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Surface plate support quick question

Discussion in 'METROLOGY - MEASURE, SETUP & FIT' started by expressline99, Mar 15, 2017.

  1. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    Oh I know. I lived over there in the 90's and early 2000's. It's more of a let's try to over do it comment. :abnornal:At 10 Richter scale nothing survives. Probably much lower down the scale even.
     
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  2. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Actually I worked with a company near me that had a design that used a cross meshed array of pure Boron wires that functioned as bearings under each column supporting the building. The idea was to allow the foundation piers to move around while the building slide around on the bearings allowing the inertia of the building to keep it more or less stationary.

    And as far as all the "big guys" using it, they don't always go for the ideal solution due to cost constraints. That and ease and speed of installation. And of course people are creatures of habit, so once a method is established, it can be hard to change. I'm not saying that method would be perfect, but I think I see some advantages to it.
     
  3. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    There are very good reference works on surface plates and mounting them. If you 'fully' support the bottom by potting it in the floor compound, it will not be as per the calibrated accuracy. If you only care about thousands, and don't mind your plate being a few thousands out, then got for it! A good surface plate (not shop grade) will be locally accurate in tens of millionths of an inch, and over the entire surface in tenths of a thousand of an inch.

    I am trying to answer the question -correctly-, there are a lot of ways to 'make do'. A pivot is on one side of the plate to ensure that all four contact points have exactly the same pressure on them. This way the plate will experience minimum flex if they are on the 'Bessel' points. I prefer to build once, the best I can, so I don't have to do it over. Oh and yes, an improperly supported plate will flex and you can see it on a tenths indicator.
     
    Last edited: May 23, 2017
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  4. Ulma Doctor

    Ulma Doctor Infinitely Curious Active Member

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    (sorry OT)
    i worked on a crew doing a few seismic retrofit projects.
    on one job in San Francisco at the PG&E building, on Market St, i was out there for eight months.
    we took a 17 story building and one at a time chopped out a 6' hunk out of the base of each column and inserted seismic vibration dampening devices , kinda like giant hockey pucks.
    the work was the most interesting stuff i have ever been a part of.


    as far as supporting your surface plate on 3 points, it's a sound principle and i use the method myself
     
    Last edited: May 24, 2017
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  5. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Dabbler, I was only thinking out loud and rambling. I'm a firm believer in following the Federal guidelines for mounting plates. I've been around them quite a while and some large enough to park cars on. And even cases where more than one plate had to be aligned with another accurately enough to be used as one. I'm very familiar with them.

    If one were to be bedded/potted, it would have to be lapped back into flatness specs anyway since that's not how it was held for manufacture, and I doubt temp variations in an average shop would let you get away with it to any great precision. A cleanroom maybe, but since there are established support standards that everyone accepts, that's what should be used. They all move when you load them anyway, it's just a matter of how much and whether it makes it possible to maintain the desired accuracy. I've done some testing with instruments on plates with and without loads and found that they move more than most people probably think.
     
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  6. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    Tony,I misunderstood your previous post as a recommendation - I apologize for sounding heavy-handed. The friend in my above comment is a highly skilled toolmaker that ignored the recommendations that you endorse. It is all too easy to follow the recommendations, if a little picky.

    I agree that if you do an alternative, some recalibration will be required. I've heard of aligning plates to use as one, but have never seen it in any shop I've worked.
     
  7. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    It might make for an interesting experiment someday, if I get the instruments again. I kind of doubt it, but in an odd sort of way it makes a bit of sense to me to give it 100% support and have it lapped. I may shoot an email to a couple of old friends and see what they say about it. Obviously it's nonstandard, but there is something nagging in the back of my mind (where it probably ought to stay) that if you used some high durometer/Shore RTV about 1/2" thick to set the plate in, it would spread any load over a larger area than the standard points. I know for a fact that if you load the corner on the single pad end of a plate, it moves the entire plate. It would seem to me that there would have to be a positive effect to have some support under the whole plate. Setting it in while the potting is still setting would prevent uneven pressure from underneath, so how could it not help? What am I missing?

    But, I'm just rambling again.
     
  8. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    If you were to try the potting experiment, it would be vital that the foundation under the RTV would support the RTV evenly as well... Perhaps a network of vertical ribs?
     
  9. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Well, yes and no I think. That's where I get a little unclear about it. Since the objective would be to distribute the weight away from the typical 3 (or 4) points, you would have to gain some ground on how much the plate distorted under load by spreading the weight. It would definitely be different from needing a strong support under a single one of the 3 (or 4) points. In my mind, a reasonably heavy steel plate with perhaps 4 equally spaced angle, channel or I-beam cross bars would be needed, depending on the size of the plate, of course. I could see that quickly the construction complexity and expense would exceed the normal approach. What I am mulling over is whether it would actually be superior in any way. I'll have to keep thinking about it.

    And then there is another angle. Instead of "soft" (relatively) RTV, use something self leveling, but pourable, like concrete or maybe just portland cement. Sort of like grouting in a machine. That wold give 100% support, conform to the underside of the plate, and not be very flexible.

    Mentally, I may be simply trying to get around using soft supports on the common points, knowing that they do allow some weight driven distortion. The obvious answer is to just get a thicker plate.:)
     
  10. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    I once found a 'free' surface plate on the internet.... > 2000 miles away, weighing 12t. --but it was 16" thick! Of course I passed. Having had a little experience in ferrocement boats, concrete flexes more than you might think. Perhaps a solid concrete base, say 30" high? Now that would be rigid!
     
  11. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Yeah, shop planning would become paramount. Kinda tough to move a plinth like that.

    Oh well, I just enjoy metal exercise and hypothetical situations.

    That "free" one would not have stayed free when you hauled it!
     
  12. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    What about upgrading a good plate by using RTV to support it with an old worn out or cheap shop grade plate? Doubling the thickness will halve the deflection, no matter how it is supported.
     
  13. ezduzit

    ezduzit United States Active User Active Member

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    This is about the only way I see to make the bedding idea (possibly) work. Otherwise the surface plate is being used to try to stiffen an inferior structure. But it would likely create an even bigger problem with the 2 bonded plates acting as bi-metals and each warping the other from thermal changes.
     
  14. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    Seems you'd need to use a powdered granite epoxy slurry to bond them, and scarify top and bottom. By the time you went through all that you could probably buy a nice, thick Starrett pink:)
     
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  15. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    touche.
     
  16. projectnut

    projectnut United States Active Member Active Member

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    Most cast iron surface plates come with a 3 point bearing pads. I have 2 surface plates, a 10"x18" no name, and a 24"x36" Challenge. Both have 3 load bearing points built in. Here's some text from the Challenge Precision website concerning their cast iron surface plates:

    "Three point surface plates provide a precise reference surface for tool making, layout, or checking the accuracy of other surfaces. The three point bearing pads and rigid ribbed construction permit support by less accurate surfaces without disturbing the surface
    accuracy. Wooden cover supplied with each plate."

    Here's a link to the Challenge Precision cast iron surface plates:
    http://www.challengeprecision.com/products/plates/threepoint.htm

    The 24"x36" plate came from our machine shop at work. It weighs 475 lbs. It has spent it's entire 50+ yearlife on an angle iron stand with a 3/4" plywood top. You would have to run into the stand with a fork lift or Mack truck to tip it over
     
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  17. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    So do they diamond lap cast iron plates? Or how do they get them to the proper grade? I have a decent understanding of how they do this on granite.

    I can't believe they have handles for those huge plates... there is even a handle for the 900+ pound version. Must be machine lifting points?

    Paul
     
  18. Wreck™Wreck

    Wreck™Wreck United States Active User H-M Supporter-Premium

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    My hovercraft is full of eels.
     
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  19. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    Since 2008, Standridge has been using 3 point instead of 4 point supports (which it did previously) ... Now this has gotten me really interested! I'm going to call Standridge tomorrow and see if I can find out why. If it makes sense, I'll relay the conversation here.
     
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  20. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    That's curious.
     
  21. projectnut

    projectnut United States Active Member Active Member

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    I'm not sure how the "precision ground" surface plates are finished. I do know a hand scraped one costs about 1/3 more than the same size precision ground one. The list price for a 24" x 36" precision ground one is about $2,000.00. The same size hand scraped one is about $3,000.00. As for the handles, there is typically a lifting tool that slips over them. The tool is then raised by a crane or lift. Here's a link to a smaller plate available on e bay. Note the lifting tool that comes with it.

    http://www.ebay.com/itm/CHALLENGE-1...urface-Plate-Very-Nice-W-Hanger-/351891804736
     
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  22. BROCKWOOD

    BROCKWOOD United States Active Member Active Member

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    Short answer is the table that the surface plate & it's '3 point rest system' can sit upon any table supported with as many casters as you like. The 3 point leveling should only be of such height as to avoid any interference from said table top. 2" should be plenty. Carry on!
     
    Last edited: Jun 12, 2017
  23. ezduzit

    ezduzit United States Active User Active Member

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    That would be 50% more. ;)
     
  24. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    Okay, here goes: I called Standridge Granite today, and talked to one of the technical people... Apparently in living mempry they have always used 3 points to calibrate and mount surface plates. My information from a website misidentified Standridge where it was another company.

    Now for why: This all goes to a Federal standard. If you do work for the government (military, etc) you must use a calibrated surface plate that meets very specific criteria - chiefly that it uses a 3 point support system. Standridge and others calibrate and prepare surface plates to use this system. For extra money, Standridge will calibrate to 4 points - which is not allowable in the Federal standard.

    Mathematically, 4 points is better. From a business POV, 3 is better. For a small shop, what-you-want. My new plate will be calibrated and supported on 4 points. My Mitutoyo, it turns out, must be on 3 points (!).

    There is a lot of good stuff on cleaning and maintenance in the standard, so by the grace of Standridge (who sent it to me) here it is:
     

    Attached Files:

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  25. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    OK guys my 24x24 plate is on it's way from Standridge. I need 3 levelers to build into the top of a stand I'm going to make for it. You guys may have pointed me to a place to buy them but I can't find it for the life of me. .125 walled box tubing should work for the stand shouldn't it?

    Paul
     
  26. Tony Wells

    Tony Wells United States Vice President Staff Member Administrator

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    You can use machine leveling screws upside down, and they are available with rubber cushions or solid delrin type plastic. I would avoid that because od the slippage factor. The rubber feet would grip and not allow the plate to slide around on the mounts. Easy enough to weld nuts the crossmembers of the frame. And yes, that tubing is sufficient for your stand. You gave only the wall, but I am guessing it's at least 1 1/2" square.
     
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  27. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    Sorry about that. Yes I was thinking either 1.5" or 2.0" square. I'm watching Shandon HKW make his now. Looks pretty straight forward. Also he mentions there are stand instructions included on the back of the Standridge certification.
     
  28. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    It occurred to me today that leveling my plate could be a challenge as my garage drink/beer fridge has been out there for years and is so out of level the door swings open after you let go.... :concerned::eagerness:
     
  29. Dabbler

    Dabbler H-M Supporter - Premium Content H-M Supporter-Premium

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    If you aren't doing static balancing on it there are a few woodworking (yes, woodworking) levels that are accurate to .004 per foot. I ended up buying several over the years, but I have now forgotten which of my woodworking levels are that accurate! (I guess I could check if you can't find one). Buying offshore machinists levels have some problems: the one I bought was so far out and nearly impossible to make right. It took a bunch of hours to free the gimbals, and more to calibrate it, due to the very coarse adjustment screws.
     
  30. expressline99

    expressline99 H-M Supporter - Premium Content H-M Supporter-Premium

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    Bob has offered to lend me his level short term to level the lathe once I get the legs and the pan together. Although like the plate I'm leaning towards buying one. I'll have to level the mill soon as well. So I will need it several times. Sounds like enough to warrant buying it. ....and maybe a 4"...a 6"...an 8"...a 12" :)
     

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