Pin gages - plus or minus?

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Alan H.

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I want some pin gages to measure small holes. I need to do some interference fits, 1/4" for example.

I took a quick look at availability and there are plus sets and minus sets. There are Shars to Starret to Meyer in terms of cost and I assume, quality.

Which would you buy, minus or plus, and why? I am hoping the answer is not "both".
 
Every place i've ever worked (including the QC lab I'm in now), and all the sets in my own shop, are minus sets. Minus gauges are .0002" undersized.

If you use a +pin to size a hole and it fits, it's already too big.

The sets at work are Meyer. The 3 sets in my shop are import (.011"~.5").
 
Yeap, looks like Meyer is top of the heap are they not?

I am looking to buy that same range.
 
Both. You can check it doesn't fit with one and that it does fit with the other, not both without both.

When you say small holes what do you mean? I have an inside micrometer that goes down to about 0.2 inches. I also have some angled gage blocks that I can measure with an outside micrometer, and of course there are telescoping gages you can measure with an outside mic for larger holes.

I have a set of B rated square gage blocks from Shars, and for what I do they are just fine. I imagine their pin gages would be about the same. I do have some pin gages, but they are for larger holes.

What are you pressing in and what is the tolerance? For example .250 dowels pressed into .246 holes in aluminum just fine (they shave a little metal), but for even soft steel I'd want .248-.249, and for something fragile like a nickel plated rare earth magnet I like .249 in aluminum and .2495 in steel.
 
Work today is a hardened .249" pin in a crank of 1018 steel. I want an interference fit of the pin in the crank and that will be lubed and locked with Loctite 648. Same pin to ride in a scotch yoke with bronze slider.

Other examples yet to come, who knows?

I have Starrett small hole gauges, Starrett and Yuasa telescoping hole gauges, and Mitutoyo gages for larger holes. The small hole gauges are a PITA, so I want some pins to augment my ability to measure smaller holes.
 
Checking a hole with a two point measuring tool and checking a hole with a gage pin tell you two different things. My accurate holes are almost always made for something cylindrical to fit into them, tight or loose or sliding. I have pin gages from .011 to .625", all minus tolerance, and they do the job for me, quickly and giving all the answer I need.
 
Thanks folks for the feedback.

I decided to go with a moderate cost approach and ordered three sets of gages from Shars (via ebay) as a start. I ordered minus sets, .011" to .500". Shars was kind enough to combine the shipping so the costs are reasonable. I am hoping my belief that the quality will be adequate is correct.
 
For a first set, I would go with minus pins. However, a standard set will only show you the diameter to .001". If you are fitting a minus pin to a measured .250" (go .250"/no go .251" The hole can be anything between .2498" and .2508". By having a plus set as well, you now have pins that increment .0004" and .0006" You can now bracket the hile diameter to between .2498" and .2502".

This can be handy when trying to make a precision fit. My .060 -.250" is a plus set, ordered by mistake. My .251 -.500" set is a minus set. At some point, when I'm thinking about it and when there's a good deal offered. I will order the two complimentary sets. My main measuring instruments are calipers, micrometers, and pin gages in that order. I use the pins extensvely for small hole verification.
 
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