I'm no tapping expert and I will not attempt to teach basics, just some general tips for difficult tapping jobs.

1) When tapping ornery materials, I recommend the upper end of the tolerance on the hole size (make the hole as large as possible). If the hole is small (lower end of the allowable tolerance) we will approach 75% of effective thread depth and the torque required to drive the tap goes up enormously while giving very little added strength to the thread. Look in the Machinery's Handbook to determine the maximum hole diameter for a given thread.

2) If you are experiencing problems with materials that are prone to work hardening (ie: alloy steels, stainless steels, titanium, etc.) try drilling the hole 10-15% under size then finish the hole with the proper drill size using liberal amounts of coolant. This procedure will cut out any heat affected or work hardened material that may be causing problems with your tapping operation.

3) Believe it or not, surface finish of the hole to be tapped will affect the performance of your tap. In some of the most difficult situations, I have reamed or bored the holes to size before tapping.

4) Tapping copper - this stuff will also work harden. It is a gummy material and some of its alloys are extremely abrasive. Watch the land on your tap drill right above the outside cutting edge. This part of the drill has a tendency to wear back and up the drill resulting in a tapered hole. If the drill isn't changed, one of two bad things will happen; 1) the drill will break or 2) the tap will break.

Final note about tapping copper - I have heard this so many times and with such enthusiasm that it must be true. Use milk as a tapping fluid when tapping copper. They didn't specify whole, 2%, or fresh squeezed so it's up to you.

5) Tapping plastics - when tapping some of the softer plastics, I have noticed that the threaded hole is prone to being undersized or tight. A simple remedy is to place a piece of paper over one of the cutting edges. This will push the tap off center a couple of thousanths and produce a looser fitting thread. Experiment on a practice piece.

6) If you don't have a tapping head (ie:tapmatic), many times you will have to power tap into a blind hole (bottom tapping). On a manual mill or lathe, it is very difficult to determine just how deep we can go without causing problems. The next time you have to do this type of tapping operation, drive the tap as deep as you feel comfortable with and stop the spindle. Take the machine out of gear and drive the tap by hand either by spinning the drill chuck on the mill or the three jaw chuck on the lathe. You will know when you approach the bottom by the torque required to turn the chuck. Back the tool out one to two turns and reference your depth on the digital readout or on the z-axis scale. If you don't have either of these, mark your depth on the machine with a magic marker. Sometimes I will use layout ink on the tap itself and scribe a line on the flutes of the tap to show me how deep I can go. Another alternative is to wrap electrical or masking tape around the tap several times with the bottom of the tape identifying the depth to be tapped.

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