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Do length standards grow over time ?

hotbluechips

Aluminum
Joined
Jan 6, 2012
Location
winchester virginia
Do length standards grow over time ?
I recently sent for certification 2 very old length standards,. a 1" and a 2".
Made by Pratt and Whitney, Nice velvet lined wooden box.
In excellent shape. I assumed because of their age and wear they would be on the minus side.
Both were long, the 1 " was .00007 over and the 2" was .00017 over, almost 2 tenths.
They were just certified for a friend and wont be used, just done for the knowledge of the actual size.
Thoughts about why they are long, made that way or did they grow ?
 
We have length standards that are over 20 years old, which we send to an outside lab for verification every two years. They have never changed during that time frame. Suspect the ones you have were probably off before and simply used anyway.
 
Back in the 19th century Johansson explored the phenomenon of steel growing with time as he was developing the idea of standard gauge blocks. He may not have known the metallurgy of retained austenite, but did figure out how to minimize it. Given that history, I can't imagine that Pratt and Whitney would have been caught out with items that continued to grow for that reason.

The two other possibilities, that P&W didn't lap them to size very closely, or that the cert lab screwed up the measurement seem unlikely as well. You might be hard pressed to measure the variation in the 1" standard, but the difference in the 2". compared to any other 2" standard or gauge block you have should be visible with a decent mic. You might compare, just for your own satisfaction, but unless a discrepancy stands out, it would have to just be one of life's little mysteries.
 
I don't know diddly about gauge blocks. But this question and others that I have seen on PM about gauge blocks are answered in this paper from NIST. Just to encourage you to thumb through it, I'll leave you with a quote:

"No material is completely stable. Due to processes at the atomic level all materials tend to shrink or grow with time. The size and direction of dimensional change are dependent on the fabrication processes, both the bulk material processing as well as the finishing processing."

Note that it says "shrink or grow". It states that the growth rate for 52100 gage block steel is positive, which I think means it grows over time.
 
I don't know diddly about gauge blocks. But this question and others that I have seen on PM about gauge blocks are answered in this paper from NIST. Just to encourage you to thumb through it, I'll leave you with a quote:

"No material is completely stable. Due to processes at the atomic level all materials tend to shrink or grow with time. The size and direction of dimensional change are dependent on the fabrication processes, both the bulk material processing as well as the finishing processing."

Note that it says "shrink or grow". It states that the growth rate for 52100 gage block steel is positive, which I think means it grows over time.
Thanks for the NIST link. I will pass it on to the owner of the Standards.
 
Thanks for the thoughts on this. I didn't measure them myself and have been returned. They were sent for certification purely out of the owners curiosity. He is a 90 yr old retired machinist friend of my boss. I might try and get in touch with P&W and get their thoughts and reasoning for the oversize condition. Just has me wondering, why.
 
I don't know diddly about gauge blocks. But this question and others that I have seen on PM about gauge blocks are answered in this paper from NIST. Just to encourage you to thumb through it, I'll leave you with a quote:

"No material is completely stable. Due to processes at the atomic level all materials tend to shrink or grow with time. The size and direction of dimensional change are dependent on the fabrication processes, both the bulk material processing as well as the finishing processing."

Note that it says "shrink or grow". It states that the growth rate for 52100 gage block steel is positive, which I think means it grows over time.
NIST does some neat stuff. see:
https://www.youtube.com/watch?v=uCyU97MoHFM
thanks for the link
 
To wish my length standard could grow over time.....
In the first 20 years yes. In the next 40 not so much.
Oh wait. That would be different things.


Just met with a customer. Husband/wife outfit...
Pappa was out, and I was dealing with Mamma.

We had a printed sample of a part that we are looking to git a mold built for.
Was looking for approval, and if there is anything that they would like to change.

Mamma thought that the unit was too thick, and I fully agreed with her.
OK so - where doo you think we want to be?

So she held her tape measure up to the part, declared it about an 1" thick, and maybe we will drop to 3/4".
And I agreed that 3/4 of what was there would seem to be appropriate for the app to me as well.

So I git back, and I inspect the part myself to verify the 1" measurement, and it was 13/16".

I am :bowdown: at "Pappa".


----------------------

Think Snow Eh!
Ox
 
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I have a 400 mm engraved scale, Made by the Swiss firm SIP. It came from one of their measuring machines. Made around 1960, and came with SIP calibration. I had this calibrated at NIST, came back with an k=2 uncertainty of about 0.3um and the measured dimension matched the SIP values very closely. That all said; I think it depends on how the metal was made about stability. (and the Swiss are very good at this!)
 
There was a moldmaker where I once worked who had previously worked for a pen company. They farmed out a lot of their tooling, but the toolmakers were responsible for inspecting incoming tools. The company had gotten so fussy about measurements that many outside shops would no longer bid. So the toolmakers were also trying to hang on to the last outside shops.

He said that they might measure a part and find it was just a tenth or two large. So, sauntering over to the water fountain, the part might hang out in the cold water a few minutes and get remeasured. Similarly, if it was just a TINY bit too small, standing around holding the part for a few minutes might correct that problem.
 
I think the issue of tool-steel growth is addressed in the most recent Moore book. From what I recall, it's rare but it can happen.
 
I think the same people that made the blocks also are responsible for calibration of thermometers over the last century. -More likely than people with an agenda torturing data to prove global warming.
 
I worked for a couple injection molding companies many years ago and at that time, all of the moldmaking was up north. We once had a shop in Denver make a mold for us and they shipped the cavity/core inserts first, followed by the mold base a few days later. It was the middle of winter and very cold in Colorado. We tried to assemble everything upon arrival but nothing fit. The lead engineer called up the mold maker in a hissy fit but the reply was to just let everything soak for 24 hours at ambient temperature. Lo and behold, all the parts slipped together with no issues. Thermal growth is a real thing…
 
There was a moldmaker where I once worked who had previously worked for a pen company. They farmed out a lot of their tooling, but the toolmakers were responsible for inspecting incoming tools. The company had gotten so fussy about measurements that many outside shops would no longer bid. So the toolmakers were also trying to hang on to the last outside shops.

He said that they might measure a part and find it was just a tenth or two large. So, sauntering over to the water fountain, the part might hang out in the cold water a few minutes and get remeasured. Similarly, if it was just a TINY bit too small, standing around holding the part for a few minutes might correct that problem.
I never quite understood this. If you're going to do something non-compliant with the standard to intentionally fudge the numbers, why not just pencil whip it and be done?
 
I would think a standard would shrink as the ends wear off a few atoms with each time it is measured. Rust or oil would make it grow. What is the 100% correct cleaning and drying method before comparing it?
Bill D
 
I worked on a tool for punching out the rubber plugs on intravenous drip bags. The plugs were compression moulded in a sheet then punched out using soapy water as a lube. The maximum punch and die clearance was .007 mm and the stripper and punch were out of alignment. The tool was made by a very reputable German company who shall remain nameless. They sent a engineer out to South Africa to inspect it and when I asked what the reason was for the change in sizes he said he thought the change in atmospheric pressure during the flight was the probable cause. I said I think you are talking shit. I do know he couldn't tell me whether they had done the double temper and Cryo treatment as specified for the material to increase material stability.
 








 
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