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Surface Plate Calibration | Chicken & Egg

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jacobfw

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So to start off with, no I am not a machinist, mechanical/materials engineer, etc. Just someone who loves understanding the solutions to problems.
So I first found out about surface plates about a year ago after going on the "how do you make a precise part without a more precise part" quest.
Read up on surfaces plates and how they're made. Awesome, really clever, love it...

but.......

Last week I found some of the original videos I had watched on surface plates again, and at one point asked myself, "how do they know"?
I understand how the 3 plate method is able to produce a flat plane, but if someone was to just had you an arbitrary surface plate, how would you
determine it's flatness?

So this led me onto a new quest looking into the various ways surface plates are measured and graded, but frankly it seems like I'm right back at square one,
where all the parts used to calibrate a surface plate seem to require a part already just as precise if not more. It's almost like someone figured out how to
measure one independently, then everyone promptly forgot about it since they had a surface plate they knew was good.

So my question is, how do the devices which measure surface plates work without being calibrated against a surface plate of a known flatness,
or by being made out of parts which themselves had to have been calibrated against a surface plate at some point during their construction?
How did Maudslay & Whitworth do it? (I did search for this but wasn't able to find an answer).

To anyone who's just going to bark out "lasers/digital sensors", as far as I'm aware those do suffer from the same problem (if I'm wrong about that then please let me know).

Appreciate any answers/insight anyone can provide!


P.S. Just in case the tone of my post comes across the way I didn't intend it, I think this stuff is really amazing and fascinating, and I'm just frustrated that what seems
to be such a huge linchpin doesn't seem to get discussed that much.
 
So to start off with, no I am not a machinist, mechanical/materials engineer, etc. Just someone who loves understanding the solutions to problems.
So I first found out about surface plates about a year ago after going on the "how do you make a precise part without a more precise part" quest.
Read up on surfaces plates and how they're made. Awesome, really clever, love it...

but.......

Last week I found some of the original videos I had watched on surface plates again, and at one point asked myself, "how do they know"?
I understand how the 3 plate method is able to produce a flat plane, but if someone was to just had you an arbitrary surface plate, how would you
determine it's flatness?

So this led me onto a new quest looking into the various ways surface plates are measured and graded, but frankly it seems like I'm right back at square one,
where all the parts used to calibrate a surface plate seem to require a part already just as precise if not more. It's almost like someone figured out how to
measure one independently, then everyone promptly forgot about it since they had a surface plate they knew was good.

So my question is, how do the devices which measure surface plates work without being calibrated against a surface plate of a known flatness,
or by being made out of parts which themselves had to have been calibrated against a surface plate at some point during their construction?
How did Maudslay & Whitworth do it? (I did search for this but wasn't able to find an answer).

To anyone who's just going to bark out "lasers/digital sensors", as far as I'm aware those do suffer from the same problem (if I'm wrong about that then please let me know).

Appreciate any answers/insight anyone can provide!


P.S. Just in case the tone of my post comes across the way I didn't intend it, I think this stuff is really amazing and fascinating, and I'm just frustrated that what seems
to be such a huge linchpin doesn't seem to get discussed that much.

.
Rahn Repeat O Meter used to indicate flatness, used when lapping worn surface plates, often checked yearly depends on shop.
.
its like a wood hand plane with indicator to measure waves, if flat indicator doesnt change, got to check in different directions. that and a precision level which will measure gradual arcs 0.02mm/m is common and 4" and 8" long, obviously surface needs to be level to use level or bubble is off scale. in practice Repeat O Meter is fast. Precision level is much slower for bubble to stabilize. many levels the center third is relieved so it only makes contact on ends (less rocking) Swiss and German levels its common Wyler makes levels. Swiss ones are expensive. Wyler catalog explains levels
.
High-precision spirit levels, various measuring ranges & sensitivities
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Repeat-O-Meter Reading Gage
 

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I think the OP is asking about how it might be done going back to first principles. It's pretty hypothetical, but just to kick around, suppose you received just a surface plate and had no other reference. Well, in that case you're probably SOL

If you have the surface plate of unknown accuracy and a known accurate straightedge you could explore the plate by mapping out the contact going different ways.

But suppose you have an UNknown plate and an UNknown straightedge. Well, a VERY insightful and experienced scraper hand could again map contact using different directions and reversals to tease out information about the errors of each piece. I'm not sure it's possible to actually write up an error analysis of the unknown plate from the unknown straightedge, but it would give you a first fuzzy view.

To really get back to accuracy from a presumed worn condition, it should be theoretically possible then to start correcting both parts the way the 3-pl;ate solution develops. For instance, start scraping a strip along each edge of the surface plate to match the straightedge. You can reverse the straightedge and come closer. Then correct the straightedge from the contact with the plate and continue alternations. I suppose if the surface plate is a uniform convex or concave curve to start with you're screwed just as the Repeat-O-Meter could be fooled without his autocollimator buddy. OTOH, reversing the straightedge in the same position should start to make that clear.
 
Repeat-o-meter sits on 3 points with with a 4th point moving to show drop of one corner via an indicator. 3 points self level so you are just calibrating the 4th I've never calibrated one but I'd think you could calibrate the indicator to read '0' by averaging any error you see from measuring a somewhat known surface. Its not so much telling you flatness as it is a drop from the other 3 points, so any error in it is going to be in the linear drop accuracy, not in how level it sits.

Autocoluminator works similarly but also involves the fact that light moves in a straight line (at least at the tolerance we're after). You can linearly measure the height of the cross hairs from the surface for calibration and It's just measuring straight lines between 3 points so similarly any difference in those 3 points is linear, not plainer.

Both tools don't tell you the 2 dimensional measurement of flatness, they tell you 1 dimensional measurements and it's up to you to map it out and know what you are looking at. You are trying to find how much error is in several measured lines, then put that together to deterimin if there is a twist, bow, hole, etc. and how high or low it is.
 
I believe that, out of all the tools mentioned so far, the level is the only self-checking one (i.e. you can rotate it 180° and it will read the same; if not, calibrate it until it does).
Therefore, if you have only a surface plate (or a straightedge) of unknown flatness, your best approach would be to use a precision level to map it (also differential levels are self-checking and they eliminate errors due to shifting weight on the floor, while operator moves around to take measurements).
A repeat-o-meter or tools working on that principle need to be calibrated on a known flat surface. Otherwise they could read as flat a concave or convex surface with constant radius.
I do not have much experience with autocollimators and am not sure if you can easily self-check that a mirror is perfectly perpendicular to the surface. However, if you do not move the autocollimator during the entire test and you moce only the mirror, it should yield results similar to a level even if not perfectly calibrated.

Paolo
 
With a planetary lapping machine you can make three optical flats that can be proven flat. Can't do it with two, need three. From that reference you can do most anything, though size is always an issue. There's a technique for measuring lathe beds and such using a taught wire and microscope. Not sure how far you can take the precision, but you could map a surface plate the same way with enough patience.
 
Here are a couple threads I started a little while back along the same lines (calibrating without a known flat surface). My last hold up was to get a piano wire test done on the un-calibrated straight edge before I began re-scraping it. Since then, I've also been fortunate to get a 6' long B Grade granite surface plate to speed up the scraping process, but I still want to go back through the other methods after the straight edge is resurfaced to see how they compare. Our new big surface plate is in need of a recalibration too to get it up to Grade A standards, so that will also give a known standard to compare the other methods too.

Large Straight Edge Questions
72" Brown & Sharpe Straight Edge Tests
 
The simplest way I can think of is to compare the surface to itself

You need a dti and a (guage) that you can invert on the plate
Look at fma page 28
 
son of gun, I have not done this my self, but the companies I worked for had to have a 6 month and or yearly surface calibration.
if the company is ISO 9001 compliant or certified this has to be done. just as any other tool surface plates wear.
and these guys are pro's , they physically lap the surface plate on the spot. remember recorded .000005 lapped surface or something crazy
like that never thought about picking their brain, but the use equipment right on top of the plate. surely it should be able to
google it for purchase but be ready to spent dollars. cheaper to have them surface lap and calibrate good for a year.
Stay Safe
 
son of gun, I have not done this my self, but the companies I worked for had to have a 6 month and or yearly surface calibration.
if the company is ISO 9001 compliant or certified this has to be done. just as any other tool surface plates wear.
and these guys are pro's , they physically lap the surface plate on the spot. remember recorded .000005 lapped surface or something crazy
like that never thought about picking their brain, but the use equipment right on top of the plate. surely it should be able to
google it for purchase but be ready to spent dollars. cheaper to have them surface lap and calibrate good for a year.
Stay Safe

As to what and how often depends on what has been written in the Quality Manual and inspection/calibration procedure. Too many write too much to impress customers rather than what is necessary for themselves. That can be expensive.
 
As to what and how often depends on what has been written in the Quality Manual and inspection/calibration procedure. Too many write too much to impress customers rather than what is necessary for themselves. That can be expensive.
I believe to the ISO certification it has to be minimum yearly, and when getting certify they nick pick every thing and yes agree very expensive but most third tear and second tear manufactures like Lockheed, Honeywell, Boeing require this it's
part of the cost doing business , a small commercial shop do what you like. but forth tear shops are always looking for small business to do their off load work. so it does pay to be at least compliant, but not certified.
 
I believe to the ISO certification it has to be minimum yearly,

If you can find anything re ISO certification where specific times/intervals are given for calibration I'd like to see it and probably others too.

Back in the day when I specified calibration intervals on "stuff" it was always based mainly on use frequency. A thread gauge used frequently on SS will wear faster than on brass.

With something like a surface plate calibration intervals would very much depend on what it was made from and how it gets used. I can't imagine a customer or ISO dictating calibration intervals.
 
If you can find anything re ISO certification where specific times/intervals are given for calibration I'd like to see it and probably others too.

Back in the day when I specified calibration intervals on "stuff" it was always based mainly on use frequency. A thread gauge used frequently on SS will wear faster than on brass.

With something like a surface plate calibration intervals would very much depend on what it was made from and how it gets used. I can't imagine a customer or ISO dictating calibration intervals.
yes we did use a lot of thread gages, spline gages, master gears, gage blocks and on , and had a master set and a working set.
I agree they wear out fast, I can say a customer did not like gages that were worn. , some times it had to be
calibrated at a more frequent cycle as not to be accepting discrepant parts, the shop was accepting parts with worn gages.is very bad, so it depends on the equipment, every surface plate had to be recalibrated in the entire shop.
I don't have the latest ISO specification but yes a lot of it could have been customer driven like boeing has very stringent
requirements in their internal requirements. I being out the loop a few years, like you said every thing has to be written
in Q.A. manual and it gets nick picked apart as well as every other quality procedures, and it can be the customers discreation
what the minimum requirements as you know ISO is very broad,
When I have time we can start an other thread and pick it apart, Inspection equipment if not used frequently could be
calibrate before use, but surface plate are used often day in and day out, so it wears rapidly.
if I get a chance I will look at an old copy I have.
 
yes we did use a lot of thread gages, spline gages, master gears, gage blocks and on , and had a master set and a working set.
I agree they wear out fast, I can say a customer did not like gages that were worn. , some times it had to be
calibrated at a more frequent cycle as not to be accepting discrepant parts, the shop was accepting parts with worn gages.is very bad, so it depends on the equipment, every surface plate had to be recalibrated in the entire shop.
I don't have the latest ISO specification but yes a lot of it could have been customer driven like boeing has very stringent
requirements in their internal requirements. I being out the loop a few years, like you said every thing has to be written
in Q.A. manual and it gets nick picked apart as well as every other quality procedures, and it can be the customers discreation
what the minimum requirements as you know ISO is very broad,
When I have time we can start an other thread and pick it apart, Inspection equipment if not used frequently could be
calibrate before use, but surface plate are used often day in and day out, so it wears rapidly.
if I get a chance I will look at an old copy I have.

Auditors do tend to "nit pick" on measuring equipment and calibration. Probably because it's easy to do and mainly looking at paperwork. The more detailed a calibration procedure is the more it gives an auditor the chance to "nit pick".

I repeat. A Quality Manual is for the company. Making one for a customer is daft.

What I've always wondered is why most customers keep auditing after a company is ISO certified. Makes it kinda pointless.
 
Auditors do tend to "nit pick" on measuring equipment and calibration. Probably because it's easy to do and mainly looking at paperwork. The more detailed a calibration procedure is the more it gives an auditor the chance to "nit pick".

I repeat. A Quality Manual is for the company. Making one for a customer is daft.

What I've always wondered is why most customers keep auditing after a company is ISO certified. Makes it kinda pointless.
from my reading you are correct there is no where it specifies intervals of calibration.
looking at the Starrett gage blocks web site they recommend 1-3 years recalibration.
so I stand corrected. so it is either before use or interval that maintains accurate reading of inspection equipment.
and this system is ungodly expensive. so it may have been a compromise by the quality manager, and the customer.
Customer quality engineers trying to justify their jobs I guess.
 
from my reading you are correct there is no where it specifies intervals of calibration.
looking at the Starrett gage blocks web site they recommend 1-3 years recalibration.
so I stand corrected. so it is either before use or interval that maintains accurate reading of inspection equipment.
and this system is ungodly expensive. so it may have been a compromise by the quality manager, and the customer.
Customer quality engineers trying to justify their jobs I guess.

Two comments :)

1. "Customer quality engineers trying to justify their jobs I guess." Sad but true in most cases.

2. Your quality manager (ex?) made a huge mistake if your Quality Manual was made in cooperation with a customer. A Quality Manual must be a company manual, not a customer manual. I wonder how much that mistake has cost your company?
 
Regarding Aerospace/Military - this was true upto 4 or 5 years ago.
Now if you are an integrator (Tier 1) for the likes of BAE or AgustaWestlands, the flow down of requirements is immense. ISO or AS9100 is a "start", but no-where near meets their specifics. Certain things will have to go into your manual, or you won't do business with them.
Even things like sub-contracting of the treatments - it is a requirement (mandate) that you flow ALL of BAE/AW etc terms and conditions to the sub-contract house.
And you are audited to ensure you have done it (although things like this are covered on their PO terms and conditions)...

What you are referring to isn't what I regard as a Quality Manual. More like having to meet specific customer company requirements.

I'm sure a sub forum such as Shop Management and Owner Issues would be more suitable for your post. Why not start a thread there and educate us with all your know how?

Constantly commenting my posts makes your intentions suspect.
 
This forum used to be about passing on info and knowledge and having discussions.
Obviously i'm now not allowed to post where I want to heh?
You start it over there if you want to - I was only replying and correcting your out of touch statement regarding Aero/Military.

You are wrong,AGAIN.
 
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