MDF with Plywood

[spalm]

I have been doing a lot of thinking about incorporating torsion boxes into CNC machines.

I love Baltic Birch ply, but the cost starts to add up. Any thoughts about using MDF as the inside matrix with a 1/4” Baltic Birch skin? Does the humidity problems with MDF need to be considered with the more stable ply skin? I am afraid the MDF will want to move, and the ply won’t let it. I believe that MDF only grows in “thickness” and if I cut my struts (matrix) “on edge” that I will be OK. Is this true?

Steve

[ger21]

MDF is a lot heavier, but shouldn't be a problem. You could seal the faces with shellac, and make sure you have a good coat of glue on all the edges and you should be fine.

[ignatz]

Hi Steve, I think you might have MDF and Plywood backwards as far as expansion goes but I'm not an expert. Look in this thread for "Dave's_Not_Here"'s posts regarding sealing MDF and using Plywood. The sum (that I got) from his posts seems to indicate that properly sealed MDF won't change dimensionally whereas the only way to completely seal plywood is to wrap in plastic (Tyvex), otherwise it will change dimensions with the relative humidity changes. I hope this helps.

[ViperTX]

The only thing that MDF will do after it's sealed is cup (sag) on you if the underside is not supported.

[spalm]

Ignatz, pretty sure I got it right. Someone else please chime in.

Dave was talking about solid wood. It will swell and shrink quite a bit through out the year. Not a good choice for making a machine. Furniture makers do all kind of tricks to allow this movement. Do not try to stop it; you will end up with cracks. Solid wood also has the problem of edge (long) grain and end grain. End grain can not hold glue. This is why dovetails and biscuits and rabbits and dados etc. are used to always try to glue edge grain to edge grain or to add mechanical strength because the glue will not. On the plus side it is beautiful.

MDF will grow in thickness when it absorbs moisture and not return to its original size. It is compressed and always wants to become un-compressed. It is also too heavy for its own strength and needs to be supported. On the plus side it is cheap, consistent, and stable if sealed.

Plywood is the most stable of these three. Every other layer has the grain running in the opposite direction. It does not change in size (for all practical purposes). Baltic Birch ply (as I have always known it) is the king of plywood. It has nine plies (layers) to the half inch. This is almost double the count of normal plywood. It is guaranteed to be “void free” meaning that there are no knots or missing pieces even in the internal layers. It mills extremely well. You can see it by looking at Ger21’s machine. Woodworker’s use it for drawers and jigs. I even think the edge looks pretty. You have to buy it from a wood store. The stuff I get is metric.

Steve

[ger21]

Originally Posted by spalm MDF will grow in thickness when it absorbs moisture and not return to its original size.

Smaller pieces will shrink and expand. I used a 3/8" wide strip for a runner in a table saw cutoff sled. I CNC'd the panel square and with a dado to accept the MDF runner. At the most humid time of the summer, it's too tight to slide. And in the winter, it's very sloppy, so it does shrink back.

If you actually get it wet, however, it will not shrink back at all.

The biggest problem with using MDF for the frame is keeping it flat while you glue up the panels. But once you get it glued up, you shloudn't have any problems at all.

Originally Posted by spalm Plywood is the most stable of these three. Every other layer has the grain running in the opposite direction. It does not change in size (for all practical purposes).

Actually, MDF is more stable than plywood. A sheet of MDF, lying flat, will stay flat. A sheet of plywood will amost always warp. With the exception of Baltic Birch. I cut parts for my router out of Baltic Birch, and they stayed flat for almost 2 years before I got around to assembling it.

[gdc]

Steve has it mostly right. Wood however, is normally dimensionally stable lenght-wise (along the leght of the wood grain). It will expand dimensionally perpendicular to the grain of wood. The lenght of expansion depends on the wood. That's not to say there will not be lenght-wise expansion, just not as much as in width. Sealing the wood will limit the amount of expansion, but it's going to happen and perhaps more than you would want (for a glued and screw structure).

MDF is the most dimensionally stable. But it's heavy and has to be sealed. On the edges you can use a slurry of wood glue and water. Sand to 220 and coat (laquer or poly). The surfaces must also be sealed. Using a laminate is quick and clean. Leaves a nice slick surface you can draw on and is tough.

I use torsion boxes for the bases of my moving table and cabinets. Dimensionally stable and will support a great deal of weight. But they are HEAVY.

Plywood is also dimensionally stable (but not as much as MDF). No real reason to worry about expansion as it is neglible. It is still advisable to seal it.

[spalm]

I agree with both of you.

Cheap plywood will certainly warp because of the crap that they put in the middle. I have had excellent results with Baltic birch and pretty good results with marine grade ply.

The difference of expansion in length vs. width of hardwoods has been beaten into me by Fine Wood Working magazine for the last 23 years. (I’ve kept every copy)

The MDF that I have stored for a long time standing on my shop floor has absorbed the moisture from the floor and the edges are all flared. This is what I was referring to. I could see the winter/summer thing too.

I need to refine my definition of dimensionally stable, kind of reminds me of accuracy vs. repeatability.

I can not express the good qualities of Baltic birch enough. Anyone that has not tried it needs to get some and play with it. Throw a piece of 1/4 inch on the CNC and route a clock gear or something. You will be impressed.

Now back to my torsion boxes….
I want to desing a machine like Gerry's only smaller. (I want to get rid of the flying pipes in the worst way.)

Gerry I know what you are talking about with keeping the webbing flat while you are gluing up the panels. I have been trying to come up with a “system” that I could share with others. MDF seems so heavy when you are caring a sheet around, but cut it into two or three inch strips and it almost floats.

One thought for the gantry walls was to route a webbing into a 3/4 MDF board (leaving some for the important connection points) and then skinning it with 1/4 inch ply. Pretty strong and light, but I’m not sure if it is worth it. Do you think it would be stronger than solid MDF? I am only talking 8” by 22”.

Steve

[2muchstuff]

I agree that Baltic Birch plywood is great stuff and very stable. I use it to build radio controlled scale boats. To seal it up I use a 2-part epoxy thinned with acetone (2:1) for the first coat. Thinning it slows the set-up time slightly and allows it to soak into the wood deeper. The second coat is done without thinning. I have never had any problems with water or moisture getting into the wood. All of my boats are as true as the day they were built.

[spalm]

2Much, pictures please. Do you CNC them?

I built a canoe out of okoume ply. Sealed with epoxy.

[ger21]

Originally Posted by spalm Gerry I know what you are talking about with keeping the webbing flat while you are gluing up the panels. I have been trying to come up with a “system” that I could share with others. MDF seems so heavy when you are caring a sheet around, but cut it into two or three inch strips and it almost floats. One thought for the gantry walls was to route a webbing into a 3/4 MDF board (leaving some for the important connection points) and then skinning it with 1/4 inch ply. Pretty strong and light, but I’m not sure if it is worth it. Do you think it would be stronger than solid MDF? I am only talking 8” by 22”. Steve

I have a large, relatively flat outfeed table on my table saw. It's slightly lower the the actual saw top (it was level at one time ). My baltic birch frame ewas very flat by itself, so I laid one end on the cast iron table saw top, and shimmed the rest to make sure it stayed flat everywhere. (As flat as I could check with a 4 ft level. I then glued and stapled the top skin, flipped it over and did the same to the other side.

As for skinning a 3/4" MDF web, it should work pretty well. A piece of MDF with plastic laminate on both sides is very stiff, should be similar.

[2muchstuff]

spalm,

Beautiful looking canoe. We have a guy in our Kansas City branch that also builds canoes.

Due to time constraints with the 'boss lady", kids, work and the house, I just don't have the time to go downstairs and spend hours cutting out wood for my boats. Three years olds make that tough to do. For years I've wanted to build a cnc router but never have until now, well still gathering parts though. I figure that I could set everything up, cut parts and bring the parts upstairs to assemble while still watching the little one. Sounds good in theory though.

Pictures, pictures, pictures everybody wants pictures. I don't have a digital camera but there are pictures of one of my boats on the web. The C.S.S. Albemarle, radio controlled, 1/32 scale (3/8"=1') scratch built, 60" long, weighs 50 lbs., twin screws and over 5000 rivets in her, YES I said 5000.

http://www.stlouisadmirals.com/admir...boat.asp?id=14
http://www.stlouisadmirals.com/admir...ure.asp?id=102
http://www.stlouisadmirals.com/admir...ure.asp?id=103
http://www.stlouisadmirals.com/admir...ure.asp?id=121
http://www.stlouisadmirals.com/admir...ure.asp?id=122