Liquid joinery

[ThomD]

Just been perusing the various router designs. A lot seem to use MDF or plywood, and this inevitably leads to comments about the stiffness of these materials, alignment, load take-offs, and the moisture resistance.

I did a search on "epoxy" and came up with mostly stuff on captured nuts. If this is a repeat of already existing material, then I apologize.

Liquid joinery (LJ) is a collection of techniques used to connect plywood and solid wood into boats and other assemblies. Normally referred to as "stitch and glue", The less common term of LJ refers to a wider range of techniques than those used simply to join curved panels.

The epoxy used in these techniques is a boatbuilding grade product that is 100% solids when dry, and thin enough to be spread by squeegees paint brush, etc... There are additional product, like cabosil and q-cells that can be mixed into the glue to give it more body, right up to a stiff putty consistency. The better grades of glue may not be necessary for this kind of project, so builders could consider bargain brands like RAKA, though I haven't used that brand myself I have heard good reports. Particularly over MDF, I wouldn't waste money on higher yield strength products. When epoxy is combined with plywood it can give materials like carbon fiber a run for the money.

I don't know enough about routers tables, but I would venture a guess the main issues load wise could be divided into 5 areas:

- Weight of the machine;
- Weight of the workpiece;
- Loads from moving the machine, racking and such;
- Operating loads like accelerations and loads developed from cutting;
- Point loads where hardware attaches etc...

Here are a few LJ applications and other wood epoxy uses:

1) Coating the wood. Epoxy glue that is 100% solids, waterproofs ply and slows down the passage of moisture. If you coat a piece of MDF with epoxy it will become waterproof, but if you put it in an extremely humid area, over, time the internal moisture will adjust to environmental moisture. However, that could take days weeks or months, and so seasonal variations are greatly buffered.

Applying these coatings can be done for speed or looks. If one has a pile of parts one can simply coat them and after they have all been coated, coat them several more times in series. Even though the epoxy never dries the repeated coatings build, and one quickly gets a thick coating. Flat surface scan be flow coated for a thick protective shinny surface. Epoxy can then be painted. or it can be the final finish if UV exposure is at indoor levels.

Coated pieces need to be 100% sealed. Therefore special care needs to be take with every screw, or wear point. With lots of pieces and lots of care needed, perhaps another material choice would be easier, but many people are uniquely at home with wood.

Solid wood like 2x4s can also be coated, but the material should be reduces into 3/4" max thickness pieces and then reglued and coated if a moisture resistant and stable result is required. Epoxy may not be able to adjust to the normal mobility of larger solid pieces of wood.

2) Filleting. Q cells and cabosil can be mixed with epoxy at a 3-1 ratio to create stiff putties that can weld plywood together. The use of a "tongue depressor" with a 3/4" radius at one end will allow "welds" to be inserted into the plywood/MDF box corners for far greater stiffness and strength than hardware can achieve. The hardware can also be used and the butts could be glued with epoxy to ensure maximum strength and envelope integrity. The fillets will not be the full strength of most 3/4" sheet stock, but they usually don't have to be if the box is properly configured. Any two surface you can bring together can be filleted and so stiffeners etc... can be added at will. If you need full strength, then fiberglass cloth can be added, but that would seem overkill here...

3) Hardware bonding. For neat functionality and maximum strength, hardware can be bonded into oversize holes filled with epoxy this increases strength and allows threaded studs, etc... to emerge in perfect alignment from various elements. Any time one joins metal and wood, one has to be realistic about the strength of the joint, but in beefy enough parts, hardware bonding can deliver full-strength load take-offs. In other uses it's just cleaner and far easier to align parts, and keeps the envelope intact.

4) Bedding. Perhaps the best example of this might be bedding rifle actions in target rifles. Properly inletting a rifle action into a stock is very skilled work, and when complete, the wood will shift and the action will get torqued, and the rifle will not shoot to peak accuracy. So you simply support the barrel at the end of the stock with a yoke glued on. You remove the two action screws and cut away wood from one contact point and fill with epoxy putty, coat the action with release wax, and screw the action down so it is supported by one screw and the yoke. repeat for second screw. This give perfect stable support for the action. Applications for rails joined to panels abound. If you can temporarily support it where you want it, build a dam of plasticine around it, you can bed the part perfectly first time out.

Any time one can support a piece of wood with hot melt glue, drywall screw, wires, zip ties, there is usually a means for securing with epoxy that is filleted or cast in etc... This greatly reduces the woodworking skill required to get perfect fits, and the end result is better.

5) Casting. Similar to the above tool holding parts etc, can be cast in epoxy putties, with a high degree of strength and finish detail. One can add materials like cotton or glass fibers to improve tensile strength, these parts can in turn have hardware mounted in them, or be bedded to other parts.

6) Surface treatments. Not sure the applications here, but anything you add to epoxy by the rule of measure adds it's characteristics in proportion to the amount mixed in. There are low friction additive, wear additives, fireproof additives. etc...

7) Vacuum bagging cored panels. Epoxies will bond anytime two surfaces can be brought together. This means that large stiff panels can be manufactured by using stuff like your shop vac and visqueen to bond together two layers of ply over say Dupont insulating foam. This creates walking strength panels of great light weight that might be appropriate for work surfaces. There are some key details here I won't go into given the size of this post. You can also use sandbags etc... to bring these panels together.

Epoxies are pretty safe, but you need to carefully follow instructions. You need a dry, warm, well ventilated space, proportional to the areas you will be coating in epoxy, and use a cartridge filter respirator if you can. You need to avoid skin contact, use gloves or barrier creams. "Cured" epoxies are still sensitizers for a while after then first harden, so avoid early sanding or grinding, in fact these techniques should be completely avoidable on this type of project. Large amounts of mixed epoxy can be a fire hazard in a low surface area container. Do not burn epoxy. Do not inhale the additives like Q-cell, or cab. This is not intended as a complete safety overview. In fact none of it may be true for your product or use so inform yourself and take responsibility for your actions. I generally limit this kind of work on any large scale to warm parts of the year.

None of the above is intended to imply current designs are need these techniques. It's just something you can have in your toolkit if you need it

[Oldmanandhistoy]

Hi TomD,

Excellent work you have done on this thread I am surprised you have not got any feed back with this. The subject may have been covered in great detail previously but I have not come across it.

I do have a project in the pipe line that in my opinion would greatly benefit from the addition of an epoxy coat and also improving strength in the joints. This project is a little in the future but I hope you will be around for me to pick your brains on the subject when the time comes.

In your opinion what thickness of coat on say MDF or plywood would be required to make a noticeable difference with stiffness? For instance if I brushed on a coat would it increase the stiffness by 5% 10% or more? Just a rough answer will do to give me an idea so I can gauge the benefit of using epoxy.

One last thing do you think that over time the epoxy coat may crack due to the flexing of a router frame?

Thank you for your time and trouble,

John

[ThomD]

Thanks. I was watching OCC last night, which these days is a pretty unlimited festival of product placements, and thought that perhaps people thought the article was a giant shill!

You won't unfortunately get much increase in stiffness from coating the ply, the main advantages of coating are waterproofness and moisture resistance, which depending on where you keep your machine may be no advantage at all. The coating properties are sufficiently strong that in certain special case this kind of thing has been used to manufacture water tanks or even fuel tanks (this last app was found not to meet regulatory requirements though it worked in practice, and the same applies to potable water tanks, worked great but the epoxy companies (and me too) don't want the liability. It does show, however, how this kind of thing transforms the properties of the wood, compared to an enamel or latex coating.

In terms of stiffening, stiffness increases to the cube of the thickness of the materials involved, so lamination or creating rolled edge details would be a good bet. Consider how a machine base in sheet metal is structured. Let's say you had some very thin sheet metal, almost like a playing card, and there is a motor access door. The metal in the area of the door may be bent twice at the edge creating a J shape this greatly support the edge. A similar detail can be made in wood epoxy, or for that mater with regular glue and fasteners, however with the right epoxy strategy you would be able to get the whole structure to act almost as though it was molded in one piece.

Here is something relatively quick you can do to strengthen a wobbly piece of MDF, or even better, ply. Take a fly cutter and rip a slot in the edge deep enough to accept a piece of rebar or threaded rod. Fill with enough epoxy glue rendered to a peanut butter consistency, or a little thinner, so when the rod is dropped in, it will/can be fully covered. You could allow the threaded rod to overhang the ends: Instant fastener. High strength and dimensional stability, no particular skill required to get perfect alignment and accuracy, no special tools.

http://epoxyworks.com/indexprojects.html, scroll to "non-marine uses", and pick "beam me up". In addition on the Westsystem.com site, search for the article on "Boatbuilding on a grand scale Testing large bonded-in fasteners
By Brian Knight, for info on hardware bonding.


In addition there are high tech fabrics that can be laminated to wood to increase stiffness etc... but those kinds of things tend to be a little pricey for what is required here. Usually just playing with thickness, integrating 90 degree parts so they support each other and act as a unified structure, or possibly bonding in stronger materials and fasteners will give you what you want.

Thick epoxy coating will crack if impacted. If impact were an issue, then a fiberglass cloth coating would be appropriate, this thickens the epoxy coating by stabilizing a thicker layer, and it supports the coating against cracks by improving it's tensile characteristics parallel to the surface. Normal flexing in wood, which is a very stiff material, will not crack the coating. MDF is just sawdust and glue, and is quite flexible, however these machines are not designed in such a way as to induce large bends in the finished parts so I don't believe cracking in epoxy coatings would be a problem in normal use. There is a lot of talk about more or less hard coatings, as though softer coatings were an advantage to reduced cracking. This is not generally true with wood deflection levels.

"due to the flexing of a router frame?" Key is to eliminate this through design.

Again, I am not trying to suggest people overcomplicate simple functional projects. On the other hand, I read one thing where someone said a machine would benefit from having all aluminum parts in the elements supporting the router. Aluminum adds weight, and properly structured wood is very efficient, think of violins, accoustics aside, I don't think aluminum would be much of an improvement. High strength, but stiffness to weight ratio is not all that wonderful. I also read comments about moisture problems "don't build it in irreland", and comments about ply machines being just a first step, "you will want to upgrade later".

[ger21]

One last thing do you think that over time the epoxy coat may crack due to the flexing of a router frame?

If you use a thin epoxy like West System, brushed on MDF, almost all of it will soak into the MDF, so there won't be anything to crack. You end up with MDF that's more moisture resistant, and has a harder surface. Easier to paint, too.

The West System manual has a lot of good info on using epoxy with wood. http://www.westsystem.com And be sure to subscribe to their free magazine.

[ThomD]

Good news, their free magazine is called epoxyworks, and they are trying to get it online from a certain point, at epoxyworks.com.