Deep Hole Tapping

[jocat54]

I need to tap 4 holes 2 1/4 inches deep to 3/4x10 in some 1018 steel.
Would a 50% thread work okay or should I shoot for 60%. Don't think I can tap a 75% thread---too much torque required. Looking at various charts there doesn't some to be much torque difference required in 50 to 60, but a lot at 75%
I am using a HSS tap(new)
I have never tapped this large of hole this deep and looking for real life experience.

 

[Tony Wells]

Just get a plug or taper tap and a bottoming tap and go with the book size on the drill. That's soft material and will tap easily. Just go deep as you can with the plug or taper, then use the bottoming tap. Repeat if necessary. Use a good quality tapping fluid. With only 4 holes to do, you won't appreciably dull the taps so that it becomes difficult.

 

[savarin]

After breaking so many 4mm taps in deep holes that I lost count I found this site.
http://theoreticalmachinist.com/
His calculator for drill/tap sizes and relative strengths of fit would have prevented me from destroying so many of them.

 

[Bob Korves]

Only the first few threads are doing any cutting, so the loads will be close to the same as for a shallower hole. You will need to make sure the tap is long enough for the job. Thread loading depends on the number of threads actually engaged as well as the percentage of thread depth.

 

[JimDawson]

This is how I tap large (over 1/2 inch) holes. Saves a lot of work.

No, the lathe was not in gear when I was doing this!

 

[jocat54]

That's soft material and will tap easily.

I did 2 holes 5/8x11 using the theoretical 75% drill size and they weren't what I would call easy, doable yes but not easy
Work piece was wanting to turn in the lathes chuck.

 

[Tony Wells]

Then John, you need a bigger tap wrench. Use the Machinery's Handbook thread data instead of a drill chart. That will give you limits of the hole size within each standard class of thread.

 

[FLguy]

Just get a plug or taper tap and a bottoming tap and go with the book size on the drill. That's soft material and will tap easily. Just go deep as you can with the plug or taper, then use the bottoming tap. Repeat if necessary. Use a good quality tapping fluid. With only 4 holes to do, you won't appreciably dull the taps so that it becomes difficult.

This will work very well, also consider a thread form tape and drill to 50% effective thread; it'll be as strong as a 72% thread and overall less work.

 

[bfd]

3/4-10 is a substantial tap and if you use all 3 types taper,plug and bottom you should have no trouble. use plenty of good cutting fluid. better than the three tap set is to use a gun tap also known as a spiral fluted tap a much better tap great for thru holes the chip flows out the open hole and clears itself. on a closed hole you will need to remove the chips with air or needlenose pliers. then use a bottom tap. on regular taps one turn and back up to break the chips most tap breakage in because the chip winds up in the flute and jams causing excessive force to be used or you start the tap crooked and put on a side load. I do most of my tapping with a battery powered drill. using the above spiral tap. bill

 

[Tony Wells]

I've never had much success with form taps by hand. Most tool reps won't recommend it either. They never say why, but their training probably is from the mfg. I love form taps in a machine though. Used them on literally 10's of thousands of holes. If you try it on this 3/4-10, be very sure you are straight. It's more critical on a form tap, as is the tapping fluid. You are actually displacing the material, so lots of pressure in a small area is seen. And if you do happen to break a form tap (in general) they are quite a pain to get out. Even an EDM or tap disintegrator is a bit limited, since there are no flutes even if you manage to burn a hole down the middle right at the minor diameter, now all you have is a hollow broken tap in the hole. It's brittle so if you burn as big as possible you can chip away at it, but it's kind of messy.