(courtesy Norseman Drill and Tool)
1. If holes are “Undersize or oversize” check against the setup instructions. Check the tap for correct Pitch diameter limit for desired “Class of thread“, and the “Drill for correct size“.
2. If the tap is “Producing rough thread” it generally means the tap is pushing or tearing the metal instead of cutting it. The tap may be dull, have galled threads or may be running at too slow a speed. If none of these, increase the hook angle of the cutting face.
3. If the tap is “Loading (metal clinging to the tap) or Galling” check the lubrication. Here too, the tap may be dull, or it may be producing too high a percentage of thread. Shift to a surface-treated tap.
4. If “Tap breakage” is encountered the tap may be cutting to high a percentage of thread, be insufficiently lubricated or have too little clearance for chips at the bottom of the blind hole.
If none of these conditions exist, Increase the speed, except in hard or tough materials. In these materials, lower the speed.