Why Do They Make Gage Pins Minus And Plus

[Andre]

I will be building gauge pins soon, a small set. I am only grinding them to a very tight tolerance because I want to make them the best I can and maybe even impress some CNC junkie in the future who thought these old machines were garbage

I've never heard of measuring a precision bore to two tenths using a hard gage, I believe a bore indicator or bore micrometer set with standards would be better suited for the purpose.

 

[Holescreek]

I worked in a jig grind room (40+ Moore jig grinders) for several years. Everything we ground had a .0002" tolerance. All size measurements were done with Deltronics gauge pins sized in .0001" increments. Drawers and drawers filled with gauge pins. I didn't see a bore gauge until I left that area to run a horizontal boring mill where the tolerances opened up to .0005".

 

[planeflyer21]

I worked in a jig grind room (40+ Moore jig grinders) for several years. Everything we ground had a .0002" tolerance. All size measurements were done with Deltronics gauge pins sized in .0001" increments. Drawers and drawers filled with gauge pins. I didn't see a bore gauge until I left that area to run a horizontal boring mill where the tolerances opened up to .0005".

Oh yeah. I forgot all about the cabinet after cabinet of 2" deep drawers filled with thousands of gauge pins.

And how almost without fail only one of the pins you needed would be there, the other checked out to a different part of the shop for a hole they were making.

 

[mmcmdl]

I worked in a jig grind room (40+ Moore jig grinders) for several years. Everything we ground had a .0002" tolerance. All size measurements were done with Deltronics gauge pins sized in .0001" increments. Drawers and drawers filled with gauge pins. I didn't see a bore gauge until I left that area to run a horizontal boring mill where the tolerances opened up to .0005".

Moores , Deckels , Pratt and Whitneys and Lindner jig bores and grinders here . Most of them had optical scales . 2 tenths was the norm on diameter and true position . NASA and DCAS would come in and inspect parts while still on these machines as CMMs were not up to that kind of accuracy yet . I miss those days .

 

[cvairwerks]

For some things, pins are much easier and accurate than trying to use a bore gauge or hole mic. Ever try to use a bore gauge on a part that is only .060" thick? The ability to get an accurate check and repeatability often justifies the pin over the gauge. We've got stuff at work that gets checked with nothing more than a set of go-nogo gauges ground and polished to very tight tolerances. With them, it takes the average structures guy about 5 seconds per hole to verify it's within limits. Multiple that by several holes per assembly and the reduced production time pays for the tools quickly.

 

[mmcmdl]

Bore gages , trimics and hard gages all have their applications where they are the best solution . We made countless gages that we called shop aids for machining and inspection . When they were not in use , they were dipped in Cosmolene (sp) and kept in the QC dept . I will bore everyone with another experience I encountered while running a 10 foot TOS cylindrical/internal grinder .

We were manufacturing tapered mating parts for nuclear subs . The parts were approximately 16" in diameter by 18" long with a 1 1/2 degree included tapers . Male and female tapers had to achieve over 90% engagement checked with blueing . The print requirements stated that at 700 lbs of force applied , the male had to protrude out of the female +/- .010 . Our younger estimator saw this wide open tolerance and bid the job accordingly . When you sit down and take the time to really find the needed tolerance on the tapers , well , we lost our arses on this job but completed them per spec .

We got to go down to Newport News Va. and watch them while installing these parts in the subs . They were beating these things together with 50lb. malls !!! LOL .........so much for close tolerance .

Now that I've bored everyone for 2 minutes , I'm off to a retirement party for a co-worker . One more machinist out of the workforce as of today .

 

[RJSakowski]

There is an issue with using pin gages to measure a bore. The gage only tells you something about the minimum distance across a diameter. Any irregularities such as lobarity (is that a word?) a barrel shaped hole, etc. will not be picked up with a pin.

We had an instance where we had precision glass syringe barrels made; approximately .09" diameter but with a +/- .0001" tolerance. We had pin gages in .0001" increments to check the diameter, as did the manufacturer, but they really didn't tell us what we really wanted to know.

Profiling the glass cylinder properly could have been done with an air gage which measures the back pressure of two or more opposing jets of air to make accurate measurements. The accuracy depends upon the size of the gap between the probe and the part but for a reasonably close fit, accuracies better than .0001" are achievable. With different probe styles, taper, lobarity, bore straightness, squareness, can all be measured.

Unfortunately, the vendor did not want to spring for the cost of the instrument and they were the only game in town, or for that matter, anywhere so we had to be content with our pin gages.
Fortunately, for most of the work we do as hobbyists, we don't need that kind of accuracy. If we are measuring a bore for a clearance fit on a shaft or a bearing housing for a proper interference fit, a ten thousandth out of round is of no real concern and pin gages will do the job.

 

[Andre]

I guess you learn something new everyday. Thanks for the info!

 

[gregg]

Thank you all. And love storys.;-)