newbie: plywood or mdf?

[dandydon]

I plan to build a wood constructed cnc router (about 14 by 16 inch). I was originally thinking of using a combination of hardwood and plywood but I see that most people use mdf. Other than price, is there a reason that mdf is so popular? Is it less prone to warping and size changes due to humidity etc.? Thanks in advance for your comments and suggestions.

[fireballcnc]

I'd go with MDF. Your machine should be kept away from excessive humidity
no matter...in most cases...what it's made from. The steel parts rusting
are the first indication that humidity is too high.

I've built around 50 small desktop routers with MDF. If you keep it small and
tight, don't screw into the ends of it, drill and bolt everything together,
it'll last and be very reliable. It should be sealed and painted.

I wouldn't build a machine from even high grade plywood myself. It'll
change size. And if it changes even a few thousandths...un-noticeable
in a nice cabinet...it could throw off your alignment and cause binding.

I have MDF machines that have run for years. I've disassembled some
of them to use parts for others and the axis still moved as smoothly as
the day they were built. I've just started making a small gantry model
and I even use it to make parts for all my other machines.

http://www.fireballcnc.com

If it's carefully designed and Not used for milling metal, an MDF machine
works very, very well and is highly reliable. Especially a small one like you want
to make.

Don't flood it with coolant and you'll have no problems...

John

[Mr.Chips]

One other thing about MDF it is like a sponge when you paint, I found this suggestion in one of my Wood Working Books.
Mix White Glue and water 50/50 and brush this on as a primer, this seals the MDF up and ends having to put on so many coats to get it to look good.

I did not know about this tip when I built mine and I used a commertial paint primer, it didn't seal all that well and it still took about 3 + coats of Oil Base paint to get is in good shape.

Another tip about MDF is to put a couple of drops of Super Glue in the holes and let it dry where you will be putting in screws. It will prevent the threads from stripping out.

[fireballcnc]

Yep! That's 1000%!...

FWIW...here's a .pdf file I made a while back to go with my
unpainted machine kits....

http://www.fireballcnc.com/cnczone/Painting_MDF.pdf

It's taken many, many, many tries to come up with an easy to do
*system* of painting that's cheap, quick and still looks good.

Best I've come up with ...yet...
John

[under-dog]

Although plywood is stronger it has a tendancy to warp which I would assume to be bad for something like a CNC which you are looking at accuracy for.

MDF is more stable but as said need more support than plywood as it is not as strong.

[dandydon]

Thanks for the feedback and tips on painting. I really appreciate it.

[carlnpa]

I asked this same question a few years ago. IMHO plywood is the better way to go. The machine I built in plywood and painted is still going strong. MDF is very sensitive to water and high humidity (swells up and decomposes), the edges get soft over time and flake. I like the plywood, it is far stronger, if you use MDF get the melamine coated stuff, is only slightly more expensive, and holds up far better.

[2muchstuff]

I would spend the extra money and use aircraft ply/ baltic birch veneer plywood. The plys are veneer thin with no voids in the wood and very solid.

[under-dog]

What about something completely synthetic. I worked.......years ago at a sign shop when I was in HS anc college. We were beginning to use a closed cell foam that they were using in aircraft wings as filler and such. It came in a variety of densities and strangths. Very stable.

Not that this is the answer but there may be some sort of synthetic, stable, strong alternative out there.


I was graduated HS in 1990 so i am sure since then technology has advanced as well.

[Mr.Chips]

Originally Posted by carlnpa I asked this same question a few years ago. IMHO plywood is the better way to go. The machine I built in plywood and painted is still going strong. MDF is very sensitive to water and high humidity (swells up and decomposes), the edges get soft over time and flake. I like the plywood, it is far stronger, if you use MDF get the melamine coated stuff, is only slightly more expensive, and holds up far better.

True MDF is very sensitive to moisture, that is if it isn't sealed and painted. My machine is 4 years old and has three coats of Oil Based Paint, and not any signs of swelling or deforming. So it is a good workable product but it must be understood and used properly.

[fireballcnc]

There's one sure thing about MDF. Those who don't like it rarely ever
will. I used to ~Hate~ the stuff myself. But I kept seeing others making
excellent use of it. So in 2001, I built my first machine from it. I was
truly amazed at how flat and stable a piece of new store bought MDF
was...as opposed to the free scraps I'd used before.

I upgraded to an Aluminum machine. It didn't work any better. I built
a machine from Very high quality cabinet grade plywood....it warped.
Just *barely*...but enough to cause binding and headaches galore.

There are many very well thought of machines on this forum built from
MDF. (Almost) Any machine shouldn't be exposed to moisture.

A first attempt at building a machine is likely to lead to at least one
second attempt. It's pretty easy to see how you'd have done a few
things differently...so MDF is a lot cheaper to use than most materials.

MDF bolted together(instead of just screwed), painted and sealed, and
kept away from moisture(you wouldn't leave your drill press or table
saw out in the rain, would you...) will make an extremely durable,
rigid, and Highly reliable machine. A Very, Very Precise machine too!

I've built about 50 machines. MDF has proven itself to me...

John

[fireballcnc]

Here's a video of my new machine making an adapter for this
high precision spindle....

http://stores.ebay.com/Wolfgang-Engineering

http://www.fireballcnc.com/cnczone/W_Adapter3.wmv

This adapter has to be right the first time. It's being cut with a
Rigid trim router using a 1/4" carbide upspiral router bit.
Speed of 40 IPM...slower for precise cutting...

Material is cast rigid polyurethane...same as my z axis is made from.

The finished product....

http://www.fireballcnc.com/cnczone/W2007.jpg

It fits in my Dremel model 300 tool holder....

John