Other Dimensions: Mind Your Master

December 22, 2009
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Masters are critical for all comparative measurements and should duplicate the work to be measured as much as possible.



Masters come in all shapes and sizes and are the peak of precision-at least they’re supposed to be. When measurement problems arise they are usually the last item to be considered as the cause of errors. But they can be, even though they are as accurate as they need to be for a given application, if the wrong type of master was ordered.

The perfect master is one that duplicates the work to be measured in every critical respect, including material. Such a master can be very expensive, however, and not last very long. So we break things down to a more practical level on a feature by feature basis. Measuring a bore? Set the bore measuring instrument with a master that has the same feature, a plain ring gage. I’m sure you already figured that out, so what could go wrong? Everything.

Assuming the master ring meets the appropriate standard, that standard allows errors in geometry-such as roundness and taper-of up to 50% of the tolerance for size. This is where the digital age becomes the digital curse. If we’re dealing with a ring made to Class X tolerance, the size tolerance will be within 40 microinches or 1 micron, which just happens to be the resolution for digital indicators on most bore measuring instruments. Placing the bore gage at different positions around the ring can cause the last digit to change.

One way to get rid of this annoyance is to ignore or block off the last digit on the display. Another is to use a setting ring to a tighter tolerance to reduce the effect, but you would be paying extra for this solution and you might not be any further ahead.

If using a three-anvil bore gage you may get worse variations because it will detect lobing in the ring’s bore, a condition that a two-point gage or calibration device will miss. I would guess that 99.9999999% of all rings are made and/or calibrated using two-point contact, so you should assume they come with this potential for problems.

Another problem with three-anvil bore gages is caused by anvils that contact along a line in the bore being measured as compared to a point. Taper in the setting ring will cause inaccurate settings, but we’ll leave that problem for another day.

You may decide to go straight to the most accurate standard you own-your gage block set-and create your own setting master for the bore gage. Assuming your blocks are in good shape and have been calibrated recently, you’re not going to get anything much more precise than that. Problem solved-or is it?

Most bore gages have spherical measuring contacts, but if there are worn flats, setting the gage using gage blocks or other methods using flat surfaces may not reveal this condition, and the setting will be in error. A setting ring will not reveal this situation either, but because it is a cylindrical surface like the bore to be measured, it won’t matter for all practical purposes.

Knowing the speed and accuracy with which air gages can measure bores, many folks decide to use them due to disagreements encountered with other bore gages. The setting masters for air plug gages are plain ring gages but the geometry of them could cause problems unless you buy a three-jet gage head. The masters used should not cause problems beyond what we’ve already noted, but there are a couple elements that could cause disagreements.

The first is the fact that mechanical gages have spherical contacts that provide point contact while the air jet senses over an area equal to that of the size of the jets. Similarly, if the surface finish on the component is poor, the air gage will average a size over the sensing area rather than detect an undersize condition.

When checking outside diameters, similar problems could occur so the master used to set your comparator should be carefully selected. If using a gage block buildup as a master, you could run into problems if the comparator worktable is not in pristine condition. This is due to the blocks spanning grooves worn into the worktable. The ideal master to avoid this would be a cylindrical master such as a setting disc.

Masters are critical for all comparative measurements. When disputes arise, keep in mind that masters for one situation may not be suitable for one that appears to be the same.

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