High porosity grinding wheels increase productivity and avoid burning

Citation

(2005), "High porosity grinding wheels increase productivity and avoid burning", Aircraft Engineering and Aerospace Technology, Vol. 77 No. 2. https://doi.org/10.1108/aeat.2005.12777baf.003

High porosity grinding wheels increase productivity and avoid burning

High porosity grinding wheels increase productivity and avoid burning

Keywords: Grinding, Porosity, Materials

One of the largest abrasives manufacturer, Saint-Gobain, has spent three years at its R&D centres in the USA and Germany developing a new technology called Vortex for the manufacture of highly porous, permeable grinding wheels, which may have either a vitrified or organic bond structure. First to be launched is a range of wheels up to 915 mm diameter, 400 mm wide, comprising a widely spaced, aluminium oxide abrasive grain in a vitreous matrix. They are manufactured in the UK and are available through the group's Stafford subsidiary (Plate 4).

Plate 4 A Saint-Gobain Vortex wheel grinding the fir tree root form of a turbine blade

Vortex is said to be especially suitable for processes requiring aggressive removal of material without burning. Wheel diameters go down to 200mm, allowing creep feed grinding of sticky, long-chipping, nickel-based alloy components such as turbine blades on a machining centre. Such operations require the wheel to be preloaded with coolant. Saint-Gobain Abrasives' new wheels are reported to be ideal as the high porosity absorbs copious quantities of coolant at unprecedented levels and diffuses it readily in the cutting zone, taking heat out of the chips and discouraging clogging of the cutting edge. The trade-off between the wheel's porosity and hardness can be adjusted during manufacture by varying bond percentage and pressing force, allowing the possibility of manufacturing bespoke products suited to certain applications.

The manufactures state that it is noteworthy that Vortex technology produces naturally occurring porosity, so it is not necessary to use artificial, environmentally unfriendly porosity inducers, as are employed in the manufacture of all other porous vitrified wheels.

Saint-Gobain Abrasives is no stranger to machining aerospace and other exotic materials, as open-grained wheels from the company's other ranges, notably Altos, Sirius and Poros Two, are already used successfully in production. Vortex technology wheels are, however, expected to outperform these products. Trials are currently in progress at a number of customer sites on machining centres and grinding machines. Results are expected in early 2005, but early indications are that metal removal rates will be higher than for Saint- Gobain's other products and indeed competitors' wheels.

Vortex technology wheels are also said to be well suited to use on cylindrical, gear and surface grinding machines, where the open grain allows high metal removal rates without clogging and consequent burning, which can lead to metallurgical damage especially in nickel-based alloys and other heat-sensitive materials. The wheels excel in applications where the area of contact with the component is large. Creep feed grinding by definition is an obvious use due to the large depth of cut. So too are gear, surface and roll grinding as well as some angle-approach applications. The ability of the wheels to machine “difficult” materials efficiently well suits them to toolmaking and automotive applications as well as to aerospace and land turbine work.

Insofar as grinding machine performance is concerned, examples are available comparing the performance of Vortex wheels with that of competitors' aluminium oxide wheels previously used by various manufacturers, the names of which have been with held for commercial reasons. All are creep feed applications employing continuous dressing of the wheel (Plate 5).

Plate 5 Photomicrograph magnified 28 times of a Vortex vitreous aluminium oxide grinding wheel showing the open grain structure

In one example, a stage-1 gas turbine stator vane has an 8 mm deep by 10 mm wide slot ground with a 500 mm diameter wheel on a Blohm Profimat. Vortex is reported to have resulted in a more than threefold increase in the Inconel parts produced, up from 17 to 54, before the wheel needed replacing. Contact (cycle) time was said to be halved from 7.28 to 3.64 min. Most importantly, the competitor's wheel caused extensive burning of the alloy, leading to time-consuming testing after machining to ensure metallurgical integrity. There was reportedly no burning with the Vortex wheel and post-grinding inspection has been eliminated.

Another component example at a different user, this time a nickel alloy heat shield for a land turbine, showed a doubling of wheel life with Vortex from 13 to 26 parts, using a 240 mm diameter by 60 mm wide wheel on a Maegerle grinder. Cycle time was not an issue owing to the overall TAKT time for the process, but in any case the contact time was down by 18 per cent.

At the factory of a customer in the automotive industry, a steel steering component has a 6 mm groove ground with a 400 mm diameter wheel. Vortex increased parts per wheel by 150 per cent and reduced production cost per part by 27 per cent.

In addition to high productivity, additional benefits are said to derive from the free- cutting characteristic of Vortex wheels. One is the reduction in the amount of dressing needed, extending the life of the both wheel and the dressing tool, and shortening cycle times in applications where continuous dressing is not used.

Another factor claimed is that 20 per cent less power is needed to drive the wheel for a given metal removal rate, which saves energy and reduces the amount transmitted to the component, minimising its tendency to move within the fixture during machining. The results are stated to be higher grinding accuracy and reduced residual stress in the material. For the same reason, a less rigid grinding machine can produce better results with a Vortex wheel.

For further details contact. Saint- Gobain Abrasives. Tel: +44 (0)1785 222000; Fax: +44 (0)1785 215463; E-mail: james.doqqett@saint-qobain.com