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In-process gaging improves grinding quality.

This special in-process gage is designed to measure workpieces with interrupted external diameters, such as gears.

In-process gaging has only recently been recognized by US manufacturers as a way to meet quality and cost objectives. With current demands for improved quality and tighter tolerances, in-process gaging has found its place on the production floor.

The two basic types of in-process gaging systems are contact and non-contact. Contact gaging, where carbide, CBN, or diamond gaging tips actually ride on the workpiece, is useful for OD measurement, workpiece positioning, stock removal from faces, and ID grinding. It's also used in match grinding, where the system measures part ID and communicates the data to the machine to set the zero for grinding the matching OD component.

Pneumatic non-contact gaging is used in the same kinds of applications as contact systems, but instead of contacting the part directly, the system uses precision air nozzles to measure differential pressure, comparing resistance to flow with a nominal air input. Pneumatic systems eliminate the tip wear inherent in contact systems, are self-cleaning, and contain fewer mechanical parts than contact gages. They also can measure IDs down to 0.060", although they are not as precise as contact gages in some applications.

Hydraulic systems are similar to pneumatic systems, but use an emulsion or oil instead of air. Applications for hydraulic systems in addition to grinding include honing, turning, milling, and use on machining centers in transfer lines.

Advantages, limitations

In-process gaging is used most often on external grinders and ID grinders to locate workpieces, identify and control grinding wheel infeed rates, compensate for wheel wear, and measure and control part size. In-process gaging constantly monitors part size during metal removal. When the workpiece reaches the preset finished size, the gage automatically stops the machine, almost eliminating out-of-spec parts.

Productivity improvements resulting from reducing the amount of time an operator spends controlling the grinding operation are another benefit of in-process gaging. Operators can tend several machines or spend time setting up the next operation.

In-process gaging effectively extends the capabilities of the grinding machine. A machine capable of producing parts to |+ or -~0.0002" can, with in-process gaging, generally produce parts to better than |+ or -~0.00001". Repeatability of 0.000004" is possible in production grinding operations using in-process gaging systems.

Systems also are useful for pre-process inspection. A "pre-process" gage measures the workpiece diameter as the piece is loaded into the machine. If the workpiece is undersize based on a preprogrammed size range, the gaging system signals the grinder to remove the part and load the next one.

Although in-process gaging reduces the need for direct operator input, changing ambient conditions means the operator still must maintain a certain amount of control over the production environment. Process variables that can be partially compensated for by in-process gaging include:

* Coolant temperature, which can affect part size control. In practice, good results can be achieved by keeping the coolant temperature stable and just above the ambient temperature.

* Workpiece temperature. Workpieces awaiting grinding should be placed next to the machine for approximately two hours before the grinding operation.

* Workpiece temperature variations can be overcome by extending sparkout time to dissipate the heat generated during roughing.

Overseeing production

Grinding cycle management is another important way in-process gaging can improve quality and productivity. To save time in a typical grinding cycle, the wheel approaches the workpiece rapidly. At some point in the approach, the infeed rate changes to the coarse grind rate, and the machine then removes a predetermined amount of stock at that rate. The in-process gaging system then switches the wheel to a fine grind infeed rate, usually 0.2 to 0.8 |Mu~/sec.

During coarse grinding, pressure between the wheel and the workpiece causes the workpiece to flex slightly. The fine grind infeed rate allows the part to straighten and cool while the wheel removes stock more slowly to reach finished size. The slower rate also allows the machine to stop more precisely when the gage communicates that it has reached the on-size condition.

When the gage signals the machine that the part is on-size, the machine usually allows for spark-out--dwell time of the wheel that lets the part continue to straighten and cool and produces a finer surface finish.

Gap control

Grinding operations are more efficient when the wheel is quickly brought up to the workpiece before grinding begins. Without gap control, the wheel will crash into an oversize part. Gap control stops the infeed before a severe collision occurs and signals the machine to begin operation as soon as the wheel touches the part, regardless of beginning size.

A gap control system can also locate the workpiece and signal the grinder to remove material until the part gets to a nominal size where the in-process gage can be effective. An acoustic gap control system using a piezoelectric transducer senses wheel contact with the workpiece and signals the machine to switch to the slower, coarse grind infeed rate.

An acoustic system can also determine wheel condition by analyzing the sound of dressing. It can detect breakage or rounded corners and signal the machine to continue dressing until specifications are reached.

In some applications, an acoustic system can optimize the grinding cycle. Once the system is tuned correctly, it can signal the grinding machine to either increase or decrease infeed rate to achieve maximum metal removal performance by listening to the sound of the grinding load.

Is it right for you?

Do you need in-process gaging for your grinding operation? That depends. The economies of the technique are realized when you need high precision over large production runs. A 100-piece part run is usually the decision point, although as few as 10 pieces could justify in-process gaging if you're producing complex parts with many diameters. It depends on how much each part is worth.

Another factor to consider: machine operating characteristics can change significantly over the course of a 100-part run, causing production of out-of-spec parts. Wheel loss is one of the most critical factors, and without in-process gaging, compensation for wheel loss and other process variables is difficult.

When deciding whether in-process gaging is right for your operation, ask yourself if--given your production rates--you can significantly reduce the number of out-of-spec parts you produce. If the answer is yes, then ask if your grinding machines are capable of responding to a precision gaging system. The performance of a worn or abused grinder will not be enhanced by in-process gaging. If your machines are in good order, you should start looking for an appropriate in-process gaging system for your operation based on the following criteria:

* The gaging system should be capable of greater precision than the tolerance requirements of the parts being measured.

* The gaging system should be quickly adjustable to accommodate each new production run.

* System operation should be simple and straightforward, with an easy-to-calibrate and easy-to-read electronic display.

* The system must be built to withstand the production environment.

The best approach is to contact manufacturers of in-process gaging systems to request an evaluation of your particular requirements. That way, you can be assured that all of the variables common to any in-process application are accounted for in a system recommendation.

James R Osborne General Manager Movomatic USA
COPYRIGHT 1993 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Author:Osborne, James R.
Publication:Tooling & Production
Date:Mar 1, 1993
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