03-06-2012, 11:25 PM
Commercially produced surface stands do support plates in 3 places. Some installers use a vibration isolator mount. Some use a total of 4, one on each side of centerline about the Airey point line, about 2/3's of the width of the plate as a separation distance between the 2 mounts on that end of the plate. On the other Airey line, on center, some use a pair of mounts. Others use a single pad there.
Flatness of plates is often quantified by laser instruments. It is true that most plates are finished on one side only. There is no specification for parallelism between the top and bottom surfaces. If there were, any time one side was relapped or reground and lapped, it would need to be verified as to parallel with the other side. Not practical for field processing. There is also a repeatability specification that has a special gauge specifically for the job. The sides are only ground, not lapped. Not precision.
Surface plates are "calibrated" or verified in the field and when found out of tolerance for the grade required, they are lapped. It can be quite a bit of work. I've assisted a plate specialist a time or two, and it's pretty labor intensive. Diamond dust and a heavy cast iron lapping plate. At times, I have had a plate lapped to higher standard than I really needed so that perhaps next time it would still pass as the next grade down and I wouldn't have to pay for lapping. It didn't take long for the guy to figure out that he was having to do the same amount of lapping, just all at once. I started maintaining the high grade all the time after that. I wasn't saving all that much.
I prefer to have my plates both flat and level. That makes setting up some parts much easier, and you can use a precision level in the inspection process as well as other instruments. Besides, your parts won't roll off and acquire new, undocumented features.
Some educational reading here:
http://www.qualitydigest.com/aug03/artic...icle.shtml
Flatness of plates is often quantified by laser instruments. It is true that most plates are finished on one side only. There is no specification for parallelism between the top and bottom surfaces. If there were, any time one side was relapped or reground and lapped, it would need to be verified as to parallel with the other side. Not practical for field processing. There is also a repeatability specification that has a special gauge specifically for the job. The sides are only ground, not lapped. Not precision.
Surface plates are "calibrated" or verified in the field and when found out of tolerance for the grade required, they are lapped. It can be quite a bit of work. I've assisted a plate specialist a time or two, and it's pretty labor intensive. Diamond dust and a heavy cast iron lapping plate. At times, I have had a plate lapped to higher standard than I really needed so that perhaps next time it would still pass as the next grade down and I wouldn't have to pay for lapping. It didn't take long for the guy to figure out that he was having to do the same amount of lapping, just all at once. I started maintaining the high grade all the time after that. I wasn't saving all that much.
I prefer to have my plates both flat and level. That makes setting up some parts much easier, and you can use a precision level in the inspection process as well as other instruments. Besides, your parts won't roll off and acquire new, undocumented features.
Some educational reading here:
http://www.qualitydigest.com/aug03/artic...icle.shtml