Joining MDF

[PJC]

Hey everyone,

I am working on building a rolling cart for my MS. I am using MDF. It will be a basic box with a shelf in the middle and locking casters on the bottom. This is my first time working with MDF, so I am not sure of the best way to join the top and bottom to the sides. I was thinking of using rabbet joints because I have never done a rabbet joint and MDF is cheap enough that I don't have to worry if I screw it up and need to buy more. Another way that I was thinking of joining the pieces was with biscuits. Does it matter which method I use? Is there a better method to joining MDF? If a do use a rabbet join, should I rabbet the side pieces and lay the top and bottom pieces into the rabbet, or do I rabbet the top and bottom pieces and lay them on top of the side pieces?

As for joining the shelf, I was just going to cut a dado on each side. Again, is this a good join for a shelf with MDF?

Finally, when gluing up the pieces is it ok to secure them with brad nails, or is MDF too unpredictable as to where the brad will ultimately wind up?

Thanks....I will clarify anything if needed.

Pete

[bigsteel15]

Hopefully Rod Kirby will chime in, but I would think biscuits are the way to go.

If you use rabbets, I would cut them into the top/bottom pieces and use screws, not brads. And pre-drill the screw holes extra deep.

[JimD]

Pete,

I will disagree with Brian but not to be disagreeable. I would dado the sides because it will make the joints less visible and you can cut the shelf at the same time. I would probably make the dados for the top and bottom deeper, 1/2 inch or a bit more, so that the fasteners could be installed on the top and bottom, again so they are less visible. The dado for the shelf should be no more than 1/2 the thickness of the MDF (I am assuming 3/4 material so no more than a 3/8 dado, I would probably use a 1/4). Screws are a good idea but not near the edge because MDF loves to split. I would probably screw the middle but use clamps near the end, or if you do not have big enough clamps, brads would be a good idea.

Biscuits are also OK but MDF isn't the strongest material so I would go with dados, I think it will be stronger (but I like biscuits just not for this application).

Jim

[Pappy]

I will give you a 3rd idea on part of it. Like Jim, I would use 1/4" dados for the shelf and the bottom. Cut the dado for the bottom 3/4" up on the sides and back and add spacers to mount the casters.

Cut a rabbet for the top to set in and then add a top sheet of either 1/2" or 3/4" MDF over that, flush with the outside of the cabinet. This will add strencth and give you a finished look around the top edges.

[crokett]

I built a similar cart for my drill press. I rabetted the bottom for the sides and also added a piece of ply on the bottom for strength. The top was simply glued and screwed - I was adding laminate to the top anyway. My thinking was I wanted fulll thickness of the top sitting on full thickness of the sides.

<edit>
correcting speeling errors and now it all makes more sense
</edit>

[Jeffrey Schronce]

I think I agree with the others. I'll biscuit 'bout darn near anything, but MDF has such a porous disposition and will be competing with the biscuit in attempting to suck up the glue. If the MDF sucks up more of the glue than the biscuit does, then the biscuit is not living up to its holding potential.
Titebond II is my standard and as a modified PVA glue it should work fine with MDF.
Screws are ok but make sure to screw away from the edges and use a coarse thread screw.
Dado would definately be the best way to go IMHO.

[PJC]

Thanks for the reply's everyone. I will try and go with the 1/4" dados. I was going to rabbet the top and bottom, but I like Pappy's idea of a dado 3/4" up with spacers for the casters on the bottom.

I do, however, have a question for Pappy (or anyone else who understands): For the top, you are suggesting that I rabbet the sides, lay in the top piece and then add on another top piece that spans from side to side? That's easy enough for me to do, I just want to be sure that I understand it correctly.

Finally, can MDF just be glued or does it need to be glued and screwed?

Pete

[LarryG]

Finally, can MDF just be glued or does it need to be glued and screwed?

It can just be glued. The main reason you'd also use screws (or nails) would be if you aren't able to get a clamp onto the assembly and need the mechanical fasteners to hold things together until the glue cures.

To get a good glue joint, the edges of MDF should have "sizing" applied. "Sizing" is nothing more than an initial thin coating of glue that you apply, scrape off most of the excess, and allow to dry for a couple-three minutes before applying glue as usual. The sizing semi-seals the edges and keeps the material from sucking up all the glue like a sponge.

(End grain of regular wood, and the edges of plywood, should also be sized. Some people dilute glue with water to make their sizing, but I just use regular glue full-strength and take care not to use too much.)

[PJC]

I had never heard of "sizing". Thanks for the insight Larry. I will do that when I glue the boards together.

[Pappy]

Thanks for the reply's everyone. I will try and go with the 1/4" dados. I was going to rabbet the top and bottom, but I like Pappy's idea of a dado 3/4" up with spacers for the casters on the bottom.

I do, however, have a question for Pappy (or anyone else who understands): For the top, you are suggesting that I rabbet the sides, lay in the top piece and then add on another top piece that spans from side to side? That's easy enough for me to do, I just want to be sure that I understand it correctly.

Finally, can MDF just be glued or does it need to be glued and screwed?

Pete

You got what I meant, Pete. I would rather beef up the top a bit too much than find a problem with it sagging under the weight of the tool. 1/2" and 3/4" glued together will give you a stable top.

[PJC]

Thanks for replying Pappy, I will do that then. I'll post some pics when it's done.

[scorrpio]

A number of magazines recently ran a feature on MDF joinery far as screws go. If you decide to use screws, here are some pointers:
1. Use uniform-thickness shank screws, not tapered. With coarse, deep threads.
2. The part you are fastening to: drill a pilot hole slightly deeper than the screw will go, and same diameter as screw shank.
3. Part that is being fastened - drill a hole just wide for the screw to 'slip through'. Countersink on BOTH sides. The bottom countersink will contain the 'mushrooming' of MDF from screw threads.
This will produce a very tight-fitting, split-free joint.

[Gary P]

request for sketch

My builders left me with a pile of MDF that I would like to use for garage cabinets. It would help me get started if I could see a sketch of the joins mentioned in this forum. Nothing elaborate, just a quick sketch to show me what you are talking about. I don't even need dimensions. Thanks.

[Black wallnut]

My builders left me with a pile of MDF that I would like to use for garage cabinets. It would help me get started if I could see a sketch of the joins mentioned in this forum. Nothing elaborate, just a quick sketch to show me what you are talking about. I don't even need dimensions. Thanks.

Welcome to BT3Central. Here ya go. Choose the 8th one down for good drawings. It is best to start a new thread with a question like yours rather than bring back one from several years ago.

[Gary P]

request for sketches answered

I didn't even notice the date. I'll look for that first in the future.

Thanks for the assistance. It will help get this project off the ground.