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Tapping ID of this steel tube troubles


Jul 1, 2011
Brusly, LA
I’m installing screw in chokes on several shotgun barrels ( yes I measured them) with tooling from Manson. These are 12ga tru choke pattern, one piloted reamer and one piloted tap. Steel is cro moly barrel steel so 4140 or 4130 I’m guessing.
Reaming goes smoothly , but tapping it feels a little tight but goes all the way to the bottom. . When I go to instal the choke tube it’s very tight. I began inserting the muzzle into a 3 jaw chuck, tightening it, and re tapping which seems to help but still requires 3-4 tapping sequences to get it where I like.
Is this the symptoms of a full tap or just what I have to deal with with thin wall barrels ?
Serial taps go better in thin wall components too as there is much less expansive stress. About the only good option if its something that can't go in a collet or have some sort of sleeve support.

Young Clive once put an old style, long taper, first cut tap of ahem "dubious" history into a thin brass tube and expanded it out to clearance real nice. Which wasn't the idea!

As to new but not sharp, I have a set of 12 mm x 1 supposedly mid range ones, that managed two holes in alloy and one in steel before giving up. Suppler refused refund. So glad it was a paying job with tooling costs built in. Kept for practcie if I ever decide to learn how the Clarkson tap reprofiler thing is done.

Those really fine threads should be a breeze to cut.

I suspect the tooling is funky. Really look at the tap grind. Confirm the reamer is cutting to the correct minor diameter before tapping.

If all that looks good best thing to do is support with spilt bushing. You can clamp bushing in three jaw.
I thread-milled some M22x.5 threads in thin walled aluminum for some sight globes. They were too tight, so I decided to try the custom tap I had made to open them up. It goes in tight, comes out tight, and makes little to no difference in the thread size.

I thread-milled after not getting a decent looking, or on size thread with the tap.

Didn't think of trying to measure distortion of the part while the tap was in there. That might have been interesting.

Another think I might try someday is regrinding the flutes to a spiral point and increase the rake. Otherwise, it's just a cool looking $250 paperweight.