Measuring small ID

[lwalker]

A few days ago I needed to make a small pin. I don't have a lathe, so I cutoff a 1.5" length of 12L14, put it in a 1/2" collet and placed that up in the spindle of the mini-mill with a lathe tool held in a vise.

Final diameter was about 0.185 or something like that. I carefully measured the hole's ID with dial caliper and got to work turning down the bar. Stopped a few times to measure, double checked measurements against hole dia, and when the pin was about 0.002 oversize I took it out to double check fit since I figured I could use sandpaper to get the remaining bit down and have a nice snug fit.

The pin slid easily into the hole! Remember it should have been .002 over at this point. When I was done cursing myself, I measured again. Everything checked out. Hole was 0.185, pin was 0.187 ??? WTF!!!

OK, so figured out later that the caliper was not measuring the real diameter of the hole because it was touching on the edges of the (inside) jaws, not the flats. So my question is what tool should I have been using? I have seen inside calipers for large holes, should I get a small pair for tiny holes, or is there some other tool that's better for this?
Couldn't use the outside jaws of the caliper because the hole is chamfered on both sides.

[DR]

For small holes like that a set of gage pins are the only practical way.

BTW, calipers are notoriously inaccurate for precision measuring.

[jimmstruk]

Check out E-BAY #280165035379 for an inexpensive set of pin guages. JIM

[tattoomike68]

I have had good luck with small hole gauges.

http://cgi.ebay.com/4-PC-SMALL-PRECI...QQcmdZViewItem

[mechanicalmagic]

I have an inside micrometer, MUCH better than a caliper.

http://www.kbctools.com/usa/Navigati...fm?PDFPage=629

But, they only go down to .200"

Pin guage is about the only way to go at your diameter.

Or, you can make a stepped guage.

[heavysteamer]

I too use small hole gages. That way I can use the same mike on the gage and the pin.

[Carld]

I too use Starrett small hole adjustable gauges. They are a split ball with an expander in the middle. Use them like telescoping bore gauges.

[SGW]

I also use the Starrett expanding ball gages, although it takes a while to develop the proper feel for them -- just like the telescoping gages. It helps to drill/ream a few test holes you know the diameter of, and measure them for practice. The feel you get in a particular hole depends in part on the surface finish, too, which adds another variable into the equation.

DR's right -- the most accurate way is with a set of pin gages. Even with those though, if the hole surface is rough...what "diameter" are you measuring?

[Bmyers]

what is the difference between plus tolerance and minus on pin gages?

[rkepler]

what is the difference between plus tolerance and minus on pin gages?

The pins gage isn't exactly on size, it's going to be a little over or a little under based on the class of gage. For a ZZ class gage the tolerance is .0002", so a 1/2" plus gage in ZZ will be between .5000 and .5002", a minus would be between .4998 and .5000"

There is a type of gage for holes smaller than the normal ball end gage will measure:



This is one from a set by Moore & Wright (I've seen them in the SPI catalog as well) and is intended to allow measuring holes from .0625 to .080, the rest of the set extends the range to the bottom end of the ball end gages. These are similar in that there are balls and a tapered pin that drives them apart to engage the sides of the hole. It takes a pretty delicate feel with these little guys.