Best tap for hard steel.

[Davisa]

Hi, got a steel plate with occasional hard areas. I mean I'm drilling 1/4" holes going along great in one area and a 1/2" away and it eats a m42 bit like nothing. I have to tap these holes and am wondering what type of tap to use. Any and all suggestions would be appreciated. I will be doing this by hand.
Tanks in advance.

[ChipMaker4130]

There is no tap I know of that will cut threads in truly hard material. Inclusions are often as hard as the ball-bearing they likely came from. My prediction is that you'll break as many taps as you try, and discover a whole new vocabulary in the process. Threads can be EDM'd if its important enough to spend the money for that, but the best solution is probably to get another piece of material.

[tornitore45]

Plan A Get some quality stock
Plan B Try Annealing the spot or the entire part
Plan C Gun Taps are 2 flutes and therefore the strongest, If the material is hard then 50% thread (larger hole) should make the job easier.
Good tapping fluid is a must. I found Boelube is the best, is more than a wax that an oil.

[Davisa]

Thanks for the info. I have a gun tap so I'll try that first. If it breaks then I'll get some new steel. This project is taken way to long and has fought me all the way.

[Harold_V]

Gun taps are not easy to start by hand due to the two flutes. I recommend you make a bushing that holds the tap erect and allows you to bear down on the tap while starting. It will also be quite useful in keeping your tap erect while tapping through the plate. There's a real benefit beyond just helping start the tap, as the bushing (guide) also limits the tap's ability to walk around, yielding a thread that is more likely to be on size. I like a large area so it can't tip easily. It need not be metal. I often use a piece of particle board that has a laminate covering. It makes a good drill guide too when drilling by hand.

H

[Russ Hanscom]

If the work is portable, a hand tapper is a great project. I made this one years ago and it has saved me a ton of taps. Most all the parts came from the recycle pile except possibly for the tap chuck.

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[tornitore45]

I stopped breaking taps after I got it through my head that after 3 or 4 threads the tap is not going to straighten and start cutting way too deep.
Another thing is to use a thread percentage commensurate with hardness and number of engaged threads. No point in having the female thread many times stronger that screw striping load.
I do not have a tapping head but always use a set up in the mill or lathe for alignment.
Something I learned in awkward position to give a thread free point to the screw or to the hole to facilitate alignment. Is not always possible to do but can save a lot of bad language during assembly.

[ChipMaker4130]

Well- did you get it done?

[Inspector]

Any way to press in threaded flanged bushings from the backside?

Pete

[knifemaker3]

do a search for carbide taps for hardened steel. They look weird, but do work. MSC, Grainger, and others carry them. I think will tap up into the 50's on rc scale. EASY to break and cost a fortune

[Fasturn]

Not the best tap but the best lube. Moly Dee is what you use. Coated cobalt taps long tapers work best. If it's not a thru hole, progressive tapping with a 50% then 75 % thread. In the lathe you dont thread in one pass ?? Good luck.

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