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  • From: Lawrence London <lfljvenaura@gmail.com>
  • To: machinist@lists.ibiblio.org
  • Subject: [machinist] Requirements for surface plate support?
  • Date: Wed, 5 Jul 2017 11:20:06 -0400


Requirements for surface plate support?
http://www.practicalmachinist.com/vb/general/requirements-surface-plate-support-335859/

Read this thread for a diagram for laying out the airy points for supporting your plate...

Square surface plate Airy Points

Thread: Square surface plate Airy Points

http://www.practicalmachinist.com/vb/metrology/square-surface-plate-airy-points-270949/

I have acquired a 36" square (6" thick) Rahn surface plate. Are the mount pad locations for a square plate calculated the same as for a rectangular plate?
Yes. A square plate is rectangular by definition. The support points are 1/4 to 1/5 of the side dimension.

The mount pads have to be, where they were during inspection and fine-tuning the inspection plate. Not where you think they should be (unless you want to repeat final scraping/lapping).

The reason for using three support points at defined locations is to duplicate the factory environment, so the plate will meet spec.

Surface plates bend, even thick ones. Proper support at defined locations is critical to proper performance.

The mounting points should be marked for you for the reasons that Nick describes. You probably can't see this marking until you get a good look underneath. I'm not sure about the really low cost Chinese? granite plates but others that I have seen have such marking. Small deviations from these marked positions should not make a measurable difference to the plate, but if you get them wildly different it could.

Got a chance to look closely at the plate today. It is supported at three points. Didn't measure but it is a Rahn plate so I suspect they are correctly located. The plate is coming off the stand when I get it to the shop. I intend to fab leveling screws at the airy points. Just as a wag I'd estimate the points are at about the 1/4 distance value.
BTW. I just started a thread looking for surface plate data reduction software.......


Granite surface plate airy points are not important in an unloaded or lightly loaded surface plate as a specification-meeting plate deflects an insignificant amount (for all practical purposes) due its own weight even when "improperly" supported. You will learn this is true if you read and calculate enough and I tested this out on a plate with a friend who had very fancy measuring equipment. (Before someone starts shouting about the fact that EVERYTHING is flexible, please note the word "practical" in the prior statement.). Airy points do help manage deflection with significant loading. If you review prior threads, the appropriate specifications are cited and this whole discussion has been thoroughly reviewed and debated and reviewed some more.

You want to have your 3 points 1/5 to 1/4 in from the ends. I just had a frame made for my by tru stone for my 48" x 96" and they used 1/5.

I as well just had to modify the plate for my 72" x 144". I used this same method. That plate calibrated to within .0008", the guys were tired and did not want to go more as it was in spec. I have asked them to come back and it will be interesting to see if it held before they start again.

A 24' x 36" could easily be supported by a small table or even rolling workbench.

https://www.instagram.com/p/BMEbf1RAWTU/
We are modifying the base for the 72" x 144" granite I bought a while back. The previous owner had it set up on a 12 point system with jack screws. We are changing this to the proper 3 point system per the Federal Spec GGG-P-463c.
I can not find any documation to support the idea to have more points than 3. I know some on @practicalmachinist forum have used 5. But I can find no data on how to space using 5 points.


A .pdf file: Federeal Specification: GGG-P-463C 1961
plate, surface, granite (inch and metric)


Inline image 1

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  • [machinist] Requirements for surface plate support?, Lawrence London, 07/05/2017

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