View Full Version : Small hole measurement accuracy

Lucky Shooter
03-16-2011, 12:05 AM
I have a question for the good machinists.

How closely can you measure small hole ID's with the Starrett Small Hole Gauges ?

I'm having trouble getting consistent readings-----plus-----since I don't know the hole ID, I don't have a reference to learn the feel of this.

I'd like to pick up a small ring gauge so as to know when I get it right---and maybe develop a consistent feel.

I have some ring gauges in the .700" to .900" range and can do pretty well with the Telescoping Gauges.

I also have a set of Starrett Webber gauge blocks and can keep my micrometers calibrated and can get good consistent measurements.

The Small Hole Gauges are eating my lunch and I'd appreciate any advice anyone might offer.


A. Weldy

03-16-2011, 01:40 AM
How small is small and how accurate do you need to be? I good set of pin gages will get you pretty darn close but they go in steps of .001". I think to consistently do better than that you'd need a dedicated bore mike with reference ring, and they don't come cheap. Depending on what you are measuring and if it is a through hole, you can tap a soft lead ball into the hole and back out and measure with an outside micrometer and probably be within .0002" if your mic is that accurate.

03-16-2011, 02:08 AM
What are you trying to measure? What approximate ID?

If you are doing chamber work, and lets say you're trying to measure a neck. Most are tapered, so, you need to know where you are on the part. Same goes for case bodies. They all have taper (if talking about stuff we use here).

I try to go for a just touching feel. If you want to know how much that is, use your caliper or mic's on the thing as you are checking. The amount of pressure you create on the gage when you use with the mic is the amount of pressure you should feel for when you set the gage on the part. If you use a vise grips on the mic spindle when you set it to the masters, then you should use a vise grips on them when you measure the part too! Lol. All joking aside, consistency is the key. For a good test, wrap the handle in some cloth and chuck it in a vise to hold it. Now, with the spindle where you can reach it and the tips where you can measure, put a caliper on the gage and set the thing so you get a feel for how much torque you need on the gage spindle to feel with the caliper at the same time. I hope that makes sense. In other words, don't put the gage in a part, crank the screw as hard as you can, then pull it out and gently put calipers on it. It's gonna measure way big that way.

As for how close can you get. I'd like to think you can get as close as your instrument can repeat. Within reason of course. I should think a couple tenths would be achieved with good technique and on a part that is super nice (internal finish, and round). I'd like to think I can do that, but, being honest, I don't have all that many parts that I've checked which have then gone to inspection to see if I was correct or not. If I'm trying to be super close, I use a dial-bore, and have a bushel basket full of em (literally). Certainly, 5 tenths should be easily doable.

Fyi, I have the Starretts, and also have an off brand but nice set that's a good bit longer. I really prefer the longer ones. They've got a really nice hard polished ends on em and are real slippery in holes. They don't trick you with friction like I think the Starretts do. They might be SPI's, I don't recall off hand. They weren't real dirt cheap if I recall correctly. Cheaper than the Starretts, but that doesn't take a lot. I bet they're 4.5 to 5" long. Tips resemble chrome, but may not be. They seem a lot more positive to me.

PS, for clarification, the vise grips thing really applies to setting mics to masters. Vise grip on the master, vise grip on the part! :D

03-16-2011, 11:42 AM
Yesterday after I posted I thought too, when you measure, you have to keep the gage inline axially with the hole being measured or the measurement will be oversize. Within some degree, you can be off to the side without affecting things too much, but up/down must be right on. This assuming you have the contacts opposing vertically.

Lucky Shooter
03-16-2011, 11:54 AM
I should have mentioned I was interested in hole sizes in the .200" to .400" range.

The idea of setting an accurate micrometer for a hole size has enabed me to get the feel for using the small hole gauge.

I appreciate your help.

A. Weldy

Butch Lambert
03-16-2011, 01:49 PM
I use Deltronic pins that come in .0001 increments to measure barrel bores.

03-16-2011, 06:57 PM
Getting the feel is the key, and your method of learning is the proper one. Starrett type expanding guages tend to measure across a small area and they can read tool marks, etc., which plays hell with your confidence factor. With good feel and technique they can read to a few tenths. I prefer now to use precision ground pins where ever I can - you can get the two most useful sets (? up to .250 and .250 to .500 in one thousanth increments) from ENCO for under $100 total. Bore specific guages to the tenths are helpful once you settle on a few bore sizes, and the Deltronic types are good. I once purchased about a thousand used Deltronic guages from a failed firms QC department. Need a precision spacer set - whack a Deltronic pin in half! The expanding bore guages still have their uses in that they will measure bulges and surface imperfections that the solid bore pins slide over - they will show jug shapes in a hole that won't be detected otherwise.