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?
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.
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 -.0002”. Therefore a .9998 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