Help with tapping problems - breaking taps

[shorton]

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

[Geof]

Cold rolled is variable so it could be that you are just unlucky with this batch.

Also you are being greedy wanting a 79% thread in cold rolled.

What kind of tap are you using? For a through hole it can be a spiral point machine tap; these are much stronger than the spiral flute type.

On occasion I have had success on difficult to tap steel using taper taps intended for manual tapping.

Also have you tried tapping partway through, coming out and going back in? I will admit this sometimes fails because the tap does not re-enter the thread correctly but with luck it can work.

[shorton]

Originally Posted by Geof What kind of tap are you using? For a through hole it can be a spiral point machine tap; these are much stronger than the spiral flute type.

Spiral point machine tap.

On occasion I have had success on difficult to tap steel using taper taps intended for manual tapping.

OK, I may try one

Also have you tried tapping partway through, coming out and going back in? I will admit this sometimes fails because the tap does not re-enter the thread correctly but with luck it can work.

I did and I thought it was helping, but it took me 3 holes to break a tap.

Someone in another forum thinks my hole is too small. They said their USCTI chart advised:

Tap drill size     Theo. % thread
--------------     ------------
5/32                    83
#22                     81 
#21                     76


For an M6 threaded hole:

Tap drill size     Theo. % thread
---------------    ------------
#10                      84
#9                       79
5 mm                     77

If that is right, their smallest drill is bigger than what I'm using. May be part of my problem..

[mc-motorsports]

And use good taps. I have never had a problem with OSG taps.

[shorton]

Originally Posted by mc-motorsports And use good taps. I have never had a problem with OSG taps.

Thanks, I'll get some of them for the mext try.

[g-codeguy]

Originally Posted by mc-motorsports And use good taps. I have never had a problem with OSG taps.

Got to agree with this statement. Our purchasing agent is famous for buying the cheapest product he can get. Can't blame him as that is what the company is asking of him. Plus I think he gets a bonus for saving money.

Anyway...for us a cheap tap may give a few good parts (sometimes as few as 0-3) before cutting oversize/breaking, etc. while one from OSG (or similar quality) will often run the whole job. Hard to get the company to understand this tho.