Holroyd has employed its super abrasive machining technology to reduce the cycle time for producing turbo charger stators from 5½ minutes to just 46 seconds. These improvements have been achieved using Peel Grinding of the stator shaft, removing as much as 5mm a side in a single cut. In addition, the super abrasive wheel used in the Holroyd process lasts up to 200 times longer than tool inserts used with in twin turret turning, providing a life of at least 10,000 parts to minimise tooling and machine set-up costs.

One of the direct results of Holroyd’s expertise in helical rotor production is that the company has established itself as one of the leading suppliers to the turbocharger and supercharger industries; both as a manufacturer and supplier of high precision machine tools, and as a sub-contract manufacturer of components using the self-same machines.

Recently, Holroyd was approached by one manufacturer concerning a cycle-time problem with the machining of turbocharger stators. The stators comprise an Iconel impeller attached to a mild steel shaft. Previously, the time to machine the shaft, and the faces and OD of each impeller was 5½ minutes; this being achieved using twin turret lathes. However, there were a number of problems with this machine set-up. First, the cycle time was too long to enable the manufacture to achieve his desired component cost. Second, although a twin turret lathe was used to ensure full depth of cut on the shaft - without bending, this did not account for the resulting upset of the shaft during the deep machining process. Third, the depth of cut required – 5mm a side - from the twin turret lathe, and the resulting chattering of the tools in operation, meant that the life of tool inserts was extremely poor, with commensurate high costs for replacement.

Holroyd’s approach to solving these problems centred on its super abrasive machining capability: in particular the use of an Edgetek Super Abrasive Turning (SAT) Centre.
The Edgetek SAT employs CBN wheels to achieve productivity rates far in advance of those obtained with hard turning. It is designed to remove stock from the OD of cylindrical parts (slot, groove, and forms) in all types of materials (from soft to hard) at rates 40% faster than a conventional lathe.

Because productivity with the SAT system goes far beyond that typically obtained with hard turning, the machines are ideal for machining Inconel and heat treated powder metal workpieces with interrupted cuts. They can be used for such typical turning applications as machining slots, interrupted cuts on impellers, gears, splines and facing cylindrical parts.

The use of thin CBN wheels means that the SAT machines are especially well suited to Peel grinding, where high wheel speeds and high work-piece speeds result in very high stock removal rates with low grinding forces. Thin CBN wheels used in this way have cutting edges similar to turning tools, thereby reducing the cutting forces required between the wheel and the work piece. Compared to conventional grinding processes, Peel grinding typically removes three times the stock with half the force within the same time frame, and allows contour-grinding of a part in the same way that conventional machining does.

Holroyd applied its SAT Peel grinding technology to the tubocharger rotors with impressive results. The overall cycle time for machining shaft, faces and impeller OD was reduced from 5½ minutes to just 46 seconds – a reduction of nearly 88%. In addition, the improved burn threshold qualities of the Edgetek SAT machine result in a lower finished surface temperature of the workpiece, which means no upset and no undesirable friction weld rings generated during the super abrasive turning process. Finally, toolwear and, consequently, tooling and set-up costs, are much lower using the Edgetek SAT machine: the life of the CBN wheels extending to an average of 10,000 parts before re-tooling and machine set-up is required.

"We are not surprised by these findings," said Neil English, Sales Director of Holroyd. "In every case where we have performed machining trials with our Edgetek machines we have achieved at least a 40 to 50 percent increase in throughput, and in many cases it is much, much more.
"In one recent application in the aerospace industry, an Edgetek super abrasive machine replaced seven conventional milling and grinding machines, and achieved a reduction in the throughput time for machining a highly complex and extremely hard Inconel part from 8 hours, to just 12 minutes.

"The large depths of cut and accompanying lower temperature work-piece characteristics that can be achieved with our Edgetek machines - even across difficult to work materials, are the key to achieving improved productivity, reduced process times and improved quality in highly competitive global markets. There is no doubt that Edgetek technology is the future for machining, with the added benefit that it's available now."

Holroyd manufactures the Edgetek machines in all types and configurations to suit specific applications. The machines can also be supplied as complete turnkey solutions as part of a manufacturing cell. They are supported by a worldwide service network and benefit from manufacturer developed preventative maintenance plans.