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.