.0001" Depth Mic

I'm curious why 0.0001" resolution depth micrometers don't seem to be common. Nearly all the depth mics I can find are 0.001" resolution.

Any thoughts as to why this is the case?

coyotekid said:

Any thoughts as to why this is the case?

Click to expand...

Pressure.

Depth mics normally rely on finger pressure to hold the anvil in position against the reference surface.

It's difficult to discern the amount of pressure required to move the anvil just a hair off its normal position.

It can be a challenge getting really repeatable readings even at .001" resolution.

- Leigh

Also conventional screw micrometers cannot be used to measure accurately to 0.0001" in any case. Appendix 1 of my third edition copy of Engineering Tolerances by H.G.Conway cites an experiment made to compare the measurement accuracy achieved by experienced users using a 0.0001" vernier micrometer with that obtained by from an ordinary 0.001" micrometer by estimating fractions of a thou between divisions. Statistically the ordinary micrometer gave better results! The test object was a cylinder ground to exactly 0.37675" diameter. Average error with the vernier micrometer was -0.00016 and with the ordinary micrometer -0.000085. Vernier micrometer reading errors ranged from +0.00025 to -0.00045 whilst that for the ordinary micrometer covered ± 0.00025. Clearly estimating gives more reliable results than trying to set the micrometer consistently so the vernier reading is accurate. Such results are no surprise to anyone who has had to make repeatable measurements to very high accuracy. A frustrating business.

Of course the tenths vernier is great for comparative work if you have a good feel. Better than half a tenth variation can be found. Which tenth of course being another matter entirely!

Actually most people are far too sanguine concerning the accuracy of measurement instruments. Conway comments that the expected accuracy of a micrometer reading with a skilled user can be taken as ± 0.0002 so it is therefore useless to measure parts whose tolerance is less than 0.002 with a micrometer because the error in reading represents at least 20% of the tolerance. Far too many users will be happy with the first part of the previous sentence but without appreciating that the second part follows on directly.

As Leigh says, given the relatively imprecise manner in which a depth micrometer is presented to the work, there is no chance of sensible tenths thou results. Even a thou is pushing matters given that a depth mike usually has to be used in measurement rather than comparator mode.

In my wage slave days I often had to create accurate measurement systems for various obscure scientific purposes and rapidly learned that life was much less frustration if everything could be done on a comparative basis rather than by trying to calibrate everything in sight.

Clive

Well as someone thats only had a depth mike for a couple of weeks ill secound the feel thing. playing with a known good optical flat, its obvious when the anvils touching it yet i cant get it to repeat to much better than 1/2 a division. Thats a metric mic though so is calibrated at 0.01mm ie 2/5ths of a thou. Comparetively i could relyable hold less than half plus or minus a division. Ie im confident i could mesure something to the nearest half thousanth of a inch with it. I could guess nearer but im guessing not mesuring then. Precision mesurment to me is getting the dimension of the part as accuratly as i am certain i can. Claiming a tenth or even a few tenths with a depth mike i think your kidding yourself.

Its like using a ruler and claiming to mesure something to the nearest thou, it just aint going to relyably happen.

Thanks for the info. I thought it was just my inexperience causing somewhat inconsistent readings with a depth mic, but it sounds like I'm not doing too bad. Of course, practice, practice, practice!

Clive's post brings up an interesting point. What SHOULD you use to measure a tight tolerance feature, say something that is less than +/- 0.002"?

I would bet that most of us would reach for a tenth micrometer if the feature could be measured conveniently by that method. Zeroing a tenth DTI on a gage block stack might be more accurate, but it's not very convenient for general shop use.

Thoughts?

Don't believe everything you read

Clive603 said:

Appendix 1 of my third edition copy of Engineering Tolerances by H.G.Conway cites an experiment made to compare the measurement accuracy achieved by experienced users using a 0.0001" vernier micrometer with that obtained by from an ordinary 0.001" micrometer by estimating fractions of a thou between divisions. Statistically the ordinary micrometer gave better results! The test object was a cylinder ground to exactly 0.37675" diameter.

Click to expand...

Erroneous results from a flawed experiment.

A micrometer is not designed to measure the diameter of a cylinder, although it can be used for such. It's designed to measure the distance between two parallel planes.

Aside from the geometry problem, the cylinder was only a ground finish, while gage blocks are lapped. An order of magnitude difference in surface accuracy between the two.

My tenth-reading mechanical mics repeat to the tenth on master-grade gage blocks.

My .00005" resolution Mitutoyo digital mics repeat within .0001" or better under the same conditions.

- Leigh

Clive603 said:

In my wage slave days I often had to create accurate measurement systems for various obscure scientific purposes and rapidly learned that life was much less frustration if everything could be done on a comparative basis rather than by trying to calibrate everything in sight.

Clive

Click to expand...

That's Poetry! and stated so elloquently.

At the heart of Tool-Making practice.
the three most important factors.
Fit & Function. Fit & Function. Fit & Function.

at the heart of ISO 9000 1/2/3/4/5/6/
documention, and lots of jobs for all those that went thru college
and now they need to make positions for them.

Accept in gage-making.........And shut-off in molds.
It is the nature of nearly all mechanical assemblies.
That "depths" be uniform and consistant.
More so than to a given dimention +/- tenths.

Leader-pins need to be In location, BIG TME.
They need to have their axis paralell "near to perfection".
The C'bores for their shoulders are meaningless as long
as they are below the surface.
Extreem exageration but you get the drift?
m1m