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Surface plate grades

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Redmech

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#1
When a surface plate is graded. Over what length is measured when it’s graded?
If a grade B is +/- .0001, is that over 6” or 12” or across the entire plate? I’d guess it’s a standard length, through some searching on the inter webs and the forum I couldn’t really find my answer.

Something that has my curiosity. Thanks for info everyone.
 

Tony Wells

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#2
Flatness is defined by the difference between the highest high point on the plate and the lowest low point. IOW, it can be represented as 2 parallel planes that are perfectly flat, one containing the high point and the other the low point. The distance that separates the planes is the flatness.

https://www.qualitydigest.com/aug03/articles/03_article.shtml
 

Redmech

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#3
Thanks, That’s a good link with a good read.
 

Redmech

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#5
Thanks everyone.

I thought I had read somewhere that many times the grade is measured over a set distance, and not over the entire plate. But now I can’t find that anywhere, or where it says that it is measured over the entire plate.

One other interesting thing I found out about my 9x12x2” grade B Woodcraft brand granite plate is that a neodymium magnet will stick to it. Makes me wonder if my china granite plate isn’t quite granite. When I have the plate setting on its side the magnet will stick to the side and not slide down by gravity.
 

Tony Wells

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#6
The link I posted says specifically that ALL points on the plate are considered when establishing a flatness figure. That pretty much means the whole plate, not over a given distance. If it were a distance limited thing, it would be a little meaningless if you think about it.
 

Redmech

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#7
The link I posted says specifically that ALL points on the plate are considered when establishing a flatness figure. That pretty much means the whole plate, not over a given distance. If it were a distance limited thing, it would be a little meaningless if you think about it.
Thanks, I missed it.
 

Wreck™Wreck

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#8
The two most important considerations in hobby machining are,1 the accuracy of your surface plate, 2 the three point method of supporting it and what material you make the protective cover from.

The three most important considerations in hobby machining are, 1 the accuracy of your surface plate, 2 the three point method of supporting it and ,3 what material you make the protective cover from and the cleaning solutions that you use.

The four most important considerations in hobby machining are, 1 the accuracy of your surface plate, 2 the three point method of supporting it and ,3 what material you make the protective cover from and, 4 the cleaning solutions that you use.

Do not even ask about leveling, this is a whole different tin of haddock.

When I was a young man I began as a hobby machinist, I began with a granite surface plate and quickly realized that it wasn't perfectly flat so bought a better one which also wasn't perfectly flat. I tried different mounting methods, different covers and cleaning solutions none of which worked. After purchasing 8-10 different plates none of which were perfect I gave up. Couldn't afford to actually buy a machine and make imperfect parts.

Hope this helps
 

Tony Wells

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#9
Glad you came to your senses and gave up.
 

T Bredehoft

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#10
I leave perfection to those hobbyists that grind telescope lenses. My machine work is adequate, good enough that the tooling I make does what it is intended to do. If I need real accuracy, I buy bearings.
 

Redmech

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#11
I appreciate the help in this thread, the rest of it well.....never mind
 

Wreck™Wreck

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#12
Glad you came to your senses and gave up.
I tried stacking surface plates without success, the stack became to tall for ease of use. Tried using 4 random support positions underneath one but this caused errors, finally settled on leaving them in the shipping crate which the manufacturer obviously knows something about, this worked best and had the least error yet still not perfect. In recent years I simply take my parts to the surface plate factory and measure them there before the inaccuracy begins.

They hate me
 
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