I have a set of Mitutoyo micrometers with .0001" resolution. They are the most basic model, no lock or ratchet. In order to get accurate, repeatable results at the finest resolution, I'm trying to develop a sense of feel for their use.
What is the correct amount of pressure to apply to the part being measured? How much is too much?
Best thing you can do is buy a "standard" aka guage block...if your mic is 1"-2" get a 1" or 1.5" the guage block will be accurate to .00005 or better....if the mic doesn't measure what the block is...then your mic is off or your feel is off......
Use the block to check your mic...(which you should do anyway from time to time) get your "feel" from keeping it at the size of the block....
Oh and "too much" is when the threads on the mic stretch or strip or you require pliers to undue it :-)
After reading again....if you have a "set" of mics....buy a 1", 2", 3", etc standard...then you can test your mics and feel at the top and bottom of the ranges (use the 2" block on your 1-2 mic AND 2-3 mic)....
Guage blocks will be solid steel....standards will be rounded ends with plastic or aluminum centers....you will probably only find standard for 3" and above....plus standards should be cheaper.....
Did your mics come in a case? Did the case come with a box full of standards? A lot of mics/sets come with them....
My 0-1 came with a .1000 guage block
My best description would be "snug".
Normally you hold a mic in the palm of you right hand with the little finger outside, two fingers through the middle, the thumb and forefinger on the spindle.
In that position you tighten the spindle with your thumb and forefinger until it stops.
You really can't apply too much pressure using this technique because the digits aren't very strong.
First off never use the ratchet or friction thimble. I always advise my people to use the lightest pressure that will allow contact with both anvils of the mic. In other words you should be barely touching the part especially if it is round and there is only line contact, any amount of pressure will give a false reading. You can practice with precision plug guages and see what pressure it takes to get the proper reading. Try sliding or rotating the part around a little while you gently close the anvils, you will get a feel for when the anvils are just contacting the part. When you do it correctly you should be able to reliably measure to less than plus or minus .0001.
The best advice I can give is to calibrate the mic against gage blocks at whatever torque feels right to you. Thereafter you will have a degree of muscle memory for where the "sweet spot" is.
A mic shared between multiple people is a different challenge. There it might be appropriate to use a torque wrench (a darn sensitive one) or load cell to calibrate against so that all instruments are consistent.
First of all thank you all for the quick replies.
Yes I have a complete set micrometers up to 6", and a set of standards 1"-5". I have tried the using the standards as xdmp22 suggested. The problem is a couple of the mics take more pressure to zero than the others. (it feels excessive to me)
Sounds like my first step should be to adjust them so they all read zero with similar pressure.
Before adjusting them, make sure they are clean and add a drop of oil to the threads......
Also, not sure your situation, but if you work at a shop, they may be willing to have them sent out and calibrated....once they are calibrated, you can find the feel with your standards.....
The other thing to do would be to ask a few fellow machinist to try your mics on the standards just to double check you before you tinker with them.....
I would not make it too snug. Just enough to cause a little resistance.
Even though my mics plain thimble I'm curious about this statement. Are they inconsistent or unreliable in someway?
Originally Posted by JG400
If you were measuring surface ground parallel flat stock, thimble or ratchet is probably fine, but as soon as you have inconsistant material, especially rounds....the ability to judge "feel" goes out the window with a thimble or ratchet....
Plus if the tool is dirty or the part is oily it throws off the ratchet or thimble....
We are talking about measuring .0001......not .001
The ratchet stop as some call it is like having a built in impact wrench. It can provide too much torque in giving you a "false" reading.
When I check my mikes, I usually use a mike standard to calibrate my mike to. I take the standard and put it between the measuring surfaces and gently adjust the thimble until the standard is "grabbed" be the measuring surfaces. Next, I slightly rotate the standard about 90 deg and gently re-adjust the thimble until there is slight restance to rotating. At that point, I make note of the measurement. If it is not regersting "zero" adjust according to manufactures instructions for making adjustments. Recheck for "zero".
That's an excellent way to abrade the standard and throw it out of calibration.
Originally Posted by 4GSR
IMO, the thimble is for people who are not good with the 'feel'. I dont particularly like the ratchet style thimbles. Regularly check your mics, and calipers for that matter, to a standard. For general shop purposes a gage block or mic standard is plenty accurate. As already mentioned, check it often enough and you will develop a feel. It should be a free moving action with a light load on the tool, not applying hardly any torque, when it stops you will feel it. If not, it at least requires a good cleaning, and possibly an adjustment.
Originally Posted by Jtommr
Simple physical fact is you can make the tool read what you want by varying the pressures, this, however, will not magically make your parts be the right dimension!
It seems everyone has a different feel honestly, atleast from what I've seen. I've seen guys crank on 300$ mics like they got them from tractor supply and I've seen some barely touch the part until they are barely snug and subtract a tenth or two .
Personally I have all ratchet and I use that first then snug up the thimble just a little bit and go off that. Just watch it not to tighten it a little more or less when real close to the edge of a tolerance, because the next guy won't.
Originally Posted by The real Leigh
I turn it just enough to get the standard "wrung" in. Not 90 deg as I first said.
Good advice thus far.
Only thing I'd add is that developing a method of consistent measurement is key, regardless the type of measuring tool.
I don't particularly like 'ratcheting' types, but that's more a personal preference than anything. However, if you calibrate them to some standard method... say, light approach, then 3-clicks... then as long as you use the same methodology when measuring, you should achieve accurate results.
I prefer friction thimble and even then, I'm a bit picky about the amount of friction. We have a set of Starretts in our metrology lab that I absolutely abhor. They require way more torque than my Mitutoyos and require a bit of getting use to...
When measuring a standard I prefer to use just my index finger to turn the thimble with the mike in the palm of my hand or resting on my belt.
Also be sure the standard and mike faces are very clean.
When measuring round parts the feel is happening both in the alignment of the mike and the pressure on the thimble.
Last edited by The real Leigh; 03-05-2012 at 07:41 PM.
Micrometer standards are not designed to be "wrung", nor are micrometer faces.
Originally Posted by 4GSR
The faces have nowhere near the fine surface finish required for wringing. Most micrometer faces are carbide, which will abrade the facing surface if
any pressure and rotation are applied.