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.