Ideal Grade for Surface Plate used for Scraping
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    Default Ideal Grade for Surface Plate used for Scraping

    Hi, just picked up an old 3'x5' Mojave surface plate with a nicely welded steel base. I did a search and could not find any information on what the ideal grade for a granite plate used for scraping would be.

    Is the highest grade the best? The plate is in good shape, but does have some dings on the surface.

    Having my plate calibrated and lapped by Standridge this week and I wanted some help determining the grade it should be lapped to, i was thinking A.

    Thanks,

    Marco

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    Smarter people than me will comment but, in broad strokes, extra accuracy is not a problem. I’ve always been told that having a AA plate requires temperature/humidity control (laboratory). Unless you have that kind of setup and demand that level of accuracy, it might be a moot point to go more than A (inspection grade).

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    Depends on how mouch money you want to pay, lapping to AA takes longer then to A.
    Temperature is not the big problem when you are looking for flatness. The most important is the differenc of the Temperature under the plate schoud be the same as obove the plate.
    The temperature differenc in the Plate over the thickness schoud be nearly zero

    Greatings Franz

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    Yeah I think the point that medsar was trying to make is that unless temperature where the plate is kept is always constant and controlled, there will always be a temperature difference as the room constantly heats and cools. It takes quite a lot of time for temperature to change and stabilize in a big slab of granite. So something like AA would be way overkill if the plate is kept in a garage or anywhere without a stable temperature... Outside of a specific clean room type of environment, a very good place would be a basement in a house with A/C and heat always set to a similar temp level. A temp controlled shop would be good too of course. Away from windows and open doors ideally.

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    At the end of the day, it needs to be better than what you are scraping on. I was happy with my second hand 36"x48" surface table when I rebuilt my milling machine, but I know (from autocollimator measurements) that it won't be good enough to rebuild my surface grinder. hence, it'll get re-lapped and calibrated to an appropriate flatness before that job.

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    Probably just about any plate will be better than what one is scraping on short of a surface grinder or jig grinder, etc. So yeah, even grade B will probably do fine for most stuff.

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    Best to talk numbers to see if OP’s needs grade A or B plate as a scraping ref. For his proposed size Federal ref states total flatness will be within 5 tenths if grade A. Can be twice that tolerance at 1 thou for a B class.

    If I was doing, say a Bport, I think I’d want an in spec grade A. IMHO, less chasing one’s tail with inconsistent prints is worth every penny spent on a better plate.

    L7

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    Two things.
    First, I think that what Franz intended with his comment is that factors light irradiation from lighting could introduce differences of temperature between top and bottom of a surface plate even in a temperature controlled environment: if temperatures vary a bit, but you have good "ventilation" both top and bottom would be exposed to the same amount of heat or would lose heat at the same rate, making it less of a problem for the plate bending.

    Second, the specs for grading a plate define also the maximum deflection when loaded with a specific weight at the center. Therefore, even if you were to lap a thin slab to AA standards, it cannot be graded more than B (or at most A) due to its flexibility.

    If I recall correctly, pink granites flex more (i.e. you need a thicker slab at parity of grade) than black ones.

    Paolo

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    Yeah I think pink is harder and more wear resistant, black is stiffer.

    In terms of temperature change, unless your plate is in a fairly regulated area, it's almost certain there will be a temperature gradient from the top to the bottom of the plate - just as there will be in the room as the air stratifies unless it's kept moving pretty vigorously. I read somewhere that a one degree difference from top to bottom of a 36" × 48" × 8" plate will throw out the top surface by something like .0003" - which would make something like AA way outside its tolerance. Just something to keep in mind no matter the grade of plate.

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    I used to sell plates for Tru-Stone of St. Cloud MN that Starrett bought. I would say buy what you can afford. I prefer the closer the better. Be sure they mark where they place the 3 - points on the bottom and you need to be sure to place or set it on those 3 points. I have ordered new plates for my shops and I always spend the extra $ and get AAA Lab grade. Spend the extra money and have them give you a print out chart. You can buy off the net or from Moore Special Tool a GREAT book on precision - The Foundations of Mechanical Accuracy by Wayne Moore that explains temp in shops and plates.. Another thing many people make the mistake by not rotating a plate no matter what grade. Most set it against a wall and only use that side in the middle. That area gets worn. Pink wears less but it is more brittle. I buy Pink. They can fill the chips if they are to deep. As far a temp goes. The parts on the plate change faster then the plate. I would be more concerned with a cast iron straight-edge being placed on a granite plate then a thick granite surface plate. As others say, don't set it next to a window or under a heater, next to a overhead door that is opened when it is 30 below F, common sense. Be sure to have a good solid stand too. Ask the experts at the granite lapping company.

    Publications at Moore Tool: Precision Machining Technology, Precision Tools

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    Thank you all, I will probably go with having it lapped to A, but who knows it could be an A already. Its a 36x60x8 black granite from Mojave, its on a very nice solid-steel stand.

    I spent a few minutes getting her level yesterday, I believe the stand is called a modified three point, four points, but two are right next to each other. My Starrett level didn't very much at all at the extremes, I leveled her in the center of the stone.

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    What is the price difference between A and AA?

    I use an AA in a non-temperature controlled shop. I find it is fairly repeatable if I am careful, i.e. I only trust it in the mornings because the temperature is somewhat constant overnight. Also position it away from heat sources. Not ideal, but there is some small benefit of the AA.

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    Its about a 50 dollar jump from B to A, then another 50 from A to AA.

    Cheaper to go to AA now instead of deciding to jump to AA from A in the future, as it will require another visit. My garage stays cool for the most part, the plate is located under my mezzanine, under the portion of the shop that really doesn't get direct sun. I plan on taking some temperature readings just to see what the gradient is.

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    Please take some photo's of the lapping and share it with us. It would be fun to see how it is done and tested. I have used larger rubber pads for the 3 points and use perimeter bolts around the edges to finger tight so if I set a heavy part on the edge it doesn't drop. Ask them about the 2 center points and let them decide as they are the experts.

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    Will do! Thanks for the help guys.

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    Quote Originally Posted by Richard King View Post
    I used to sell plates for Tru-Stone of St. Cloud MN that Starrett bought. I would say buy what you can afford. I prefer the closer the better. Be sure they mark where they place the 3 - points on the bottom and you need to be sure to place or set it on those 3 points. I have ordered new plates for my shops and I always spend the extra $ and get AAA Lab grade. Spend the extra money and have them give you a print out chart. You can buy off the net or from Moore Special Tool a GREAT book on precision - The Foundations of Mechanical Accuracy by Wayne Moore that explains temp in shops and plates.. Another thing many people make the mistake by not rotating a plate no matter what grade. Most set it against a wall and only use that side in the middle. That area gets worn. Pink wears less but it is more brittle. I buy Pink. They can fill the chips if they are to deep. As far a temp goes. The parts on the plate change faster then the plate. I would be more concerned with a cast iron straight-edge being placed on a granite plate then a thick granite surface plate. As others say, don't set it next to a window or under a heater, next to a overhead door that is opened when it is 30 below F, common sense. Be sure to have a good solid stand too. Ask the experts at the granite lapping company.

    Publications at Moore Tool: Precision Machining Technology, Precision Tools
    Do you know what they use to fill in the chips? A future project of mine is to lap my plate, it also has some chips that I haven't decided what to do with. My concern is that all products I can think of for filling in would be softer than the cast iron lapping plate and would possibly get grit from the lapping embedded in it. I know the chips don't affect the accuracy of the plate, but it's so many of them that it's sometimes hard to feel with my hand if there is dirt on the plate or if I'm just feeling the chips.

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    That was 30 years ago and honestly I don't remember. I could call them on Monday and ask them. I did Google it and there are several products out there. https://www.amazon.com/granite-fille...e+filler+epoxy

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    That would be great if you could ask them what they use. It seems to me those products listed are for kitchen countertops etc. in which embedded grit wouldn't be an issue.

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    i have a middle sized aa plate and every time i get out an instrument graduated to 0.25µ i wonder if im measuring the plate or the part. get it lapped to the highest grade they offer. in the long run it works out to less then 1 cent per day.

    get a map of the surface. e.g. mine was high in the middle, so i supported it on the four corners i hope its aaa by now.

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    I found this page and it has a lot of answers. Tru-Stone frequently asked questions: The world's largest manufacturer of custom precision granite
    Nothing on material for fixing pitted plates though. I will call them today and ask.


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