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