How do I figure out the diameters so I will have a nice screw fit?

For beginners, 20 TPI doesn't sound especially fine to me. Perhaps

A final comment: The size and length of engagement sound as if this is a one of a kind project. Therefore pick whatever is easiest for you!

Good luck!

Should be able to do this with very little math. If you have one of those little, arrow shaped threading gages; look on the side and it will tell you the "double depth of threads". In case you don't have one, I will tell you it says you can use any thread finer than about 13 tpi for your parts. So you can use 20 or 32 or any that you like the looks of. Start with part A. Chuck it up in the lathe. Set your threading gearbox to the number you want. Start cutting the internal thread. Take light cuts until you have a nice thread with a slight flat on the tips of each thread. Now with the tool sitting at the depth of the last pass you made for the thread, move the tool into the thread like you were going to bore out the entire thread. Only go about .010 deep. This will give you a counterbore that is the OD of the thread. The purpose of this counterbore is so that you can measure the OD. Now subtract .010 to .015" from this diameter and you have the OD of part B. Turn part B to this OD and start threading. Take light cuts. You should end up going IN ABOUT the same amount you came OUT for the ID thread. Keep checking fit with part A. Use oil. And DO NOT screw them together very tight until you have the fit you want. If the fit is too tight the aluminum will stick to itself and you will never get them unscrewed. Good Luck

Machinery's Handbook shows that an internal 1 and 13/16 - 20 thread has a minor diameter of 1.7584 so that might be a good choice. You will only have to open the ID of the tube by .0084 so you won;t lose hardly any wall thichness.

Therefore your OD of part B will be 1.8125. The minor diameter of the outside thread will be 1.753. If you do not have thread wires with which to check the thread you could turn a short length of the part to the minor diameter and then thread until the tool just kisses the

If it were me I would make the part B first and use it as a thread plug to fit up part A. Your fit will depend on the care you take in the final few cuts. Remember, threading is an exercise in patience. As you get close to finish size advane the tool only .001 at a time, take several spring cuts and check, check, check. Rushing will not accomplish anything.

You might try using mineral spirits (as in paint thinner) as a lubricant when cutting aluminum. Just a suggestion.

Good luck and let us know how it turns

Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech

860 774 8511 x1811

Hey LP,

Without commenting on anything else, using a thread of 1.75 X 20 will give a pitch of 0.050. If the thread is a standard 60 degree, all 3 sides of the triangle formed will be the same 0.050", and the depth of thread will be 0.050 X 0.866 = 0.0433", or a "diameter change" at the roots of 0.0866". In your case, this is a "raise" from the 1.75 ID of Part A to 1.8366, so you make the OD of the "screw-in" piece to be that size (1.8366).

Taps and dies in "unusual" threads are expensive. For instance, Wholesale Tool wants $115 for a 1.75 X 20 tap, but only $33 for a 1.75 X 10, while the norm is NC 1.75 X 5. Of course, you can't do a 1.75 X

Take care.

Brian Laws>I have a piece of 6061 AL tube, 2"OD, 0.125" wall, 1.75" ID, 1.25"

I guess that I had "assumed" that both parts would be single point threaded. No expensive taps involved.

Brian your depth of thread calculation is for a sharp V thread. The formula for a standard thread with a 1/8P flat at the crest and root is .6495/N. This gives a depth of thread of .0325 for 20 TPI and that is what I had based my thinking on.

Errol Groff Instructor, Machine Tool Department H.H. Ellis Tech

860 774 8511 x1811