No need to ‘tap out’ when it comes time for threading holes

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HASCO Hasenclever GmbH + Co KG

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Overfeed/Underfeed

In addition to the difficulties previously noted, tapping can create oversize threads that will ruin a part. On some occasions, threads will disappear at the entrance of a hole. When a tap advances inside a hole, it should advance a pitch in a revolution. If not, it will create either overfeed or underfeed. If the pressure of the feed is great enough to allow overfeed due to any elements – this applies to the leading flanks of taps – it makes them shave material on the leading side of threads that shouldn’t be removed. If the pressure is great enough to allow underfeed – this applies to the trailing flanks of taps – it makes them shave material on the other - trailing - sides of threads.

Overfeed and underfeed can happen when a tap reverses and exits a hole because of the spindle backlash, the poor condition of the tapping attachment or from other factors. In cases such as this one, overfeed shaves the trailing sides and underfeed shaves the leading side of threads.

As a result, whether through overfeed or underfeed, the shaved surfaces make the root width too wide but the crest too narrow. The inside diameters, major diameter between the roots and minor diameter between the crests, are not influenced at all. The thread gauge doesn’t inspect these. It checks the pitch diameter of threads, in other words, the width of space and thread. The amount of shaving, i.e., the amount of over/underfeed, per length of internal thread determines whether the No-go gauge can pass through the internal threads or if it engages a few threads at the entrance of internal threads (bell mouth). The oversize, or bell mouth problem, is almost always caused by the shaved thread flanks.

In most cases, the oversized conditions come from the tapping equipment, or reaction or axial force as a result of flute geometry and the difference of wedge angles between the leading and trailing flank, or both.

Furthermore, the tap is easily broken and the machine needs more power consumption when oversized conditions happen. That happens because the chamfer threads and all of full threads cut the material, and more chips are produced than with normal tapping.

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