Pin gages - plus or minus?

I was always a big fan of Deltronics . If you buy a set , lets say a 1/4 set . It comes in a nice plastic case with the .2500 pin in the center with 10 pins on each side of the nominal 1/4 pin each of the pins on the side steps .0001 . In the last yrs. of my shop I got into honing , these sets were indispensable to me as I found them more economical to buy as opposed to a air gauge . I was getting them off the Bay . You can buy them individually also .
 
Pin gages will only tell you the largest diameter cylinder that will fit your bore. They tell younothing about variations in diameter, roundness, or cylindricity. An air gage can tell you all of that.

Some years ago I was involved in a project the required high precision glass bores. Precision glass bores are made by starting with oversized glass, inserting a precision ground mandrel, applying a vacuum to one end and heating the glass to shrink it around the mandrel. The specification that was usually quoted was adopted from the specification for grinding the mandrel. The resultant bore will necessarily be larger than the mandrel. Otherwise the mandrel couldn't be removed. But the question was how much larger and were ther bulges in the wall where the glass hadn't shrunk down properly.

We used tenths of thousandths pin gages in our incoming inspection but always had that gnawing question as to what undetected imperfections were present. An air gage would have answered those questions in an elegant fashion.

Unfortunately we couldn't convince our supplier to purchase the equipment or even to send out samples for inspection. I think the mantra was ignorance is bliss in that if we discovered some ugly truths, they couldn't claim the level of precision that they did. Also unfortunately, the number of vendors for this type of product is extremely small so, if we wanted the product, we had to capitulate.
 
Oh, very informative. Thanks

I guess that's why they are only hardened to RC60 -62.
 
Pin gauges are fairly inexpensive in small sizes, you will need 2 for bores, if say the target diameter is .2495" +-.0001"

A minus gauge will be GO at say .2494" which is within your +-.0001", if it goes in the hole is big enough.
A + gauge is NO GO, a .2496" pin will not go in the hole, if it does the hole is to large.
 
The discussion of gage pins and tolerances has been around a long time. Some people swear by them, others at them. But it's important to understand them. One often misunderstood feature is the actual size of the pin when discussing plus and minus sets. I pulled this from a tooling site:

Class ZZ Gages - These gages have an inch tolerance of .0002” and a metric tolerance of .005mm.
They are practical where good precision and speed are important.
Class Z Gages - These gages have an inch tolerance of .0001” and a metric tolerance of .0025mm.
They are one half the deviation of the ZZ for a better fit.
Class X Gages - These gages are available with an inch tolerance of .00004” and a metric tolerance of .001mm.
These gages are used where greater precision is needed.
Class XX Gages - These gages have an inch tolerance of .00002” and a metric tolerance of .0005mm.
They are one half of the deviation of the X.
Great for your most critical needs.

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What is important to note is that the nominal size isn't necessarily what they are, nor are the automatically plus or minus whatever tolerance they are specified to be. What this means is a 0.2500 minus pin in class ZZ(most commonly found in shops) does not have to measure 0.2498. That would be ok, and the pin perfectly acceptable in class ZZ, but most definitely not in class XX. A 0.2500 minus pin might measure anywhere between 0.2498 and 0.2500, but only within those limits. I've gone through many sets with a bench micrometer and found worn pins on the shop floor, and verified the size (to the limits of my equipment) of brand new sets and I can tell you that the manufacturers do definitely use their tolerances.

So don't depend on the pins being over or under their nominal size; maybe they are, and maybe they aren't. Check them for yourself if it is really critical. But also bear in mind things that affect whether something actually will go into a hole. Is there clearance? Of course there is if you can put a pin in a hole. Zero clearance is NOT a hand fit. It always takes a few tenths to allow you to hand fit a pin. Ask any engine machinist about wrist pins, for example. They fit close indeed, but there is clearance in any floating wrist pin. There has to be for the lubrication. But ask them how they measure that hole. Odds are it isn't with pins. Granted, that is larger than the pins under discussion in this thread, but the point I am trying to make is that the pin that "goes" only tells you that the hole is large enough to pass that pin. Of course, as previously mentioned, it tells you nothing of the other conditions of the hole, such as roundness, taper, egg, etc. . Normally, if you want an off the shelf dowel pin to press fit you would want a nominal size hole, because the dowels are deliberately ground oversize. Usually about 0.0002, so how much "press" or interference you need would dictate the desired hole size. In thin or soft material you may want a tighter fit, so you would shoot for an undersized hole. So to fit a standard 1/4" dowel, a 0.2500 hole might be specified. So what gage pin would you like to see go and not go? Remember, either one (minus OR plus) could actually measure right at 0.250000000______ but is only guaranteed to not be over(minus pin) or under(plus pin).
 
It is true that there is a tolerance associated with both minus and plus pins and there is no guaranty that a minus pin will measure in at nominal - .0002" or a plus pin at nominal + .0002".

I have to say though, I have never measured a pin that wasn't exactly .0002" under nominal for a minus pin or .0002" over for a plus pin based on a micrometer measurement. Just luck, I guess.
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In many instances, I just want to know if a hole is .250" rather than .249" or .251" and the tolerance band is good enough for that. For any critical dimensioning, I always measure the pin with a micrometer.

"Trust but verify."
 
Sorry, just suggesting that there was no definitive information or message other than innuendo.
 
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