(03-06-2012, 05:12 PM)stevec Wrote: Arvid, I hope your questions have been answered, Sometimes the jocularity obscures the legitimacy.

So, to reiterate, because I'm curious, are some surface plates ground to the same flatness and surface precision on both sides?

(03-06-2012, 06:12 PM)EdAK Wrote: Just some FYI from the Starrett web site.

"How should my surface plate be supported? Does it need to be level?
A) A surface plate should be supported at 3 points, ideally located 20% of the length in from the ends of the plate. Two supports should be located 20% of the width in from the long sides, and the remaining support should be centered. Only 3 points can rest solidly on anything but a precision surface.

The plate should be supported at these points during production, and it should be supported only at these three points while in use. Attempting to support the plate at more than three points will cause the plate to receive its support from various combinations of three points, which will not be the same 3 points on which it was supported during production. This will introduce errors as the plate deflects to conform to the new support arrangement. All Tru-Stone steel stands have support beams designed to line up with the proper support points.

If the plate is properly supported, precise leveling is only necessary if your application calls for it. Leveling is not necessary to maintain the accuracy of a properly supported plate."


I have a message in to Starrett asking if their surface plates are ground and polished on both sides. Based on what I read on their web site, I doubt they are but I'll post their response once I get it.

Ed

(03-06-2012, 05:12 PM)stevec Wrote: Arvid, I hope your questions have been answered, Sometimes the jocularity obscures the legitimacy.
You've had one reply that "One side will be ground flat and the other is usually kind of rough."
Is your plate like so? (I doubt you would have asked the question if it were blatantly obvious).
The other point of parallelism you've answered yourself and I agree.
So, to reiterate, because I'm curious, are some surface plates ground to the same flatness and surface precision on both sides?

(03-06-2012, 11:25 PM)Tony Wells Wrote: 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/articles/03_article.shtml

(03-06-2012, 11:40 PM)Tony Wells Wrote: You're welcome, Jerry. I'm doing pretty good. Getting around to all my favorites places to visit this evening. How are you?