Modifying a surface plate stand

SirRage · Jun 5, 2017

I recently purchased a 36x48" surface plate with a stainless steel stand on casters from a tool liquidator. The surface plate was a little pitted but not too much from what I can feel. I'm planning on getting it calibrated again, but while transporting it I realized there was a problem with how the stone was being supported. The stand doesn't have cross members to support the stones three contact pads. Well, there are 4 contact pads, two of them are button up against one another to act as one contact point.

So I need to add those cross members, I think I'm supposed to weld nuts into place and use those to level the stone. But seeing the stone is on casters I don't think leveling is an issue. Also thinking about putting a support brace in the middle of the 4' span, so the weight is more evenly distributed. The stone does weigh 1200 lbs but whatever is on top of it.

I'm assuming its made from 304 stainless so I'm wondering if I should be using stainless for the cross member or can I get away using hot rolled steel? Stainless is pretty expensive once I started pricing out what I would need. 2" angle, 0.375" tick.

Adding photos:

View attachment 200432 View attachment 200433 View attachment 200434 View attachment 200435 View attachment 200436

It's actually two inch, quarter inch walled angle iron.

ewlsey · Jun 5, 2017

If the stand is supporting the plate at the same points that it was supported when it was originally lapped, it's probably OK. Can you post a picture?

SirRage · Jun 5, 2017

Sure I can post some pictures when I get home. If I can get away without modifying anything that would be great.

swatkins · Jun 5, 2017

You support the plate on Airy Points.. Do a google search for this term "Airy points surface plate www.practicalmachinist" and you will find a recent thread with a diagram on where the supports are suppose to be located.

SirRage · Jun 5, 2017

Ah, well I got the airy points from this document:
http://precisiongraniteusa.com/pdffiles/4919 INSTALLATION INSTRUCTIONS FOR SURFACE PLATES.pdf

Because it's a half inch thick 3-4" wide pad if I should just have them rest on the angle iron or if I should put something like a half inch screw and nut there for them to rest on. I don't think its standard to put two pads in the front insted of one but maybe it is given the weight of the surface plate. It does weight 1200 lbs. My truck was glad to be rid of it when I finally got it out.

Currently it's a on a makeshift pallet I built out of 4x4 and 2x4.

Mark Rand · Jun 5, 2017

People keep doing that...

If you are going to specify where to support a plate for best flatness, it's the Bessel points not the Airy points.

The Airy points result in the ends being parallel, the Bessel points result in the closest approximation to flat.

<PEDANT MODE>
It's worse than that Jim:-The minimum deflection occurs when the deflection at each end is the same as the deflection in the middle of the beam. For a Euler-Bernoulli beam, this minimum deflection occurs with supports placed 0.2232 L from each end.
</PEDANT MODE>

ewlsey · Jun 5, 2017

The difference between the Airy points and Bessel points is 3%. On a 4' long plate, that's .72". If you can measure a difference in flatness by moving the mount 3% on a granite surface plate, I'll fly over to the UK and buy your lunch.

Mark Rand · Jun 5, 2017

No excuse to use the wrong terminology though...

ewlsey · Jun 5, 2017

You did warn us of the impending pendanticism...

SirRage · Jun 5, 2017

Ok here are the pictures. So the frame is made from two inch angle iron, quarter inch thick wall. Some of the wellds on the frame show under cutting. I didn't take a picture but it was something I noticed, again after the stone was off.

I am being very carful around that pallet because that is a lot of weight to come crashing down, even if its just 8 inches off the ground.

ewlsey · Jun 5, 2017

It might work if you put a cross member to contact each pad. Looks a little spindly to me.

I bought my plate mostly because I like the stand. It's made by Rock of Ages. Basically they use 4 plates to make the stand. 2 plates are the same size as the granite and one sits right below the granite, and the other sits on the floor. Then the other 2 plates are vertical and they are lined up so that they form an "X" at the single support point and the long legs of the "X" each pass under the other 2 support points.

CalG · Jun 5, 2017

The stand for my 4'X5' DoAll is but 17 inches off the floor. Add to that the thickness of the plate, and it's a very comfortable height for larger work. A bit low for small work, but for that I keep an 18X12 granite on the bench.

Does anyone else feel that some triangulation would be good, the thing is on wheels!

ewlsey · Jun 5, 2017

One has to also consider the weight of the objects you might put on the plate. I know I've craned objects weighing a ton or more onto larger plates.

Monarchist · Jun 6, 2017

CalG said:
The stand for my 4'X5' DoAll is but 17 inches off the floor. Add to that the thickness of the plate, and it's a very comfortable height for larger work. A bit low for small work, but for that I keep an 18X12 granite on the bench.

Does anyone else feel that some triangulation would be good, the thing is on wheels!

My grade A Herman 30" X 48" stand has four iron wheels, but.. a step-on / toe-release jack at one end, mid-beam, that turns it INTO a 3-point once put where one wants it to be. No provision at all for leveling, though.

BFD. Only have a couple of projects where it needs moved anyway. Can just shim the 'usual' location, each work area.

Nice thing about the 30" width is it easily clears the 3-foot fire doors between chip-making area and layout/metrology/tool-storage room.

Monarchist · Jun 6, 2017

ewlsey said:
It's made by Rock of Ages.

Got a tour of the Rock of Ages quarry and finishing operations as - what a six year old? Dad's interest, not mine.

Rock of Ages were advertising tombstones in magazines we took at home.
That age I hadn't a klew what a 'surface plate' was.

Went puzzled for YEARS thereafter as to whointhe Hell rated a pink granite tombstone six feet wide, ten feet tall, a foot or more thick!

Later figured it had to be Douglas MacArthur - just to keep his ego from getting loose.

lienjohn · Jun 6, 2017

SirRage,

I'm glad you posted those pictures. My Starrett 36x48 plate also has the two rubber pads touching. I thought it was a mistake. Maybe that's the way they do it or we both have messed up plate supports.

Thanks, John

Monarchist · Jun 6, 2017

lienjohn said:
SirRage,

I'm glad you posted those pictures. My Starrett 36x48 plate also has the two rubber pads touching. I thought it was a mistake. Maybe that's the way they do it or we both have messed up plate supports.

Thanks, John

Herman plate, same deal.

I "suspect, but cannot (be bothered to) prove".. that the game is to have about the same mass resting on a given surface area of elastomeric "puck", such that the single support at one 'end' of the pattern does not find itself unduly compressed because it is in the 'right place', but has twice the mass to carry as TWO 'pucks' at the other end.

JM2CW

The real Leigh · Jun 7, 2017

The proper surface plate support points are as shown in this diagram from the federal standard:

Note that these are the support points on the underside of the plate itself.

They have nothing to do with the number of legs the support structure has.

- Leigh

Modifying a surface plate stand

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