Form tap or cut tap 1/2-13 in 12L14?

If it's not broke, don't fix it. I'd stay with the cut tap as you've already had success with that method.

Best, Steve

I've seen people grind away most of the back threads on a tap, so that only about 1/2" worth of threads do the cutting. Actually, proly only a few threads do the cutting, right?

Good idea? Bad idea?

I've noticed that OSG has gone to a similar idea on their taps, a small amount of threads and the rest is relieved down to just under the minor dia. If they're doing it on their standard lines I see no reason not to.

I like interrupted taps for pipe taps but a quick search didn't turn up those for the larger standard taps. I'm curious as to why?

Best, Steve

The major question is how often it will be removed and replaced in service. The form tapping will work-harden the 12L14 and orient the grain so it will be stronger and more wear resistant.

If there is no requirement to increase the strength (and if you are using 12L14, I would suspect that strength is not a primary concern) then keep using the cut tap.

Enjoy, DoN.

I hear this "formed threads are stronger" argument a lot. My thinking is if your design is such that the small difference between a cut and a formed thread makes a difference between success and failure maybe you should rethink your materials or design.

In 12L14 I think it's a mute point.

Best, Steve

Perhaps because it is for soft and stringy metals (according to the web description) which describes pipe materials more than most other metals. And for those which do fit the description -- that is where thread forming taps make sense in straight threads (though I'm not sure how well they could handle tapered pipe threads).

Enjoy, DoN.

Mebbe someone used the wrong tap drill??? :) :)

Apropos of Garlic et al comments, it would seem the diff is closer to 10%.

Another machinist I know did this same type of test, got a similar result from his recollection, and provided two cites, which show a max of 10%.

I don't know the specifics for individual materials. When I was a tooling editor I was supplied with a variety of studies and test results, and they did show wide variability with material types and states of initial hardness.

One phone call to a major manufacturer of thread-forming taps is worth ten days of guessing and speculation. The guys at Emuge were always especially helpful to me.

I dunno.... somehow I think if that 1200 vs 2600 lb result were typical, cut taps would be falling out of favor/use REAL fast....

I don't think it *is* typical.

The basic reason for form taps is that they're more productive. They last longer, and they don't produce chips -- the bane of production machining.

The stronger threads, initially, were an incidental benefit. Soon they became one of the prongs of their marketing story. When you deal with a company that makes high-quality examples of both cutting and forming taps, such as Emuge or any of several others, you should get an accurate picture of when to use them.

I'm curious if anyone has encountered a drawing where the use of a form tap is mandated because of the increased thread strength?

Best, Steve

I use thread forming taps all the time, especially in aluminum, because they don't tend to go off angle if you are not using a fixture to keep the tap perpendicular to the work. If tapping holes in something large, this can be a big advantage.

Joe Gwinn

When we tap holes in a complex part we use cutting taps because they can be sparked out.

DanP