Custom Gaging
Temp. Comp.
SPC Guide



The Custom Gaging

Sometimes "design
and build" is the only way to go.

by Steve Cindric


Everyone knows that off-the-shelf gaging offers excellent performance at the lowest possible acquisition price. Nevertheless, there are times when "design and build" (i.e., custom) gaging is the only alternative that makes sense.

 Consider this scenario: As a manager in the quality or engineering department, you're responsible for some aspect of quality assurance in your organization. An important customer has given you a big new contract that comes with some very specific inspection requirements. All of your research to find a suitable off-the-shelf gage to perform the inspection has yielded nothing. You have lots of fat catalogs from all of the gaging vendors, but no product is jumping out as the solution to these requirements. You're up against the wall.

 Here's another one: It used to be all right to have an operator walk by the machine tool every once in a while to sample a few parts and make some adjustments to keep the parts in tolerance. Now your customers won't accept a batch of parts with even a single one out of specification. So your company has decided that it's time to move up to the next level of manufacturing excellence at your plant. Senior management has asked you to investigate a system with automatic gaging and machine tool feedback to keep the parts in tolerance without operator intervention.

 These are just two of the potential situations in which off-the-shelf gages won't take you where you need to go. What will do the trick is a package of one or more custom-built gages designed to efficiently fulfill your specific inspection requirements while resolving issues involving unique critical dimensions, tight integration of inspection with manufacturing processes, inspection throughput requirements and shop floor environmental conditions.

 Is custom gaging more expensive? Are the specification requirements more complicated? Is there a potential for you to overshoot your gaging hardware budget and miss critical deadlines if you don't handle this project carefully and methodically? Yes, yes and yes. Should this be cause for alarm? No.

 What follows is a step-by-step guide to a process that will bring you through the design-and-build gaging selection and procurement labyrinth to improved manufacturing precision and higher yields.

 Where do you start? If you've been purchasing gages as a part of your normal job function, you should have the names of several vendors who may be qualified to provide you with the gages you are looking for. Choose a good one to partner with in this process. But if you're new to design-and-build gaging, keep in mind the following considerations when selecting a vendor.

Danger Zone

Fuzzy input about material handling for design-and-build gages can produce unhappy consequences.

 Material handling is a major part of any design-and-build automatic gaging system. It is also a danger zone where misinformation is less likely to be questioned until late in the project. Don Plonka, a Mahr Federal design engineer, relates a case where "fuzzy input" about material handling requirements threatened to cost a great deal of money and cause tremendous delay on a custom gaging project.

 "We built a series of automatic gages designed to be loaded by an overhead gantry," Plonka explains. "The gantry, a standard unit for loading and unloading parts, was to be supplied by the manufacturer of the horizontal turning machine. After machining, the part had an inside diameter (ID) bore that we had to check, and the gantry was to place the part over a variable reading plug gage.

 "We began by asking for the X-Y repeatability of the gantry," Plonka continues. "We didn't quite believe the machine tool builder's statement of 0.0004 in. repeatability, so in our initial calculations we substituted 0.003 in. Based on this, we felt very comfortable using our standard-catalog Dimentron gaging plug to do the ID bore measurement. This standard product would be incorporated into a gaging setup that would be used with multiple machines, saving the customer a lot of money compared to designing a special bore measuring system.

 "We were two weeks away from the delivery of the first units when we received a 'panic call' from the machine tool builder. It seems there had been some problems with the gantry and the machine tool builder was revising the repeatability specification. Instead of 0.003 in. clearance, they now required 0.015 inches on the diameter of the Dimentron plug for clearance.

 "The gantry would also need more clearance to swing past the tip of the gage before moving in to position the part. The potential downside of this information was the need to design and build custom bore gages at a cost of tens of thousands of dollars and the loss of eight to 10 weeks on the delivery cycle for the gaging systems.

 "Due to the now incredibly short lead time, we did some quick studies. Fortunately, the part was being held in the gage very accurately on the centerline by other means. We calculated that the Dimentron plug's body outside diameter could be reduced without having any centralization difficulties. To achieve the swing clearance, we were able to substitute a longer 'reach' Dimentron with a highly increased measuring range, and still maintain gaging accuracy.

 "This turned out well in the end, but only because of sheer luck that the part was being centered precisely for another measurement function," Plonka concludes. "In a situation where the ID bore was the only measurement, this entire gage concept would have required redesign with severe consequences in cost and delivery time. This potential for disaster could have been avoided entirely if all the right information about the material handling system had been available upfront."

Basics first

 As always, the starting point for any gaging acquisition is writing a specification. Immediately we encounter a potential trap dubbed the "I might as well have them add this and a little of that" syndrome. After all, how often do you get a chance to write a specification for the gage of your dreams? But just remember, everything you put on the page adds cost, complexity and delivery time to the end unit. If you want to know how much extra a few goodies will cost, have them quoted as options to the gaging system.

 Be ready to furnish a full blueprint of your part, including tolerances, even if it's a single feature on the part that's being measured. An engineer trying to provide an adequate quotation will need to know the overall measurement envelope and the materials involved.

 If you're unsure of how to proceed, give the potential vendor a call. Most provide their sales engineers with simple forms for quoting design-and-build gaging. These forms give the basics required for a good quote, and you should be able to borrow a blank one for your use.

 The basics required for quoting the price of a bench-style gage are:

  Part identification--The part name and number, along with a full blueprint.

  Critical features--The feature(s) to check, with the nominal values and tolerances. If possible, highlight them on the blueprint.

  Due date --The date the quote is due. Also determine when the gage is required in the plant. This date is usually quoted as "ARO" (After Receipt of Order).

  Pricing detail--Do you need firm pricing, or is a budgetary figure enough at this point?

  Completeness requirements--Do you expect to get enough information in the quote to place a direct order for the gage, or are you simply checking the feasibility of your project from a technical and/or budgetary perspective?

  Gaging method of preference--Do you have a preference for the method used to gage the part? (Are you a real fan of air gaging, for example, so that you prefer this method to an electronic system? Or will you leave it up to the potential vendor to suggest the best method for providing the results?)

  Master--Do you need a master to set the gage, or do you already have one available? If you're buying a new one, do you need a certificate of compliance to a specific standard for your record-keeping requirements?

  Runoff requirements--Are you going to require a gage repeatability and reproducibility test to accept the gage? If so, in what format? And at what level are the results considered "passing?" (Although this seems like a straightforward requirement with today's standards and specifications, don't forget that conducting these tests requires both time and personnel. If you're dealing with a reputable gaging vendor, you'll pay for the labor necessary to provide this data.)

  Other considerations --Do you have any other specific items you feel you should point out to the person preparing the quote? For example, if you've had problems in the past with gages marking or burnishing your parts, you may want to point this out by including a note stating that this is an unacceptable condition.


Advanced preparation

 If you're looking at an automatic system, even more information is required:

  Material handling responsibility--Who is to supply the material handling? Are you going to provide a system that places your part in an exact location, in a repeatable attitude? Or is it up to the gage vendor to take your parts from the current production process, in the random attitude this process creates, and handle them through the gaging process? (A quick word of advice here: If you break down a quotation for an automatic system, you will soon realize that the part handling is usually more expensive than the gaging portion in the overall system. Material handling components capable of staging parts properly for gaging can become quite costly. There are ways to control these costs, such as using manual loading of a semiautomatic gage, but be prepared for costs above what would be considered "normal" for simple gaging alone. Refer to the sidebar on page 31 for more information.)

  Part discharge--How should parts be discharged from the gaging system? Is there a single category for rejected parts, or are rejects sorted by type for further processing?

  Gaging environment--What is the environment in which the gage will have to perform? Is there grime, grit, coolant and other debris to be cleaned prior to the gaging station? Is this to be incorporated into the gage? (You want to measure your part, not dirt, right?) Are there extreme temperatures during certain times of the production year? Do you have sources of heat on the shop floor, such as heat exchangers from other equipment, that are directed at the area where the gaging station will be located? (For a related article, see "Do You Have a Temperature Problem?" on page 41.)

  Gaging throughput --What production rate is required? What is the duty cycle? Are you running on the day shift only, or is yours a 24-hour operation? Does the gaging process need to be capable of variable speed? This would be the case, for example, when a second production source adds to inspection volume at the gage during peak periods. This might call for additional material-handling equipment such as a "makeup table" system. Who would be expected to provide this?

  Unique plant specifications--Do you have your own plant specifications regarding such items as special paints and colors? Do you require the documentation to be on your paper with your title block? Do you require the prospective vendor to use certain brands of electrical, hydraulic or pneumatic components to satisfy your maintenance operation's standardization requirements? What utilities, electrical and pneumatic, will power your gaging system, and are there sufficient outlets nearby?


Select an experienced guide

 All of the checkpoints detailed above are made with the understanding that you're working with a trusted design-and-build gaging vendor who can help you over the rough spots. If you aren't, then you'd better find one. When examining the credentials for the prospective vendor, here are some important considerations:

  Gaging systems--Does the vendor offer multiple gaging systems based on the component and tolerances being inspected? Are electronics, air gaging, electrical, simple mechanical and computerized systems all available? Can they be intermingled to design a system with optimal functionality and performance? (If not, you either may not get the resolution required or may pay too much for intricate systems that aren't required.)

  Construction --Does the vendor use durable components? You're buying a system meant to last the life of a specific program; make sure the samples you see from the prospective vendor are engineered to do so.

  Experience --Ask your potential vendor for a list of customers for whom the company has supplied design-and-build gaging. Review this list carefully. In most cases, you should see names you recognize, maybe even your competitors'. Ask about specific applications. Can the potential vendor show you examples of similar gages it has built in the past? (On the other hand, if the vendor offers too much information, such as divulging proprietary competitive information, don't forget that it might do the same to you.)

  Technical help--With whom would you deal locally if you need technical assistance? Does the vendor have skilled technical field representatives who are available to guide the process to a conclusion? Or are you going to communicate by phone and fax during the design and approval stages? Can you meet directly with the engineers assigned to your project?

  Guarantee --Expect to see a written guarantee at the time of the quotation. Know the difference between components that are covered and items that are considered "wear parts."

  Service --Should you have a problem, will service be prompt and thorough? Even though you don't expect personnel to be onsite 24 hours a day, you should be comfortable with the service response time quoted. Where is the local service coming from? Is it by factory-trained, direct employees or a third-party service? Are the service personnel qualified in all aspects of their jobs?

 Once you've compiled a short list of potential vendors, it's time to choose one. Put your specific requirements down on paper and let them be your guide in making an apples-to-apples evaluation of the contenders. However, don't become so attached to your list that you're closed to new suggestions and considerations from your prospective suppliers. These can be very valuable.


The home stretch

 When you place your order, it's a good idea to specify various milestones in the project, with a timeline for your use in ensuring that the project is on track:

  Approval--The first stage would be an approval process. You should receive full blueprints of the gage system assembly for your review and acceptance well before a single chip is cut in the manufacture of your product. Review the drawings and explanations carefully. Questions should be fully answered at this point. Once you sign off on the approval drawing package, manufacture begins.

  Changes--Any items you decide to change after the approval stage usually result in a change order to the program at additional cost to you. If, on the other hand, changes are required for technical reasons not caught by the vendor during the design phase, make sure you have updated drawings and full explanations for the change, costs of which should be the vendor's responsibility.

  Manufacture--During manufacture, don't hesitate to call the vendor on occasion to inquire about progress.

  Runoff--Select the runoff date well in advance. If parts are required for runoff at the vendor's facility prior to shipment, make sure they're there on time. During the runoff, remember that you already did your homework, so the testing and buy-off procedure is already outlined. If you wish to do additional tests, be prepared to cover any of the vendor's additional expenses.


 It would be far simpler and less expensive to purchase off-the-shelf gaging, but doing so denies you the particular tools you need to achieve excellence in this manufacturing situation. By working together with your customer, members of the manufacturing team and your vendor, as well as covering all the points on the checklists, you should arrive at the gaging system you need within reasonable costs and timeframes. Once you've done so, congratulations to all are in order. But don't overdo it: It's time to start making good parts.


About the author

 Steve Cindric is the director of design and build operations at Mahr Federal ( www.mahrfederal.com ), where he is responsible for all activities of the business unit. E-mail him at scindric@qualitydigest.com .

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