All,
I have seen several threads start to deal with tapping problems. I am starting this topic to collect information in one place to ease searches for support in this arena.
I have a couple of thing posted at
http://www.scribd.com/Lew%20Merrick that may be of interest to you. One is the paper
Pilot Hole Considerations which details how to calculate the size of tap drill you should use to provide a given
percentage of full thread and how you can calculate the effect a change in the percentage of full thread will have on your joint. Another paper,
UN Thread Data Chart provides information on full thread strengths for most Unified National (inch-based) threads. I have
not done this for metric threads as there is still no
single set of tolerances and allowances needed to complete the calculations for them. Using the largest pilot (tap-drill) hole you can for any given task makes tapping a whole lot easier. This is
rule zero for tapping.
Rule one is to buy good quality taps. The cheapie taps are cheap for a reason (in most cases). They cost you dearly when trying to remove a broken one from a hole!
I am partial to OSG taps, but I designed and built some grinding machines for them years ago and can buy them direct from the factory -- which has something to do with my bias.
Rule two is to keep them
sharp! If you do not have the set-up (or access to a set-up) to sharpen them, be very careful as it is easy to make them worse trying to hand sharpen them. When in doubt, throw it out.
Rule three is to be sure that you are driving them
straight into your part. Very few people have the hand strength to torque a (sharp) #10 (5 mm) tap to the point of breaking. A six-year-old can break such a tap in
bending very easily. Tap guides are cheap at twice the price. A
Tapmatic type head is golden (and there's an inexpensive Indian-made head being sold by Harbor Freight and the like that is reasonably good so long as you disassemble it, clean up the burrs, lubricate everything nicely, and reassemble it -- I have seen them on sale for less than US$75 several times in the recent pass and I have "cleaned-up" three of them for friends in recent years).
Rule four is
use the right lubricant!
Tap-Magic Gold is my current favorite when dealing with obnoxious materials (hardened stainless, titanium, nickel alloys, etc.).
Moly-Dee is my more general purpose (steel) lubricant -- though I still use axle grease for plain carbon steels. I use cetyl alcohol (wax) for most aluminum tapping (though
Tap-Magic Aluminum works better on non-heat-treated aluminum). I often use lard when tapping copper-based materials. I usually tap cast iron dry.