View Full Version : Adjusting depth micrometers?

08-23-2008, 04:03 PM
Do they make standards for calibrating depth micrometers? It's easy enough to adjust the 0-1" rod, but I don't have a reference for the others. I suppose 1-2-3 blocks and a surface plate would work, but what is the proper way to do it?


08-23-2008, 05:45 PM
I would not count on 1-2-3 blocks to be accurate enough to use as a standard. If you could mic them or otherwise determine their actual dimension, thay could be used.

Jo blocks could be used as a fixed standard, or a planer gage set to a known, accurate dimension with mics as a substitute.

08-23-2008, 08:18 PM
I suppose 1-2-3 blocks and a surface plate would work...

That is exactly what I would use.

08-23-2008, 09:00 PM
How about taking the standars used in setting 2", 3", 4" or what ever mike, check them with a mike and then check the depth guage. riceone

J Tiers
08-23-2008, 09:45 PM
you'd need two, and some care, but why not?

The 123 if mic'd would be OK.

How about the 12" rod (or larger) for an ID mic? THERE is an awkward thing to cal, I'd guess, at least in the home shop.

08-24-2008, 12:28 AM
How about taking the standars used in setting 2", 3", 4" or what ever mike, check them with a mike and then check the depth guage. riceone

The length standards would be accurate, but how would you hold the depth mike and the standard to be square with something like a surface plate? A slight out-of-perpendicularity would introduce a large error for calibration purposes. I believe that I've seen step blocks for setting, but I don't think I'd pay real money for one for just occasional use. Gage blocks would be my choice.

08-24-2008, 04:00 AM
Twin stacks of gage blocks well wrung would be my choice, though a single would do if careful. Or take a calibrated mic and check the required dimension of a 123 (or other) precision setup parallel, particularly one with a hole, and then use to calibrate the depth mic. For a quick sanity check when I swapped the 2" rod into my Starrett DM, I took just took out a "known good" B&S 123 block with holes, wiped it down, wiped down the granite surface plate, lay the block on the surface plate, and checked that I got 2.0000. Seems like it was maybe about a "fuzz" off (tenth or two?), but what did I care? All I wanted to do was make sure there were no gross errors before checking the bottom of a bore shoulder.

08-24-2008, 10:05 AM
How about taking the standars used in setting 2", 3", 4" or what ever mike, check them with a mike and then check the depth guage. riceone

I can't imagine holding the micrometer steady on top of one of those standards. I think the 1-2-3 block idea would work for my purposes. I can just mic the dimension of the block and make sure I get the same reading on the depth micrometer. Thanks for letting me pick your brains!

08-24-2008, 10:11 AM
I don't use a depth mic, but I'm a tech.
So.... I'd use some sort of continuity test.
Meter, buzzer, LED.

Of course, you'd need a standard.
Or was that what you were asking for?

edit, sorry, you were asking for a standard.
hmmmm, gotta be a way

08-24-2008, 10:41 AM
I presume the OP's depth micrometer is 0 > 4".

Let's not make it too difficult or expensive.

If you have a reasonable 0 >4" outside micrometer and a set of "checking sticks" that came with it, you are half-way there.

Get some precision hot-rolled/extruded tubing - any size or OD between say 1" and 3".

Cut it off and face it both ends to lengths of anywhere between 1" and 2", between 2" and 3" and between 3" and 4".

Length doesn't matter but having the ends faced-off and parallel to within 1 or 2 tenths (or better if you can) certainly is. If you only have a solid shaft/rod, drill a hole - any size bigger than the depth mic. rods.

Material doesn't matter - as long as its metal - steel, brass, stainless etc.

Use your pre-set/pre-checked 1">2", 2">3", and 3"<4" micrometers to accurately measure the lengths of those pre-prepared tubes and note their lengths - perhaps engrave (by hand) the sizes on them. If they are damaged they are easily restored or replaced.

Use them to check, and if necessary (re-)set your depth micrometer.

Put the tubes away for future use or reference.

It is NOT important the the tubes be any particular length.

Sometimes too much is made of what is basically a simple requirement and a simple job.

08-24-2008, 06:23 PM
Just measure your 1,2,3 blocks with good micrometer and what ever they are use to set depth mic to that dimension . For home shop use more than good enough.

08-25-2008, 02:57 PM
I presume the OP's depth micrometer is 0 > 4".

It's actually a 0-6" set (Starrett), but with two 123 blocks you can get any combination up to 6 inches with them.

08-25-2008, 08:41 PM
Thanks hornluv.

I'd agree with your intent in theory.

The practicalities are that the 1-2-3 blocks have tolerances as regards measurements, say with a micrometer (which are usually mentioned) and also for flatness and "square" (rarely mentioned).

These can be additive or subtractive or full of partially self-canceling - or anywhere in between.

Slip guages address this problem pretty well.

These problems and the need for "1-2-3" blocks, expecially if you don't have them, are the main reason I suggested the "tubes" in a previous post in this thread.
[Close edit]

So, what to do?

Use all combinations on those 1-2-3 blocks and measure them individually and record them for future use.

While the usual accuracy of imported 1-2-3 blocks is quoted at "within a tenth" that's OK for one measurement on your depth mic.

But if say in mearuring with two 3" sides to get 6" it is possible that in one combination or two 3" blocks each "one tenth under" the actual size will be "2 tenths under" at a nominal 6".

The reverse applies for two 3" sides that are each "one tenth over" as the error will be "2 tenths over".

The difference between the two limits of the 1-2-3 blocks that are within tolerance is "4 tenths".

That has gone from a perhaps 1 or 2 "tenths" error to a quite significant possible "4 tenths" between the limits.

It can be worse if any of the faces are twisted of "cupped" as a micrometer will not detect it.

There can easily be measuring errors with outside micrometers measuring the 1-2-3 blocks, singly or in combination as well as with the depth micrometer. These too can have similar outcomes as previously.

So, what to do.

It is relatively simple to get a pretty good consistent result with setting or checking your depth mic. if you accurately measure the 1-2-3 blocks, check that they are flat (indicator and a good flat surface), and consistently use the same combinations.

If my depth mic was "out" by more than 2 tenths, I'd be looking for an error in replacing the rods or cleaning of the under-side of the depth mic. anvil/bottom face. Depth (and most mics.) are pretty well "set and forget" unless something quite unforeseen or important or serious has happened. Most often the cause is lack of cleanliness.

I have deliberately avoided "standard conditions" as ambient temperatures in the shop is most usually the case - as "temperature co-efficients of expansion" etc. can and will be talked about a lot but in every-day use in the HSM shop is either not important (enough) or can be discounted or ignored altogether. There will be "special cases" of course - but that's all they are.

If you want accuracy of between "2 tenths" and "half a thou" this should see you right.