3-Plate Method, Surface Plates?

Best argument for granite: dropping items on it cause chips that have minimal effects on accuracy.
Argument against cast iron: Dropping items raises up a mini volcano that affects accuracy until you locate them and stone them flat.
 
I'm shocked at the level of accuracy capable from a surface plate. In the video the guy was even standing in the same place on the floor to take readings because he said there could be some deflection! Holy cow! On that stone he had about 0.001" of concavity over 4 feet and considered it unacceptable. I find that fascinating!
 
I just got My surface plates lapped, it took me a while to find someone to do it for me..... I found him by calling local tool & die machine shops and asking them who does their plates.... Didn't take me very long to get a phone number to a local company, he came to my shop and knocked them out very quickly.... Did two 24x36" plates in less than an hour.... Within .00008" now.....
 
A friend bought a very nice looking pink Starrett 24x36" surface plate, I told him it was nice, but make sure it is tested and calibrated as necessary. He finally did so, and it had a .005" low area in the middle of the plate. A proper test is worth a million expert opinions...
 
Bob Korves said:
A friend bought a very nice looking pink Starrett 24x36" surface plate, I told him it was nice, but make sure it is tested and calibrated as necessary. He finally did so, and it had a .005" low area in the middle of the plate. A proper test is worth a million expert opinions...
Click to expand...

That's called a soap dish.....
 
Bob Korves said:
A friend bought a very nice looking pink Starrett 24x36" surface plate, I told him it was nice, but make sure it is tested and calibrated as necessary. He finally did so, and it had a .005" low area in the middle of the plate. A proper test is worth a million expert opinions...
Click to expand...
A good straightedge and a flashlight would show that.
Was it able to be lapped out and get it into 'A' or 'B'?
 
middle.road said:
A good straightedge and a flashlight would show that.
Was it able to be lapped out and get it into 'A' or 'B'?
Click to expand...
Yes, it was put back into grade A condition by Standridge Granite. Not such a big deal for them, everyday work. This is not something that can be tested or re-calibrated by the typical hobby machinist, or even a pro machinist. An unknown surface plate is just that, and we are just guessing. The brand name on it and the calibration label only show what it was when it was last calibrated. Unless it has been under your personal control since that date, it is pretty much just a Wild A$$ guess what you have, rated only as a potential tombstone...
 
Bob Korves said:
Yes, it was put back into grade A condition by Standridge Granite. Not such a big deal for them, everyday work. This is not something that can be tested or re-calibrated by the typical hobby machinist, or even a pro machinist. An unknown surface plate is just that, and we are just guessing. The brand name on it and the calibration label only show what it was when it was last calibrated. Unless it has been under your personal control since that date, it is pretty much just a Wild A$$ guess what you have, rated only as a potential tombstone...
Click to expand...
We have two definite tombstones in the recent batch. They resemble the surface of the moon. Looks like a toddler went at them with a mini rock hammer...
Found a sticker on one for a company between Knoxville and Nashville that had not showed up in searches.
They at least mention lapping and have pictures on their website which is more than some of the others.
I'll have to get in contact with them after the holiday.

I wonder if Starrett over in Mount Airy, NC does maintenance... That's only (4) hours away. :grin:
 
Is it possible that a plate can be lapped flat even though there's chips through-out the surface? A surface gauge would have a big enough base to float over at least some chips, right?
 
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