Why would industry use undersized taps?

[jscarmozza]

About 20 years ago I acquired a large quantity of taps, some were obviously for a power tapping machine since they were spiral tipped and didn't have a square end for a tap handle, all of these taps were undersized. What is the purpose of using an undersized tap? I think these taps came from Curtis Wright and had been used to manufacture aircraft engines during WWII.
Thanks, John

[10KPete]

Threads can have various fit tolerances for different applications. These will require different taps and dies.
The most common 'different' taps are oversize for use where plating is involved.

Pete

[GlennW]

Some aircraft engine manufacturers of that era used standard size studs screwed into undersize threaded holes in castings to obtain an interference fit to retain the studs. Others used standard taps with over sized studs to obtain the desired interference fit.

Over sized taps are also used on some alloys where heat treating is needed after the holes are tapped. The holes constrict during heat treating

[jscarmozza]

I knew about the oversized dimensions for coated fasteners but undersized had me puzzled. Your explanation makes sense Glenn, the last thing you would want on an aircraft engine is to have the studs vibrate out. Thanks, John

[spro]

They were worn out. Of course they were undersized.

[tornitore45]

They were worn out. Of course they were undersized.

Taps get dull long before they wear undersized.

[Harold_V]

They were worn out. Of course they were undersized.

Taps get dull long before they wear undersized.

Yep! I dare say that the size of a tap goes unchanged by the time it has dulled beyond being useful. That might not be true if a guy is tapping blocks of aluminum oxide, however.

H

[Steggy]

Back in my railroad days, the company for which I worked was producing steam generators for mounting in Amtrak's first- and second-generation Diesel-electric locomotives (E-units and the SDP40F)—steam-heated cars were still common in those days. The steam generator used a feedwater pump with a cast-aluminum crankcase. Undersized taps were used to tap the crankcase holes into which studs were to be placed.

Studs were inserted by heating the crankcase to where the tapped holes would expand a few thousandths, allowing the studs to be threaded in and seated. Once the crankcase had cooled the studs were solidly locked in place. If a stud had to be removed as part of a pump rebuild it was necessary to heat the boss with a torch to the point where the stud could be turned. In practice, that need seldom arose.

As I recall, the company purchased standard "gun" taps which the tool room ground very slightly undersize, producing a thread with a major diameter slightly smaller than nominal. I once by accident used one of those taps to tap a hole into a fixture I was building. Imagine my puzzlement when I discovered the bolt that was supposed to go in the hole wouldn't fit.

[Steggy]

Yep! I dare say that the size of a tap goes unchanged by the time it has dulled beyond being useful. That might not be true if a guy is tapping blocks of aluminum oxide, however.

Now, I must say I had seen an awful lot of oddities over the many years I was in the railroad supply business, but can't recall encountering aluminum oxide in block form. Trying to tap something like that would be akin to trying to tap the center hole of a grinding wheel.

[tornitore45]

Trying to tap something like that would be akin to trying to tap the center hole of a grinding wheel.

And that is the recommended way to undersize a class H3 tap.

[jscarmozza]

These aren't worn out, they're unused old stock. Attached is a photo of just a small sample of what I have...and the're all undersized

[Harold_V]

As I recall, the company purchased standard "gun" taps which the tool room ground very slightly undersize, producing a thread with a major diameter slightly smaller than nominal. I once by accident used one of those taps to tap a hole into a fixture I was building. Imagine my puzzlement when I discovered the bolt that was supposed to go in the hole wouldn't fit.

Hmmm. Something doesn't sound right here.
For starters, class 5 threads (interference fits) are not created by altering major or minor pitch diameters. If one cares to explore the H28 handbooks, there's an explanation why.

Interference threads rely on the pitch diameter. To not do so encourages failures

H