Causes of Oversize Tapped Holes

One of the most common tapping problems is oversize tapped holes where the NOT GO gage threads into the hole 4 or 5 turns or more. The National, Federal, and Military gaging standards allow three turns max. on the NOT GO.

The first obvious cause for oversize threads is misalignment between the drilled hole and spindle, particularly when the hole is drilled in another station or machine. Under these circumstances, it is extremely important that the fixturing be maintained and is clean when loading the part to insure accuracy. Slight misalignment can be compensated for by using a “radial” float holder, which allows the tap to freely move from side to side to align itself with the drilled hole. In a properly maintained CNC machine, where the hole will be drilled, chamfered and then tapped, a radial float holder is generally not necessary.

A less obvious cause for oversize tapping, but one that accounts for the vast majority of oversize tapped holes, is “shaved” threads. Shaving is a condition where the tap chews or “shaves” away the sides or flanks of the threads in the part. The teeth become thinner or narrower which allows the NOT GO gage to enter the hole beyond the three turns. This “shaving” is caused by a slight mis-match between the lead of the thread on the tap with the feed of the machine. If overfed, the tap is pushed faster than the taps thread lead, which shaves away the front flanks of the threads in the part. These would be the flank surfaces you would see when looking into the top of the hole. This is the most common form of shaving. If the tap is underfed, it will shave away the bottom flanks.

Shaving can occur with all types of feeding mechanism, including the most sophisticated CNC’s with synchronous or rigid tapping. Permissible tolerances in both the tool and machine may cause a slight mis-match of lead and feed, which may result, is shaving. When this occurs, the tap should be fed slightly less (approx. 98%) than the thread lead and use an “axial” float holder (pull out-tension/push in-compression) with tension to allow the tap to follow its own lead.