Plywood or MDF for a Jig

[BeauxTi]

Hey guys - I am building a jig to allow me route slots on small stock - as the slots will go all the way through the wood I will need some sacrificial material under the stock. The top guides that the Router will rest on and use a guide is 3/4 MDF Melamine. I started with the Idea of using 2 sheets of 1/2 plywood but got to thinking about MDF. I am guessing you all have far more experience with this type of work than myself. If you have opinions - I would love to hear them before I wrap this bad boy up. I am thinking about weight (Not too important) and warp stability.

[chopnhack]

As far as sacrificial material goes, the only real differences I see would be the potential for plywood to tear out some and for mdf to dull your cutters a little faster? Either choice would be fine. If the sacrificial piece is to provide structural support to the jig you might opt for ply.

[dbhost]

Keep in mind that your location lists Texas, but not where in Texas. If you are NOT in the western (Desert) part of Texas, keep in mind that MDF swells up like someone with allergies to a bee sting when exposed to humidity over the long term... I personally hate the stuff.

[chopnhack]

Db, have you had that happen to you? I have had mdf sitting in the garage for years and always feared it would happen and never used the stuff, but I see no noticeable difference after all this time.

[cabinetman]

As far as sacrificial material goes, the only real differences I see would be the potential for plywood to tear out some and for mdf to dull your cutters a little faster? Either choice would be fine. If the sacrificial piece is to provide structural support to the jig you might opt for ply.

+1. Either would work as a substrate. How well they would hold up might depend on how much they're used and how they are handled. If the through slots will be in the same place, the plywood may only get furry once.

I don't see MDF reacting much to humidity per se. Here it can be 50-60% at 9 A.M, and at 2 P.M. can reach 95%. It will respond to direct moisture contact.

.

[BeauxTi]

Thanks guys - I am in the north Austin area. I think the tear out would be okay and I think I would prefer it to dull bits. I too worry about MDF deforming, but recall years ago my son built some speakers out of the stuff and the remains of that effort hung around the garage for what seemed like ages without deteriorating. Go figger ;-)

[chopnhack]

Exactly, that has been my experience too... the stuff seems to be dead stable, unless you dip it... then it swells and becomes spongy after it dries.

[LCHIEN]

if you don't need real clean exit side of the slots, then maybe you could back it up with a couple of layers of corrugated cardboard or even non-corrugated cardboard. Cheap and easily replacable.

[pelligrini]

Is the jig is intended to be a one time thing, I'd use MDF or maybe some B/C ply. If it's meant to last years, I'd use some good baltic birch.

I've had MDF swell too, but not terribly.

[cwsmith]

I'm not "picturing" your jig, so I don't know the components, any fastening points/methods, or any of the stresses that you may be exerting on the jig.

Personally, I can't stand MDF for anything. It's nothing more than glued dust particles and I find it not only a mess to work with (dusty and bad smelling), but there is very little integrity to it other than compression. (It sags, warps, and fasteners pull out much too easily.)

A good grade of plywood would be preferable over MDF (for me), but I don't find plywood all that attractive either, for both weight and some of it's holding failures, especially edge. It is stable though.

I much prefer solid wood, as it is usually worked better and is more attractive. Provided it has proper moisture content and is properly sealed, my esperience is that it is pretty stable.

Of course if this is just a temporary jig, probably ply would be more cost effective.

CWS

[dbhost]

Db, have you had that happen to you? I have had mdf sitting in the garage for years and always feared it would happen and never used the stuff, but I see no noticeable difference after all this time.

Yes. I can cite at least two specific cases.

Case #1. My old Grizzly H7827 drill press table that I bought of of LCHIEN. It came to me just fine, but within a year, the Melamine coated MDF swelled up like a balloon and was anything but flat. I particular it swelled around anywhere that had metal.

#2. Another example would be a jig I made a few years ago. Again melamine coated MDF. It was the infeed / outfeed tables I made for my Ryobi planer. They swole up within 6 months and made me tear the whole shebang apart and rethink my approach.

#3. My first Thien baffle was made of MDF. It lasted a year and a half before it swole up, again where metal contacted it.

[pelligrini]

David, Was your MDF problems happening more in the cooler months? With your statements about it especially swelling near metal I'm guessing that the metal was attracting a lot more condensation and affecting the MDF. You've got some insane humidity issues being so close to the Gulf too.

I like working with MDF, except for the dust which I can handle fairly easily now.

[dbhost]

Keep in perspective just how close to the Gulf I am...

1 mile from my house is Galveston Bay.

During hurricane Ike, the high water line was at the front of my subdivision. I am on high ground for my area, which is why I bought the house I did...

I am fairly certain sweating / condensation issues have a LOT to do with my MDF swelling problems. I do not experience the same issues with plywood. Just what I am experiencing...

The RH right now is 65%, and that feels relatively dry to me...

I am sure my insulation project will go a LONG way to helping out with humidity problems. I get condensation all year long outside, so damp it is hard to notice the inside sometimes...

I guess it is like living in Seattle, but the rain is in constant suspension in the air we breathe...

David, Was your MDF problems happening more in the cooler months? With your statements about it especially swelling near metal I'm guessing that the metal was attracting a lot more condensation and affecting the MDF. You've got some insane humidity issues being so close to the Gulf too.

I like working with MDF, except for the dust which I can handle fairly easily now.

[chopnhack]

Yeah, thats wicked DB, but the swelling was only occuring at the metal fasteners and no where else? I too am not far from the gulf, but I haven't seen the humidity cause as many issues. I did have one speaker box that I made out mdf over 10 years ago, been in the garage for at least 7 before I broke it down, assembled with those brass colored machine screws with no swelling.... the garage is well insulated. Hope this means you will find more utility from your current project :-)

[woodyb]

are you sure you are using MDF with melamine ? I have never seen that. here is central texas, the only melamine I see is laminated onto particle board which really is crap. I have MDf sitting outside on the screened in porch for the last couple of years. even in the fog it has not swollen or shown any problems at all.

however, the last sheet I brought home in the truck got some water spilled on it and it took me a while to sand off that bubble.


robert