I was drilling and tapping bunch of 1/4-20 holes in mild steel and I stopped the machine after I thought drilling sounded little different than usual just to find that drill got little dull and it drilled one hole 0.22". No problem I thought, I just tap it with a form tap. I have to say I use form taps very seldom. The hole is fine when you check it with a thread gauge but dia reminded 0.22". Are form taps not recommended for mild steel? I used rigid tapping at 1000 RPM. Any thoughts? Jerry
Form tap
Apr 28, 2008
5 Replies
Jerry, Did you use the form tap after the hole was already tapped? If so, the minor diameter won't shrink because there is no metal left to displace by the tap. Fred
No. I stopped it before taping.
Form taps do work in carbon steel, stainless, too
Check a manuf website for more info.
(I don't have the cajones to do ss that way, but have at it.)
We form tapped thousands of holes in electrical outlet boxes, they are the ticket in thin sheet.
I just looked closer at the material today and it does say mild steel on the drawing but it was customer supplied material so I checked the hardenes and it is 22 RC so I think it must be something else. Maybe this was the problem. Our inspector didn't have a problem with it. I guess it is only a hole for a guard. I still wonder what happened. Jerry
They work great for threading in modeling clay.
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