Tough Materials Continue to Drive Creep-feed Grinding

Today's difficult-to-process workpiece materials and complex, high-value parts have sparked resurgence in the use of continuous-dress creep-feed grinding. At the same time, the capabilities of advanced grinding machines, CNC technology, and grinding wheel materials have boosted the cost-effectiveness of the process.

As compared with traditional reciprocating grinding, creep-feed grinding provides the means to remove signifi­cant volumes of metal in a short time and still generate top accuracy and finish. While reciprocating grinding removes smaller amounts of material in multiple faster lighter passes, creep-feed grinding applies the wheel in single passes that are 0.300" or more deep and at slower feed rates.

When first put to use in the 1950s, creep-feed grind­ing utilized relatively soft, porous grinding wheels and flood coolant to minimize generation of heat. Although creep-feed grinding could cut cycle time by half in some cases, the process was limited in application because wheels wore quickly, restricting the length of cut possible before wheels required dressing.

Creep-feed: Back to the future

The advent of continuous-dress creep-feed grinding in the 1970s addressed the issue of rapid wheel wear. As the name indicates, in continuous-dress creep-feed grinding machines dress wheels without interruption throughout the grinding process. A diamond dressing roll with a mirror-image form of the desired part profile maintains constant contact with the grinding wheel to keep it sharp.

Because the wheel is always sharp, long lengths of cut are possible. Plus, this consistent wheel perform­ance results in lower average grinding forces and leads to more efficient abrasive use, shorter cycle times and increased production rates.

Use the right equipment

According to grinding machine builders, gaining the maximum benefits of creep-feed grinding requires the use of machines and grinding wheels engineered specific­ally for the process. Admittedly, some shops perform a variant of creep-feed grinding involving deep passes on workpieces with narrow cross sections. Such operations are possible on standard grinding machines at somewhat reduced table speeds using conventional grinding wheels. But doing so depends on factors that include workpiece material, depth and width of the grinding pass, rigidity of the machine and nature of the grinding wheel.

A major source of complication is the continuous-dress process. For continuous-dress applications, a grinding machine's basic three CNC axes are supplemented by a fourth axis controlling the infeed of the diamond dressing roll. The roll turns in the same direction of the grinding wheel and feeds into the wheel in increments of millionths of an inch per revolution。

As the grinding wheel's diameter shrinks during con­tinuous dressing, the grinding machine's programmed wheel spindle increases rpm to maintain the surface speed required to grind the part. Likewise, a variable-speed servomotor constantly changes the dresser roll's rpm to maintain the correct pace of dressing.

New wheels, better performance

Wheel durability as well as grinding results themselves are dependent on the proper application of coolant in creep-feed grinding. Coolant prevents buildup of heat in the part and grinding wheel, and also removes swarf from the wheel contact zone to prevent marring of the workpiece. Sufficient coolant flow also prevents the pores of the grinding wheel from becoming filled with swarf, a con­dition that reduces the wheel's cutting effectiveness and further reduces the coolant's temperature-control benefit. When temperatures rise too high, the wheel may burn the workpiece and swarf may weld to the wheel, negatively affect­ing workpiece dimensions and finishes.

Successful application of creep-feed grinding requires investment in appropriate equipment includ­ing specialized grinding machines and grinding wheels, but for the right situations and materials, return on the investment will far exceed a creeping pace.--iAbrasive report