Gaging Improves Automatic Grinder Control

May 8, 2003
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Automatic gaging lets manufacturers produce short runs quickly and take measurements on the fly.

In the world of part production, companies are faced with smaller part runs, tighter tolerances and shorter cycle Arial. Whether it's for making bearings or ball joints, camshafts or connecting rods, dedicated machines are few and far between. Suppliers must be agile and lean. In the world of gaging for grinders, the gages must be precise, reliable, quick to set up and easily reconfigured for each new part run.

Four major trends are affecting the grinder gaging industry, and they all relate to quickly testing a variety of parts. The trends include improved speed and resolution, reduced part changeover time and an emphasis on low life-cycle costs, measurement "on-the-fly" as parts leave the grinder and increased use of statistical process control (SPC) for automatic grinder control.

Improved speed and resolution. In an in-process measurement environment, speed and resolution of contact-type gaging are vital, and thanks largely to advances in electronics, improvements are being made on both fronts.

The heart of virtually every contact-type part gage is a linear variable differential transformer (LVDT). While LVDTs have been in use for more than 50 years, recent advances such as improved signal filtering continue to improve the resolution of LVDT-based gages, and theoretical resolution limits have yet to be reached. Resolution of 1 microinch (0.000001 inch) has been achieved and resolution to 0.15 microinch is under development.

Data acquisition speeds are likewise improving, due to increasing capabilities in digital signal processing (DSP) chips. The current generation of gaging amplifiers and controllers can capture up to 100,000 raw readings per second, while process control is updated at rates of 500 to 1,000 times per second. The sampling rate can be bumped even higher if an application requires. With DSP chips, special filters can easily be customized to meet specific applications.

Improved speed and resolution are important as tolerances get tighter and parts move more quickly through the gage. Some manufacturers of bearings are grinding workpiece outer diameters with a total grind cycle of three seconds, for example, a cycle time that was previously unheard of. And while grinding tolerances of 0.0002 inch with a Cpk of 1.33 might have been considered tight only seven years ago, one recent application for in-process measurement of small compressor crankshafts called for a tolerance of 0.0002 inch with a Cpk requirement of 2.0. A Cpk of 2.0 means that 68% of the parts will be I33 millionths of an inch of nominal.

Reduced life cycle costs and quick part changeover. Purchase price is just the beginning when it comes to calculating life cycle costs (LCC) of factory equipment. Operating costs including air, electricity, maintenance, setup and downtime continue throughout the useful life of the product. And in some cases, there can even be significant disposal costs once the equipment's useful life is over.

Reducing LCC of grinder gages is tough. The environment for an in-process grinding gage is harsh. Precise measurement must take place while the gage is subjected to coolant and swarf; day after day, month after month, year after year.

The task of LCC reduction has been particularly difficult for air gage manufacturers. Compressed air is expensive. The air consumption of air rings is not particularly onerous, but the air blow-offs required to clean the part before it enters the air ring could be costly. Contact gaging, on the other hand, does not suffer from air consumption costs. Carbide, diamond, polycrystalline diamond and even ceramic contacts have been developed to insure long contact life without part marring.

But air gaging is still most suitable for small ID's where the diameter being measured is inaccessible to contacts. Part changeover times can play a surprisingly important role in LCC. Moreover, reducing part changeover time is more important than ever as more manufacturers implement lean manufacturing and just-in-time production philosophies. The cost per hour of downtime can range from $500 to $2,000 per hour. Costs are rising because several machines often have to be reconfigured when a part changeover occurs. For example, at one plant, the production line was retooled approximately 15 times per month. One manufacturer's gage required an hour of setup time while a second manufacturer's gage required six minutes. Annual savings derived from substituting a fast part changeover gage for a slower one ranged from $150,000 to $250,000 depending on the number of machines involved. In this manufacturing environment, the quick setup gage paid for itself several times over in the first year of use.

Measurement on-the-fly. One of the latest variants in the use of gaging for thrufeed centerless and disc grinders are gages that measure workpieces "on-the-fly," as they move between machines. Gages are available for measurement of bearing rings, shock rods, struts, brake pistons and many disc-ground parts. The parts are measured in a dynamic fixture so that part movement is not arrested as it flows from machine to machine.

A recently introduced "ring gage" measures the outside diameter of centerless ground rings. As the parts leave the grinder, they are separated and knocked flat as they move down a conveyor. An adjustable side guide locates each part laterally as it moves toward the gage. A fixed side guide then brings the part back on center. The gage mechanism floats on two linear bearings. As a part enters the gaging area, the fixture is forced upward and two carbide rails force the part to traverse the gaging portion of the fixture without tilting.

The gage itself pivots on a precise flexure to "find" the part. The single gaging finger is also mounted on a flexure. An LVDT probe measures gage finger movement and transmits part-size data to a statistical controller, which can be used for automatic grinder control based on real-time workpiece measurement.

The dynamic fixture measures each part individually. Part-size data is displayed on a control chart and, when necessary, individual out-of-tolerance parts are diverted from the part stream.

A similar device is available for measurement of workpieces as they leave a disc grinder. The parts are captured between polyurethane belts and transported through a gage fixture where they are measured by two "single-finger" LVDT based gages. Once again, the part size measurement is used for grinder control and out-of-tolerance parts can be diverted to insure that bad parts do not make it to the next station.

Real time SPC for automatic grinder control. The ability to measure parts on-the-fly has enhanced the capability of manufacturers to employ real-time statistical process control for parts produced on disc and centerless thru-feed grinders.

Based on real-time part measurements, a statistical controller continuously updates an internal control chart for a given process. The control chart is broken into sigma zones.

Compensation will occur if a certain number of consecutive plots fall within or above a zone as defined by a trend table. The amount of compensation is proportional to the distance from nominal to the most recent subgroup mean. As the process improves, the sigma limits will tighten.

As sigma limits tighten, the statistical controller calls for compensation more often and in smaller amounts. The reduced variability caused by the smaller, more frequent compensations will allow the control limits to be further tightened. This iterative process tightens the control limits until parts are produced at the highest Cpk attainable with the grinder. Proportional compensation significantly improves grinder process control. For example, a process that has a wheel wearing at a rate of 0.001 inch every 1,000 parts will yield less variability if the compensator makes 100 corrections of 0.00001 inch rather than two corrections of 0.0005 inch.

Use of statistical grinder control can allow manufacturers to grind much more aggressively while maintaining part-size control. In some cases, grinder performance can be improved so much that a second pass through the grinder or the need for a semi-finish grinder can be eliminated completely.

Grinder gage manufacturers are responding to the challenge of short part runs, tighter tolerances and their customers' needs for automation to cope with high labor costs. There has been more change in the gaging industry over the past five years than in the previous 25. The emergence of sophisticated electronic gage controllers has helped achieve improved speed and resolution as time and tolerance requirements tighten. Dedicated machines are disappearing and the dedicated gages that they used are disappearing too. The current generation of grinder gages can be quickly reconfigured to measure a wide range of parts and require very little direct labor for supervision or maintenance.

Tech Tips

  • Gages for thru-feed centerless and disc grinders measure the workpiece on the fly as parts move between machines.
  • Statistical grinder control can allow manufacturers to grind much more aggressively while maintaining part-size control.
  • Quick changeover of gages can make short runs more cost effective.

This quick setup gage is used for in-process, interior diameter testing. Quick changeover times for gages make short runs more cost effective. Photo: Control Gaging Inc.

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