Tapping drill rod/silversteel... is it just a PITA?

[TroyO]

I'm making up some short rails for a powerfeed out of 1/2 drill rod. The ends are tapped 1/4-20... or well, they eventually will be.

I haven't broken a tap off in it yet, but this stuff is just not easy at all. Is it just the material? As far as I know the taps are OK...I've used them on other stuff without issue. I've tried the 2 I had in that size with the same result. To the extent they were machined that seemed to go OK... face 2 ends and part off 4" chunks.

Maybe it would be easier to just get some 1/2" mild steel and re-do them... I just happened to have the drill rod on hand, and it looked all shiney like it was just begging to become a slide rail, LOL.

 

[Metal Butcher]

Hi TroyO, Its very hard to say what the problem may be, and if you really have a problem. I guess it depends on how much experience you have had with tapping this size, and tapping with the tap that your currently using. And I'm sure you know that as a tap size goes up, so does the required force. If you tapped aluminum or brass with the same taps, they are much easier materials to tap and require less force.

If both taps are from the same batch or of a questionable quality that could be a reason. Are you using a cutting oil, steel should be tapped with a suitable tapping oil.

What size tap drill did you use, trying to tap a thread with a high percentage requires considerably more force than a smaller percentage using a larger drill.

Unless you have a reason, a 50 to 60 percent thread should be adequate.

For a 50% 1/4-20 thread drill with a #3 drill, for 60% #4 drill, and for 70% ist's a #6 drill. I wouldn't even attempt 70% unless I had a definite reason, and wanting a supposedly stronger thread is not a good one.

Drill rod (silver steel) is relatively easy to work with as far as I'm concerned, but I haven't had the opportunity to tap very much of it.

-MB

 

[TroyO]

Ya know, I still don't know what the problem was. As I sometimes do when things are going wrong I "took it all down" and tried again. Same setup (7/32 hole, setup in the lathe with a spring center, using a Nix-Stick for lube... a GP beeswax type cutting/tapping compound) It wasn't a top quality tap, but a decent one.

Anyway... after putting all back in place starting from square one I tried again. A few more turns of high pressure turning and something loosened up and the rest went just fine? Multiple holes so I can't figure it just had a hard spot in the metal or something. Unless I managed to work harden it while drilling?

Anyway.... whatever it was, I got though it and now have the parts I need and the tap seems just fine.

I am still curious... I have used the same setup in several materials without issue.

Thanks for the help though, I just wish I could add what the solution to the issue WAS for future searchers of the forum, but I got no damn clue, LOL.

 

[tel]

I suspect you may be right - a little bit of local work hardening.

 

[Mainer]

Drill rod isn't my favorite stuff to work with, but it shouldn't be *that* bad. With a 7/32" tapping hole, you should be in good shape. That's larger by a bit than a #3 drill, certainly larger than the #7 drill usually recommended for 1/4-20. It must have been some localized condition.

 

[arnoldb]

Like Tel said, most likely work hardening.

I love working with silver steel - but it wants to be machined at a slower speed, with sharp toolbits and with a "positive" feed; don't let your toolbit or drill slide against it; it must be making chips all the time. A lot of the time I just machine it dry, especially when turning, but for tapping you definitely need a good tapping compound. Same goes for parting; it likes a bit of lube for that.

Regards, Arnold