MDF vs Plywood

[RockBoy]

I've just finished building my first MDF machine, things were going well until i noticed that the humidity of summer has caused parts of the machine to swell. The biggest impact this has had is the fact that my x-axis lead screw now has a bow in it (just threaded rod) and the machine is vibrating.

So now i'm about to start my second build and I'm against the idea of MDF, it just isn't working for where i live. I don't have the equipment to work with steel/aluminium so I'm wondering if plywood is a viable alternative. Why do i rarely see it being used by members of the zone?

[gmfoster]

I've just finished building my first MDF machine, things were going well until i noticed that the humidity of summer has caused parts of the machine to swell. The biggest impact this has had is the fact that my x-axis lead screw now has a bow in it (just threaded rod) and the machine is vibrating.

So now i'm about to start my second build and I'm against the idea of MDF, it just isn't working for where i live. I don't have the equipment to work with steel/aluminium so I'm wondering if plywood is a viable alternative. Why do i rarely see it being used by members of the zone?

Plywood moves as well. Seal what ever you are using, that is keep the moisture out. In fact most all wood moves with temp and humidity.

Garry

[ger21]

The only plywood I'd recommend is Baltic Birch. It's what I'm using for my first machine, and the next one I'm working on. It's the only plywood that'll stay flat, and it's extremely strong. But it is 2-3 times more expensive than MDF.

[Drools]

What Gerry said +1

[bogiestl]

From my speaker building experience, I prefer MDF for dimensional stability, and elimination of possible "lose" vibrating pieces... And yes, I've also used Baltic Birch...

But everything that is wood/porous will change with weather. You could try disassembling it, and sealing it... Or maybe make a rough-cut (you don't have to machine it...) frame/platform of angle, and then attach it to the MDF, and screw to other stuff to it...

[drhanger]

The only plywood I'd recommend is Baltic Birch. It's what I'm using for my first machine, and the next one I'm working on. It's the only plywood that'll stay flat, and it's extremely strong. But it is 2-3 times more expensive than MDF.

I'm making mine from Baltic Birch plywood as well, what is a good product to use to seal it?

[ger21]

Epoxy, followed by polyurethane.

If you skip the epoxy, a lot more will soak in, especially on the end grain edges, and you'll need more coats for a good finish.

[bobsch]

Epoxy, followed by polyurethane.

If you skip the epoxy, a lot more will soak in, especially on the end grain edges, and you'll need more coats for a good finish.

Zinsser SealCoat http://www.rockler.com/product.cfm?page=10310 also works well to seal MDF before applying poly.

[PropNut]

I have several machines made of MDF, including my 4x8 CNC router. I seal them with garage floor (concrete) paint, I have had no problems with the very humid west Michigan weather. Gerry is correct on sealing plywood though, you really need a sealer coat on the end grain when working with that material.

A bit off topic but isnt it ironic that the cheapest plywood that I use in my cabinet shop has always been baltic birch and now it is the most expensive? I used it for all my cabinet carcase assemblies and then veneered it to whichever species that the customer desired (outside finished ends, etc). In the last couple of years it has become that most expensive. Supply and demand I guess, and a building boom in east asia.

Edit: I used BB and BBBB grade but even those are getting too expensive these days to show a profit on unless I want to raise my prices, and it is already hard to compete with cheaply made cabinet from the big box stores.

[Big-tex]

I have several machines made of MDF, including my 4x8 CNC router. I seal them with garage floor (concrete) paint, I have had no problems with the very humid west Michigan weather. Gerry is correct on sealing plywood though, you really need a sealer coat on the end grain when working with that material.

A bit off topic but isnt it ironic that the cheapest plywood that I use in my cabinet shop has always been baltic birch and now it is the most expensive? I used it for all my cabinet carcase assemblies and then veneered it to whichever species that the customer desired (outside finished ends, etc). In the last couple of years it has become that most expensive. Supply and demand I guess, and a building boom in east asia.

Edit: I used BB and BBBB grade but even those are getting too expensive these days to show a profit on unless I want to raise my prices, and it is already hard to compete with cheaply made cabinet from the big box stores.

I remember days when baltic ply was coming from Finland then from russia.
Now days we have plenty of imports coming from China and indonesia.
Be careful this stuff is just like Chinese toys with lead paint. Formaldahide, barium, arsenic. Arsenic wasn't that what Napoleon was poisoned with?
Now day we kill our population with same chemicals....my $0.02.

I agree with Garry all wood products should be sealed at end grain this is where moisture gets absorbed. As per any kind of ply all edges since ply construction is odd number of layers for balancing and cross grain layering for warping.

[dkirtley]

I really can't see messing with MDF. The only real thing that it has in its favor is being cheap and fairly uniform. It doesn't hold fasteners well. It doesn't glue very well. It is really weak in comparison to most things in just about any direction except compression. It has all kinds of nasties in the binder that just are not very healthy. Even with an epoxy coating, it is not impervious to moisture. Epoxy will slow absorption, not stop it entirely.

Some places carry MDO, which is the stuff they make signs out of. Some places use it for concrete forms. There are some phenolic laminated plywoods that are pretty nice but they are hard to glue together into thicker members. If you have it available, marine grade ply is really nice also but tends to get pricey but still cheaper than a lot of materials. It is put together much like the old baltic birch you used to find. BS 1088 is the nicest grade. It does not have any voids.

Hardwood plywood is much better suited to structural use. If you seal it well, plywood will last as long as just about anything else and it is much stronger per unit weight that just about anything you can name that is commonly available.

Don't discount using real honest to goodness solid wood as well. If you orient the grain properly, the expansion is not really an issue. It doesn't change much at all in length, moisture makes it expand radially.

[PropNut]

To each his own I guess. Everyone has an opinion, as I said, MDF works very well for me. My machine has been in operation for about a year now under pretty regular use. Could I have built it out of aluminum or steel? Sure, but I wanted to build it cheap (part of the fun for me) and MDF fit the bill (pun intended). No, it is not a perfectly precise machine, I will never get it to .0005 accuracy but it is close enough to cut cabinet parts and PCB's for hobby electronic projects. I was working with 1/2" aluminum the other day and am very happy with the results. Does this mean that I am "stuck in the Dark Ages"? Perhaps, but perhaps there is more than one way to get the job done.