Renold Power Transmission (Holroyd Machine Tools & Rotors): Rotor Milling Machines

Abstract

The use of helical rotors offers considerable advantages over reciprocating systems for the compression of air and gases. The first patent on these had been taken out in 1946 by a Swedish firm known as Svenska Rotor Maskiner AB (SRM). High-precision requirements for the manufacture of rotors, which had to rotate in intersecting pairs, meant that up to 200 hours of hand-finishing work was necessary, making helical rotor compressors very expensive. Following an approach to John Holroyd & Co. of Rochdale, a firm experienced in the manufacture of worm gears, a milling process based on that used for gears was developed. In the version current at the time of the Queen’s Awards blanks were held in position while three stages of machining were applied. Initially, the grooves or lobes (for female and male rotors respectively) were rough-machined into the required spiral configuration. For the next stage similar cutters were used but with a specially shaped blade. In the finishing stage an original approach was adopted. Holroyd had realised that with a large multibladed milling cutter, minute differences would exist in the heights of blades.