Thoughts on meniscuses and mix ratios.
dfro,
Welcome to our pot of stone soup ;) Thanks for the tip on small quantities of teflon powder.
You will be the first one on this thread to try to cast parts of a precision metalworking machine so I would encourage you to be careful as most of the work here is still rather experimental. Quickly, Epoxy granite is only good to 1000-3000 psi in tension/flexion in AustinT's test so big pieces of it or reinforcement will be required for it to survive high tension or flexion loadings. If you need true metalworking precision stable to .0001, some large cross sections will be needed if you want to forgo reinforcement.
As for casting an epoxy level surface, I will first say that I'm the thread's theoretician and I'm just about to start pouring epoxy. What I say about the leveling epoxy is theory which I believe to be correct but haven't yet tried.
You will want to roughen the surface of the bed as much as is practical before applying the leveling coat. From past post by others on this thread, it's probably also a good idea to cast this while the epoxy in the bed is still achieving a cure which is going to be sometime within hours of the original casting; How much it matters if any is unknown by me.
I don't think that standard epoxy will turn out to be all that hard compared to tools and clamps etc. so I've been thinking about how to reinforce it yet still have it flow well. For the standard epoxy, It would be wise to wait a week, preferably at summer temperatures, before placing any pressure on the leveling coat as these epoxies take a long time to achieve full cure and accuracy will go down if you clamp something against the leveling epoxy before it is as hard as it is going to get. You will also want to level the ways (hopefully with a precision level) before you pour the surface coat so that you have a uniform thickness and hopefully level ways.
The meniscus that forms will depend on the ratio of the surface energy of the epoxy to the surface energy of the mold. If I am understanding the theory correctly the meniscus will be concave towards the edge if the epoxy has a lower surface energy than the mold edge and the meniscus will be convex if the epoxy has a greater surface energy than the mold edge. If they are equal then there should be relatively little or no meniscus. While I haven't tested this, I suspect that if the mold edge were made of hardened epoxy cast earlier that that would be the best you could do other than slopping it over the edge like Larry suggested.
Larry is the thread's expert on actual epoxy so I have to defer to his advice as being better than mine here: I do personally suspect, although I haven't tried it, that the shopmaninc 635 epoxy than many members have used will be okay as a leveling material provided it is mixed accurately.
On the topic of mixing, I think I have identified a bit of a problem with the directions for the 635 epoxy. Larry pointed out blush issues for laminating epoxy but it is my understanding that imprecise mixing is the number one cause of blush. In short, the ideal mix ratio for the 635 epoxy is not 2:1 by volume like the directions state, it's actually 2.06 to 1 by volume for a 3% error.
In detail, the Epoxide Equivalent Weight (EEW) of the A component (Reichhold 37-127) is 197 with a specific gravity of 9.2 lbs per gallon. The Amine Hydrogen Equivalent Weight (AHEW) of the B component (Reichhold 37-606) is 85 with a specific gravity of 8.2 lbs per gallon. 197/85 gives a ratio 2.32:1 by weight between part a and part b which equates to 2.06:1 by volume.
Since the directions specify a 2:1 volume mix ratio, there is a slight excess of hardener in the epoxy mixture produced via the uscomposites directions. In itself, the 3% error might not mean much but given any further error on the part of the user, it might in theory cause batches that do not set properly or are a bit soft.
see http://www.reichhold.com/docs/litera...ochure2007.pdf
For Reichhold's explanation of epoxy mixing.
So, welcome dfro and good luck with your machine. If any of us can help, please make sure to post and ask.
Regards all,
Cameron