[SolarSystemsLLC]

Lurker here with a question for the masses, how would you build a 24"x36" surface plate?

The critical aspects to my plate needs are flat, smooth, and hard. There are a couple obvious things that need to be addressed first. Flat mean granite surface plate, grade B or better over the whole surface, not just the middle. Smooth means like mirror glass, maybe some very fine pinholes but would prefer glassine. Hard is relative, but something that won't scratch easily compared to hardwood or unfilled plastics/epoxy/poly-urethane.

There is some talk of using an epoxy system from Precision Epoxy, and I like the notion of high accuracy. But, none of these systems are going to hit the hardness requirement.

So, I'm off to build an Epoxy-Granite surface plate bigger than the one I already have...

Here's what I have to work with.
1 granite grade B surface plate, 18"x24",
2 2"x2" ground cast iron right angles
1 small set of parallels
24"x36" glass mirror with the reflective surface stripped and etched off
West Systems epoxy, 404 filler, 1/32" milled fiberglass filler, carbon black pigment, cab-o-sil, and microspheres
fiberglass mat, 7781 cloth, and 0.75oz/sq yd cloth
kevlar mat, limited 5oz cloth
carbon tissue, 3K graphite tow (the GOOD stuff)
0.5" structural PVC foam
play sand & .375" granite
misc hand held tools (including Dremel)
vacuum bagging supplies, carnuba wax, PVA
0.0005" polyethylene (mylar) film (McMaster-Carr)
mail-order computer science degree and a couple patents

Some limitations:
No mill, surface grinder, or drill press
No $ for mill or surface grinder (kinda what this plate is for anyways)
I don't want to destroy or wax the granite surface plate
No air compressor for a cup gun to shoot PVA
Wife's purse strings made with Kevlar

I have been operating on the assumption that the best way to go is to make a single high-accuracy replication of the granite surface plate. This replica would be purpose built for making more replicas in some Epoxy-Granite technique. Then, multiple Epoxy-Granite plates could be coupled together to make an arbitrarily large plate. The key is getting a single high-accuracy replica which can be waxed and used as a mold for future plates.

I don't want to risk curing epoxy onto the granite surface plate, nor do I want to contaminate the plate with wax. This means the first surface that comes into contact with the plate has to be already cured or otherwise solid and pinhole free (epoxy-proof). If it's already cured, then it has to be thin so that it can be forced to comply to the flat surface of the granite under the weight of the Epoxy-Granite pour on top of it. If it were thick, it wouldn't comply cleanly and the resulting surface would be wavy.

To date, I've tried three experiments:
1) Wax the clean mirror on a leveled 0.75" MDF backer. Pour a slurry of pigmented black epoxy and 1/32" milled fiberglass onto the mirror. Spread out a single layer of 7781 fiberglass cloth. Work out the excess epoxy, bubbles and cure.

2) Pour about 0.5 mL of isopropyl alcohol on the clean mirror. Place a sheet of 0.0005" mylar on the glass and spread the alcohol out under the mylar. This holds the mylar down to the glass with surface tension. Score the mylar with 1500 grit sandpaper and layup 0.75oz fiberglass on the mylar.

3) Pour plaster of paris directly on surface plate and let harden. Back up the plaster with structure to avoid sagging and cracking.

Experiment #1, it's too thick to comply. The surface finish was smooth with limited pin holes and relatively hard compared to neat epoxy. It just doesn't comply without a vacuum bagging, and even then it's not reliable. The other issue is that after three coats of wax it didn't come off the glass cleanly, leading to spots stuck to the glass or creases in the sheet where it came clean, but over-stressed the epoxy. This might be where PVA would come in handy. With a clean release, it might not need to comply since the mirror is so flat to start with; I think the lack of PVA meant I warped the fiberglass trying to get it off the mirror and that's why it won't comply. How do you get PVA to take a mirror finish before you laminate on it? PVA always has surface finish roughness if brushed and I don't have a cup gun to shoot with...

BTW, pigment doesn't do much in a layer this thin.

#2 - Even after using water/alcohol surface tension to hold down the mylar, it still didn't level well. The mylar has 0.05" dia 'dents' (~0.005" deep) in it after the epoxy cures. These dents turn into domes when placed on the granite and don't level out without vacuum. How does one ensure the mylar takes on the first surface of the granite plate? Vacuum with water under it and let the water squeeze out, then layup?

#3 - Don't know how that turned out, it's curing now. I cast a 6" round slab of plaster of paris ~0.75" thick on the surface plate with no release film. It needs to cure and dry completely or the moisture will act like a glue to keep it held to the plate.

OK, so given the above it looks like I need to either figure out how to get a glassine smooth layer of PVA on the glass, or use vacuum to get the mylar to lay perfectly flat before doing a layup on it. I'm kicking around the idea of epoxy infusion to get the first layer void free, but still have to address the release system.


For those that might care, this is indeed for a commercial application. But I learned so much from the Epoxy-Granite thread that I wanted to document on the Zone how to make an Epoxy-Granite surface plate as karmic payback. Too bad Walter isn't still active...

What do you folks think?

Cheers,
Thom

[greybeard]

...........it has to be thin so that it can be forced to comply to the flat surface of the granite under the weight of the Epoxy-Granite pour on top of it. If it were thick, it wouldn't comply cleanly and the resulting surface would be wavy.

......... Pour a slurry of pigmented black epoxy and 1/32" milled fiberglass onto the mirror.

Hi Thom.
Is this where the problem with the 1st trial lies ?

While I don't have a practical method to suggest, would it be better to try to evolve a method that allows you to make an almost "film-like" layer of pure epoxy ?

Many years ago, at ICI, when casting some of the first perspex sheets onto plate glass, they found that the slightest contamination on the surface of the glass would lead to the two surfaces bonding. If it was ultra clean, then the perspex released ok.
I'm guessing, but I suspect that the final rinsing of the glass would need to be done with distilled water, not deionised, so evaporation left nothing behind, and water, not anything organic - ipa or such.

Perhaps think in terms of a temporary support on the back of the epoxy film to give it sufficient strength for removal from the glass, rather than mixing anything into the resin, which you then remove before pouring on the permanent backing.
No idea what, but I'll carry on thinking about it

John
ps restoring paintings that need new canvas was done by covering the paint with beeswax, then tissue paper ironed onto it as a way of supporting the paint layer when the canvas was removed from the back.
Perhaps something similar in principle ?

[harryn]

The challenge is an interesting one.

The self leveling epoxy sytems sold by Precision Epoxy are quite amazing in their "low level of curvature" -certainly one of the simplist and lowest cost ways to achieve a very flat area. The flatness approaches the curvature of the earth, which is sufficient for many applications. The fact that it can be performed by literally anyone is just icing on the cake.

The main downside of unfilled epoxy is of course scratch resistance. It is useful to note that it is used in auto repair shops every day, so for some applications, it's hardness is sufficient. For $ 100 - 200, you can easily find out.

As you have noted, float glass (literally, glass floated to the top of molten metal) is also quite flat. It is readily available in various thicknesses - possibly one sufficient for your needs, especially if you back it with some epoxy.

You can also buy LCD quality glass from corning and schott, or perhaps they have a way to provide a flat blank like a telescope blank that had bubbles.

I would be tempted to just leave that nice granite plate alone and try to build a "glass faced" version by replicating the self leveling epoxy instead.

This is just an idea, but perhaps:
- Make a high quality self leveling epoxy plate on the floor as your "master".
- Lay a piece of 1/8 in float glass on the surface, preparing the "face down" face to be a near perfect match to the epoxy plate.
- Stiffen up the glass with your various approaches to making epoxy resin / glass mix.

I still see a couple of potential challenges though:
- It will be more sensitive to temperature changes than granite.
- It might be necessary to put some grooves into the epoxy plate to prevent stiction from preventing release.

Have you approached any of the suppliers of granite surface plates about your application, or looked into the used equipment market ? In this market, people can be pretty financially creative, even if the banks are not. You might find that someone will even rent one to you.

[ckelloug]

Hi Thom,

If you are trying to get a 24x36 plate, my first question is why not buy a used one. I have a 48x60 I bought from the local machine tool dealer and it cost $600 plus $300 for the local crane and rigging company to move it from the machine tool dealer to my loading dock.

24x36 Surface Plates at HGR Industrial Surplus can be had for about $200 plus shipping.


Here's my thoughts. The E/G surface plate technology is an open question and a lot of us would like to know how to do it. The following are my opinions on what techniques I would tend to believe:

Firstly, I'd ditch the PVA and go with a spray on silicone based mold release. The raw PVA that I have tends to want to be thick and not as uniform as I'd want for a precision surface. I have some silicone from Mann Release Technologies but it is the matte stuff and you'd probably want the stuff for a shiny surface. See http://www.mann-release.com/

Glass fiber is problematic because it will want to telegraph it's texture through to the surface. I think you are going to want either a pure epoxy that sets hard or a particle filled epoxy with a relatively high fill ratio and micron sized particles as the smallest particles.

Cabosil will tend to make the epoxy thixotropic which isn't good. Carbon black may also cause this problem.
Leave these two alone unless you have some high end mixing equipment and a lot of test time. I'd look more at a filler like IMSIL or zeeospheres.

You probably want a thin epoxy, thick stuff won't self level well. You also probably want a deairing agent like BYK A500 or A525. Ideally, you also want a vacuum chamber to vacuum deair it although Walter's reports on the deairing additives was promising. I'm still waiting for the samples I ordered last week.

I'd be skeptical of mylar for being flat and I wouldn't think the surface tension method of sticking it to the glass would lead to great uniformity because of the possibility of dents caused by uneven pressure and thus uneven thickness of the fluid layer. Once you get out the sandpaper and start sanding on the mylar, I'd be even more skeptical.

As far as I know, plaster of paris isn't dimensionally stable and may contract in unpredictable ways.

Given what you have to work with, I'd try casting epoxy directly on the desilvered mirror with as thin a layer as possible of a well chosen Class A finish mold release from the silicone family.

I'd then try casting other epoxy surfaces from the well siliconed epoxy surface you cast from the mirror.

The way that we've discussed on the E/G thread is to cast a slick of low viscosity epoxy. Barring problems like surface tension, viscosity, and blushing, this should conform to the earth's equipotential surface on the top layer and make a surface that's theoretically good to about .0005 inches per foot.

Another crazy technique I considered the other day was to cast a sheet of epoxy on top of a layer of something like cerrolow-117 alloy see http://cerrometal.com/lowmelt.html. This should entirely cure the mold release problems although it has the same theoretical accuracy of the free surface of the epoxy idea above. This is approximately how float glass is made. Unfortunately cerrolow is really expensive.

Once you've cast some kind of primary plate, you can always cast a plate with appropriate fillers against the primary. I think it will turn out important to keep the filler loading of the product pretty high because I have noticed a bit of shrinkage away from the mold in my vacuum cast parts.

Best of Luck in your endeavor.

[SolarSystemsLLC]

John,
Hmmm, I'd love to be able to make a thin film of epoxy and have it come off the glass cleanly. I was hoping to get the final surface plates first surface to be harder than pure (neat) epoxy. Epoxy's just not as hard as I would like. I figured that adding the milled fiberglass would toughen the first surface up and reduce any shrinkage with temperature. That was my thinking in experiment #1. I do like the resulting sheet, just need to get it to release cleanly and still remain smooth, thin, and locally flat so it conforms better to the granite plate.

I'll look into trying a clean surface, but I found that the epoxy stuck like mad to the glass where the wax wasn't. This implies to me that I really have to have a release media of some kind. Although I can see where that might not be true for all materials cast on glass...

I do use distilled water to clean with. It's cheap at the grocery store in small quantities.

As for a temporary backer for a thin epoxy film, that would be interesting. Something like a sacrifical layer of structural foam that could be laminated between two glass/epoxy layers, then dissolved after it's cast on the glass, perhaps?

Thanks!
Thom

[SolarSystemsLLC]

Harryn,
Very interesting. This would have a size limited only to the size of glass plate that could be acquired... The hardness is also limited to that of the glass. CTE doesn't really concern me too much since I can control it's environment, and the predominant CTE factor would be the 3+ inches of EG under the plate glass.

Do you think a compressed air wedge might be enough to pop the glass off the PE plate, maybe?

Thanks for your ideas!
Thom

[SolarSystemsLLC]

ckelloug:

I didn't want to buy a plate because for my product I'd like to integrate rail edges, bolt hardware, etc. Getting someone to machine that in Phoenix isn't really cost effective. Besides, where's the fun in buying what you can make just as well? Good point, though.

Silicone eh? I've heard so much about silicone being bad for paint shops that I wanted to avoid it. I agree with the PVA being thick and lacking uniformity. Matte vs. shiny isn't a concern, but the material is. Along that vein though, I do have a can of PTFE that I had forgotten about. What do you think of using spray-on teflon?

For the first surface, I agree that the fiberglass will print through. Experiment #1 started with 1/32" milled fiberglass in the epoxy with some pigment just for that reason. The milled fiberglass is microns thick and when mixed with epoxy just makes it more viscous. The 7781 fiberglass went on top and just soaked up some of the first layers epoxy. It didn't print at all when cured (on the front). I did buy some West Systems 404 filler to play with, but haven't used it yet.

I have Cab-o-sil, but also ruled it out for the first surface. The carbon black is actually mixed with resin to act as a pigment; it's not in powder form. I think it's West Systems #43 if memory serves. Ether way, it was too thin a section to bother with for pigment anyway.

Zeeospheres I've heard of, but IMSIL is a new one on me. What is it? I'm ruling out filler that might react to water, like unset plaster of paris, hollow microspheres (break on the surface and retain water/gunk).
zeeospheres.

I'm not quite sure where self leveling comes in. Are you referring to making a self leveling epoxy surface plate? If so, I think I'd just get the Precision Epoxy kit. But for the final product plates I'd like something more tough than self leveling epoxy. Did you mean something else?

I have a vacuum pump and plan to degass the epoxy as I get closer to building production plates. For now, I'm just letting it do it's thing. Since it's really only the first surface that needs degassing, it shouldn't be too burdensome to do.

Mylar, I agree. The mylar doesn't really want to be that flat, and the resulting surface tension inconsistancy really seemed to show in the final film. I was thinking that running the cure in a vacuum bag might defeat the mylars natural tendancy long enough to get the backing fiberglass to set up. With a little liquid under the mylar it wouldn't have a chance to trap air under it like seran wrap does. The vacuum would just squeeze it all out.

The word on the street (wikipedia, etc.) was that plaster of paris has really limited shrinkage due to the fact that it sets before the water can all evaporate. That's part of the reason that plaster of paris is used to cast molds of foot prints and the like.

The E/G thread's take on pouring a surface plate is really compelling. My only issue with it is that the poured surface probably would not be hard enough for the kind of use these plates would see. But the thread is what set me off looking in this direction for a workable solution.

Cerro, that is crazy. But it might work since only a thin layer is needed. Could the release wax be made to come off the cerro easily? If so, that would be a really interesting avenue to explore...

Agreed, the filler is really needed to keep the shrinkage down and the resulting surface hardness up.

Thanks!
Thom

[SolarSystemsLLC]

FYI,
Experiment #3 failed. The plaster stuck like mad to the surface plate. I even followed the suggestion on the box of moistening the surface (ah, like granite was going to take on moisture) to get it to release easier. It's going to take the rest of the night to clean the plaster off the granite plate.

Any bright ideas how to remove set plaster of paris from a granite surface plate?!

Thom

[ckelloug]

Thom,

I'd try vinegar to get rid of the plaster of paris. If that doesn't work, I'd wash the plate and try muriatic acid. Neither should ruin the surface plate although a quick googling shows the reaction of the muriatic acid with the plaster of paris produces sulphuric acid so make sure to wear your rubber gloves and neutralize the whole thing with baking soda once you've gotten rid of the plaster of paris.

Understood in terms of making your own. If you need one for a project buying is easier if you need many, then casting is probably a good solution when you get it to work.

Try the sprayon teflon and see if it works. I've had no problems with the silicone but I do understand that silicone contamination can craze finish in high end automotive paints. Decent mold releases are an off the shelf item so I wouldn't personally put too much effort into improvising this part. Teflon likely presents the same problems as silicone.

Cabosil is silica fume. It tends to have long strands of tiny particles bonded together and can increase the thickness a lot. IMSIL is microcrystalline silica, a white powder which is effectively just quartz very finely ground. I have a sample of the 1240 grade which was given to me by Walter. Zeeospheres are a solid aluminosilicate product that has no water reactivity to speak of. I avoid glass bubbles and other similar stuff though because they have low compressive strength.

My suggestion with the self leveling epoxy is to make an epoxy plate out of the self leveling epoxy to use as a master mold surface for a heavily filled epoxy. I haven't tried this nor has anyone I know of done an objective test yet. The physics is right but the chemistry as far as surface tensions etc could make this more difficult than desired. .0002 per foot is the supposed accuracy.

Keep in mind that low shrinkage etc is a whole new ballgame when it comes to trying to make things to machine tool tolerances. 1% shrinkage is 1/100 of an inch per inch. If you need a surface plate, you're looking for on the order 5/36000 of an inch per inch aka five tenths over the 36 inch length.

Finally, the insane suggestion with the cerrolow alloy was to cast the epoxy on top of the liquid cerrolow and let the epoxy harden while the cerrolow is still liquid. The beauty would be that no mold release is needed because you should be able to let the epoxy harden and lift it off of the still liquid cerrolow. A hair dryer should get the rest of the cerrolow off. This is purely speculative. I haven't thought through it clearly, much less tried it. I also assume that epoxy hardeners don't corrode cerrolow.

In short cast in place surface plates are a critical need to make useful E/G machines so I am interested in your progress and wish you the best of luck.

--Cameron

[harryn]

There actually is another method for you to use to make a surface plate - the intrinsic method.

My understanding, inept as it might be, is that they are made by grinding surfaces against each other. If you are patient, you can, in theory, make your own from glass, cast iron, or any ceramic.

I am not 100% sure of this, but I think if you grind the surfaces exactly in-line, they go flat, and if you make a circular motion, they will go spherical (used to make telescope mirrors).

Here is the wikipedia link. http://en.wikipedia.org/wiki/Surface_plate

I am not sure if this meets your needs or not, but for volume production, there is another option as well. I was looking for ways to obtain very flat, square, and parallel Al plate for my DIY router and found an outfit in LA called "Dix Metals". As a routine service, they supply precut, square, flat steel and Al plate.

Here is their link

http://www.dixmetals.com/services/list.html

The theory of their operation is that a lot of people would like to buy plates that are ready to be milled to final size, instead of goofing around with milling all 6 sides to square first. In other words, as a time / cost saver due to their volume. You might be able to obtain "close enough" results with their service, depending on your actual need, and certainly with an Al or steel plate, you can add your other parts as well.

I have not used their service yet, but I am planning to unless the price is just plain crazy.

[HuFlungDung]

Harryn,
The intrinisic method of producing flats is to lap 3 unknown surfaces against one another. When all three touch each other all over, then they are all flat. Would take a fair sized machine to lap a 2 x 3 foot piece, at least its more weight than I'd want to be pushing around

[SolarSystemsLLC]

Cameron,
I got the plaster off pretty easily. A credit card scraper took it down once the big pieces were broken off. A v-cut razor blade took the rest off except the little bit that snuggled into the pores. A precision flat indicated that the stuff in the pores isn't impacting the plate flatness, so I'm calling it good. There's a white ghost left, but I can live with that since I'd rather have plaster in those pores than misc. built up gunk.
I was thinking Zeeospheres anyway, just gives me another excuse to get some...

I think that if I can't get a piece of glass setup with an epoxy-granite base on top of the granite I'll double back to the poured master mold.

As for shrinkage, filled epoxy isn't going to shrink much, if at all. Since the whole plate is going to be filled with something, any layer that isn't filled (surface perhaps) is going to be dominated by the rest of the plates shrinkage performance. We'll see!

Cerrolow has me thinking. Wax popped into my head as an alternative, but I don't think epoxy is going to float on wax. I need to look up the specific density of each. Mercury is obviously out. Thick motor oil maybe? A bit of wikipedia searching is in order...

Thanks for the encouragement!
Thom

[SolarSystemsLLC]

Harry,
Yuck! Lapping a couple 24" x 18" epoxy-granite slabs doesn't sound like fun; but it might make for a good work-out.

Dix Metals has some interesting stuff and machines. They seem to indicate 0.001" as their standard tolerance, but don't say much about their accuracy. I wonder if they could compete on price with just buying another granite surface plate and using as a dedicated mold?

Still, for large stock they have a compelling business model...

Thom

[SolarSystemsLLC]

Harryn,
The intrinisic method of producing flats is to lap 3 unknown surfaces against one another. When all three touch each other all over, then they are all flat. Would take a fair sized machine to lap a 2 x 3 foot piece, at least its more weight than I'd want to be pushing around

You'll flung dung, but not a little EG?!

Thom

[SolarSystemsLLC]

OK, here's what I learned tonight:

Experiment #1, it appears that I didn't let the epoxy cure long enough. When I went to clean the stuck-on-epoxy off the waxed glass tonight, it just peeled off. It couldn't have been the same amount of adhesion as the night I tried to pull the sheet off or it would have slipped right off the glass. So even though the epoxy was no longer tacky when I tried to pull it off the glass, it still had a lot of grip to the surface it was curing on (still had a high surface energy cause it wasn't done curing, duh). This is encouraging, because it means that waxed glass is still usable as a mold surface.

So, once the glass plate was cleaned off and tack clothed, the granite surface plate was cleaned off too. Then, using about 0.5 milliliters of IPA on the granite, the glass was placed on the granite. As you may recall, the glass is a mirror with the reflective side stripped down to the glass including the metallization and rubber protective layers. The theory is that the metallized side will be the equal or better surface of the two. The formally metallized surface is now wet with IPA and nearly in contact with the granite.

This experiment was really cool. 0.5 ml of IPA is plenty to wet out the entire 18" x 24" surface area as long as both surfaces are really clean and free of fine particles. Air still gets trapped under the glass, but a little pressure with the palms of your hand or the end of a couple of sticks can get the bubbles to run off towards the edges of the granite. Use your hands on either side of the bubbles of interest and kind of funnel or guide them. Two points of pressure are needed, but those two work really really well.

Once the bubbles are pushed out (most of them at least), they can't really get back under. It does appear that if you let the glass slide, the air gets drug back under the glass. Sliding the glass is pretty easy since it's sitting on a thin-film fluid bearing (IPA & granite)! The glass is pretty flat, but this process pulls it more flat, almost to the same spec as the granite. As you work out the bubbles, you get a feel for where the glass is being pulled down and where it was pretty flat to start with.

And here's where a vacuum bag would come in handy. Putting a vacuum bag over the glass and taped to the sides of the granite plate would pull all the remaining air out. The bag technically could just cover the boundary of the IPA and let the glass act as the bag itself if needed. This might mean the vacuum bag is reduced to a thin breather ply and a layer of duct tape that hits the glass and the granite with a coupling for the pump somewhere.

But, as it is, 0.5 ml spread over 18" x 24" comes out to a whisker over 0.00007". That's 1800 nm, a factor of 2 from being the wavelength of dark red light! At this point, the quality of the granite slab is the limiting factor as I bought a cheap import. You guys with Grade A or better might want to go for the vacuum bag to max out the potential accuracy.

Now, given that the glass is pulled flat, an epoxy-granite slab can be poured on the glass (scoured of course to encourage bonding). When the EG cures, water can be shot into the perimeter of the glass/granite boundary and the two can be easily separated.

Viola! One glass faced EG surface plate with granite surface plate accuracy. Wax the glass and cast more surface plates directly from the glass faced one. The original granite plate is contaminate free (no wax, silicone, or teflon release is used on the granite).

A couple notes,
1) my mirror was waxed with carnauba three times then used for an epoxy/fiberglass layup. This wax layer may have an impact on the wet-out of the IPA. Raw glass might not behave exactly the same, but it should be close.
2) My mirror is 24" x 36" since it was bought before the granite plate. The next experiment will utilize a virgin 18" x 24" mirror to match the surface plate and allow the vacuum bag to go over it.
3) My cheap import granite surface plate is REALLY porous on the working surface. They might all be, but I don't have any others to compare it to. It hard to see the pores until you wipe the IPA off after pulling up the glass and it's still wet.
4) Carnauba wax in insanely tough once it sets up. Before all this started, I had two 12"x12" granite floor tiles that I waxed with three coats. With the edge of a razor blade, it's hard to scuff the wax surface. Moving to the tip of the razor nets only an infinitely small scratch and almost no kerf on the blade. This means the wax layer is impossibly thin too.
5) Simple Green purple formula seems to take off carnauba wax. It feels like it's gone off the one tile I tried it on. The trick is there's no really good way to measure the wax so it's hard to measure if it's gone. The surface no longer beads water and it feels remarkably less slick to the touch.
6) Granite surface plates with glass bonded to it via IPA is really pretty. The IPA and glass bring out the surface of the granite as if the plate was a Ferrari waxed and polished for a night on the town.

Thom

[Nzoldun]

Step 1:- Buy an old, machined surface only, cast iron surface plate of the size you need.

Step 2:- Using your granite surface plate as a master and 'marking' blue, hand scrape the C.I. plate until you get an even distribution of marking from the master to the C.I. plate

OR

Find some GOB like me who, did this sort of thing as part of his apprenticeship, way back in the fifties!

NzOldun

[ger21]

Once the bubbles are pushed out (most of them at least), they can't really get back under. It does appear that if you let the glass slide, the air gets drug back under the glass. Sliding the glass is pretty easy since it's sitting on a thin-film fluid bearing (IPA & granite)! The glass is pretty flat, but this process pulls it more flat, almost to the same spec as the granite. As you work out the bubbles, you get a feel for where the glass is being pulled down and where it was pretty flat to start with.

And here's where a vacuum bag would come in handy. Putting a vacuum bag over the glass and taped to the sides of the granite plate would pull all the remaining air out.

You need to be careful here. It's possible the vacuum bag will seal the edges of your glass to the granite, leaving a pool of the IPA under the glass.

[amplexus]

HOW ABOUT TWO CHUNKS OF KITCHEN COUNTERTOP LAPPED AGAINST EACH OTHER WITH GOOD SUPPORT UNDER THE BOTTOM PIECE. YOU SHOULD BE ABLE TO GET THEM PRETTY FLAT. A BIT OF ABRASIVE AND A BACK AND FORTH SETUP, MOTOR WITH ECCENTRIC ARM OR WHATEVER, SOONER OR LATER THEY SHOULD GET FLAT. MAYBE PUT THE SLABS IN A PAN OF ABRASIVE SLURRY.
AMPLEXUS

[joeybagadonuts]

I have been to several auctions lately and when no one bids on the surface plates they end up giving them away for $20.00 or less just to get the heavy bastard out of the building.

I would rather spend my free time eating a good doughnut anyway.

JoeyB

[SolarSystemsLLC]

Step 1:- Buy an old, machined surface only, cast iron surface plate of the size you need.

Step 2:- Using your granite surface plate as a master and 'marking' blue, hand scrape the C.I. plate until you get an even distribution of marking from the master to the C.I. plate

OR

Find some GOB like me who, did this sort of thing as part of his apprenticeship, way back in the fifties!

NzOldun

Both would work, I agree. But that kind of defeats the purpose of figuring out how to make the plate with epoxy-granite!

Thom