Torsion box router with a 4th axis.

[TheWire]

Hi Jerry - Your first options would have worked, maybe not up to what you think. Re: aluminium bonding with epoxy. The type of epoxy you describe is quite thin. If you laminate with it and clamp it you can create a zero thickness bondline that obviously has no strength. This sort of connection is perfect for contact cement but since you have epoxy.
1) I'd firstly do a small test piece to understand how the epoxy soaks into the timber or not and to time its gel time. You need to know that it does not leave a dry surface and when the epoxy gets tacky. You can also use a "primer" coat on the timber so it seals and allows a consistent epoxy film when you do the join. I'd do a primer coat on the timber.
2) scotchbrite is good for wet sanding the AL surface but any fine wet and dry paper is OK (240 or 320G). And yes you apply the mixed epoxy to the surface (use a V spreader). Wear gloves!! I suggest butyl gloves latex tends to tear
3) If you want to get technical you should use a glass fibre tissue or some very light cotton cloth to create a consistent bondline thickness. Glass tissue is 0.25mm thick. The perfect bondline thickness for structural components is 0.25 to 0.5mm thick. You can achieve this without a scrim by applying the epoxy to the timber, allow to get tacky then apply the wet AL to the surface. The tacky epoxy then can't be squeezed out of the bondline. Do a small scale test to get to know the timeline

In this application the bondline strength required is tiny so I'm sure if you dived in and did it it any old way it will be fine. Peter

To get a little more technical the most common reason epoxy and aluminium bonds fail is surface moisture. Epoxy does not like water. If you have ever TIG welded you will see the moisture coming out of the metal. So a waft with the hot air gun (or propane torch) before you coat the metal is a good idea but this can change the gel time. Automotive and aerospace now use flame (plasma, corona) treatment to prime surfaces prior to bonding. And laser prep have you seen laser cleaning of rusty surfaces aaaaaaaaAMAZING!! I expect Dakota is dry air however compared to where I live. The deserts in USA are full of aerospace component makers for the dry air and cheap land.

re laminating plywood and LVL - I'd prime coat these as well before laminating the result is much more consistent.

My main use for the epoxy I have is to level the bed. If my laminated gantry beam ended up if variations in it, I could pour a layer of epoxy on the side the rails attach. I would let this fully cure and then glue on the 3mm aluminum skin. But I want to use the best adhesive I can to fasten the aluminum skin.

I used polyurethane glue, which requires a slight amount of moisture to activate, on my torsion box and it worked well. It looks like people have good luck gluing aluminum to wood with polyurethane glue also. I'm using contact cement to glue the plastic laminate sides to my torsion box, but for some reason, I thought it might not be strong enough in an aluminum skin application. But after looking into it, it looks like it would work. If using contact cement I'd do a "primer" coat with the cement on the wood to seal it. I'd probably also use a bunch of screws.

I have an email into a woodshop here to see if I can find a wood laminating press to use. If I can't find a press, should I jump through a bunch of hoops to keep my beam uber flat while gluing and screwing it together or just plan on pouring a layer of self-leveling epoxy on the rail side of the beam to get rid of slight variations?

[peteeng]

Hi Jerry - Lets start at the end.
1) I think levelling with epoxy is problematic. Epoxy has the same stiffness as rubber say 3000Mpa and to level you have to do 2mm or 3mm thick to sidestep wrinkling due to crawling etc. So although flat its the same as gluing a rubber sheet to the surface, then when you bolt something to it its soft. I'd prefer to find something flat and hand sand it flat just like you would a cast iron surface plate with a lap.
2) PU is great because its moisture curing so as everything has moisture on it it sticks well. If its expanding PU it needs to be used very very thin and clamped well. Then contact works because it has an aggressive solvent in it. Both of these are predicated on starting with a flat surface. If the Egyptians and Mayans can make flat rocks you can make flat wood
3) Best adhesive for skin is all discussed, just what you have at hand and what your comfortable with in terms of the processes.

crawling is a combination of curing viscosity and exotherm expansion. Epoxy cures in random "cells". These cells get hot and expand pushing epoxy nearby into waves. Areas that aren't cured flow a bit and areas that are curing expand a bit creating more waves. To stop this very mild curatives are used to create a very long gel time. But if the film is too thin say 1mm then the curative is so mild it doesn't cure as the starting exotherm goes cold before the reaction can continue. Thin film curatives are usually aggressive to get around this but then you get severe crawling so its a chemistry juggling act. Best plan is to sand timber as flat as possible. Then epoxy prime timber to harden surface and sand flat again using a long lap. Prefer two laps so it averages out. Use a "contrast" between lapping sessions so you know what is happing. Use panel beaters dry contrast or a light spray paint mist.

Any panel beater (or boat builder) will know how to blend/fair a surface, same goes for getting one flat. Ask around? Plus whatever you do on one side do on the other. If not symmetric the timber will warp. The backside does not have to be lapped just kept to same construction. Not trying to make things difficult any handcraft has its theory and practice to master...cheers Peter

Its always best to start with a flat surface, don't try to correct something after the fact unless its a mistake.