Questions about standards (measuring rods) for micrometers

[erikmannie]

Regarding measuring rods for outside micrometers, my questions are:

(1) to what degree do you trust import measuring rods?,
2) do you zero your micrometer before using it?, and
(3) should I be concerned about buying used measuring rods?

For my imperial mikes, I have Shars standards and Shars gage blocks.

For my metric mikes, I bought used Starrett and Mitutoyo standards.

As far as what tolerance I am shooting for, it doesn’t actually matter, BUT I do take my hobby seriously and I shoot for .001” or better. I fail more often than I succeed. I am striving to be able to work to .0003” one day; like I said, it will probably never have any practical application because most of what I do is just an exercise.

I make no effort for temperature control, but I do make an effort to clean the surfaces of oil before I take measurements. That should give you an idea of what precision I am aiming for. For myself, I consider anything much over a thou out to be a failure (or should I say a learning experience?).

 

[BrentH]

Well,


If the "Standard" I have checks out with the same results on a variety of measuring tools I consider it to be just that - A good standard.

I check my micrometers out periodically or when the project needs to be very accurate. We had a company working for us and they would only use their own micrometers and they had to be ISO 9000 certified. They flew them in from Germany in a large crate....These are the ones we will use as they were all calibrated ...they said....WTF - you just flew them overseas in a large crate how can they be any better than the one I have here tested against a known standard?

For measuring accurately on the lathe or milling project that is subject to getting heated during the machining process you should allow things to cool to ambient, take a measure and then cut to final dimension - measuring hot can change a lot!

Check your "used rods" against some reliable tools you have. Most measuring rods will not see significant action to result in damage. I would see no reason not to trust them -

If possible use the same instruments for your project so you may not be 100% bang on if someone else measured with a different tool but your fits should still be on the money.

 

[erikmannie]

Well,


If the "Standard" I have checks out with the same results on a variety of measuring tools I consider it to be just that - A good standard.

I check my micrometers out periodically or when the project needs to be very accurate. We had a company working for us and they would only use their own micrometers and they had to be ISO 9000 certified. They flew them in from Germany in a large crate....These are the ones we will use as they were all calibrated ...they said....WTF - you just flew them overseas in a large crate how can they be any better than the one I have here tested against a known standard?

For measuring accurately on the lathe or milling project that is subject to getting heated during the machining process you should allow things to cool to ambient, take a measure and then cut to final dimension - measuring hot can change a lot!

Check your "used rods" against some reliable tools you have. Most measuring rods will not see significant action to result in damage. I would see no reason not to trust them -

If possible use the same instruments for your project so you may not be 100% bang on if someone else measured with a different tool but your fits should still be on the money.

A lot of good points there. I am very guilty of measuring things that are pretty warmed up, as what you recommended had never occurred to me.

Nor had it occurred to me to test a measuring rod by using it in two different micrometers and comparing those results. In a perfect world, I would have full confidence in the measuring rod and use that as the standard (as is it’s definition).

I will very often use a few different instruments to measure something. For some reason, I always consider the Starrett mike to have given the correct measurement, with the Shars mike serving as the second opinion, and the calipers to be the sanity check.

I do much the same with the measuring rods and gage blocks. I like a second opinion.

We can easily work to .005”, but rarely to .0001”, so that mere 5 thou range is where we move from pretty easy to extremely difficult.

 

[middle.road]

Hitting .001 in an non-controlled shop is going to be tough.
.0003 without climate control and other controls? That's going to be even tougher.

 

[erikmannie]

Hitting .001 in an non-controlled shop is going to be tough.
.0003 without climate control and other controls? That's going to be even tougher.

This might explain my near 100% failure rate.

 

[benmychree]

2) do you zero your micrometer before using it?,

You get close to zero and take a measurement on the rod with a very careful feel, and then reset the mike to zero if necessary. I find it difficult to use gage blocks to set mikes because it is difficult to avoid slightly off perpendicularity with the mike's anvil and spindle. The setting standard rods have radiused ends, avoiding the problem.

 

[middle.road]

This might explain my near 100% failure rate.

It's not 'failure' when you try.

 

[westerner]

We can easily work to .005”, but rarely to .0001”,

You do realize that is a 50 to 1 relationship, yes?
If you are trying to secure a NASA contract, those numbers may be relevant. I have several posts here about chasing precision that is not required. I am just a hack, but what difference does .0009 make? Or .005? What are you building?
I surely appreciate the desire to "hit your numbers". I have accepted the fact that the numbers I can hit are big 3 digit ones. I will not obsess over 4 digits at all.

 

[mikey]

Regarding measuring rods for outside micrometers, my questions are:

(1) to what degree do you trust import measuring rods?,
2) do you zero your micrometer before using it?, and
(3) should I be concerned about buying used measuring rods?

1) I use Mitutoyo mic standards and they're pretty okay for a quick check but they are not good enough to calibrate a mic.
2) I always clean the mic faces with Kim Wipes and gently close them to make sure the mic is zeroed before use. I could probably dispense with this because the mic always reads zero.
3) Mic standards (rods) are not used often so if you buy good ones used, they will probably be okay. If you really want to know if they are accurate, send them to a calibration lab and have them report their lengths so you know what they really are. Because a mic standard is marked 1" or 2" or whatever, it doesn't mean that is the actual length to a millionth of an inch. If you must know how long they are, have them measured by a lab. You just have to keep in mind that that rod was measured in a lab and its length may/will be different in your shop and in your hands ... but it will be damned close.

Your unasked question is actually about trust. Can you trust the rods to be accurate enough to give you confidence in your measuring tool? This is not about the level of work you can actually do; it is about trusting the readings you get.

Just so you know, calibrating your tools is a personal choice. Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. This is different from Certifying it, which is done by a lab; they will tell you if the instrument falls within the acceptable standards for that tool. Certification is done in a controlled environment by techs who know what they're doing, using equipment that is far more accurate than most of us own.

I calibrate my own mics and calipers. I now use a Mitutoyo Cerastone Grade A set that is specifically made for calibrating micrometers and is accurate to between 4-6 millionths of an inch. I follow the standard procedures to check my tools and this gives me enough confidence in them to trust them. I do not need to certify them or have them certified; I just need to know I can trust their readings. How accurate I can work is a separate thing.

 

[erikmannie]

You do realize that is a 50 to 1 relationship, yes?
If you are trying to secure a NASA contract, those numbers may be relevant. I have several posts here about chasing precision that is not required. I am just a hack, but what difference does .0009 make? Or .005? What are you building?
I surely appreciate the desire to "hit your numbers". I have accepted the fact that the numbers I can hit are big 3 digit ones. I will not obsess over 4 digits at all.

I get my money from my job which has nothing to do with machining, so all of the work in my shop is hobby stuff. I have the most fun when I am challenged; the more challenging, the more fun. The best day is when I fail because I make sure to learn something from it.

Generally, after I make something it goes out on the “metal” shelves which is a euphemism for the scrap area.

In machining school, they recommended to shoot for a thou tolerance and expect to fall short. They also said that if a tolerance is not written on the drawing, then assume .005”.

With the understanding that I am just having fun in the shop, my target tolerance was chosen as .001” for now because I don’t think there will ever be any temperature control out there. My best case scenario of .0003” is probably optimistic, considering the lack of a controlled climate.

So the .0003”-.001” tolerance is just a goal and not required by any means. To be honest, I would be very pleased if I hit a thou.