Minimize Tap Breakage
By Bob Daley
When tapping in the lathe, it’s sometimes difficult to keep the tap parallel with the axis of a small diameter hole in a shaft. The common method is to hold the tap in a chuck mounted in the tailstock and rotate the lathe chuck, either by hand for blind holes or with power for through holes.
I’ve done this over a number of years and occasionally have broken the tap, for whatever reason. Also, if the tap rotates in the holding chuck it wears the chuck jaws.
Here’s how to do it and not break the tap:
1. Drill the tap hole in the workpiece, then remove the workpiece.
2. Install a piece of angle (I use aluminum) in the tool post.
3. Measure the shank of the tap to be used and select a drill to suit. Mount the drill in the lathe chuck and drill a hole in the angle. Make a note of the cross slide dial just in case you need to move the cross slide.
4. Reinstall the workpiece in the chuck and lock the lathe drive so the chuck can’t rotate.
5. Pass the tap through the hole in the angle piece, attach the tap handle, and tap the hole.
The angle piece supports the tap central and parallel to the workpiece, thus reducing the risk of tap breakage, and hand tapping gives a more sensitive “feel” when tapping a blind hole.