1.375-12-UNF-3B tap through 7075 aluminum with thin wall

[charlieb3cnc]

I am attempting to thread 1.375-12-UNF-3B threads through 7075 aluminum.

The part is .25" thick. I am water jetting the profile of the part and then boring the hole to be tapped out to 1.297".

I am using a 4 fluted HS Ground Thread tap running at about 120 rpm (using a manual mill). The threads cut nicely but do not fit the go / no-go gauge that my customer supplied me with.

I believe the problem arises from the part having a thin wall around the threaded hole and the part possibly flexing around the tap instead of cutting. The OD of the part is only 1.625".

I have tried speeding the machine up, slowing it down, supporting the part with a steel cutout around the outside, and the only success I have had is by tapping at least 4 times with the part in a different orientation in my fixture.

The problem there, other than the amount of time, is that I have gotten several parts that fit the no-go side.

I appreciate any ideas, this part is driving me crazy.

[Hennessy]

Do you have a lathe? If so, I would use that..... If not I would suggest boring some soft jaws to the finish od. Use a small jaw spacer so you can get as much "even" clamping force to the od of the part as possible. Basically make a split collet out of your soft jaws.... Releave the jaws for the boring and taping ops and try to clamp the part as lightly as possible. Just not to light... Good luck

Hennessy

[charlieb3cnc]

I do have a small engine lathe. Are you suggesting single point threading, or using a 4 jaw chuck with my tap, or other clamping method with my tap?

We are by no means a machine shop so I will have a serious learning curve to overcome.

I have considered soft jaws, and I think I will look further into that.

Thanks for the reply.

[Hennessy]

Originally Posted by charlieb3cnc I do have a small engine lathe. Are you suggesting single point threading,

Yes, that would be the way I would do it. But you would need bar stock and i am assuming that you are using .250 in sheet/plate to save money. So in this case soft jaws would be your best bet.

Hennessy

[RICHARD ZASTROW]

Full diameter soft jaws AKA "pie jaws".

Dick Z

[Hennessy]

Originally Posted by RICHARD ZASTROW Full diameter soft jaws AKA "pie jaws". Dick Z

Yep... that would work to It felt like I was forgetting something

Hennessy