Drake in American Machinist

A continually compensating grinding system from Drake Manufacturing, Warren, Ohio, with a closed-loop software control, gives fabricators with demanding SPC requirements shorter cycle times, higher Cpk levels and reduced wheel consumption. The CBN wheels on the Drake GS:I internal grinders see typical G ratios, the amount of metal removed to wheel consumed, over 1,000 and can reach 4,500 under optimum production conditions. This means these wheels last longer than wheels with aluminum oxide or seeded-gel abrasives, which may have G ratios of 10 and 50, respectively.

It's the software
According to Rick Sanders, Drake systems engineer, a number of factors
assure the consistent G-ratio and Cpk performance of the Drake machines. One element is machine positioning, which has plus or minus-uin. resolution. The grinding quill, constructed of carbide, provides three times the rigidity of tool steel, and through-spindle coolant delivery means high speeds and low wheel consumption. The CNC compensation also reduces the frequency of truing procedures.

However, it is the closed-loop software control that gives the GSD:I grinding machine its unique capabilities. The machine monitors part-size data collected by a Product Control Technologies PD-4 postprocess gage. The CNC uses the data to correct part size every three pieces, if necessary, and to calculate and compensate for actual wheel wear per part. This method typically has the CNC making adjustments before the post processgage even determines a need.

Such an approach avoids a staircase effect often related to infrequent truing procedures. The system recognizes that if the wheel is losing 50 uin. every 50 parts, the grinding wheel is losing 1 uin./part, so then compensates accordingly. The dynamic adjustment, says Sanders, adds an "expert" characteristic to the control because the database expands with every use, adding to the system 's knowledge base.

Quality in, quality out
To underscore the importance of incoming Cpk values, Sanders cites a boring application having a plus or minus 0.00035-in. tolerance on a 0.356-in. bore. This application uses incoming sleeve blanks with close dimensional tolerances of plus or minus 0.00l in. and good concentricity. The GS:I grinder gives this customer a Cpk on the bore of better than 6. A rough pass removes 0.010 in. and the finish grind takes 0.002 in., both at 1,450 rpm during a 10.3-sec cycle time. Actual production parts have a finish of 10 to 12 uin., while prints spec 18.

To keep the Cpk levels focused near the mean values, size compensation by the control becomes more aggressive the greater the deviation from the mean. The result: For one customer, who previously had 20 different size classes within the process limits, the Drake system reduced this to three.