Gallery, Projects and General > How do I??

Quartz Surface Plate?

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PekkaNF:
I measured flex of two common materials:

30 mm thick granite table top 290*600 mm

10 mm thick float glass. Plate size 300*400 mm.

I placed the spesimen on top of the small surface plate. Overhang was 240 mm and measurement point c/300 mm and 200 mm overhang. DTI was zeroed when there was no load and read when a dummy weight of 1320g was placed behind/front of the central measurement point.

With 10 mm glass plate the deflection was 0,05 mm and with 30 mm granite plate there was no perceptible change. This clock had a reading resolution of 0,01 mm. I do have 0,001 mm clock, but not that good base that it would make here much real life difference.

Soo it looks like if proper Bessel points are used to support the load/surface plate it should support measurement apparatus.
https://en.wikipedia.org/wiki/Airy_points

Therefore hunt of 30 mm and/or 60 mm granite interior decoration material is on the way.

Pictures:
Initial try....too much uncertainty, but it showed that glass deflected and that measurement system was not rigid enough. Also initial 100 mm overhang produced reading on the glass plate, but absolutely not reading on granite table top.

For next round I INCREASED the plate overhang to 240 mm and measurement point to 200 mm overhang and on width on the middle of the plate. Load was placed off centre fore/aft.

Next two pictures are measured on the same setting.

Pekka

sparky961:
If you need even more rigidity, what about sandwichkng multiple layers of granite with epoxy between?

PekkaNF:
I really don't know about epoxies enough, I think I would need a speciality epoxy that does not shrink etc.

One engineer at the work suggested of adding some more vertical offcuts, bit like I-beams under, glueing them together firs then mounting the embryo surfaqce plate on to of the real surface plate and glue the whole pagage with low shrink epoxy, but most of the surface would covered with release agent....epoxy would be there just even out all the surface features between rigid member and table top. I'm pretty sure the devil would be here in details, like where to mount it and how and how accurate hard points would be....

I have to see waht is available, it might turn out most cost effective to use 60 mm thick stone if I can get it with any decent surface quality.

Cheapest real surface plates even close to this size 1000*400+mm starts to cost serious money and shipping is a killer. On that behalf a good small 600 mm surface plate + and a half decent straight edge might come cost effective.

But let's see.

Pekka

Will_D:
Ok, Its Saturday night and a wee drink has been taken!

This talk about surface plates etc leads to 2 different considerations:

1. The most stable surface I know of, (but not neccessarily flat) is an optical bench as used for holography. The home made variety consists of a concrete slab floating on a lot of car inner tubes suported by another massive structure. The keyword here is "floating" as it resists distortions caused by passing traffice and the like.

The mirrors and lenses placed onto this relativley are so light they cannot distort the "flat" surface. This allows the holgram/whatever to be created

2. If a very flat surface (like plate glass/granite) is expected to remain flat as a reference plane surface when loaded with significant loads then at a minimum a floating solution needs to be used to distribute the load evenly. That said the material also needs to be thick & strong enough to resist deflection given the applied point loads.

   A long time ago I was advised to "float" my plate glass "surface plate" on a nice piece of carpet (itself resting on plywood) in order to erect my 3 1/2" gauge loco chasis.

PekkaNF:
Will, you are right on those points. The "thing" with optical wizardy it timedomain, wibration and other time spesific disturbance would make absolute mess of holograms an othe laser stuff.

RPI (Rensselaer Polytechnic Institute) has least one sandwitch table on it's lab. I have seen one big one here in Finland it had a thick aluminium sheet peppered with holes on top and may layers of different material. No car tyres thoufh, but it was in a cave.

Anyway our needs are a little different. When lorry passes by, I'll notice it here houses are on caly and trafic nearby rumbles the house. Then I'll just repeat the measuremet. Also optic puts very little load on table. I'm planning to place about 100 kg lump of stuff and thay would distort most of the stuff I could wheel into my garage unless some consideration is placed on mounting the piece and reference.

Heck. If there were a lightweight, rigid, but weak straight reference (say honeycomb filled composite) plate I would not support any weight on it. Just use it as a reference.

I'm having a bad flu or something and I'm really to sick for the shop. I have had time to play with 10 mm glas plate and I'm starting to see where old instructions come from. Glass plate seems to have pretty good surface quality when it comes to measurement with 0,01 mm accuracy on short distance - say 100 mm. But it bends easily under load. It needs realatively flat surface under it. This understructure could have relatively rough or even abrassive, or soft surface that reders it uselees to measuremet. Glas does not respond well on local stress, therefore there has to be something flexible between the glass and this "rough" surface. However too much flex is not a good idea either, it could allow the glass to bend under piece you are trying to measure. Also, I don't think gluing the glass on any other material is a bood idea.

One idea that proably would work is to have rigid base, injection compound on top of it to even out all surface and conform glass undulation. This has few unknowns to me. Another is how accurate glass really is? How to prevent glass from gluing and still keep it on it's place. Is the glass waviness such that it will not mess up measurement due to thermal expansion or othe type of movement? This would need some iteration on smaller scale.

I'm checking the "rock" next and try to find some more samples. It looks like it would offer me beter chance for success than glass.

Pekka

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