A Rotary-Table Surface Grinder Speeds Production, Simplifies Setup

This large bearing manufacturer needed an improved process for grinding the races in bearing sets that range from 3.5 to 11 inches in diameter. The preload of their angular-contact ball-bearing sets is determined by the relative widths of each pair of races, which are machined to final dimensions by flat surface grinding. The labor and skill required to measure one race and then grind the other race to suit, within a total tolerance envelope of 80 microinches (0.000,080 inch), was excessive. The company wanted an automated process that was flexible enough to grind each race to an individually programmed width, with quick setup and high (1.67 Cpk) repeatability.

The races are machined from 52100 bearing steel, Rc 62-64. Total stock removal ranges from 0.005 to 0.006 inch. One race is ground and measured; from that measurement, the amount to grind from the face of the other race is calculated.

Amada Wasino met the production and accuracy requirements with its SS-501N, a one-axis CNC rotary surface grinder with 10 microinch minimum resolution. With hand scraped ram slideways, hand scraped table bearings with metered lubrication, and special long-axis, rigid spindle bearings with independent pressure controlled lubrication, the SS-501N is well suited for the flatness and accuracy required in this production application.

The bearing manufacturer wanted in-process inspection to minimize operator intervention as much as possible. They also wanted to perform 100% part inspection. The SS-501N was able to hold the required tolerances without the use of in-process gaging, but a gage was necessary to meet the customer's other needs. A unit made by Control Gage was selected for its rugged, compact design, its accuracy in wet grinding environments, and its easy setup and operation.

Process control and reliability were proven by means of a runoff conducted on 40 each of two different workpieces. Repeatability was in excess of the required 1.67 Cpk.

An Amada Wasino star pole chuck provided powerful magnetic workholding for the various ring sizes. The machine's patented constant-surface-speed, gearless table drive system maintained the optimum surface footage for each of the various ring sizes by adjusting table speed as the ram stroked across the bearing race. This feature produces an excellent, consistent surface finish.

To minimize setup time required to change part sizes, Amada Wasino and Control Gage engineered a simple stand that allows the gage to access the entire grinding envelope. To position the gage, one bolt is loosened and the gage head is adjusted for height and location. The gage provides a comfortable "window" of adjustment, and it features a simple and accurate "zero out" button. Once the gage was repositioned and zeroed out, it's ready to grind.

The amount of grinding required for each race is entered into the SS-501N's conversational, canned cycle screen. As the final dimension is approached, the CNC automatically switches to fine-feed and then to sparkout. When the target dimension is reached, the wheel automatically retracts and the ram is parked.

The new process is easier to operate and faster to set up; it greatly reduces the scrap rate; and it increases productivity.