Hi guys:
I have a small manual mill that I use with a tapmatic head to make some steel parts. My parts are initially cut for me, from 1/4" thick cold rolled material with a waterjet. I finish drill the holes to be tapped on the mill, then tap.
I use a non-sulpher cutting fluid manually applied to tap and hole. I've made hundreds of these things over the past coupel years, but the last 2 or 3 batches have given me a lot of trouble. Today while trying to figure out how to solve it, I've broken 4 taps.
I am drilling a 10-32 and a M6 through hole.
I used a guide (my notes say from USCTI) to choose the drill size. I wanted a high percentage thread.
For the 10-32 I chose a #23 drill (0.154) which the guide said my probable hole size would be .1578. That would give a 79% thread.
I haven't changed anything in my process except obviously the steel may be slightly different from batch to batch, but it is cold rolled. I could have changed drill bits due to wear, not sure there, and the resulting finish hole could be different. I have bought new taps to be sure tap wear wasn't the problem, broks all 4 of them today . I don't have a good way to gauge the true hole size. The #23 bit is a snug fit by hand.
I do not know what speed is best to use for this tap and material. I used to run at about 500-600rpm and didn't have problems, but as I was having problems lately, I tried to slow the speed. Initially that seemed to help, but then I broke some more taps. I do the same holes in aluminum and don't have any problems in it, only the steel.
I woudl venture the problem appears to to be this... the tap seems to want to hang part way through the cut. Once it hangs, it's difficult to get out.
Can someone tell me what the correct rpm is for tapping a 10-32, and M6 holes, in 1/4" cold rolled should be?
And if my drill selection looks wrong, can someone offer any advise on the correct hole size to produce a high percentage thread that is close tolerance/tight?
Thanks,
Scott