Work Hardening Effect!

It has been observed that alloy steels having high carbon percent can pose a problem of work hardening of the skin because of drills that work at high cutting parameters with emulsion as a coolant. The margins (or cylindrical land) are wide in size (see photograph 1) and these burnish/rub on the cut surface. This action in the presence of the coolant leads to work hardening.

This is particularly harmful for the subsequent finishing operations.

Tapping of connecting rod bolt hole could not be done on an HMC after a full carbide internal coolant hole drill performed the core hole drilling operation. Tapping after this operation was not economical because of very poor tool life. Various tap geometries/tap tool material could not improve the tool life. The only remedy was to introduce a low speed intermediate operation (for cutting the hard skin) that produced the final size of the pre tapped (core) hole. Thus for tapping of M12x1.25 4H/5H class tolerance, the original process of diameter 10.75 mm drilling followed by M12X1.25 mm tapping was changed to 1) Diameter 10.5 mm hole drilling 2) Diameter 10.75 mm hole milling by HSS tool and 3) M 12x1.25 mm tapping!

Almost similar were the symptoms in crankshaft (shank end) drilling and tapping. Here too, an intermediate tool for removing the hard skin restored the tool life during tapping!

In case the productivity is a prime concern then this intermediate tool can be a full carbide tool (a reamer or a hole mill) that can be used at 40 to 50% of the drilling RPM! This tool can be used without coolant (air jet will help) if the taps continue to show poor performance due to effects of skin hardness. These finishing tools have a very small supporting land (or margin, see photograph 2) that does not lead to skin hardness problem.

By Surendra Datar
Tata Motors Limited
Pune