Why Are Roll Form Tap Recommended “H” Limits Higher Than Cut Thread “H” Limits?
Roll form tapping is a great way to produce internal threads in production applications. There are many benefits to this process including the absence of chips.
Some factors need to be considered before proceeding with roll form taps such as the material to be tapped, the hole size before tapping, machine capabilities, and the class of fit requirement after tapping.
In the case of “class of fit”, the requirement for the finished thread is the same regardless if it is a cut or rolled form thread. Use a 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. When 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 fit recommendations.
The reason for this larger value is due to 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 for it by forming a slightly larger thread which shrinks slightly after the tap is removed.
Some other factors may interfere, but in general the recommended “H” limit values published for cut or roll form threads will give acceptable gaging results.
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