? about accuracy of mic & indicators

In MSC theres a Mar Test 800SGM/801 SGM Test Indicators. there is an Erro span fe 10um/.0004 and under that a Hysteresis fu 3um/.00012. Just what the heck does that mean? Also what is the Federal or DIN specifications for micrometers?

I've never seen "Erro span fe" at all, and I've never seen the hysteresis of a system given in a ratio as there's nothing about hysteresis that lends to being described that way.

Are you sure these aren't typos or a misprint?

It's not given as a ratio, it's the metric immediately followed by the inch value. I.E. 10µm=.0004". I'm thinking that the fe and fu are just notations for error and hysteresis.

I think MSC has the specs screwed up. Mahr lists the 801 SGM DTI (inch) as having an accuracy of fe .00012" and fu of .00008". The 800 SGM is the same DTI but graduated in mm's. The accuracy is the same at 3 microns and 2 micron.

What the numbers mean is that throughout the whole range +/- .004" the total positioning accuracy is no more than .00012" and the hysteresis or lag when reversing direction is less than 80 millionths.

In practical terms this means that if you put the DTI in a calibration stand and checked the accuracy at all 80 positions (.008"/.0001") no single measuement would be off by more than 1.2 graduations. When you reverse direction the measured movement won't lag by more than .8 graduations. (Think backlash)

This is the maximum allowed tolerance, normally when new the DTI will be a bit better.

My editorial on DTI's would be that the Mahr is very good and if the price is right (cheaper than a Compac or Bestest by a bunch) then go for it. The best DTI in my opinion is Compac, then B&S Bestest, then Interapid. The Interapid needs to be used at 12 degrees to be accurate. So I find it a PITA to use inside a machine but handy enough on the surface plate and bench centers. I'll never buy another one though. It's too tricky to get accurate readings. The Bestest is a little light and doesn't survive being dropped very well. But the good news is they are easily repaired. The Compac is a tank and very accurate. Mitutoyos are just alright and not a great value for the $$. Everything else is junque. Especially Starret. Very disappointing.

DTI's are best if made in Switzerland. Given that it is a lot like a clock, I suppose that only makes sense.

I forgot about your micrometer Q. There are more than one federal and DIN spec relating to micrometers.

The federal specs are fairly meaningless when you are buying a micrometer anyway. None of them say that a micrometer has to be this or that. They deal mainly with calibration, certification and traceability procedures. The manufacturers specifications are what matter as that will be what you are getting.

You need to decide what level of precision you need along with which features are important, The quality of the materials and construction, and of course the price you are willing to pay. No federal or DIN spec can help you there.

At minimum you need to find out what the manufacturer's specs are for the micrometer and make sure that they provide certification that the micrometer meets or exceeds those specs and that their measurements are certified traceable to NIST.

What you need to know as far as specs are the accuracy throughout the range. The flatness of the anvils, the parallelism of the anvils, and the spindle run out.

NIST is a good place to learn a little more. Start at the Engineering Metrology Tool Box here - http://emtoolbox.nist.gov/Main/Main.asp

There's hysteresis and then there's stiction. I understand hysteresis in mechanical movements is represented by the difference in the contact force in the postive direction Vs the negative. If an indicator develops 10 grams of contact force going towards the work and 9 grams going away then 1 gram is the difference and represents the hysteresis.

Stiction is the product of elasticity in the measurement system deflected by hysteresis.

Whenever I try for refined measurement using apparatus with mechanical movements or with set-ups having some elasticity I've been taught to "dither" it; that is apply a small vibation or shick to settle the system to its lowest energy state. My usual dithering tool is a wood pencil I use to tap someplace on the set-up.

If you know the calibration error, the behavior of the instruments and apparatus under various conditions, and have a good sense of Kentucky windage it's possible to work well within the minimum advertised specifications of the available equipment.

Jo blocks for example are calibrated and their errors are logged in the calibration sheet. Using the calibration sheet and a little arithmetic, the stacking error can be determined. Simply add the errors correspnding to the blocks assembled and add/subtract from the stack.

I've used plain vanilla dial indicators in large awkward horizontal set-ups. If the weight of the assembled dial indicator set-up is known it's possible to determine deflection from gravity using a sensitive spring dynamometer.

So if you are looking for ultimate in accuracy and reliability, remember that much depends on how the apparatus you're considering is employed and what measures you take to compensate for system errors from mechanical stiction, temperature, elasticity, gravity, etc.

All that said Mahr makes first rate equipment and its accuracy and reliability is in part measured by the price.

I have a 50 millonths graduation 0.0020-0-0.0020" dial indicator, a clone of the famous Mahr "Comparmess"(?) made in China sold by Shar. As near as I can tell using my Jo blocks and a granite base comparator stand the Chink indicator is on the money going in 0.00005 increments. And it const $115 Vs over $400 for the good one. Is it any good? I don't know, fine apparatus is a little out of my general expertise.

So what you buy depends on how long you expect to use it and what confidence you require. And don't forget annual calabration and serviceing if you have to meet documentation and third party verity.

High accuracy, high confidence readings is not for the timid or the poverty stricken.

Swisspro and Forest thanks that’s what I wanted to know. I wrighting a paper on what can be expected from today’s instruments accuracy wise. In other words is what we see really what we are getting using the commonly available, not the high dollar stuff that only NASA can afford, measuring instruments.

Kurt,

You might want to check out this paper:

http://www.hn-metrology.com/isogum.htm

You can see how pretty quickly a 0-1" micrometer has influnces that add up to .0002" worth of uncertainty in the accuracy of the measurement being taken.

I just knew Forrest would have the answer. Ididn,t
understand the question or the answer.
Old Bill