Question About Inside Micrometers

[ctwo]

Just buy the set you can afford and trust it, until you can verify otherwise. It's not likely to go out of tolerance any time soon, especially if you take care of it.

I'm just glad to have one "good" gage block set that I will trust to cal my mics. That setup is inside.

I may then be able to say I can split a tenth on my good mic. I might rejoice in knowing I can at the very least call a tenth heavy or light. Who's gonna say I'm wrong?

I'm pretty sure even commercial shops do it this way. They may even lapse a bit on the cals if they are not required to show them, or make such claims.

This begs the question though, how important is it really on this scale? I mean, if gage block size is drifting around, then so is the mic, so is the machine, and so is the part you are making. If Harold's block was half a tenth out of spec, maybe just wait a couple years for it to come back in... (I don't actually believe the block moved half a tenth without some help, that's 500 millionths, or about 500 times more than it should have been able to move).

[SteveHGraham]

Harold actually said his block was out by half a thousandth, which is considerably worse than I would have thought possible.

[Harold_V]

Harold actually said his block was out by half a thousandth, which is considerably worse than I would have thought possible.

No, not a half thou. .000050" (fifty millionths). Doesn't sound like much but the set was to be within ± .000002"

H

[Harold_V]

If Harold's block was half a tenth out of spec, maybe just wait a couple years for it to come back in... (I don't actually believe the block moved half a tenth without some help, that's 500 millionths, or about 500 times more than it should have been able to move).

You had it right with the first comment. A half tenth is 50 millionths, not 500 millionths. That's a half thou. The block had no help moving. It was not damaged in anyway, nor had it been abused. My blocks have never been used by anyone but me, so I know what they have seen and done. Were it not for submitting them to Univac for calibration, they had never been handled by anyone but me. Unfortunately, I do not recall if the block grew or shrank. This was many, many years ago. (Univac had a certified secondary standards lab)

Such movement is the nature of alloys----which appears to be one of the hardest lessons for many to learn. Metals are not inactive. They have stresses that cause them to move about. That's why I talk about roughing and finishing operations. If you follow that procedure, you can limit the amount of movement. Note I did not say it can be eliminated. It can't, but it can be made insignificant.

Harold

[Harold_V]

Okay, but...how do you get your home shop mikes CLOSE to a tenth without paying for metrology every year? Let's cut to the chase. Most of us won't be doing A+ work, so what do we use to get to B+?

The harsh reality is that you most likely don't have the capability to work beyond the degree of precision of the block set you have, even if it's not of high precision. Further, unless you are concerned with producing interchangeable and very tight tolerance work for others, it's not really an issue. So long as you work to your own standards, right or wrong, the work you do will be just fine.

It's important for you to keep in mind that the degree of error that is being discussed is beyond the ability of most folks to detect, including myself.

Even if your set of blocks happens to have an error of .000050", so what? Such precision is rarely required for typical work, where even a tenth or two doesn't make a difference. Just strive to use good work practice, and calibrate your instruments with the same devices, so you work with the error, such as it is.

The toughest work I ever had to do had a tolerance of .000020". A set of ring gauges. I was unable to determine the size. That was accomplished by the lab. The gauges were honed near the finish size, then were "seasoned" in the lab for a day, then measured. That process was repeated a couple times until I achieved size.
No, they were not "perfect". I had to use some of the tolerance. It's the luck of the draw that you stop before going oversized.

If you find yourself pursuing high precision work, such as fine grinding, or jig borer work, yeah, you may need greater precision. Then you invest in a high quality set of gauge blocks, just as I did. Then, as required, they must be calibrated on a schedule. I'm having a very hard time trying to determine how the home shop would have to worry about that.

Harold

[SteveHGraham]

Okay, now that we know you omitted a zero, the world makes more sense. So if I get an AS-0 set, I should be able to go on with life.

[warmstrong1955]

Okay, now that we know you omitted a zero, the world makes more sense. So if I get an AS-0 set, I should be able to go on with life.

Whewwwww......
Glad you finally figured that out.....

[ctwo]

You had it right with the first comment. A half tenth is 50 millionths, not 500 millionths. That's a half thou.

Well, I was torn between reading your mind and what you wrote, and Steve was confusing me, so I went with half a thou. I mean, it was written on the page, twice then.

Anyhow, reading that you could lap a block and it could get longer was a bit revealing as to why.

[GlennW]

Okay, now that we know you omitted a zero, the world makes more sense. So if I get an AS-0 set, I should be able to go on with life.

Whewwwww......
Glad you finally figured that out.....

That was easy!

Next question?

[Harold_V]

You had it right with the first comment. A half tenth is 50 millionths, not 500 millionths. That's a half thou.

Well, I was torn between reading your mind and what you wrote, and Steve was confusing me, so I went with half a thou. I mean, it was written on the page, twice then.

Yeah! For that I apologize, and I have since made a correction. As much as I understand the numbers, occasionally I screw up. That was a good example. Sorry 'bout that!

Anyhow, reading that you could lap a block and it could get longer was a bit revealing as to why.

I'm not convinced that one could lap and expect growth. In my case, I was working with bores, so getting larger makes sense. However, over time, as the alloy in question moves about, yeah, you can expect growth, or even shrinkage. It won't be much, but if a long block is in question (mine was either the .950" or the 1.000" block, and I can't remember which it was), it's not unreasonable to experience such a change.
Does it occur commonly? Probably not---but that doesn't exclude the possibility.

Harold

[Cary Stewart]

In the metrology world the rule of thumb is the measuring device should have an accuracy of one order of magnitude greater than the tolerance to me measured. Example: If you are measuring a part to + or - .001" then you gage or measuring device should have an accuracy of .0001". As the number of zeros get larger the ability to comply with this "rule" becomes harder and harder. A fellow once asked be about temperature changes and how it affects measurements I told him basically that if the mic. or what ever he was using was the same temperature as the item he was measuring all would work out OK. I told him that if he thought what he was doing was so critical then he should convert his garage into a fully temperature and humidity controlled, dust free lab. and keep that way 24/7/365or 366. He went home and a couple of years later he showed up with a very nice running locomotive.
Cary

[SteveHGraham]

Whewwwww......
Glad you finally figured that out.....

All you have to do is pin people down and keep pointing out the wobbly bits in their posts, and sooner or later someone comes forward with the truth.

Sometimes.