10 Questions You Should to Know about pin gauge set

Question:

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We recently received a complaint from a customer who claims a diameter hole is oversize. The method of gauging the diameter is with minus gauge pins.  The part is a plastic molded part (the material used is PBT). The diameter is .150 +.004 /-.002.

The method question is we do not force the maximum pin in the part, we use the weight of the pin to fall into the opening using no hand force pressure except to guide the pin over the opening.

Our customer is using a method of hand pressure to force the maximum pin in the diameter opening.  If the gauge pin begins to enter they continue to try and force the pin and record the hole as oversize.

Are there any instructions on the proper method for using gauge pins in regards to hand pressure, force entry, and gauge pin weight?

Thank you.

Response:

This is a question that comes up often.  To begin with, let me say that a gauge pin should never be forced into a machined hole.  The largest pin that can be fully inserted and extracted using only light finger grip on the sides of the gauge is what will determine the hole size.

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Most gauge pins used in industry today are Class Z. These can be either 'Plus' or 'Minus' pins.  Those most commonly used are the Minus pins.  They are tolerance up to -.'. Therefore a . gauge pin might be actual size but it is generally referred to as a 1.000' pin (The size shown on the pin).

It is common practice in American industry to use a GO/NOGO pin set up.  The size you mentioned, .150 +.004/-.002 would require a GO pin of .152 and a NOGO pin of .154.  If NOGO pin will not fit but, the Go pin can be fully inserted without interference, the part is acceptable on the low end of the tolerance.  If the NOGO pin fits without interference, then the hole is oversize and the part should be rejected.  To touch on that just a little further, keep in mind, if you have a 1.000 hole, a 1.000 pin cannot be inserted into it. That would be a size-on-size interference fit.  However, a 1.000 Minus pin might slip in without difficulty.

One other thing to keep in mind is the surface finish of the holes.  A hole that is out-of round could also introduce fit problems.

The Machinery's Handbook shows the American National Standard Tolerances for Plain Cylindrical Gauges.  However, there really is no documented standard (that I am aware of) which tells you how tight or how loose a gauge pin should fit.  The common practices noted above should help you there.

You mentioned that 'if the gauge pin begins to enter they continue to try and force the pin'.  It is not uncommon for the beginning of a machined hole, or a hole in an injection molded product to be slightly larger near the surface.  Various machining and/or molding practices would eliminate that.  Yet, it is the 'full' insertion and extraction of a pin, without forcing, that determine acceptance criteria.

Thank you for the good question.

Bud Salsbury, CQT, CQI

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Calibration of Pin Gauges

I'm the QAM at a company that makes plastic parts. We do not have the money to have an outside source conduct yearly calibrations on all our gauges. I have read about a few different methods mentioned in the forums such as calibrate before use, etc. Could someone explain this method a little more, and how it is documented. Or any other methods that they use to pass the Calibration section of ISO for pin gauges. Thanks in advance Our pin gauges are used for final acceptance on alot of our product, so I know that they need to be calibrated. But what do you suggest for calibrating them, a calibrated super mic? I have decided to coat them with wax to help indicate which gauges have been used since the last calibration. I truely need help in the calibration process. We have had the set of gauges for several years and I don't believe we can get a certificate from the manufacture. Calibrate before use is intended for circumstances where an instrument or gauge is not used for some indefinite period of time. So to save calibration costs, it is placed in a controlled area where it cannot be used without authorization with the status CALIBRATE BEFORE USE, meaning simply that.

When it is determined that the unit is to be used, it is submitted for calibration at that time.

It sounds like CALIBRATE BEFORE USE may not, however, be the appropriate status you are looking for. Any gauge you use to determine product must have a known accuracy. Whether you send to an outside calibration supplier as the method for assuring that known accuracy is not critical. What is a must is to know that when you make a measurement of a product parameter, that it be correct.

Some questions to ask perhaps would be as to whether all of your gauges are used to measure product specified parameters. Another question is as to whether all of your gauges are used on a regular basis. Are some of them sitting in cabinets for indefinites periods of time? Have you researched what measurement instruments would be adequate to calibrate the pin gauges inhouse? Are the required specs being measured less demanding on some sets of gauges than others. There may be some variations on how some sets are calibrated (meaning it also needs to be well documented and labelled to preclude using the wrong sets on the wrong process).

Just a few thoughts. Any further questions let me know.

Jerry Eldred said: Some questions to ask perhaps would be as to whether all of your gauges are used to measure product specified parameters. Another question is as to whether all of your gauges are used on a regular basis. Are some of them sitting in cabinets for indefinites periods of time? Have you researched what measurement instruments would be adequate to calibrate the pin gauges inhouse? Are the required specs being measured less demanding on some sets of gauges than others. There may be some variations on how some sets are calibrated (meaning it also needs to be well documented and labelled to preclude using the wrong sets on the wrong process).

Just a few thoughts. Any further questions let me know.

Not all gauges are used for specified parameters, but some are. Some gauges are not used at all and do sit for long periods of time. We have alot of very common sizes like .250, .300 that are used on a daily basis. As for the instruments to calibrate with, we have not decided on this issue. We do have some parts that are critical and need gauges that are very accurate! We usually use pin gages to do go/no go testing of parts with a tolerance of +/- .004" or more. We calibrated those gages to start every month, but time proved we could go to 3 months with no problem on most of them. We use micrometers to calibrate them and set the allowable tolerance based on the tolerance of the spec. we're testing -- i.e. we may require the pin be +/- ." from nominal if the tolerance is +/- .004" but we may allow +/- ." if the tolerance is +/- .010" etc.

The thing to remember is you cannot get a ." pin into a ." hole --or even a ." hole in most cases. We used ID micrometers with the pin gages to determine experimentally how much "clearance" was required to get a pin into a hole in our parts, then wrote some rules on what size pin was needed for a given size hole. For example, the "go" pin for a ." hole may need to be ." - .".

Hope this helps. Many pin gage sets in use are minus-toleranced (approx. ." under the marked nominal size for an inch set). These pins have gagemaker's tolerance like a no-go plug gage. Plus-toleranced pin sets (like go plug gages) are also available. If you have several sets, you should determine the tolerance direction and mark them so pins are uniquely identified for calibration traceability.

As for the critical ones requiring great accuracy, check out air gages from companies like Federal or Western. The accuracy is there, parts aren't damaged and the air spindle won't wear. One master ring gives easy setting, and you will have variable data for a better understanding of your process and it's output. It will come with a particle filter, but you will have to add an oil removal filter to the air supply line (compressor oil will get into the line). You don't have to buy it from the air gage manufacturer, filters suitable for instrumentation are sold by many industrial supply houses. Mine cost less than 50 dollars and I installed it myself.

Hope this helps.