MDF butt joints

[Joren_Wendschuh]

How can you join MDF at right angles to itself, without it splitting due to the screw wedging the ply's of paper appart? I'm trying to build a box with the two 'large' sides being 3/4 inch MDF, with the four 'connecting' sides between the two 3/4 inch peices, being something like .3-.4 inches thick.
This box needs to be able to be strong enough to not be totaly messed up when dropped from a couple feet high, the outer, heavier boards, being dementions of about 8" by 24"

[Source4]

Make sure you pre-drill and counter-sink your holes for your screws and make sure its straight. If you do that there should not be a problem with spliting. And to make a strong joint with MDF I use lots of glue And about 3 2½×10 Coarse thread screws per foot. Hope that helps you out.

[ship]

S-4's idea will work, but I would take a different tact to it.

MDO does not have a paper face much less paper layers. It's particle board that's a bit more strong in using finer saw dust making it up. Are you thinking MDO plywood that is a good grade of plywood with a paper face on one or both sides? Good stuff, very strong. Or are you talking about some form of hardboard that uses less dense layers of paper or fibers and is available in thicker sheets?

In any case, the MDO should work at least decently for your purpose when as with S-4 says, glue and the 2" or 2.1/2" screws to a point. That point is the stress you put on the joints when it's dropped. Given you will want to pre-drill and countersink the holes. The 6" on center is decent but I might look towads 9" on center for the screws and 2" screws instead. Reason for this is the more screws you have and the deeper you get in after a point, the more they are going to act as if a wedge in tearing apart the layears of plywood when dropped or stressed. Given 3/8" thick plywood attached to the 3/4" plywood, a 2" #6 screw would be about the maximum I would use. Even a #8 wire size screw will be creating a larger wedge even if a #6 is stretching it in sheer/breaking strength. Perhaps a 8" on center to make up for it.

This is given plywood or MDO faced plywood. If you are really using medium density fir or fiber pine, it holds up better than particle board to abuse but still is not what I would tend to want to put a screw next to the end of, much less would trust a 3/8" side panel to in the possiblity of there being something under it as dropped. Just might break thru the MDF and be a hastle to repair once glued.

MDF will have much more trouble in splitting between layers than plywood that has more fiber to absorb the screw. It also will show more dings than plywood when it's dropped and there is a possibility that the 3/8" thick stuff will crack or split around where the screws are once stressed, especially if too close to the edge.

How about plywood that's sanded, laminated or sethat you install a paper or fabric face on? A fabric covering or even fiberglass covering will add a lot of support to it.

The best engineering plan for any of the sheet goods would be a sub frame inside the box to attach to. This way you are going into the side grain of lumber, not the end grain of sheet goods, and your attachment screws will be further away from the edge of the material. It will also give much more support to the box in providing a larger glueing surface for attachment. The sub frame will also support the thin materials sides as they butt up to each other. Note also that the 3/8" plywood without support will not support a lot of weight or falling abuse without a frame. What's the length of the box anyway given the 3/4" thick sides are 8"x24"? Given this, 8" thickness and no matter what the lumber type I might look to using 1.5/8" screws instead of even 2" screws.

[Joren_Wendschuh]

MDF being Medium Density Fiber Board... Essentialy a paper like substance that, if 'flaked' will come off in peices that look like paper pulp.

Hope you dont mind a bit of input from UK, but we use mountains of MDF.
The best way here in my opinion, is to rout rebates down both sides of two of the faces - say just under halfway through. It will obviously be the same thicknes as the board.
You now have glue on 2 faces of each corner instead of one face on each corner. This is much stronger but you still need plenty of screws - but they would now need to be smaller.
The routing involved here is very simple and diy routers can be incredibly cheap now.
Also, whichever method you use, do try to use some sort of reinforcement to the joints. Theres lots of things created especially for use with MDF - blocks or angle brackets etc.

[Joren_Wendschuh]

Hehe, no, I dont mind I apreciate all input!
routed 'rebates' ... what are those? are they equivolent to "dado's" or, essentialy a grove cut into one board, the other board's thickness wide, for it to slip into?
When doing Dado's, being that I dont have proper 'tools' if you will, I use an assortment of approximatly the same cutting blades on my table saw... the width of the cut I need. I just go slowly, having the finer toothed saw blades on the outside, and the 'wider' toothed blades on the inside, to rip.

[How2Junkie]

Great thread guys. I'm not trying to hijack this thread buy what if the MDF joined at an angle less than 90degrees. I'm working on a book case of sorts that calls for a back connected at right angles to sides which are beveled at 22.5 degree angles at the front to fit flush with another piece which is beveled at again 22.5 degrees on both ends this time with the front end connected to a face board.

For a better description, basically it's almost like an octogon cut in half. I was considering using dowels and glue and nail gun for the joint, but after some investigation, blocks on the inside used to anchor like a bracket could be a posibility. The unit may be moved alot etc, so I was trying to get a feel for the best joint for the application.

It's not going to be dropped or anything, but it may have to support some weight (app. 150lbs). It will have a fixed shelf at the top which fits flush with the walls and a fixed shelf at the bottom about 4.5 inches from the bottom. There will be 3 adjustable shelfs within the unit. It will be in a highly visible area, so clean angles are a must. I'm also kinda lazy, so the easier the better.

Thanks in advance for your help.

[ship]

Biscuit and block it would be my idea.

[Mayhem]

When building speaker boxes I have used a variety of methods outlined above and for simple box design I have found that PVA glue and a nail gun has provided just as good a joint as screws and dowels or biscuits. In fact, I have tested the bonding strength of PVA glue on several types of board in which I have glued two pieces together and once dry, hit one piece with a hammer. On all but one occasion, the joint held and the break occurred either above or below the joint. I guess the type of joint fastening that I use depends on the box. On many boxes that I build, I use corners (either moulded plastic or metal) that screw onto the box and will add strength. Sub or bottom end boxes will also have braces to improve strength.

I work for a custom/semi-custom fish tank mfg. I build MDF cabinets to support the weight of 200+ gallon tanks. We simply wood glue and brad nail our joints (corner, lap etc.). The brads are used mainly to hold the joint together until the glue dries. The wood glue creats a strong joint and we've never had a failure that I'm aware of.

Hope this helps.