Surface plate stand

Hi

I’m going to need to make a floor stand for this 620×400 granite surface plate that I have. The commercial stands cost almost twice as much as the plate! no problem, I have an arc welder and I made quite a few stands. I’ll get some angle iron and bobs your uncle.

The question though, is what jacks to use. I’ve seen some commercial Jacks online at $2500. There are also some fairly common Indian machinists jacks, which are still expensive at about 50 quid if I want five of them and would need welding on. Can’t I just tap some holes in a mild steel frame and use some bolts?

If anyone has any tips or thoughts on making such a stand, would be appreciated.

Cheers.

Steve

Edited By Steve355 on 12/11/2022 08:04:45

Edited By Steve355 on 12/11/2022 08:08:43

Primarily a question of stability.

Simple commercial bot in a tapped hole is a bit wobbly and when locked up with a nut you can't be sure how much it shifts. You need a longer hole. Ideally, close tolerance bolts in a close tolerance hole. In practical terms a decent bolt will usually be 6g which will screw into a 6H tapped hole, 6H is the common tolerance for good taps, with minimal wobble. On a thread more than about 3 bolt diameters deep the pair will frequently go stiff and almost bind due to tiny pitch errors et al. Serial taps are better for this sort of thing.

Slightly techie description about what is going on :- **LINK**

https://www.boltscience.com/pages/screw8.htm

You may be overthinking things. My surface plate sits quite happily on a levelled bench top made from a cut down old style Civil Service lino type desktop. Advice from the inspection and science types at RARDE / DERA  said this would be perfectly satisfactory unless doing things that even Inspector Meticulous might consider excessively pedantic.

Hard felt pads under the mounting points would have been about all the "better" that might reasonably be considered. But my plate is cast iron so has feet at the support points.

I imagine your granite plate came with suitable support pads. Was told rubber, plastic and similar was not good. Felt being better long term.

Clive

Edited By Clive Foster on 12/11/2022 09:44:46

Many years ago at one company I worked at we had a similar size granite surface plate in the inspection area that was mounted on a stout timber frame, never encountered any problems and it was easy to move by forklift, not that you would need to do that once set up. Dave W

This recent paper looks promising: **LINK**

https://www.researchgate.net/publication/351113457_Simulation_of_Kinematic_Supports_of_Surfaces_Plates_for_Optimum_Flatness_Tolerance

There is no direct download, but I have requested a copy.

You will probably choose to ignore most of it, but there may be some inspiration within.

MichaelG.

A great deal depends on how accurate the surface plate needs to be. I'm at the lowest possible end: I don't own a surface plate at all, instead making occasional use of a pane of float glass plonked on my moderately sturdy bench. The problem is any weight placed on the pane and bench causes both of them to bend, so my surface isn't very trustworthy. But it's 'good enough' for my needs, at least so far.

Next step up would be to buy a small granite or cast-iron plate, probably new, and use that on my bench. Now I'm getting serious enough to worry that a second-hand plate has a bad history, and they are all untrustworthy unless calibrated. Calibration may not matter - for many amateur purposes, a sub-standard plate would be 'good-enough' because we work to relatively low accuracy, ±0.02mm, about a thou or worse. If so, any reasonably stiff unbending stand and a sub-standard plate in reasonable nick would do the job.

An ordinary practical approach isn't good enough if the full accuracy of a surface plate is really needed by the owner. Commercial surface plate stands are expensive because they're carefully made to support the plate accurately and not flex under load. The picture, pinched from Eley Metrology, shows a typical example:

It features a stiff stubby box-section construction (not angle-iron); levelling jacks; three pad mounting; and the blurb says the thing is made to close tolerances. As stands go they are particularly well-made, but this is for high-end measuring and may not be strictly necessary.

How good the stand needs to be depends on the accuracy required of the surface plate. In my case, not much, but a tool-room making jigs, fixtures and gauges needs considerably better, and a metrologist would go to extremes.

I think for most amateur purposes any reasonably sturdy frame will do unless one is doing something special. Therefore the jacks could be made from ordinary bolts, preferably fine threaded. Again, how accurately does the plate need to be levelled? My pane of glass isn't levelled at all. An ordinary builders level isn't sensitive enough to show jacking errors caused by bolt thread inaccuracies, but an engineering level would be easier to adjust with proper jacks with accurate square or buttress threads.

Metrology is 'quite interesting' because it gets exponentially harder to achieve improvements. Easy to measure to about 1mm, 0.1mm requires a coarse tool, 0.02mm is about the limit of ordinary engineering, and 0.001mm or better requires special equipment and precautions.

Dave

The stand in SOD's pic above looks like it has three-point support on what appear to be square pads, plus the two jacks that presumeably would be just brought up to meed the surface plate, to provide stability when heavy objects are rested on the edge of the plate etc, rather than to support it full time per se. You should be able to use large nuts and bolts suitably arranged rather than buy special expensive machinists jacks.

I cheated for my main use surface plate - It is sitting on a 4'x2'6 cast iron surface table...That plate is a 450x450 granite one. It doesn't have support point markings, so I figured that supporting if on a flat was the best thing.

My 2'x2' Cast iron plate (I use it as a welding table) is on a fairly weedy looking stand made form 1" box section. That supports the plate on its 3 feet, with additional 'anti tip' supports just off touching (like a feeler gage off)

Dave

It is worth looking at (12) on this page: **LINK**

https://www.starrett.com/precision-granite/precision-granite-faq

MichaelG.

3 points of support define a plane unambiguously.

Starrett plates have the 3 points marked during manufacture to enable the end user to support the plate as it was made. This removes potential changes because of the supports.

The bottom of my plate ( and I imagine many other Chinese plates) is not marked for where the supports should go. It is however quite flat, so I figured a flat thing on a flat thing is probably ok. The top surface checks out too as flat as I can reliably measure in an un temperature controlled garage so Im happy with it.

Dave

Dave S

I wasn’t arguing, just chipping-in to the general discussion

I thought it might be helpful that Starrett gives the preferred locations for the three points.

… sheer coincidence that my post followed yours

MichaelG.

Edited By Michael Gilligan on 12/11/2022 18:12:45

If you get your supports somewhere near to the bessel points ( the points where the factory would put the food pads) you're not going to go too far wrong. If the plate is supported around the edges rather than at those points then it WILL bend under it's own weight and certainly if you put a heavy part in the middle, but the amount is very very little for a home shop sized plate. I could see it happen with my autocollimator (1/2 second resolution) but only just. When you consider that most sensitive levels will be in the 2/5/10 second range this means that it's so slight that it would not affect normal work.

With an idle half hour, I played with the beam calculator I have used before.

Assuming the plate (620 x 400) is 75mm thick, and that granite has a Youngs modulus of 70,000 MPa* (about 50% of that of cast iron and a third of that of steel**), the calculator suggests that

With the plate treated as a beam supported at the 20% and 80% points (as per Staretts) of the 620mm length, a point weight of 1000Kg (a Tonne) dead centre will cause the centre to depress by 0.0027mm. At less than 400kg point load the calculator gives up and says no deflection (probably less than 0.001mm).

For any reasonable hobby use I suspect anything that supported the plate reasonably well would be good.

*Granite :: MakeItFrom.com

**This surprised me somewhat when digging around. A cast iron surface plate is apparently stiffer than a granite one!

Edited By Peter Cook 6 on 12/11/2022 18:57:14

.

Perhaps not too surprising, when you think that granite is a filled thermoplastic

… admittedly only ‘plastic’ at volcanic temperatures

MichaelG.

+1 for what Peter says.

If it was polished when flat rather than on the Bessel points supporting it in the proper manner using calculated positions will cause it to self bend under its own mass. Making flatness worse.

Any granite plate without suitably load carriers at the Bessel points marked must be assumed to have been polished flat. Either one side at a time or set on edge with both done simultaneously.

Flat polished plates need to sit on a rigid, nominally flat, surface with a suitably stiff resilient material between to take out mutual deviations. Stiff felt is good as its fibrous nature effectively makes it millions of close packed teeny, tiny springs allowing a little deformation where needed with only very small deviations in local support force.

Bessel point support is used with high quality surface plates because the results are totally predictable. Any self mass induced bends are taken out during manufacture. Supporting it on those points after installation replicates the self bend so the plate remains in specification and can be checked at any time.

Clive

Well, thanks for all the contributions. I made a stand at the weekend. The choice now have is to use some bolts, head upmost, to support the plate (with some thick felt pads like one uses for chairs), or I have a 4mm sheet of steel I could place on the top. The plate seems to be designed for the “Bessel point” arrangement, as commercial stands are available for it (at great cost). I’m currently considering using the bolts.

I worked in engineering for 45 years and all the cast iron surface plates were mounted on simple angle iron stands or bench tops. It sufficed for all practical engineering purposes in the real world.

Have you considered using adjustable height feet, but inverting 3 of them, rather than bolts and pads.
The 4th one out of the packet, the correct way up, would ensure that your stand will always sit firm on the floor if it's uneven.
https://www.screwfix.com/p/suki-levelling-foot-m8-x-60mm-2-pack/838GX

Lots of other sources, and sizes, are available.

Bill

Edited By peak4 on 21/11/2022 20:53:35

We made granite surface plate stands with 5 bolt supports the heads of the bolts were shallow domed to contact the bottom of the plate and screwed through pads welded to the frame and used a fine thread and lock nuts. The plate was levelled with three bolts one at one end and two at the other end. When level the other two were just wound up to support the other corners to prevent tipping if a weight was put on the corner.

David

Great idea Bill. I’ve ordered some.

Steve

When I got my 18" by 12" granite surface plate I read all the stuff about how to mount it but in the end just plonked it down on a flat worktop (old kitchen worktop ) with an 18" by 12" sheet of cushioned vinyl flooring material under it. I reckoned that for a small plate any bending under its own weight would be negligible. There is no way I will ever know for sure if this was the ideal choice but I have never lost sleep thinking about it.

Doug