Cutting tools made from tough high speed tool steel or a cemented tungsten carbide such as Ceremet, are sharpened by grinding the surface with an abrasive wheel.
This works well for tools up to, say, 10mm diameter. But, for larger tools the heat generated means the grinding wheel must be replaced frequently, perhaps every two hours, to avoid damaging the tool.
Dr Richard Hall, a researcher with grinding specialist Unicorn, says this is undesirable, with machine uptime an important issue.
Unicorn has developed and patented a method of making grinding wheels that can safely grind larger tools and other workpieces more than 50mm in diameter. The process can be tailored to produce wheels matched to the grinding application.
At issue is the amount of heat that a component will tolerate without damage. Heat will harden, or in some cases soften, metal. The higher the heat tolerance the faster the grinding wheel can run, leading to higher stock removal as well as improved productivity.
The Unicorn process reduces heat build up between the wheel and the workpiece by introducing pores into the structure to improve the interface between wheel, component and cutting fluid.
Unicorn grinding wheels are made from a polyimide resin loaded with nitrogen to make the material stable at high temperature – 400iC compared to bakelite resin which degrades above 200iC.
Cubic boron nitride or diamond particles embedded in the resin do the grinding work. But both have crystal forms that make them highly heat conductive. As the crystals heat up they start to soften the resin matrix and then fall out, leading to premature wheel wear.
Wheel performance is improved by making the resin porous. For thermally sensitive components, says Hall, you need more pores; but if you want the productivity and the workpiece can get warm, you do not want any pores.
Varying porosity and pore size are achieved using additives but mainly by controlling process parameters such as heat, pressure and time at temperature during the polyimide cycle.
By Sue Stuckey