Abrasive peel grinding reduces cycle time by 88 per cent

Holroyd's super-abrasive machining technology has been employed to reduce the cycle time for producing turbo charger stators from 5.5 minutes to 46 seconds, which is an improvement of around 88 per cent. This has 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 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 same machines.

Recently, Holroyd was approached by a manufacturer about a problem with cycle times when machining turbocharger stators. The stators comprise an Iconel impeller attached to a mild steel shaft. Previously, the time to machine the shaft, the faces and the OD of each impeller was 5.5 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 manufacturer to achieve the 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 per 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.

Super-Abrasive Turning Centre

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 per cent faster than a conventional lathe.

Because productivity with the SAT system goes far beyond that typically obtained with hard turning, the machines are excellent 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 workpiece 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 workpiece.

Compared with 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.

88 per cent cut in cycle time

Holroyd applied its SAT Peel grinding technology to the turbocharger rotors with impressive results. The overall cycle time for machining shaft, faces and impeller OD was reduced from 5.5 minutes to just 46 seconds – a reduction of nearly 88 per cent. 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, tool wear and, consequently, tooling and set-up costs, are much lower using the Edgetek SAT machine, with the life of the CBN wheels extending to an average of 10,000 parts before re-tooling and machine set-up is required.

Neil English, Sales Director of Holroyd, comments: "We are not surprised by these findings. In every case where we have performed machining trials with our Edgetek machines we have achieved at least a 40 to 50 per cent 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 eight hours, to just 12 minutes.

"The large depths of cut and accompanying lower temperature workpiece 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 is 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 systems as part of a manufacturing cell. They are supported by a worldwide service network and benefit from manufacturer-developed preventative maintenance plans.