In-process gaging optimizes grinding: on two high precision chuckers for processing stamping dies.

Two high precision, high production chucker-type CNC grinding machines, combined with the latest in-process gaging, have been built by Aldridge Grinding Machine Co, Huntsville, AL, for processing close-tolerance stamping dies. The 3-axis OD-type chuckers utilize a 30 deg angled grinding wheel with left-hand face and radius to grind diameters on 12 dies ranging in size from approximately 11/16[inches] to 1 5/8[inches] dia. The operations require no wheel change and only one CNC wheel dressing program.

To optimize their performance, the chucker grinders are equipped with a precision air chuck, precision vision-based part alignment system, electronic part touch probing, and a programmable in-process multidiameter gaging system that provides in-process size control of critical part diameters. Using this equipment, each grinder can grind to size dies of different diameters in a continuous run, with only a change in part programs. No gage mechanical alterations are needed, so long as part size differences fall within a 20mm range.

Both the in-process gaging and touch probing systems used on the Aldridge chucker grinders are products of Marposs Corp, Auburn Hills, MI. The Marposs AB72 multidiameter in-process gaging system is available in models that cover various ranges of grinding diameters. Each individual system can then be user-programmed to automatically measure up to four different diameters within a 20mm range, eliminating the need, within that range, for any gage changeover. Zero settings for different diameters are made automatically, following a teach process utilizing master workpieces.

For the chucker-grinder die application, a single multidiameter gage head is mounted on the grinder bed so as to approach the die crown at a 45 deg angle. A Marposs Mida T36G touch probe, used to inspect part length before grinding, is also mounted on the grinder bed, below and to the right of the in-process gage head.

The gage head is electronically paired with a Marposs E39 gage amplifier, by which it is programmed to automatically adjust for diameter measurements on different dies that fall within a 20mm range. Actual measurements during the grinding cycle are performed by fingers on the gage head that are moved up and down by tiny servomotors inside the head. Automatic feedback of size data to the grinder control signals corresponding microadjustments to the grinding wheel, allowing the gaging system to successfully hold specified finish diameter size to within [+ or -] 30 millionths of an inch.

Gaging/grinding process

In current production use of the two Aldridge CNC chucker grinders, six smaller-body dies are processed on one machine, and six larger-body dies on the other. The dies are ground one at a time in two separate chuckings, with manual part loading and unloading. The grinding cycle for each of the six small-body dies is accomplished by the following steps, which include two gaging/grinding operations.

First, chuck inserts are selected to accommodate the small-body die neck size to be ground, and installed in each of three chuck jaws. The operator then opens the chuck jaws via the precision air chuck, inserts the appropriate small-body die, neck-first, and locks it into place for grinding. The proper program for the part to be ground is called up and verified, and the grinding cycle is activated.

To begin the grinding cycle, the workhead containing the part to be ground advances laterally until it contacts the Marposs touch probe. If the measurement of part length does not fall within a 0.005[inches] window, the part is rejected and the grind cycle aborted. The operator must then replace the rejected die with another.

When probing confirms a die of acceptable length, the workhead returns to its starting position and begins rotating. At the same time, the grinding wheel, already in rotation, advances toward the part in a rapid infeed, with accompanying activation of coolant spray. Grinding of the die butt then takes place in successive rough, medium, and fine modes, reducing the part length by up to 0.005[inches].

Subsequently, in the same part chucking, a nongaged flange diameter is ground to precision size tolerances ([+ or -]0.000 15[inches]). This is followed by a fiat grind on the flange periphery, which is generated by circular interpolation.

At the conclusion of the fiat grind, the die is manually reversed in the chuck, so that it is gripped by the chuck inserts at the opposite, or flange, end. (The chuck inserts have two counterbores, one for the flange and one for the neck.) In this chucking, the die body and neck diameters are ground in cycles controlled by the Marposs in-process multidiameter gaging system.

Before beginning the reverse-end grinding cycle, the operator once again verifies that the proper program is in place for the die to be ground. The cycle is then activated, and a rough grind is performed on the die body. At a point where approximately 0.002[inches] has been removed - leaving an additional 0.002[inches] to be ground - the Marposs sizing gage jumps in to control the grinding to finished size. Generally, four part revolutions of dwell occur prior to zero. When final sizing is reached, the gage levers open, the gage retracts, and the grinding wheel moves back 0.010[inches] and holds for the next grind command.

In the final gaging/grinding sequence, the workhead, still in rotation, shifts to the left to position the die neck with respect to the grinding-wheel side wall. The neck grinding cycle then proceeds in the same way as the body grind cycle, with the Marposs sizing gage controlling the grinding until zero part size is reached. At this point, the grinding wheel retracts to the rear starting position, the coolant shuts off automatically, the 2 backs up to the starting position, and the work spindle comes to a stop. The entire part grinding cycle, including two chuckings and part load/unload, takes about 2 1/2 min.

Grinding of the large-body dies is accomplished in basically the same way as grinding the small-body dies. Some changes are necessary, however, to accommodate two important part design differences. None of the six large-body dies has a flange, and three of them have tapered bodies.

In current production, two Aldridge CNC chucker grinders are used, respectively, to process smallbody and large-body dies. However, other production mixes are also possible. One possibility is noted by Aldridge Grinding Machine Co president Warren Aldridge Sr. According to Mr Aldridge, it may be even more productive in large lot runs to dedicate all first-chucking grinds to one machine and all second-chucking grinds to the other. "The important thing to remember is that both machines are identical in all respects," Mr Aldridge says. "Each of them can grind every one of the twelve dies without retooling." He notes, too, that a single operator can run both of the machines simultaneously without undue effort, which makes combined production of the same dies practical.

Mr Aldridge stresses that a key element in this production flexibility is the 20mm multidiameter gaging range of the Marposs in-process gaging system. "The 20mm gaging range covers all diameter gaging requirements within either the small or large die family," he notes. "You only have to reset the gage fingers, not change them, since all the size differentials are accommodated at the program level of the gage amplifier." Changing over the gaging system between the small and large die families takes only about 10 min of reset time, according to Mr Aldridge.

Aldridge also builds two other types of CNC chucker grinders. They include a Type B external/internal grinder, with a 30[inches] grinding wheel approach and bed-mounted internal grinding head; and a Type C grinder, with a straight approach and dual grinding wheels and work spindles for simultaneous grinding of two parts. Where needed, as in the case of the Type A chucker grinders used to grind the coining dies, Aldridge machines are quoted with the Marposs multidiameter in-process gaging system and part touch probing system. Marposs wheel balancers are currently offered on Aldridge cylindrical grinders and will be offered in the future on both universal/cylindrical and chucker-type machines.

For more information from Marposs Corp, Auburn Hills, MI, circle 261; http://www.mar poss.com