Micrometer accuracy in use

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refinery mike
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Micrometer accuracy in use

Post by refinery mike » Sat Dec 13, 2014 11:44 am

I got to playing with my gauge blocks and micrometers. I just can not get consistant readings. Then i hear about people who can read to the 10 thousanths . I need some more schooling.
1. As carefully as i calibrate a mic, the next day it is out by at least a half thousand.
2. When i practice i can actually see the readings change as my hand warms the frame. my shop is about 60 degrees.
3. The test bar on my new Mit. 2-3 leaves me with slightly different readings each time and about 2 thousand different from a gage block.
4. The group includes starrett, brown and sharp and mitutoyo. Friction thimbles and ratchet thimbles. some new some old. So i am convinced it is me that is the problem.
What do i need to do to improve the consistancy and accuracy of my readings.
I am ashamed to admit that i thought i was pretty good at this until i tested myself and found i stunk.
HELP!

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GlennW
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Re: Micrometer accuracy in use

Post by GlennW » Sat Dec 13, 2014 12:13 pm

How are you holding the mics? You want to have as little finger contact as possible on the frame.

Are you using the ratchet or friction features?

There is a certain amount of "feel" involved in clamping a micrometer to a gauge block, (or parallel surfaces) as the anvils need to be seated perfectly on the gauge block to get an accurate reading.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

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Dave_C
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Re: Micrometer accuracy in use

Post by Dave_C » Sat Dec 13, 2014 1:30 pm

Not to worry! We all stank at first... :lol:

Time and practice but you will need tool and material temperature at a constant if you expect to read in tenths accurately.

60 is fine but is it always 60? As Glenn said, how you hold the mic will make a big difference as well as how long you hold it.

So just build yourself a quality control lab with year around temp control and problem solved. Yeah right!

We talk a lot about accuracy but in truth, most of us can't control the material temp and room temp well enough to cut to tenths even if our machines are rigid enough to do so. Most of us are not professionals and don't make our living at this craft although we are fortunate enough to have people here who have and some still do.

We can frustrate ourselves by expecting too much too soon! This is supposed to be something we enjoy. :P

Dave C.
I learn something new every day! Problem is I forget two.

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GlennW
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Re: Micrometer accuracy in use

Post by GlennW » Sat Dec 13, 2014 2:58 pm

You might start with measuring cylindrical objects first, as they can be a bit easier as the anvils seat easier.

Gauge pins or drill blanks would be great, as they are hard and ground. End mill shanks too.

Just try to get consistent readings, don't be too concerned about what the actual size is. If you are getting consistent readings without cheating, you are developing a good technique for holding the mic and operating it.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!

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Richard_W
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Re: Micrometer accuracy in use

Post by Richard_W » Sat Dec 13, 2014 4:48 pm

refinery mike wrote: 3. The test bar on my new Mit. 2-3 leaves me with slightly different readings each time and about 2 thousand different from a gage block.
I wouldn't trust the standard Mitutoyo send with their micrometers. I have seen 3 standards of the same size all measure differently and none were exactly dead on. You need to calibrate the standard to find out if its correct to begin with.

I have found that beginners often do better learning to measure using a round standard. Mitutoyo standards a flat on the ends which makes them more fussy to check the micrometer with. The Starrett standards are crowned and easier to use.

Richard W.

morsetaper2
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Re: Micrometer accuracy in use

Post by morsetaper2 » Sat Dec 13, 2014 5:09 pm

To the OP I totally know where you are coming from, been there.

Trying to repeatably measure to .0001 increments can be tough to do as was said previously. I was able to get decent consistency by in getting my mics calibrated by getting a micrometer stand and handling the mic and part to be mic'd as little as possible. I wear tight fitting mid-weight fabric gloves to try and limit heat transfer to the gage block or the mic standard when calibrating. And with the mic clamped in the mic stand, little heat transfer to it occurs during cal & adjustment. I found that some mics took a several adjustment iterations several hours apart to get them set. With the mic & std stored in the same location to equalize their temps between handling & adjustments. Adjust, check, let sit for couple hours and recheck the cal. Repeat till the std consistently mics within a .0001 or so. Whether that's right or wrong its worked for me.

Measuring actual work its harder to get repeatable .0001 level measurements. As the workpiece may be warm from the act of removing material. And will measure less as it cools down. And the warmer workpiece transfers heat to the mic as does your hands. So the measurement is actually moving around a bit unless everything is at the same temp. Trying to measure to .001 you won't detect much variation due to temps on small pieces. At least that has been my experience.

Mr Ron
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Re: Micrometer accuracy in use

Post by Mr Ron » Sun Dec 14, 2014 12:11 pm

This was an interesting post. I was going to post a similar question. In a home shop, I think it is impossible to work to tenths due to the temperature variations in the shop, the tools and the work. That is why I have been asking about digital calipers. Since thousands are the best I can hope for, the digital caliper serves me well. I have mics that read in tenths, but if I can't attain that degree of accuracy, they don't do anything for me.
I can see in a production facility, where machining centers are used, coolants are very important. They keep the work and the tool at constant temperature, thus avoiding the temperature variations that result in parts that are out of spec.
Mr.Ron from South Mississippi

Downwindtracker2
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Re: Micrometer accuracy in use

Post by Downwindtracker2 » Sun Dec 14, 2014 2:28 pm

Machinists dismiss the ratchet, but as apprentices they spent hours getting the "feel". I worked in a wire mill, some specs were in 1/10s. There, even clown operators could measure, but they used the ratchets on the company supplied Mitutoyos. If you really want to pull your hair out, try getting consistent measurements with a fine metric veriner calipers ,imperial ones are a breeze.
A man of foolish pursuits, '91 BusyBee DF1224g lathe,'01 Advance RF-45 mill/drill,'68 Delta Toolmaker surface grinder,Miller250 mig,'83 8" Baldor grinder, plus sawdustmakers

Harold_V
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Re: Micrometer accuracy in use

Post by Harold_V » Mon Dec 15, 2014 5:02 am

Mr Ron wrote:This was an interesting post. I was going to post a similar question. In a home shop, I think it is impossible to work to tenths due to the temperature variations in the shop, the tools and the work. That is why I have been asking about digital calipers. Since thousands are the best I can hope for, the digital caliper serves me well. I have mics that read in tenths, but if I can't attain that degree of accuracy, they don't do anything for me.
I can see in a production facility, where machining centers are used, coolants are very important. They keep the work and the tool at constant temperature, thus avoiding the temperature variations that result in parts that are out of spec.
Don't take the attitude that one can't work to tenths in the home shop. One can, and it's not all that difficult.
Fact. I ran a one man operation for 16 years. In that time, I had a few jobs with just .0001" tolerance, and more than I can count with ± .0002" tolerance. I had no temperature controlled rooms, and didn't always run coolant, but I was armed with skill and patience (and the wisdom to cool parts when critical measurements were required). My work was not rejected, in spite of having to withstand critical inspection. One CAN do tight tolerance work in the home shop, but it requires skill and effort. It can't be accomplished without proper measuring, however.

If you choose to use a caliper instead of a micrometer, it's safe to say that the level of precision that you'll be capable of achieving will be far worse than a thou. That's especially true if you attempt to create proper fits, where you may have need to measure a bore, then translate that reading to a turn for a bushing or shaft. You will most likely achieve success only by chance, as the use of a caliper for determining proper size is simply not capable of providing that kind of information. That's the point, and one that is pretty much ignored by many. They really want to believe that the reading they achieve is precise, then wonder why they can't make parts that fit one another properly.

Bottom line. If you can't work to .0005", you will struggle with critical work, achieving success only by coincidence.

I don't use the ratchet, by the way, although (almost) all my mic's are so equipped. There is no ratchet on a Starrett 221.

Harold
Wise people talk because they have something to say. Fools talk because they have to say something.

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Dave_C
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Re: Micrometer accuracy in use

Post by Dave_C » Mon Dec 15, 2014 11:14 am

Harold,

I've come to know you quite well over the past 5 or so years and I respect your experience and advice. However, a newer person trying to learn this craft may be discouraged by your level of success that you describe when they find they can't turn a part repeatedly to .0001"

Most days I can't, even with the CNC conversion which will repeatedly run the same cut on the same part over and over.So why is that? Simple: My machine is not of the quality and rigidity where I can expect such results. So knowing that I have to sneak up on tight tolerances as best I can. Can I measure them and do I use good mics as you suggest? You bet.

All I am suggesting is that you mention the quality of the machine and tooling you are using to accomplish such great results so our new friends don't expect more than their machines can deliver.

Thanks,

Dave C.
I learn something new every day! Problem is I forget two.

Mr Ron
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Re: Micrometer accuracy in use

Post by Mr Ron » Mon Dec 15, 2014 12:47 pm

Harold/DrDavo, Sounds like good advise to me. I have and always will be self educated. At 80, it's too late to go back to school.
Mr.Ron from South Mississippi

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refinery mike
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Re: Micrometer accuracy in use

Post by refinery mike » Mon Dec 15, 2014 8:09 pm

I am sure i will not acheave 1/10,000 accuracy. but i feel that if i can not measure repeatedly at least 1/4 and or half 1,000s then i can not expect to work to 1/1,000 accuracy. Any suggestions would be appreciated. And the suggestions so far mentioned seem to have helped already.

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