|Sometimes woodworking involves a
little metalworking. I was trying to modify a jig a couple weeks ago by
drilling and tapping a hole in a metal bar, and I broke off a tap. I was not
But I am lucky in that I have a terrific resource in the guys at
ShopNotes magazine. So I asked them what I was doing wrong, and I
thought I'd share their tapping technique with you.
Here's how I should have done it: Start by marking the position of the hole. To
keep the drill bit from wandering, make a dimple with a metal punch. Now you're
ready to drill the hole.
To keep the drill bit from heating up and breaking, it's best to drill a series
of progressively larger holes. The final size of the hole should be slightly
smaller than the diameter of the tap. This leaves enough material for the tap
to cut the threads. It doesn't hurt to add a drop of oil now and then to help
the drill bit cut smoothly with less heat build-up.
Before tapping the holes, there's one more thing to do: To help center the tap,
use a countersink bit to cut a slight chamfer in the rim of the hole.
Now choose a tap that's sized for the screw you're using, and tighten it in a
T-handle. And add a few drops of oil to make it cut easier. And start tapping.
The goal here is to start the tap straight. This way the tap won't bind as you
cut the threads. So, holding the tap straight up and down, apply a little
downward pressure and rotate the tap clockwise about a half-turn until it
starts to cut. After the taps cut a few threads back it off to remove the
While it's tempting to cut the threads all the way down, this puts a lot of
stress on the tap, and I can tell you from personal, recent experience, there's
an EXCELLENT chance the tap will break.
So after each half turn, back off the tap to clear the waste. Add a drop of oil
and repeat the process until the threads are cut all the way down.
Finally, run the tap down the hole a couple of times. This cleans up the
threads and clears the shavings from the hole.
Go to Tip #29