12" x 18" x 4" Used Granite Surface Plates - $25

Wish I were a few miles closer :)
 
In the pictures they look like they have had rough usage over time. It will likely cost a relatively large amount to clean up and calibrate them back to a known flatness and tolerance. An unknown condition surface plate is, well, unknown, and you will just be guessing at any results obtained when using it. Caveat Emptor (let the buyer beware...)
 
Bob Korves said:
In the pictures they look like they have had rough usage over time. It will likely cost a relatively large amount to clean up and calibrate them back to a known flatness and tolerance. An unknown condition surface plate is, well, unknown, and you will just be guessing at any results obtained when using it. Caveat Emptor (let the buyer beware...)
Click to expand...
Just buy 3 and do the 3 plate method on them :)
 
Bob Korves said:
In the pictures they look like they have had rough usage over time. It will likely cost a relatively large amount to clean up and calibrate them back to a known flatness and tolerance. An unknown condition surface plate is, well, unknown, and you will just be guessing at any results obtained when using it. Caveat Emptor (let the buyer beware...)
Click to expand...

I can't resolve the conditions or durations of usage from the photo (except for a couple of small corner/edge chips). What I can see is that they are advertised as 4" thick which, at 12" x 18" area, implies they are of quality origin and intent. I think I see a 5"er in there. Most cheap plates, of that size, are 3" thick. I also see RAHN nameplates (quality supplier) and two ledge plates (not common in B grade plates).

Judicious inspection at pick-up is a very good idea. Don't forget your Autocollimator, mirror and straight edge.
Recertification is always the way to assure the condition of any surface plate.

Or maybe, one is good enough, as is, for a hobby-machinist.
 
extropic said:
I can't resolve the conditions or durations of usage from the photo (except for a couple of small corner/edge chips). What I can see is that they are advertised as 4" thick which, at 12" x 18" area, implies they are of quality origin and intent. I think I see a 5"er in there. Most cheap plates, of that size, are 3" thick. I also see RAHN nameplates (quality supplier) and two ledge plates (not common in B grade plates).

Judicious inspection at pick-up is a very good idea. Don't forget your Autocollimator, mirror and straight edge.
Recertification is always the way to assure the condition of any surface plate.

Or maybe, one is good enough, as is, for a hobby-machinist.
Click to expand...
They probably were very nice when they left the maker's plant. That says nothing at all about their condition today. Unknowns are simply unknowns until tested properly and then calibrated if and as needed. I don't know how close the nearest calibration company is to Meadville, PA, but delivering it or shipping it for calibration, or having them come to the owner, can be quite expensive. Shipping it (both ways) can result in a damaged or broken plate. I would get all those ducks in a row and know the potential costs before purchasing those plates.
 
Bob Korves said:
They probably were very nice when they left the maker's plant. That says nothing at all about their condition today. Unknowns are simply unknowns until tested properly and then calibrated if and as needed. I don't know how close the nearest calibration company is to Meadville, PA, but delivering it or shipping it for calibration, or having them come to the owner, can be quite expensive. Shipping it (both ways) can result in a damaged or broken plate. I would get all those ducks in a row and know the potential costs before purchasing those plates.
Click to expand...
Eh, for 1/4 the price of a "cheap" one that size, I'd be tempted to buy and use them as-is. You can lay them out if you need to identify which spots are good.

AND I was only sort of joking about flattening them with the 3 plate method. They are about the largest size you'd want to do so with, but at $75+ time and abrasive cost, it would probably be a fun project.
 
I have seen very nice looking surface plates with .0004+ inch "holes" in the center. You cannot eyeball them to the accuracy that surface plates need to provide, even for "hobby-machinists." But you can always "hope for the best." I have a saying that I use regularly -- "One good test is worth a thousand expert opinions."
 
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