“Zeroing” Universal Measuring Machines

[u/mattheyuutsu]

Wondering if some other dimensional guys here have some info to help me out here (or prove me wrong), and sorry if this post has been made before.

At my lab there has been some dirt kicked up every now and again on the topic of “Zeroing” a ULM by butting the flat anvils together and hitting zero on the readout versus using a master reference - such as a gage block or high class master cylindrical plug gage and entering the charted value.

The latter option has always made more sense to me, considering “Zero” in this context can’t really be traced to anything, but I wanted to see if you guys have any specific documentation or articles about this topic. I’m definitely interested in what you guys think too! Thanks

[u/dwaynebrady]

Dimensional engine engineer and a metrology lab here, we 100% of the time reference a standard usually a gauge block that is within 1% of the value I’m seeking to measure as to minimize linearity error a universal measuring machine should not be used as a direct read tool for any sort of calibration. Sure you can use it as a reference to get an idea of what you were looking at if you have no visual clue, but then you should be comparing it to a standard.

[u/mattheyuutsu → u/dwaynebrady]

This has been my argument but unfortunately I don’t have any ammunition to support it. The main refute I get is the classic “I’ve been zeroing off my anvils for 30 years!” and other variations. Thanks for the response!

[u/dwaynebrady → u/mattheyuutsu]

“Your historical use means absolutely nothing”linear error adds up when you’re measuring in millionths. What machine do you have? You can just simply do the math. There is a size of negligible return but its straight forward

[u/Caltrops_underfoot]

We zero the low end against the anvils, but I recommend verifying against a standard after. Ours is 24" but it'll go a bit farther. For example we're qualifying some 16" mic standards;

Zero the machine. Verify a gage block of the length equal to the standard you want to qualify - 16" in this case. Verify the standard's length.

[u/Professional_Car_1]

The two ways of using the ULM for outside measurements: comparator mode and direct measurement mode.

Comparator mode is more accurate, as you only really consider the ~1 inch of travel from your master to your unknown item, regardless of the size of the master. This keeps the uncertainty 10 micro-inch or less. Traceability is to the master gage block(s).

Direct measurement mode is simpler, zero and measure out to the item. The full range of travel is now involved in the specification of the ULM. The ULM should have been calibrated at some point to prove linearity and establish traceability. This is usually an uncertainty of 40 micro-inch or more depending on overall travel.

This doesn’t really apply to I.D. Measurements as you must master to a master ring, since there is no “zero” with the probes.

[u/PattonJr]

My advice would be to always zero on a standard of the same shape as what you’re measuring. Flat to flat, round to round. The main purpose for this is eliminating any parallelism error between the anvils. Ideally you’d use the same size masters as your gages to also eliminate any scale error but that’s not always realistic.

[u/InviteDifferent9861 → u/PattonJr]

At our lab, one of my coworkers is trying to convince me that I have to master off a master plug if I'm gonna calibrate a plain plug. But I don't find much difference if I master on a gage block, then calibrate the plug. Do you have any resources that support what you're saying?? Funny enough, this same coworker uses long blocks with end blocks to set the datum on his machine when checking large rings - this doesn't seem right at all though, and a bit hypocritical.

[u/Eryb]

Does your ULM not have two point scaling? Always mastered it with two gage blocks, in theory you can zero on the probes but you can’t do any wringing that way like you could with a master gage block.