Wear of Form Taps in Threading of Steel Cold Forged Parts

Abstract

Tapping of blind or through holes is a common operation in several applications in the automotive, windmill towers, general machinery, and other industrial sectors. On the other hand, tapping is usually performed at the final manufacturing stage of components, so operation errors can lead to very expensive scraps. This article focuses on the performance of different forming taps since there are no standardized procedures for evaluating form tapping, beyond the mere visual assessment or go/nogo tests. Tool behavior was studied according to four aspects: (a) wear analysis of tap forming lobes, (b) measurement of thrust force and torque during tapping, (c) metallographic study of threads, and finally (d) evaluation of coating and tap geometry. In the work presented here, tap wear and thread quality are correlated with processes monitoring parameters. A useful conclusion for tap manufacturers is that taps with hexagonal section and 5 pitches in the chamfer zone led to lower thrust forces and torques than pentagonal section ones, along with a lower wear. Hexagonal/5pitch taps led to better results due to a more gradual deformation process for producing a thread. A larger number of pitches before reaching the nominal diameter resulted in a reduction of strain between successive lobes, generating lower force and torque values.