There was also the issue of cleaning the chips out of the tapped holes. Most of my projects are precision or optical in nature and chips simply can't be tolerated. I've spent many hours with a fine wire getting the last particles out of some particularly small threaded hole.

Surface feet calculatorrpm

The size of the tap drill is very important, and cannot be obtained from the usual table. As an example, the drill size for a normal #4-40 tap is a #43, or .089". The drill size for a #4-40 "Thredfloer" is a #38, or 0.101".

Surface feet calculatorinches

You can also use the calculator to find approximate SFPM on disc grinders.  To calculate disc grinder enter motor RPM and select the area you are using on the disc pad.   Measure from center and enter in the wheel diameter box.

When you inspect the thread, it will have a double peak. This is normal and expected. In fact, you can judge the thread percentage and evaluate the tap drill size from the appearance of the peaks.

Surface feetper minuteCalculator

Finally, a machinist friend gave me a Balax "Thredfloer" tap. These are cold forming taps- they don't cut at all, but form the material into the correct geometry. The formed thread generates no chips, and strength is increased due to work hardening of the material. Naturally, they can only be used with ductile materials, but that includes most of the things I would normally tap. With a good moly oil they even work on stainless steel.

Surface Feetper minute Chart

My first reaction was amazement at how easily the taps did their job. Without the usual grooves the tap is extremely strong. For large threads the torque will be higher than a cutting tap, but for the small threads I usually do, the torque is actually less. The wear rate is also extremely low, in fact I've yet to need a replacement on any "Thredfloer" I've purchased.

For years I've used conventional taps, with the occasional foray into spiral taps just for aluminum. Though the results were fine, it seemed like the taps quickly dulled, even with good quality lubricants. When taps are called "perishable tooling" they really mean it.

It's always hard to choose a tool that costs five times as much as others, especially when everyone else does things the traditional way with what appears to be good success.

The only downside is that these taps are about $13 instead of the couple bucks for a short-lived regular tap. I've tried to save a bit by buying cheaper imported thread rolling taps, but they can't compete with the patented Balax design. You only save a buck or so, the torque is higher and the finish rougher. Get the good ones!

Use calculator to find approximate wheel surface feet per minute by entering  the fastest spindle speed RPM, the wheel diameter, and the selected drive percentage. The calculator easily allows you to compensate for wheel wear during buffing applications.