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Screw Sizes
Screw Sizes
Screws are labeled by the diameter of the outside of the threads. To make a threaded hole that fits a particular screw size, you need to start with a hole that is slightly smaller: just a bit bigger than the size of the inner core of the screw (as if the threads were all removed). Additionally, threads come in several different gauges, although usually there are only one or two standard options for any given screw diameter.
For example if we have a 3/8" screw, and therefore need to drill holes of 5/16". Sometimes this information is printed on the tap itself. The thread gauge for our screw is 16 threads per inch. The tap set had a handy little tool for measuring this, but it is also easily possible to count threads next to a ruler.
Standard Drill and Tap Chart
Standard Drill and Tap Chart
Standard Tap Chart.pdfThis Tap chart came from the Little machine shop site - Tap Chart
Metric Drill and Tap Chart
Metric Drill and Tap Chart
Metric Tap Chart.pdfPreparation
Preparation
The tap & die set should have two handles, one to hold taps (for inside threading) and one to hold dies (for outside threading). Select the tap handle and unscrew it until the tap you've chosen will fit in, then tighten it again. Clamp your metal solidly to something that won't move, or otherwise fix it in place. Run a light coating of oil over the edges of the tap screws. You're ready to go.
Twist!
Twist!
Insert the end of the tap into your hole. Keeping the tap as perpendicular to the face of the metal as possible, and exerting some downward pressure, start turning the tap slowly clockwise. Very soon you will feel the teeth of the tap start to cut into the metal. For a short while the turning will be wobbly but as the tap progresses downward into the hole, it will stabilize.
The tap works by scraping a spiral groove on the inside of the hole. The metal displaced from the groove in the hole is pulled into the grooves on the tap. You're doing this by hand, and it gets hard to cut the metal pretty quickly -- after about a quarter turn the tap gets very stiff. This is because the piece of metal already scraped out is too big now. To cut it off so it can drop out of the way, reverse the tap direction for a quarter or even a half turn. You will hear and feel the other side of the tap screw edge knocking off the metal scraping.
Keep doing this until the tap is all the way through, and turns freely. The bottom 1/4" or so of the tap, called the chamfer, should be entirely sticking out. Then simply unscrew the tap out of the hole again. Don't forget to clean off the oil and metal shavings!
One Day Build Thread Tapping Guide Block
One Day Build Thread Tapping Guide Block
Adam shows you how to make a tapping guide block to help make perfectly aligned screw and bolt threads reliably and with ease. It's a handy accompaniment to the tap set you may have in your own shops, and something that may save you from breaking taps or bolts in the future. Adam dives into the ins and outs of thread tapping, common pitfalls, and his favorite tips and techniques for using these essential tools!
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