Supporting a surface plate
I have two small granite surface plates , and I'm wondering how much effort I should put into stands or supports for them. No stand, or bench surface, is going to be as flat as the plate itself, so in theory I suppose the plate is designed to be rigid in spite of an uneven support. Even so, perhaps placing the surface plate on a reasonably flat area of my bench, and cushioning the underside with a thin layer of soft material might spread out the support area and help the plate to remain flat. Anyone have experience with this issue ?
What size's are they? Most of the time just an angle iron support is the norm.
Surface plates should be mounted on a stand with a 3 point support. Usually 3 rubber bumpers.
I recently made a rolling supporting table for a 6" thick 36 by 48 table. I used this blue deformable synthetic rubber. 1" 4 by 4". Its for vibration damping. Dont remember the name, maybe epdm or vinyl chloride, but you can buy blocks of it. I got it from some 2007 vintage process machines I had to disassemble. It deforms just enough to take up level variations in a welded frame.
Depending in size, most smaller plates can just be thrown on about any table and used. A friend if mine looked at the crystal structure of granite and indication was they are made thick enough to avoid nearly any deflection at all. I guess a 1000lb part on a 50sf table might need some forethought but smaller ones just are not that big of a deal. I have found though that it is nice to use the precision level to level out a plate so when you set round parts on it, they stay there..
There's no particular reason for an elaborate support for a granite urface plate unles you're working close to the plate's weight capacity where deflection is a factor or the resolution you require is in the 0.0001" range.
Surface plates are made to be supported on three points as was mentioned. On domestic and well made import plates the support points are either marked on the bottom or there are pads bonded to the underside. Scraped cast iron surface plates have three cast legs. Cheap import plate are seldom marked for support so you have a 50/50 chace of supporting it correctly if you guess at it.
In the absense of the standard (GGG-P-463c) a rough rule of thumb for support location suffices. On one end locate the pad centered in the width and 1/5 the length from the end. On the opposite end locate the support 1/5 the width from the long edge and 1/5 the length from the short edge. If your surface plate is in a calbration program subject to third party inspection you will have to support the plate according to the standard. If the support point pads or marks absent or missing alltogether, they will have to be re-establihed either at a calibration lab or by personnel qualifed with an autocollimator or differential electronic levels of 1/10 arc second reolution.
(segue) If you have a calibration cert for your surface plate keep it in your equipment history files. This file can be a folder in a drawer, a looseleaf notebook, a clipboard, or a manila envelope. Whatever form it may take even a small home shop should have some form of record-keeping for machine manuals, major purchase receipts, correspondance, notes, calbration records etc. (back to topic)
It's seldom mentioned but a three point support arrangement is a bit tippy. An indexing head (for example) slid to the end of a smallsh plate may overbalance the plate on its pads. I strongly suggest anti-tip jacks under each corner of the plate. Adjust them with about 0.020" clearance and also provide some sort of retraint so the plate can't shift sideways.
There's no need to level the plate unless the work you perform on it is thereby facilitated.
All that said, most intallations are safisfied with a piece of carpet between the surface plate and a stout work bench.
Theory doesn't matter. In practice it will be flat if supported by three points. Surface plates were supported on three pads when it was made. It needs to be supported on the same three points to be accurate.
Three support points under the plate define a plane (as in Geometry), so the plate will "sit" flat. Only reason for leveling the plate would be the same logic for leveling a mill. Things won't roll off the table if it's level, but it doesn't need to be leveled to an extreme.
Last edited by reggie_obe; 01-16-2010 at 10:14 AM.
The Federal Spec that Forrest mentioned, GGG-P-463C Section 3.2.5, specifies two support points located 1/4 to 1/5 of the plate Length and Width from the corners, and a third point at the other end, midway across the width and 1/4 to 1/5 of the Length from the end, as shown:
The single support should be at the end with the identifying label.
Thanks, Leigh. My copy of Fed Spec GGG-P-463C is on the hard disk of my dead computer. I forgot the pad location to the label