Cutting Tools Chicago aka General Cutting Tools along with Greenfield Tap & Die helps explain and take the mistery out of the “H” limits on taps. Roll form tapping is an effective way to produce internal threads in production applications. There are many benefits to this process, including the absence of chips. Before proceeding with roll form tapping, consider the material to be tapped, the hole size before tapping, the machine capabilities, and the class of fit requirement after tapping.
In the case of fit, the requirement for the finished thread is the same regardless of whether it is a cut or rolled form thread. Use 5/16-18 2B internal thread as an example. 2B is the designation for internal thread minimum and maximum values after tapping. It is normally checked with GO and NOT GO thread gages. For cutting this thread, the recommended tap “H” limit value is H-5; but, when roll forming the same thread, the recommended value is H-7. There are published charts in our catalogs detailing other sizes and for recommendations.
The reason for the larger “H” value is the slight shrinkage that occurs in the hole after the tap is removed. All metals have a memory and will try to spring back to their original shape. When displacing metal using a roll form tap, this condition will occur. The higher “H” limit compensates by forming a slightly larger thread which shrinks slightly after the tap is removed. Other factors may interfere, but, generally, the recommended “H” limit values published for cut or roll form threads will deliver acceptable gaging results.