Dimensional Stability MDF vs Plywood

The builders of the DIY CNC routers overwhelmingly prefer MDF as the basis for these machines over plywood because they say that plywood is not as dimensionally stable as MDF. They claim the dimensional changes in plywood with weather and time would affect the accuracy of the machine.

I am toying with the idea of building one of these machines (haven't told LOML yet), and it seems to me that cabinet grade plywood would be as dimensional stable as MDF. My view is that MDF is better for flatness and surface integrity, but with all the glue and cross grains in plywood, its dimensional stability would be about the same. Since plywood is easier to join than MDF, and I have a lot of cabinet grade scraps leftover from a previous project, I would like to use plywood.

What do you think - is MDF significantly more stable dimensionally than plywood?

Interesting problem! I have no idea for sure, but I've always assumed that mdf is more stable that ply. But, it's hard to imagine that you would be using a piece of ply or mdf for a part that is so critical for accuracy that it couldn't tolerate ANY movement. For those parts, it would seem that you would use metal or plastic.

Anyway, don't count my comment for anything - I honestly have no idea!

Angie, this question gets brought up from time to time over at cnczone.com. Not plugging another forum, but there is a lot of DIY-CNC info there. They have a large photo gallery of home built machines, for ideas and inspiration.

John

My experience isn't that extensive, but I do know that when I store plywood or hardwood in my garage, it moves around. It always expands/contracts/warps in some way, shape, or form. The MDF that I store in my garage has never moved. Not one bit.

I would guess it's because MDF is basically just a bunch of saw dust that has been glued together. Plywood is just more susceptible to moisture and changes in humidity.

I dont know the actual technical specs between the two but I would suspect that a quality grade of cabinet grade plywood should be more than stable enough. I am definitly not talking about the crap from Home Depot or Lowes.
The advantage/disadvantage of MDF is the weight. And if not properly sealed, see how dimentionally stable the MDF is when it bloats into a pile of oatmeal.
Overall, I think a combination of the 2 should be OK because I dont suspect you could measure the difference between them. I have laminated hundreds of tables using both MDF and plywood and both are exceptionally stable otherwise the large library tables I built would be stressing the solid oak edges and they would have separated after 20 some years.

MDF is highly uniform (sawdust and glue) and so it expands/contracts equally in all directions with humidity changes. (And unless you soak it down, the expansion isn't bad.) No warping.

If any of the plys in a sheet of plywood are different material, different density, etc. they can cause the sheet to warp when the humidity changes. Also, the thickness of the sheet is not as uniform as a sheet of MDF.

I have jigs made of MDF, and also some left over off cuts (I don't think I'll ever use MDF again....even with great dust collection, it seems to get everywhere). Nearly every piece of MDF I have has at least some bowing/warping. My shop lives are 75 degree and 40% humidity, by the way.

One thing I noticed, though, is that the MDF does not take a "set" like wood or plywood will. Even though it bows, it straightens out very easily with a little bit of pressure, so it seems like the ideal substance to build things like torsion boxes. If it warps a bit, it won't try to tear the underlying structure apart, yet it's very strong in shear. That's just my observation from limited MDF experience.

There is a certain kind of MDF that uses more and a better glue that
makes it stronger and more water resistant. I forget the name used
right now but I think the special water resistant extra strong
MDF would be better than plywood for critical use

I don't visit the CNC forums but, like here, I bet there is some great info from folks who have been down the road you are considering. Based on my unscientific observations of small panels of various grades of ply and MDF that have lived in or near the cutoff bin for years; MDF is more stable. I know what it is like to want something to be a certain way so you can use up some scrap material as opposed to buying what you are recommended to use but, I would learn from others experience and save the ply for something else.

Well it depends on how you measure dimensional stability and which aspects of stability you are more interested in.

Wood will expand or contact whenever it experiences a change in humidity and temperature.

MDF is a composite product manufactured where wood is refined into a fibrous material which is mixed with resin and wax and pressed under pressure and heat until it cures. Plywood is generally made by combining an odd number of plies of veneer which are orientated at 90 degree angles.

When you talk about wood movement it is important to understand that wood has three primary directions. The longitudinal direction is from the base to top of the tree. The radial direction is form the middle of the tree (pith) out to the bark. The tangential direction is along the circular growth rings. Wood expands and contracts differently in these three directions with the most movement occurring in the tangential direction, followed by the radial and lastly the longitudinal direction. This is important because when plywood is manufactured the plies are orientated with the longitudinal direction arranged at alternating 90 degree angles. This creates a composite that opposes movement. Unfortunately, when plywood is not carefully balanced (poorly manufactured), the disproportional movement will create warp.

MDF is less prone to warp because the composite randomizes the three primary directions of wood. The result is still movement (actually more movement when compared to plywood) but it is more equal in all directions.

So if you are looking at less movement, choose plywood. If you are looking at maintaining a flat surface while the wood moves choose MDF.