Custom Gages to Go: When You Want To Have It Your Way
Many standard off-the-shelf gages no longer provide the right solution for today’s parts. That’s where custom gages come in.
Today most food chains are learning to cater to their customers. Whether it’s burgers or subs, you can get them made exactly to order and “have it your way.” Successful and growing chains understand this.
Gage users are people too, and they also have unique tastes in gaging. It seems these days that if you can draw it, you can build it. Thus, many standard off-the-shelf gages no longer provide the right solution for today’s parts. That’s where gage manufacturers need to pull from their experience and make “custom gages to go.”
Dial Indicator “Specials”
Some manufacturers make custom dial indicators an important part of their business. Often, minor changes suffice to make stock indicators appropriate for many unconventional uses. “Specials” can make indicator gaging easier, quicker or more accurate for many applications.
Generally, the more specific the gage’s purpose, the more quickly it can be operated.
But there comes a point when even the most narrowly focused, manually operated gage must give way to automation.
Any shop involved in a large production run can usually benefit from some form of specialized gaging to make inspection easier and faster.
For example, in the diameter gage illustrated in Figure 1, two reference contacts provide the benefit of self-centering positioning. But this means that the indicator’s sensitive contact is measuring a perpendicular to a chord, not the diameter itself. There is a fixed ratio of 5:4 between the workpiece diameter and this perpendicular. To compensate, the indicator has a special-ratio face that reads five units for every four units of movement at the sensitive contact, allowing for easy “direct” readings. The special-ratio dial saves time for the user, eliminates a source of potential error, and requires no re-engineering, just a custom-printed face.
Faces can be shaded for different purposes. In “spotlight gaging,” the green-shaded area of the dial indicates that a workpiece is within tolerance, the yellow areas warn the user that he is approaching tolerance limits, and the red area means he is out of tolerance. Shaded dials can also be used to quickly sort parts by size, using color-coded bins to coordinate with zones on the dial. For go/no-go gaging, areas of the dial can be masked so that the pointer cannot be seen at all. If the pointer is not visible, the part is no good.
Dial indicators can also be used to measure non-dimensional units. For example, where a relationship exists between temperature and the deflection of a material, a special dial may be used to measure in degrees of temperature. The penetration of a probe into a metal sample can be converted on the dial to read in units of hardness (e.g., Rockwell scale). Special dials can be designed to read in whatever units the industry or application requires. Other examples include: foot-pounds of torque, degrees of angle, pounds of impact force, spring force or cable tension, compressibility, and even diopters.
Before a fixture gage can be designed, the engineer must understand what specifications need to be inspected and also, in many cases, the manufacturing processes that produce it.
Take for example large bearings—not the six-inch or twelve-inch variety, but those that are six or eight feet in diameter. The bearing surfaces may be curved, or at an angle, and require various surface finish parameters. However, most surface finish systems base some part of their precision drive systems on gravity to lock in their axis of motion. This means they need to be level to work properly. Since the goal is to measure the bearing races—which are square to gravity when the bearing is on its side—the drives won’t work. This is where special fixturing comes into play: fixturing that not only supports the surface finish gage square to the part, but is also able to hold parts that can weigh hundreds of pounds.
Small parts can be even trickier. Injection bores in cylinder heads are very small holes with critical surface finish requirements. They are not square to any face, but sit at a specific angle to the top of the cylinder head. This presents many gage design challenges, including how to align the probe, and how to protect it from being damaged during insertion, measurement and retraction.
By knowing the design of the cylinder head, a special fixture can be designed that aligns on the cylinder head, using it as a reference to position the surface finish probe at the correct angle to the bore. A special sleeve protects the probe and a mechanical switch keeps the probe retracted until insertion is complete and the probe is in position. To calibrate the system, a calibration fixture simulates the part and stages the gage for proper alignment to a precision surface finish patch.
Air gaging is the ultimate in custom gaging. Because of its limited measuring range and high performance, every tool is made for a specific application. There are standard configurations for air plugs and air rings, but even these can be slightly customized. Things like specific location of the jets at certain heights, jet configurations for short lands, and form checking applications all require some customization. Others include relational measurements, such as distance between centers, taper and concentricity. Along with high resolution and magnification, speed and repeatability, air gaging exhibits great flexibility.
Air gages are often simpler and cheaper to engineer than mechanical gages. They don’t require linkages to transfer mechanical motion, so the “contacts” (jets) can be spaced very closely, and at virtually any angle. This allows air to handle tasks that would be difficult or extremely expensive with mechanical gaging.
Gaging the straightness and/or taper of a bore is a basic application that benefits from close jet spacing. All it takes is a single tool with jets at opposite sides of the gage’s diaphragm. The gage registers only the difference in pressure between the two sets of jets, directly indicating the amount of taper.
The concept can also be applied in a fixture gage, to measure several diameters and tapers in a single operation. Such a fixture is much simpler than a comparable gage equipped with mechanical indicators, each outfitted, perhaps, with a motion transfer linkage and retracting mechanism. And the air gage can have an electrical interface to signal in- or out-of-tolerance conditions with lights, which are much quicker to read than dial indicators.
From this one basic concept, many options are possible. The basic principle is: air circuits operating differentially measure dimensions; circuits on opposing sides measure relational measurements or differences between features. The beauty of the concept is that you can choose to ignore dimensions while seeing only relational measurements, and vice versa. You never have to add, subtract, or otherwise manipulate gage data. It can all be done with direct reading.
Consider, for example, a fixture gage that allows a shaft to be rotated. Two air jets on opposing sides of the journal will accurately measure the diameter of the journal, even if it is eccentric to the shaft. As the shaft is rotated, the air pressure increases at one jet, but decreases at the other one. Total pressure against that side of the diaphragm remains constant, so we can obtain a diameter reading.
Custom air gaging can measure more than dimensional applications. A manufacturer of plastic nozzles and valves needed a way to verify the orifice integrity of a high volume part by meeting specific operational flow requirements. Because the holes were so small and the location was difficult to get to, there was no mechanical or optical solution that would meet the performance or part volume requirements.
While most air systems measure diameter using backpressure from restricted air jets in a fixed body lug, air can also measure flow. Since flow is typically measured between two points, it is very similar to a min/max master system used in the dimensional world. This allows the use of simple air gage systems like column gages for dedicated orifice sizes, while for a larger range of orifice sizes a full air gage PC system may be incorporated.
For the parts in question, an air gage based PC system was chosen that repeats within 0.00005 inch (1µm) and can accurately achieve readings within 0.0003 inch (~8µm) or better. The system also provides an equivalent flow rate in liters per minute (lpm) based on dimensional values and reliably handles the very low flow rates required for this application.
Here’s a good “problem” to have. An automotive Tier 2 supplier was required to perform 100% inspection for dimensional tolerances and several other characteristics on aluminum forgings for air conditioning compressor pistons. The inspection requirements included: dimensional tolerances for bend and twist; checks for fill at four positions; presence of two radii; and flash removal at two positions. The problem was the size of the contract: the supplier was obligated to deliver, and thus inspect, nearly 100,000 parts per day.
Generally, the more specific the gage’s purpose, the more quickly it can be operated. But there comes a point when even the most narrowly focused, manually operated gage must give way to automation. It’s either that or hire a whole roomful of inspectors.
Automated gaging is usually cost-justified in applications where a part must be inspected every 45 seconds or less. This figure includes not just gage operation, but the entire gaging cycle, including: placing the part in the gage; operating and reading the gage; and removing the part from the gage. Other required actions may include: recording the measurement; sorting parts into appropriate categories by size; and removing rejects from the lot.
Many additional variables influence the speed at which a part can be measured, and hence influence gaging setup, whether manually operated or automatic. These include: the number of features to inspect; the need for a dimensional reading versus simply go/no-go results; how measurement data will be used (e.g., for export to SPC or for direct process feedback); the level of accuracy required; and whether gaging occurs in-process or post-process.
With so many variables in play, it is hardly surprising that automatic gages can rarely, if ever, be bought “off the shelf.” In the case of the Tier 2 supplier, we custom-engineered a fully automatic gage, capable of inspecting one part every 3.5 seconds (i.e., 1,030 parts per hour, or 24,720 parts per day). Four identical units were built and installed, providing total throughput of 98,880 parts per day.
Reliable parts handling was obviously a key engineering consideration. Accordingly, the gage was designed to make use of the most reliable parts-handling mechanism ever developed: gravity. Parts feed in at the top of the machine and slide down a 45° chute to a dead stop, where a proximity switch senses the part and triggers a locking mechanism. An air-driven cylinder then raises the holding fixture, and a nest of electronic gage heads descends until it contacts the part. The gaging device traverses the part, checking for true position, material fill, flash, and the presence of radii. Bend and twist are checked as independent features by comparing position and diameter measurements at opposite ends of the piston.
When the inspection is complete, the holding fixture descends and the part is released to drop down the exit chute. Out-of-spec readings trigger an escapement, which diverts bad parts into a reject bin, while good parts pass straight through to the next production process.
Gage head signal conditioning, data processing, and data storage are controlled by a gaging computer, from which measurements are downloaded daily. Programmable logic controllers (PLCs) control all of the gage’s logic functions. The systems have proven to be extremely reliable, operating around the clock for months between downtime for preventive maintenance.
Few machine shops face throughput requirements even close to this, but any shop involved in a large production run can usually benefit from some form of specialized gaging to make inspection easier and faster. And for larger shops where throughput requirements are very high, and the production run will last for a year or more, customized, automated gaging may be the only practical approach to inspection.