Expectations of a granite surface plate ...

[DaveH]

(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?

In a word no, they are "finished" as Jerry stated, but a normal standard type surface plate only has it's top side lapped and polished.

DaveH

[EdK]

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

[ETC57]

(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

Thank's Ed,

Jerry.

[arvidj]

(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?

The reason for the question was to see what "was normal". I just looked at mine and it is like Ed's ... rough on one side. But then it is a Grade B import. But no one has suggested that if I had purchased a "xxxx" then both side would have be ground flat and parallel so maybe they do not exist outside of NASA.

Which makes me wonder about the sides? They are ground. I guess I will have to test it to determine if they are flat and parallel.

And for those wondering why ... I have an 8 inch precision level and 11 inches between the two items I want level. The granite plate is the closest thing that I have that is flat and ridgid enough to span the gap so I can then then use the precision level to make it level.

But my next plan of action is to level the granite plate with the precision level, then use that as a reference to see how far off my aluminum levels are, adjust them so they are accurate enough to use across the gap and then call it done. The aluminum levels pass the "flip it around test" but I am interested in the last little bit of accuracy.

And the whole thing is more of a mental exercise in the possibilities for precision than a requirement of the task ... and curiosity about what “normal” was.

[Mayhem]

Ed that is interesting information. Thanks for posting. For me it makes sense that they only finish one surface (thinking cost). I don't have one but I wonder how many people support them only at three points? I was surprised to read that failure to support a plate correctly could lead to distortion. I have seen some that size of a dining table and it is hard to believe it could lose its accuracy.

[Tony Wells]

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

[ETC57]

(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/artic...icle.shtml

Thank's Tony and how ya doing?

Jerry.

[Tony Wells]

You're welcome, Jerry. I'm doing pretty good. Getting around to all my favorites places to visit this evening. How are you?

[ETC57]

(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?

Not bad actually, the ankle is healing and I'm getting some shop time so all is well.

Jerry.

[EdK]

Here's the response I got from Starrett.

"Hello,

Your online inquiry was forwarded to me.

Our granite plates are ground on the top & bottom. However, only the top surface is finished to a specific grade. The bottom of the plate simply has a machine ground finish.

Thank you,"