How Do You Measure Your Holes?

I agree with everything you say but thread was about measuring holes rather than checking for out of roundness which as you point out correctly the only tool for that is a CMM.

Posted by MalcB on 17/12/2016 17:36:56:

Not exactly true.

I have run a precision engineering shop ( 35 yrs ) manufacturing precision plain and white metal lined bearings where measuring bores is critical. Bowers, Tesa and Mititoyo 3 point internal gauges are in fact still used used on the smaller ranges of bearing bores. They are easy to use for operators and consistantly give accurate results to some extent. However they have their drawbacks. The main one in that they take no account of the holes roundness. We do in fact have operators who can measure bores equally as well using inside micrometers that are then verified over with an outside micrometer. This is down to operator skill levels and experience though.

Given roundness issues, it means measuring has to be further verified by a precision CMM, which is also demanded by those customers that will not accept 2 or 3 point micrometer results.

The issue with all micrometers is that 2 and 3 point measurement will never be as accurate as as a good precision CMM such as the Zeis models for the reason highlighted. The CMM will give a very accurate plot of the hole, its roundness, its cylindricity, taper, its best fit minimum, maximum and mean diameters. A Bowers or similar just cannot do this.

Micrometers ( 2 or 3 point ) do not in fact have guaranteed uncertainty of measurent figures that go down to 0.0001" as there are in fact too many variables in the tool itself, the process of using and interpretating it to achieve the end result required.

The Bowers type micrometers are far too expensive and each has a limited range such that will normally take them well out of the scope of the home engineer. Good results of bore/hole measurement can be achieved by using standard inside micrometers and cross verifying over with an external micrometer, you just need to practise getting the feel right and consistant.

For the home engineer, smaller holes a good pair of toolmakers inside calipers with micrometer over can get you pretty close to final sizing, again with practise on the feel. Telescopic bore gauges can give similar. Final sizing usually done with pins, be it drill shanks, drill blanks, precision dowels, silver steel, homemade plug gauge etc.

Edited By MalcB on 17/12/2016 17:37:57

Exactly, a three point measurement will give a 'precise' result on the inside of an equilateral triangle, although it is anything but round.

Andrew