Granite surface plate accuracy

[Dave123]

Are the smaller granite plates (12 x18 x 3) square, meaning would the 12” side and the 18” side be exactly 90 degrees ?

Dave

 

[Joe Gwinn]

Are the smaller granite plates (12 x18 x 3) square, meaning would the 12” side and the 18” side be exactly 90 degrees ?

Surface plates, no. Items sold as granite squares, yes.

 

[michiganbuck]

I never even checked one to be square sides to the top..likely not.

 

[Davewitt]

Are the smaller granite plates (12 x18 x 3) square, meaning would the 12” side and the 18” side be exactly 90 degrees ?

Dave

Never saw one that was exactly 90*, the top is the only part that is precision.

 

[Milland]

Are the smaller granite plates (12 x18 x 3) square, meaning would the 12” side and the 18” side be exactly 90 degrees ?

Dave

If you truly need an artifact (granite lump) with true right angles, you need to chose at least a granite 2-side square, or go up to the full 6-side stone. Just lube your wallet first...

18 x 18 x 3 AA Laboratory, A Inspection Grade Master Granite Square 6 Side ID 6500-

 

[DMF_TomB]

granite squares get checked like anything else, 36" granite squares i use are within .0001 per 36"
.
some are marked on edge per inch with a type of ruler sticker. so you can lookup error say at 10" mark
might say its -5 or -5 millionths there at 10" mark
.
the original manufacturer can probably "fix" any worn square if really worried about the error in millionths

 

[CarbideBob]

Are the smaller granite plates (12 x18 x 3) square, meaning would the 12” side and the 18” side be exactly 90 degrees ?
Dave

No, the sides will not be square unless you get super lucky. One can clamp a precision square onto the top of the plate for a somewhat true 90 reference.
The sides also will not be square to the top no matter how nice they may look.

If you truly need an artifact (granite lump) with true right angles, you need to chose at least a granite 2-side square, or go up to the full 6-side stone. Just lube your wallet first...
18 x 18 x 3 AA Laboratory, A Inspection Grade Master Granite Square 6 Side ID 6500-

Well that seems like pocket change... and beware a 2 side and 6 side much different when using even though they look the same.
Set a two side on it's back and it may no longer be such a good 90 if real fussy matters.

Unsure the desired usage here.
Bob