Requirements for surface plate support?

Cannonmn · May 29, 2017

Getting ready to mount a 2'x3'x8" granite surface plate on something appropriate, and need to know minimum requirements, if any, beyond being strong enough so it won't collapse. For example, are "quality" granite surface plates made so they'll be just as flat when hung from both ends only, as they are when supported uniformly underneath? I'm asking because I have a great table in mind for it but I'd bought the table in order to mount some nice bench drills or other machinery and don't want to sacrifice it to hold a big rock if said rock would be just as happy on an old, beat-up wood workbench. Here's a pic of the 600-lb. table which I think may be overkill. It is shown on its way to be mechanically de-rusted.

dgfoster · May 29, 2017

This has been much discussed in the machinery scraping and inspection subforum.

Denis

swatkins · May 29, 2017

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

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

Milland · May 29, 2017

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

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

Do not follow the advice about using inner tubes or other soft suspension methods. The mounting point pads should be quite hard rubber or plastic and not compliant, as when you put a load on the plate you don't want it tilting out of level.

Area of the pads could be around 3-5% of the plate area at a rough guess, but if the plate manufacture had pads in place that are now missing they'll sometimes leave a mark on the underside, use that to size your pads.

Cannonmn · May 29, 2017

dgfoster said:
This has been much discussed in the machinery scraping and inspection subforum.

Denis

Thanks, would take a look but I can't find that subforum despite looking for quite a while. Maybe someone can post a link to it.

swatkins · May 29, 2017

http://www.practicalmachinist.com/vb/machine-reconditioning-scraping-and-inspection/

dgfoster · May 29, 2017

Cannonmn said:
Thanks, would take a look but I can't find that subforum despite looking for quite a while. Maybe someone can post a link to it.

Just put, for example, "airy points practicalmachinist.com" in Google. You will get a lot of hits. Don't try to use the search function on the PM website as Google is much better.

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.

Denis

jrmach · May 29, 2017

gravity and the Earth constantly moving I would try and solve first

Spencer in NH · May 29, 2017

http://www.tru-stone.com/pdf/FedSpecGGG-P-463c amendment 1.pdf

atomarc · May 30, 2017

I didn't read the links supplied so this may not pertain. I just purchased a plate the same exact size you mention. It's shipping weight was 417lbs so I'm sure your table is extremely capable..and then some! The plate I bought (Precision-AA) had three points permanently marked on the bottom where the pads are to be installed. Does your plate have these factory marks on the bottom side?

Stuart

Cannonmn · May 30, 2017

atomarc said:
I didn't read the links supplied so this may not pertain. I just purchased a plate the same exact size you mention. It's shipping weight was 417lbs so I'm sure your table is extremely capable..and then some! The plate I bought (Precision-AA) had three points permanently marked on the bottom where the pads are to be installed. Does your plate have these factory marks on the bottom side?

Stuart

Thanks, hope it has them, never looked underneath but will do so next weekend or so.

Cannonmn · Jul 4, 2017

Cannonmn said:
Thanks, hope it has them, never looked underneath but will do so next weekend or so.

Finally got around to lifting the plate and took a good look underneath, but found no distinct marks unfortunately, however have calculated where the three points should be and unless better info found, I'll support it there. Present plan is to use an aluminum table with top dimensions close to footprint of plate. To ensure the three points stay nearly co-planar the aluminum table will be topped by a 1" thick steel plate somewhat larger than the triangle formed by the airy points. The three support discs are then placed on the steel and the plate set down on them. Then the table will be leveled. Before all that I'll talk to the maker and see if they have the locations of the airy points used when this s/n plate was certified. Last pic shows a helper next to the plate, lifted by lifting Straps around ends, a method that works well for us.

SteveBausch · Jul 4, 2017

Buddy gave me a steel table, 24 inches square, or less.

It's nothing special, just about heavy gauge steel shelving.

Said it was originally for a surface plate.

Cannonmn · Jul 5, 2017

The tags on the plate pictured above are interesting. The plate left the maker as a grade A (the A stamping looks like an overstamp?) then in 2014 calibration at a 3rd party company only made grade B. So I'll be calling both that lab and the maker to see if I can find out anything about how it was mounted. I'm guessing the lab would have wanted to calibrate it as-mounted for use. I found one post above interesting (or maybe on one of the linked threads) that stated something like "only plates that would have significant loads need to be mounted on three points." He didn't define "significant loads." To me, that sounds like a load the average man can't lift, and would exclude about 99% of the things we'd measure on our plate.

cash · Jul 5, 2017

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.

I would keep that nice old drill press table for a heavy duty workbench.

Instagram

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

Cannonmn · Jul 5, 2017

cash said:
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.

I would keep that nice old drill press table for a heavy duty workbench.

Instagram

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

I came to same conclusion that the massive DP table was overkill, so changed to the small custom aluminum table mentioned; I liked the rolling idea but might make harder to keep level, so if the rig needs moved we'll just roll the die lifter's platform under it and lift a bit until it all rolls.

cash · Jul 5, 2017

Cannonmn said:
I came to same conclusion that the massive DP table was overkill, so changed to the small custom aluminum table mentioned; I liked the rolling idea but might make harder to keep level, so if the rig needs moved we'll just roll the die lifter's platform under it and lift a bit until it all rolls.

We have several tables we roll around. say unless you are building machine up on them or something that must be level tot he world, the overall level really does not matter.

Only my big one in the shop is level. The rest sit as they sit.

Cannonmn · Jul 5, 2017

Here's a REAL surface plate. Use the 11" wide taped-on papers to get size. I was told it weighs about 20 tons. Looks like it was made to be hung from ends. Wonder if it has those three support points underneath? Wonder if it matters? This is in a large-part precision grinding shop that does work for aerospace companies.

dcsipo · Jul 5, 2017

Cannonmn said:

Was the crappy room built around the plate?

dee
;-D

swatkins · Jul 5, 2017

Cannonmn said:
Here's a REAL surface plate. Use the 11" wide taped-on papers to get size. I was told it weighs about 20 tons. Looks like it was made to be hung from ends. Wonder if it has those three support points underneath? Wonder if it matters? This is in a large-part precision grinding shop that does work for aerospace companies.

You try hanging that from the ends and you will end up with a two chipped off ends and the plate on the floor...

Those are ledges for clamping fixtures, angle plates, straight edges and other items to.. Some people pick up smaller plates using those ledges but on the plate shown that would not work so well...

Requirements for surface plate support?

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