonehundred

Well-Known Member
I am making a Collet Closer for an Atlas lathe. I have just drilled out the 3/4" rod to .501" and need to prepare the end for the 3C collet. When i go to Machinery's Handbook, it says that the thread on the collet is a .640" x 26. So now I need to know what the rood diameter of the thread is supposed to be. The closest I can find in Machinery's is 5/8x24 which has a tap drill dia of .580 to .585. Also, a 5/8x28 is listed at .586 to .591. Since 5/8 is .625, I am a little lost as to where to bore the opening before threading. So do any of you have any idea what the root diameter of the thread should be? I am sure there is some calculation out there. It is probably even in Machinery's Handbook if I knew where to find it.

Blogwitch

Ex Bogstandard
HMEM Supporting Member
Tech,

The closest I can get it to is 5/8" x 26TPI CEI thread (Cycle Engineers Institute)

This has a core diameter of 0.584" with a thread form angle of 60o.

If you have Excel on your machine, I have attached a thread chart for almost all threads you can think of.

Hope this helps

Bogs

mu38&Bg#

Well-Known Member
The thread appears to be 41/64"-26. If we do it the metric way by subtracting the pitch from the diameter we get 1/26=.0385 .640-.0385=.602 minor diameter for internal thread.

Ned Ludd

Well-Known Member
Hi There,
Do you know if the thread form is 55 or 60 degree?
The depth of thread for a 26TPI 55 thread is 0.0246", whilst the DOT for a 26TPI 60 is 0.0205. To find core size, double the above figures and subtract from your 0.640"

You will notice that there is only about 8 thou difference between the two threadforms, so if you opt for the larger hole it will make little difference in thread strength.
As you are thread cutting this thread the final size will be a matter of repeated cut and try as there will always be some leeway in your tooling. This leeway will mainly show itself by not having the absolutely correct radius on the tip of your tool (make your own jokes here). Too small a radius will require a deeper cutting, than figures would suggest, to bring the flanks into the correct position. Likewise a too large a tip radius, cutting to full depth, will produce a sloppy fit.

Hope this all makes sense and is helpful.
Ned

jpeter

Well-Known Member
As a matter of information I made a 5c collet closer for my Sheldon lathe and used 1 inch black pipe for my drawbar. It fit great. I bored the thread for the collet and screwed a coupling (brass) on the left end for the stop. I added a nipple for an extention and an old pulley for a hand wheel. I used locktite on all the fittings to keep it from coming apart when I loosened it. I love it. The hard part was making a nose piece. I have about .001 or .002 runout depending on how carefully I insert the nose piece. I bought a cheap soft drill sleeve I intended to use for making the nose piece but I found it less accurate than the one I'd made so I never completed it. Drill sleeves are cheap, under 10 dollars so going that way for a nose piece is not a bad idea if you can tolerate slight runout.
I'm sure everybody knows this but I'm gonna mention it anyway. You need to be really careful when you install the nose piece to make sure no chips are in the spindle and also carefully using a soft hammer make sure the nose piece is fully inserted.

techonehundred

Well-Known Member
Thanks for the reply's. Diesel and Ned, Both of you come up with pretty much the same numbers. I do think that the thread is 60 degrees, but I have no way of telling. So I am going to just assume it is and go from there. So I think I will stick with the round number of .600".

Jpeter Thanks for the advice. I could not find a Iron pipe with the right dimensions, so I just drilled a solid rod. I also picked up a 8" hand wheel at Names so I will bore it to fit the tube. As for the Nose piece, I picked one up earlier on fleabay so I am good to go there. Just need to thread it and attach the wheel.

techonehundred

Well-Known Member
So, I decided to get with it and finish this thing. so I made an internal threading tool

and proceeded to thread my collet closer. I am here to say that the advice worked, and .600" was a good size. I finished the thread and put on a Milling Machine round handle. Now I have a fairly decent Custom built 3c collet closer. So here is the pic with 3c collet screwed in.

Thanks for all the help.
Anthony

jpeter

Well-Known Member
That looks pretty sweet. You're gonna love it. Got much runout? Put a dial indicator on it and tell us how its working 3 inches out from the nose.
Hey, if you get a chance check out my v8.

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