Gage blocks

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Mr Ron
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Location: Vancleave, Mississippi

Re: Gage blocks

Post by Mr Ron »

WOW! this topic has mushroomed out into space. I better not mention ceramic blocks or optical flats. It is fascinating and interesting all the same.
Mr.Ron from South Mississippi
SteveM
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Location: Wisconsin

Re: Gage blocks

Post by SteveM »

GlennW wrote: Thu Sep 20, 2018 4:44 pm This would get you close...

https://www.ebay.com/itm/Van-Keuren-Lig ... xyPc5SB3P1

Van Keurens answer to the P&W Supermicrometer.
How is one of those used (other than turning the large wheel).

There seems to be a bunch of parts on it.

What is the glass thing on the base?
VK_Lightwave.jpg
Steve
pete
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Joined: Tue Feb 10, 2009 6:04 am

Re: Gage blocks

Post by pete »

What may not be quite as obvious at first is gauge blocks give you "roughly" the same capabilitys as what good recalibration labs use when there recertifying most of the common metrology equipment used in the shop. They'd use high end lab grade blocks, but the process is much the same as we would do. But unlike what we'd be doing, it's also done by the big boys under tightly controlled cleanliness, humidity and temperature limits. 68F or 20C + - not a whole lot. About 1 degree change either way is I think the maximum limits. Block sizes are chosen carefuly so that something like a micrometer spindle when being checked is at different degrees of rotation. Then there's those optical flats for checking anvil and spindle faces for wear and alignment.

Then there's checking yourself against those gauge blocks which I think is just as important. At least for me it is. The equipment can be the best in the world, but if your methods are even slightly flawed then the numbers will be as well. Measuring accurately to a thou is fairly easy. To 10ths not so much even with mikes designed to do better than that. And it's not quite as easy or obvious as it seems on the surface. If you can't get repeatable results with the best conditions you can manage in your shop on the known flat, clean and lapped surfaces of those blocks, it's sure not going to happen under the usual shop measurement conditions. And it's easy to double check yourself. Grab half a dozen blocks of various sizes, DON'T look at the sizes and measure each one randomly 10 times and write each number down. Minimizing heat transfer into the blocks from your fingers should be a priority. I'm not as good as I thought I was after doing this, but I learned a lot about technique. Everything matters when trying for 10ths. And I'd still not be willing to ever bet large money on which 10th number I'm getting 10 out of 10 times. :-(

My apologys ctwo, Mentioning that block change of one micron per year per inch was a poor attempt at humor and I hope that didn't cause any confusion. We can pretty much ignore a one micron change. For most of us were not producing tightly speced parts that have to match the numbers as close as possible. A set of blocks is just a way of getting all the rest to measure to the same numbers or at least know how far they might be off. Same idea as you checking what should be a good dependable set of blocks against some your unsure of. The exact 1/100 thousandths or 1/millionths they measure to is mostly unimportant. Only that one shop measurement standard is chosen and everything else is checked against those. For anyone building commercial parts then they already know if/when there equipment needs checking and why that recertification documentation might be needed.
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GlennW
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Re: Gage blocks

Post by GlennW »

SteveM wrote: Fri Sep 21, 2018 1:57 pm
GlennW wrote: Thu Sep 20, 2018 4:44 pm This would get you close...

https://www.ebay.com/itm/Van-Keuren-Lig ... xyPc5SB3P1

Van Keurens answer to the P&W Supermicrometer.
How is one of those used (other than turning the large wheel).

There seems to be a bunch of parts on it.

What is the glass thing on the base?
VK_Lightwave.jpg

Steve
The quick answer is: The glass thing is an optical flat over a lapped metal block. What that does is allow you to apply the exact pressure on the micrometer anvils for each measurement. It would be used in conjunction with a Monochromatic light source and the optical flat would be pressed onto the block until the light bars visible in the optical flat display a specific pattern. Then the micrometer is zero'd. When measuring, you would apply pressure on the part being measured until the same pattern is achieved in the optical flat which would indicate that the exact same amount of pressure is being applied to the micrometer anvils and then read the handwheel graduations and vernier.

The magic is in being able to apply the exact same amount of pressure for each measurement which gives the accuracy at such fine resolution.
Glenn

Operating machines is perfectly safe......until you forget how dangerous it really is!
Mr Ron
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Location: Vancleave, Mississippi

Re: Gage blocks

Post by Mr Ron »

If the gage block and the micrometer (for example) are allowed to normalize at a constant lab temperature; a measurement taken and then allowed to normalize at a shop temperature of say 85°, would not the readings be the same, regardless of the temperature?
Mr.Ron from South Mississippi
John Hasler
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Location: Elmwood, Wisconsin

Re: Gage blocks

Post by John Hasler »

Only if they are made of materials with identical coefficients of thermal expansion.
pete
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Joined: Tue Feb 10, 2009 6:04 am

Re: Gage blocks

Post by pete »

Within average limits then there's pretty accurate calculations that can be used to adjust for size if the material type and temperature is known so when things cool down or warm up to the more standard 68f/20c specification they should be quite accurate. I can only go by what I've read about it since I've never set foot in a real metrology lab. I'd assume that 68f/20c number was picked to set a number that's comfortable for humans and having that set number it can be duplicated anywhere in the world and still get the same results. The very good metrology equipment is finished ground and/or lapped, then assembled and tested for factory certification at those same metrology lab conditions. I'm certainly no mechanical engineer or metrology expert so this is a best guess and worth about that, the metrology equipments actual shape may change that carefully ground, lapped and tested condition at temperatures even 10 degrees above or below the ideal 68f/20c? But I don't know that for sure. A micrometers C frame shape seems like it could grow percentage wise a bit differently than say a 1" steel cube would. Still only a guess though.

From what I've read it seems an industry standard for recertification labs is to let any equipment that's to be checked to thermaly stabilize within the lab for at least 24 hrs. I just ran your numbers through an online thermal calculator and for a 1" long piece of steel from 68f to 85f it seems there's an increase in length of .00013" over that 17 degree temperature change. For most of us that isn't much and maybe could be ignored? Parts do get pretty hot if a lot of metal is being removed so the temperature swings could be a lot larger, maybe enough that a good bearing fit on something of a few inches or more then it might be something to consider. A 20" micrometer should in theory change .00248" over those same 17 degrees. That could be more than enough to need adjusting for with some parts. Since I don't own any 20" gauge blocks or mikes I may never have to worry about it.

How "technicaly" correct it might be I don't know. But if I'm checking anything new against my gauge blocks I always try to set everything I'm going to be using on my surface plate so it's used as a somewhat stable thermal mass and then try and let it all stabilize for at least 4-6 hrs. Obviously a long ways from lab procedures, but everything should be no more than a guesstimated 5 degrees from that perfect 68f and only a degree or two at most for differences between items. Probably close enough to run meaningful checks for anything in a home shop.
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