What Kind of Gage Do You Need?
Sometimes only a “custom” will do.
Like every other function in modern manufacturing operations, inspection is subject to management’s efforts at cost control or cost containment. It’s good business sense to try to maximize the value of every dollar spent, but it means that hard choices must be made when selecting gaging equipment. Issues as diverse as personnel, training, warranties, throughput requirements, manufacturing methods and materials, the end-use of the workpiece, and general company policies on gaging methods and suppliers may influence both the effectiveness and the cost of the inspection process.
For example, what’s the ultimate cost of a bad part passing through the inspection process? It could be just a minor inconvenience to an OEM customer—maybe a two-second delay as an assembler tosses out a flawed two-cent fastener and selects another one. On the other hand, it could be a potentially disastrous equipment malfunction with expensive, even fatal, consequences. Even if the dimensional tolerance specifications for the parts are identical in both instances, management should certainly be willing to spend more on inspection in the second case to achieve a higher level of certainty—probably approaching 100%. One disaster averted will easily pay for the more expensive process in lawsuits avoided, lower insurance premiums, etc.
Some gaging applications call for inspecting a part for variation across a given feature, which calls for freedom of movement in at least one plane.
Other applications call for measuring a series of parts at exactly the same location on the feature, time after time, to check the repeatability of the process.
In the first instance, freedom of movement is a good thing. In the second, it’s an enemy.
Many companies have achieved economies by moving inspection out of the lab and onto the shop floor. As this occurs, machinists and manufacturing engineers become more responsible for quality issues. Luckily, many gage suppliers are more than willing to spend time helping these newly assigned inspection managers analyze their gaging requirements.
A good approach is to first define the functional requirements of the inspection task, and let that steer you towards hardware capable of performing the tasks. There is at least a baker’s dozen of key factors to consider:
Nature of the feature to be inspected. Is it flat, round or otherwise? ID or OD? Is it easily accessible, or is it next to a shoulder, inside a bore or a narrow groove?
Accuracy. There should be a reasonable relationship between job tolerance and gage accuracy—very often on the order of a 10:1 ratio, although a resolution and repeatability requirement for statistical GR&R (Gage Repeatability and Reproducibility) testing may require 20:1.
Inspection costs. These increase sharply as gage accuracy improves. Before setting up a gaging operation for extremely close tolerance, verify that the particular level of accuracy is really necessary.
Time and throughput. Fixed purpose-built gaging may seem less economical than a more flexible, multi-purpose instrument, but, if it saves a thousand hours of labor over the course of a production run, it may pay for itself many times over.
Ease of use, and training. Especially for shop floor gaging, you want to reduce the complexity of gage operation and the possibility of operator influence.
Cost of maintenance. Can the gage be maintained, or is it a throwaway? How often is maintenance required, and who’s going to perform it? Gages that can be reset to a master to compensate for wear are generally more economical over the long run than those that lose accuracy through extended use.
Part cleanliness. Is the part dirty or clean at the stage of processing in which you want to measure it? That may affect labor requirements, maintenance, and the level of achievable accuracy, or it might steer you toward air gaging, which tends to be self-cleaning.
Gaging environment. Will the gage be subject to vibration, dust, changes in temperature, etc.?
“Mobility.” Are you going to bring the gage to the part, or vice versa?
Parts handling. What happens to the part after it’s measured? Are bad parts discarded or reworked? Is there a sorting requirement?
Workpiece material and finish. Is the part compressible? Is it easily scratched? Many standard gages can be modified to avoid such influences.
Manufacturing process. Every machine tool imposes certain geometric and surface finish irregularities on workpieces. Do you need to measure them, or at least take them into consideration when performing a measurement?
Budget. What do you have to work with?
If the part is simply an OD with a generous tolerance, then a caliper, micrometer, or even a dedicated snap gage would be potential solutions. However, if your key factors include performance (accuracy and repeatability), time and throughput, ease of use and operator skill, assured part classification, and cost per piece, then a custom gage solution is one that should be considered.
Often custom gaging is thought of when a solution is needed for a high precision, tight tolerance part. Air gaging is technically a custom solution since each air tool is made to do the specific check it was built for. Being custom built it is very accurate, fast, extremely easy to use, and economical based on the results it can provide. Air is a great solution for one or two checks at a time.
But things can get complicated: there may be more checks; they may not be ODs or IDs; the process being checked might be automated and a gage user may not exist. You may even have a fairly wide tolerance but the feature being checked is so difficult that any user—even the most skilled—would be unable to make accurate and repeatable measurements. Along with the gage, you need to consider part staging.
Custom Staging for Gaging
Some gaging applications call for inspecting a part for variation across a given feature, which calls for freedom of movement in at least one plane. Other applications call for measuring a series of parts at exactly the same location on the feature, time after time, to check the repeatability of the process. In the first instance, freedom of movement is a good thing. In the second, it’s an enemy.
For example, with the custom air plug mentioned above, to inspect a nominally straight bore for taper error you would insert the plug slowly, and watch the indicator needle or readout display for variation as you go. On the other hand, if you are inspecting IDs to confirm the stability of the boring process from part to part, you would measure every bore at exactly the same height. If you do not, any taper present may lead you to an erroneous conclusion that the process is unstable. The first application requires freedom of axial movement. The second requires that axial movement be eliminated. This can be readily accomplished by installing a stop collar on the plug to establish a depth datum. This bit of staging also makes the plug a custom gage.
The number and type of datums required varies with the type of gaging and the application. Figure 1 shows a slightly exaggerated drawing of a fixture gage used to check a piston for perpendicularity of the wrist pin bore to the piston OD. The bore is placed over an air plug, which serves as a datum, locating the part both lengthwise and radially. The critical element in the engineering of the gage is in the dimensioning of the two V-blocks that establish the heights of both ends of the part. Because of the piston skirt’s ovality, the V-block at that end must be slightly higher, to bring the OD of the head of the piston perpendicular to the plug. Without reliable staging in this plane, the gage could not generate repeatable results.
Other parts that often pose staging challenges are gears. Since gears are intended to rotate and interact with other gears, both the diameter of the gear and the concentricity of the gear diameter to the ID are often the two most critical dimensions checked. In this case, staging on the ID is critical as any looseness or offset will certainly affect the concentricity and possibly the OD. It’s therefore necessary to create proper staging that can find the center of the gear accurately and repeatably. Using special centering arbors, a custom bench gage can be created that is easy to use and has virtually no operator influence.
As many as three datums may be required to properly locate a part and gage relative to one another in three dimensional space. In Figure 2, an air fixture gage checks connecting rods, cranks and pin bores for parallelism (bend and twist) and center distance. Placing the connecting rod flat on the base establishes the primary datum. Although it is not shown in the diagram, the base is angled several degrees toward the viewer: the uppermost ODs of the crank and pin bore plugs therefore establish a secondary datum against which the connecting rod rests. Two precision balls are installed on each plug, located at a height half the depth of the bores. These balls locate the part lengthwise, establishing a tertiary datum.
This is a great custom layout for a bench gage that can make multiple diameter and relational checks of the bores (parallelism, taper, center-distance, bend and twist). Adding a little computing power will get all those checks measured in the blink of an eye. However, as connecting rods tend to be made in high-volume fast-moving production lines, one of our other custom gage design factors also comes into the equation: speed and throughput. No manual operator is going to keep up with the production demands of some processes, nor accurately make the right decision 100% of the time if he tried.
Automated gaging is usually cost-justified in applications where a part must be inspected every 45 seconds or less. This time includes not just gage operation but the entire gaging cycle. While the part measurement itself may take only three seconds, the complete cycle includes at minimum: 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 any gaging setup, whether manually operated or automatic. These include: the number of features to inspect; the need for a dimensional reading vs. 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 at play, it is hardly surprising that automatic gages can rarely, if ever, be bought “off the shelf.” However, automated gages can often achieve amazing results. In the case of one 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. To equal the throughput of the automated gage, the Tier 2 supplier would have to keep 13 human operators working around the clock, at a minimum throughput of one part every 45 seconds per person.
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.
Semi-Automatics—the In-Between Gages
Maybe you don’t have such a high volume of piece parts, but you do have part volume such that good operators cannot make multiple checks efficiently and accurately enough to keep up with the manufacturing process. Turbo charger housings are a good example. There are multiple diameters and relationships machined into these housings that need to be verified. A set of hand gages or a CMM could make the checks, but certainly not with the throughput needed.
A semi-automatic gage can solve the problem: one that makes multiple checks, classifies the part, and can mark or stamp it for identification. Frequently, these are manually loaded and unloaded, but they can also incorporate a disposal system.
Semi-automatic gages can offer other advantages. Since part of the gaging cycle is automatic, they are much faster than completely manual gages. When manually loaded, they can handle workpieces that may be difficult or costly to feed or orient automatically. Manual loading also permits visual inspection for scratches, discoloration and unclean finishes prior to the gaging process.
Semi-automatic gages can check a relatively large volume of parts quickly and accurately, enabling the inspector to keep up with production. Many manually loaded gages can be operated at speeds close to one part per second. Disposal is automatic, eliminating operator interpretation or sorting errors. Most semi-automatic gages are controlled by a small gaging computer that takes over the complete gaging function—positioning gage heads, moving the part, collecting data, marking the part, and disposing of it in the correct class. Operator fatigue and misclassification can be a big problem when handling many parts. Since the semi-automatic gage is tireless and consistent in its decision-making, many of the operator-influenced problems go away.
Today there are many choices of custom bench fixture, semi-automatic or automatic gages. And design time can be faster than you might expect. Often today’s gage builders use modular components, providing both faster deliveries and lower costs. A “custom” gage built with off-the-shelf gaging components and supplemented with motion control and gaging computers can be designed quickly and delivered ready to meet its gaging challenge.
All of these factors may be important when instituting an inspection program. Define as many as you can to help narrow the field, but remember that help is readily available from most manufacturers of gaging equipment—you just have to ask.