Tapping Deep Holes??

[Pete W.]

Hi there, all,

I'm a bit embarrassed to start this topic - I thought I knew how taps work and how to use them but a recent experience has sown some doubt!!

A friend asked me to drill and tap some pieces of 10 mm brass rod, some short ones and some about 37 mm long.  The holes were to be axial and tapped M6 x 1mm and the threads were to be continuous right through the whole length.  He supplied the brass rod, I guess it was CZ121 or some such.

The short ones were no bother, they were within the capability of my standard length taps but, to do a proper job on the longer ones, I bought a set of taper, second & plug nut taps, with long shanks, thinner than the root diameter of the M6 thread.

Before these arrived, I'd started the threads from one end with my standard taps, having drilled the through holes with a number 11 drill.  Much to my surprise, when I applied the taper nut tap to the job, it wouldn't cut as far as I expected.  I'd started with the brass rod in the lathe chuck and the tap in the tailstock (keyless) chuck, turning the lathe chuck by hand, and the tap would cut about half-way up the taper section, after that the resistance was such that the shank of the tap revolved in the chuck.  So I transferred the job to my bench vice (with fibre jaws) and the tap to a Tee-handle tap wrench.  Same thing, before the tap would cut, the brass rod turned in the vice, even after I'd leaned on the vice handle.

By now, my friend was anxious for his parts.  So I tried the second nut tap and was easily able to run it right through the length of the job.  I then ran the plug tap through to complete the job.

I relayed this experience to the supplier of the taps (no names, no pack drill) who offered to send me a replacement taper tap.  That arrived this morning, complete with a short length of mild steel bar with a tapped hole in the end which he claimed to have cut with that tap.  However, when I tried the taper tap in a short length of the brass bar, same number 11 tapping size hole, it too would only turn about half the turns on the taper section before the required torque rose above what I regard as reasonable.  I opened up the hole with a number 10 drill but that made no improvement.  Trying the butt of the number 10 drill in the hole in the steel bar suggested that he hadn't used any larger tapping drill than I had.

Both tap containers had a label saying it is a 'bottoming' tap but a second label had been applied over that saying 'taper lead'.  The supplier stated that the taps had been reground from bottoming to taper in his/their workshop.

So, here I am in a deep quandary!!!  The supplier has offered a remedy but I still can't make it work!          

Surely, I should be able to use the taper tap to at least the depth of the taper section (clearing chips as required) and then apply the second tap?  Or have I been mis-using taps all these years?  If the taper tap tightens up that soon, why was I able to easily put the second tap right through the hole?  (I don't mean the second taper tap, I mean the first second tap!!)

Help!!  I'm confused!!  If I can't get a job like this right, what chance have I got?!?!  I don't want to be unfair to the supplier, I do want to understand what's going on.

[BillTodd]

Correct drill for M6  is  5mm not #11 or #10 both are too small


Bill

[vtsteam]

Looks like the old rule of thumb -- subtract the pitch from diameter to get tapping drill size works for metric, too.

6mm - 1mm = 5mm

[angus]

it does for any thread that has a 60 degree form

[Lew_Merrick_PE]

Pete W,

1) The standard M6X1 pilot drill is ø5 mm (#8 or ø.199 in) for holes < 20 mm deep and progress from there up to ø5.25 mm (#4 pf ø.209 in) for holes over 100 mm deep.

2) In most brasses, a slightly undersized pilot hole should not be a problem unless the tap is dull!  Many manufacturers' of taps assume that you will not tap for greater than 75% of full thread depth and cut sharpening corners under that assumption.  If this is the case, you may wish to change to higher quality taps.

[Pete W.]

Hi there, all,

Thank you for your various inputs.

I acknowledge that I was using too small a pilot drill.  It seems that my trusty DTC chart probably gives thread root diameter - I'll investigate that further.  It's never tripped me up before!

Here's the part of the chart I used:



However, I've done some more work today that suggests the pilot hole diameter isn't the whole story.

I put through a 5 mm drill and tried the taper tap again - same result, it cuts a few threads and then locks up!  So, I swapped to the second tap (not the second taper tap, the first second tap!) and it went through like the proverbial knife through butter.  Backed off every turn or so but the chips were falling out of the other end of the test piece OK and it both felt and sounded right.

My conclusion is that when the supplier ground the taps to convert them from bottoming to taper, they didn't back off the material behind the cutting edges of the flutes.  If I could contrive a set of bench centres I could probably throw more light on that conjecture but I'm eager to progress my microscope repair. 

[Joules]

Can you rig an airline onto the opposite end of the tube you are cutting.  My guess is its swarf getting trapped and not cleared out so getting trapped in the threads, wiggle the tap as you thread to break up the swarf and wear eye protection.  That would be my try 

p.s
     tap through a piece of cloth might keep the swarf from flying all over.