Other Dimensions: Standard or Special?

February 25, 2010
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If a custom-made gage is in your future, it is important to ask the right questions.



It comes up a lot. The method used to measure the parts is not good enough using standard instruments so the manufacturer looks for something “better.” Wading through catalogs, while interesting, is usually not going to help much because there often is more to solving measuring problems than changing the hardware involved.

From a cost-saving point of view-the only way some people look at these things-the choices from the lowest to the highest cost are a standard gage or instrument, a modified version of the standard item, or an out-and-out specially made gage or instrument.

If it appears a special instrument is in your future, this column will attempt to answer key questions to meet your needs. To do this, I’ll fall back on some basic questions: Why? What? Who? When? Where?

Accurate answers to these questions will put you on the home stretch to finding a better way-or the realization that there may not be a better way than what you’re already doing. So let’s get started.

Why?

Why are you on this quest in the first place? Is it speed or accuracy that needs fine tuning? Frame your subsequent questions around the answer. Keep in mind that higher speed may mean lower accuracy, depending on the measuring system selected for use with a specially built gaging fixture.

What?

What is it that you are actually trying to measure or check? What is the tolerance on that feature? Are there geometrical characteristics such as taper and squareness that must be determined?

What are you using now? The answer to this one question may be all that your gagemaker needs to know to put you on the right track.

What type of answers do you need? Actual values or yes/no answers? Too often the answer given for this question is: “Quote me both ways.” This is not good. Yes or no answers usually mean fixed-limit gages or fixtures. So if you need numbers, there is no sense even considering fixed-limit equipment even though they may be cheaper. A hybrid fixture may be a practical compromise.

What sort of budget do you have in mind? Suppliers need to know this, particularly if a specially made item may be involved because customers may have unrealistic expectations from a financial point of view. If you don’t have a budget number, ask your supplier for an estimated range before everyone invests a lot of time.

What type of data processing do you need from the equipment or does it have to link up with existing systems?

What is the production rate in pieces per minute?

What is the required inspection rate? 100%? Every other piece?

What gage repeatability and reproducibility (GR&R) do you expect from the process? Did you just say 10% because it is everyone’s goal or do you know it is achievable-all factors considered?

What will be the acceptance criteria-gage accuracy or GR&R numbers? Most suppliers can estimate a GR&R range, but guaranteeing it is another story since so many variables outside of the gagemaker’s control can influence the results.

What type of master is preferred for setting up gaging fixtures? Super accurate ones where every feature has near-zero errors, or less costly models with offsets?

Who?

Who is the go-to person with authority regarding changes to technical requirements and design? Will the supplier have to go through a buyer for technical information?

Who is going to be using this gage, instrument or fixture? Neanderthals, apes, unskilled humans, semi-skilled or skilled people are all types we have been asked to design gages for. Skill level of the equipment operator can be a deciding factor on design and performance.

Who is responsible for carrying out GR&R tests if they are required?

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