# Pin gages - plus or minus?



## Alan H.

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".


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## Holescreek

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").


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## Alan H.

Yeap, looks like Meyer is top of the heap are they not?

I am looking to buy that same range.


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## Robert LaLonde

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.


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## Alan H.

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.


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## Bob Korves

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.


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## Alan H.

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.


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## Dinosaur Engineer

Don't forget that a 0.250 pin will not go into a 0.250 hole without forcing it.


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## RJSakowski

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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## ezduzit

Alan H said:


> ...I decided to go with a moderate cost approach and ordered three sets of gages from Shars...



They're going to rust like a *****.


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## westsailpat

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 .


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## RJSakowski

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.


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## Alan H.

ezduzit said:


> They're going to rust like a *****.


What makes theirs rust more than others made of steel (i.e. Meyer & Vermont)?


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## ezduzit

Cheap steel.


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## Alan H.

Oh, very informative.  Thanks  

I guess that's why they are only hardened to RC60 -62.


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## Wreck™Wreck

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.


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## Tony Wells

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.

********************************************************************

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).


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## RJSakowski

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.
.  
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."


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## ezduzit

Alan H said:


> Oh, very informative....



Don't shoot the messenger. You could have asked before just buying the cheapest crap available.


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## Alan H.

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


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## ezduzit

Unsubscribed.


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## Alan H.

RJSakowski said:


> 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.
> .
> 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."


It will be interesting to spot check a few of the moderately priced pins that I ordered when I receive them in a few days to see how close they are.  Of course they are the ZZ spec. and are minus sets. (+.0000", -.0002").


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## 4GSR

Alan,

Don't worry about what ezduzit said about the Shars gage pins.  For your needs, they will last a lifetime.  Keep them oiled down with LPS-1 or Starrett M-1 or Starrett gage & Instrument oil.  DO NOT USE WD-40!.  Doing so could/will turn them brown!

Ken


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## Alan H.

Thanks Ken, yes I believe they will be fine.   Shars is certainly not Vermont, Meyer or one of the other top shelf manufacturers.  I am looking forward to confirming their specs on tolerance, edge treatment, surface finish and hardness they claim.  I have bought very few Shars items but what I have bought is okay.  

I am fortunate that my shop is humidity controlled and I don't have problems with rust.   

Your recommendations for surface treatment are appreciated and I have all of them in the shop.  I added the LPS a few months back and haven't used it yet.  I believe your recommendation against using WD40 is spot on too.  It is a great cleaner because it's full of surfactants/detergents and I use it by the gallon but never for permanent lubrication or protection.   I use it in my ultrasonic tank (in a beaker) from time to time and it makes a dandy cleaner.  

One I might add to your list for a corrosion protectant is Boeshield.  I've had great luck with it over the years.  A flannel cloth saturated with it is an excellent tool to wipe down gages and the like.


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## kalister1

The pin gages at work came with a sheet of paper giving the exact size of each pin.


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## kalister1

The pin gages at work came with a sheet of paper giving the exact size of each pin.


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## pacifica

Holescreek said:


> 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").


I agree, If too small you may be able to remove some material to get an exact fit. 

If too big you are stuck.


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## P. Waller

The diameter callout on the hole will dictate what size pins will be required.
As an example a diameter dimension of .500 -.000 +.001 will tell you that the intention of the engineer is a hole that is not LESS then .500 so buy a .5001 pin as a GO gauge that MUST go into the hole (a + pin) and a .5019 pin as a NO GO gauge which MUST NOT fit in the hole (a - pin).

This assures that the hole is larger then .500 but smaller then .502
You will notice that this example is 3 decimal places, there is no wiggle room with -.000 because at 4 decimals it is still 0, the + .001 is still within tolerance at .5019, if the call out is .5000 -.0000 +.0010 then buy a -.501 pin as a NO GO.

Much to your chagrin you will have to buy pins for each size hole in .0001 increments if you work that closely at all times.
.032 to .5 is .468 / .0001 = 4680 pins.
At .001 increments it is only 468 pins.

Good luck


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