Jig Grinding On A Machining Center

Roughing and finishing on a single machine, using a single setup, has appeal for most shops. The advantages in time savings and accuracy are obvious. Eliminating the transport of workpieces between machines, as well as the setup for those secondary operations, is a boon for throughput. Critical features that need to maintain dimensional relationships can be much more reliably produced if machined complete in one clamping.

Current CNC machining centers do a fine job of accommodating these process requirements within milling, drilling and tapping operations. But, if a workpiece needs additional processing, such as jig grinding, a second specialized machine is normally required.

That, however, may not be the case with the use of a coordinate grinding head. This attachment is designed to extend the process capability of virtually any CNC machining center by allowing it to jig grind, ID grind and profile cam grind without removing the workpiece from the machining center.

Called Ray KSK 1000, this jig grinding unit is distributed by AmeriSwiss Corporation (Scottsdale, Arizona). It's designed to fit a 40- or 50-taper, horizontal or vertical machining center spindle. The coordinate grinding head is used with an exchangeable fixing cone that is secured by the machining center drawbar. The KSK 1000 is available in two sizes capable of grinding a maximum bore diameter of 150 mm (6 inches) and a depth of 120 mm (5 inches). Additional models can grind diameters up to 310 mm (12 inches).

A set of planetary gears in the grinding head runs off the machining center spindle and is used to produce the orbital motion needed to jig bore. For profile grinding, a cam is used in place of the planetary gears.

A self-contained U-axis slide accurately positions the grinding spindle for fine finish. Accuracy is 0.001 mm (0.00004 inch). A choice of three high frequency spindles (600W, 1,200W and 1,500W) provides speeds between 7,500 and 100,000 rpm. To go from machining center mode to grinding takes about 15 minutes.

In operation, the grinding wheel is positioned within 0.001 inch of the workpiece using the machining center's positioning system. For the final size, the fine feed slide on the head takes over to grind the remaining stock. Motion control for this axis is available either as a stand-alone unit, or it can be integrated into the machining center CNC.

The control has 31 programs for different feed rate and time combinations to accomplish various grinding operations. These include determination of final grinding point, spark-out time allowance and automatic approach to the workpiece.

An additional jig grinding accessory for oscillation of the workpiece is available. In ID grinding, for example, rather than program the machining center Z axis to create the oscillating stroke, a machine-mounted table is available to accomplish this. It is driven by a low inertia motor and programmed through a control disc. The 300 mm by 400 mm (12 inch by 16 inch) table has a working stroke of 50 mm (2 inches).

Ray coordinate grinding heads are not designed as a replacement for production jig grinding machine tools. Instead, they are an alternative for the shop or tool room that occasionally has the need for jig grinding but can't justify the investment in a dedicated machine.—GCK