Internal and External Threading a Piece

[Splat]

Guys, I've never done a piece that had both internal and external threads across from each other, as can be seen in this pic:


I'm wondering about tool pressure and wall flex while cutting either thread. I'm using carbide to cut the internal thread and HSS for the external thread. My question is does it matter which threads are cut first, internal or external? Thanks.

 

[aliva]

I would cut the internal first. Then make a threaded mandrel and installed in a 4 jaw chuck, dial in, then cut the external, since it's on a mandrel there should be no flex on the internal ID.

 

[mikey]

Threading cuts are usually very small. I wouldn't worry about flex much. Do them in whichever order you prefer.

 

[Splat]

Thanks guys. I thought that, too, @mikey. I would take light cuts on either, or both threads, if needed. I think I'll do the internal first, thread a bolt into it, then cut down and thread the external. Thanks.

 

[mikey]

The cutting forces when threading are mostly axial and tangential so the bolt really isn't necessary. Do if it makes you feel better.

 

[Splat]

Sounds good. Thanks Mikey.

 

[mikey]

Just wanted to clarify my response. Axial forces are feed forces and do not contribute to deflection. Tangential forces push the cutter down and this is where the bulk of the cutting force is felt. However, given that you are taking depths of cut in the 0.002" range, they are not a major deal. Radial forces push the work piece away from the cutter when external threading and they push the threading bar away from the work piece when internal threading. Again, at small depths of cut, radial forces are not overly huge. This is why I don't think that deformation is going to be a major deal.

Years ago, I made a boring head adapter for a Sherline mill. It was 7/8-20 on the outside and 3/4-16 on the inside; that is a thin-walled work piece! There was no distortion, no problems when cutting and the fit was just fine. Just wanted to let you know that this cutting force stuff is not just theoretical; it is what happens at the lathe.