Ground vs. milled gear racks: pitch accuracy, hardness — and which axis needs which

Where the processes differ

A milled rack: straighten the bar → mill the teeth → (optionally) induction harden → finish the mounting faces. The tooth form is milled before hardening, so any quench distortion stays in the teeth; accuracy is about ISO 1328 class 7, cost is the lowest, and it is the economical choice for medium-accuracy linear motion in automation and general machinery. Wolfer also offers a hardened-and-milled class 7 rack.

A ground rack: mill with stock → induction harden to HRC 50–55 → grind the flanks on a gear grinder → grind the mounting faces → inspect. Because grinding comes after hardening, the tooth form on the hardened surface is ground in rather than locked in by distortion: pitch accuracy reaches class 6, and the flank is hard, wear-resistant and long-lived. Wolfer grinds up to 2 m in one piece; longer travels are dowelled end to end and ground after assembly, with pitch matched to ≤ 0.03 mm across the joint.

Module 1–10; standard lengths and mounting holes are on the selector pages, other lengths and hole patterns are machined to drawing; pinions can be supplied as a matched set.

Straight or helical?

This is a second, independent choice. A helical rack (Wolfer's helix angle is 19°31′42″, matched to standard helical pinions) runs more smoothly and quietly above about 60 m/min and carries roughly 30 % more load for the same face width, at the cost of an axial thrust that the pinion bearings must absorb. A straight rack is simpler and slightly cheaper and suits lower-speed positioning. All four combinations — ground straight, ground helical, milled straight, milled helical — have their own Wolfer selector page and size table, module 1–10.