- Joined
- Jul 2, 2014
- Messages
- 7,594
If you buy a used surface plate, you have an unknown. You are not likely to be able to check it with any reliability by yourself in your home shop. Buying a used plate by brand name and paying more for it is about the silliest thing I can imagine. They are all chunks of rock that look visually flat. I bought my 18x24x4", 2 ledge for $50 off a rack behind a machine shop, covered with junk and grime. I "guessed" it was in pretty good shape by chasing various indicator setups and straightedges around it and scratching my head. A group of local hobby machinists brought their plates to my shop and Standridge granite dropped by to calibrate the 5 plates. Mine ended up costing ~$103 to be calibrated to AA grade and certified to A grade. It turned out to have a .003" "hole" in the center. To be used for precision work, guessing will not do. A friend bought a very nice pristine looking 24x36" Starrett pink plate for a pretty high price, and it turned out to have a .005" hole in it, and had to be taken 400+ miles to Standridge's facility in SoCal to get it calibrated, they will not remove that much in the field. IF THE PLATE HAS NOT BEEN IN YOUR POSSESSION AND UNDER YOUR CONTROL SINCE IT HAS LAST BEEN CALIBRATED, IT IS NOT A SURFACE PLATE, IT IS "AN UNKNOWN KINDA' FLAT LOOKING ROCK." No exceptions. The idea of a surface plate is it being a known flat plane reference surface in your shop. If it is not a known quantity, it is worthless for that job. If you want to use it only for simple layout work or for lapping non critical parts, stacking parts and tools on, eating your lunch, a tombstone, or some other non precision use, then go for it. Just don't hope that it is something it is not.