MDF butt joints

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"
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.
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.
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.
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.
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 agree with Ship here.

1. I would never use MDF for a box that will be dropped and handled in a rough way. Why MDF? What advantage does it give you? Just make it out of plywood.

2. If I had to use MDF, I would biscuit and glue it, and use a brad nailer. Alternately a spline on the table saw, or an interlocking joint on a shaper. However biscuits would be the simplest. Then add some blocks inside.

As for a glue and nail joint being as strong as a biscuit joint, as Mayhem suggests, even if you properly clamp it as you must with PVA glue, with all due respect he's simply wrong. Possibly under ideal test conditions, but in practice, no. How could it be?

I had a full time cabinet business for over 15 years while I raised my family and before I got back into theatre, so I know whereof I speak. :)
BTW, the term rebate as used by our English friend would be rabbet in this country. It is a simple notch running the length of the panel- very hard to describe with out a picture.

If you set your table saw to cut 1/4" and lower the blade to about 3/8", and then run the panel through twice, once flat and once on edge, you will get a rabbet. The first cut on the flat will leave a 3/8" deep dado, 1/4 " in from the edge. And the second willremove the waste, leaving a 3/8" x 3/8" rabbet.

This joint mostly serves to hide the back for a bookcase and adds little structural strength.
Will said:
As for a glue and nail joint being as strong as a biscuit joint, as Mayhem suggests, even if you properly clamp it as you must with PVA glue, with all due respect he's simply wrong. Possibly under ideal test conditions, but in practice, no. How could it be?

To put my statement into a practical (as opposed to ideal) perspective - Over the years I have constructed about 20x 12" and horn top boxes and foldback wedges using this method. I still have 6 of these for my own shows and the rest were made for customers. Not one box has failed or come back and I know that some have taken quite a beating (mine included). These boxes were constructed out of 16mm water resistant chipboard and glued and nailed.

Every 12 months, each box gets stripped down and the wiring checked and the box vacuumed out. During this time I check the structure, fill in any chips and give it a quick lick of paint. Some of these are now 8 years old.

One of the considerations for me was the time and therefore cost involved using this method and other methods. As I have found (in my own experience) that the results are the same this is the option that I have taken.

But I take on board your experience and also agree that if a box is going to be dropped or thrown around that this method will not suit. However, keep in mind that I am specifically discussing my experiences in building speaker boxes, which I would hope are treated with a little more respect given the value of their content. Also given their size are more resilient than larger boxes or set pieces that will flex and be subjected to more forces.
Sorry I was so blunt, Mayhem, and thankyou for not taking it personally. I have also run tests with these glues, and they are amazingly strong, but you have to get just the right thickness of glue line and the surfaces must be in close contact. This means either clamping or extremely accurate machining. In practice this rarely happens.

Biscuits, however act like a perfect fitting tenon because they expand in their slot and there is much more glue surface than a simple butt joint. You are adding a third and stronger level of joinery to the glue and brads so by definition it is a better joint.

Cheers Will - it is good to have this level of discussion and as I am sure you have seen, most people on this site conduct themselves in such a manner. The aim is to provide information in a useful and (dare I say) constructive manner.

I could not agree with you more regarding biscuits providing a better joint. I would be mad not to. As you have said, you are adding a third dimension to the joint and you are also increasing the surface area of the joint.

One thing to keep in mind is that this is aimed at high school and college and as such, biscuit cutters (and even routers) may not be as accessible as in an ideal world. My aim (and I may not have worded it as well as I could) was that I have found these simple joints to be just as effective in the applications in which I have used them.

So - please keep posting as you obviously have a lot of experience in this field and feel free at any stage to challenge what I, or anyone has to say. After all, we could be wrong!


One of the problems with a forum like this and havinh high school kids asking the questions, is we rarely get the whole picture. As I said in my first post, I agree with Ship, why MDF in the first place? Especially if this is a drop box as it appears.

I almost never use MDF in the theatre. There is nothing it gives me that other materials don't do better, and it is heavy to work with. Some of the scene shops I use for television love the stuff because it is cheap, but the riggers curse them out for using it because of the weight. :lol:
Will said:
Some of the scene shops I use for television love the stuff because it is cheap

Just answered your own question Will :wink:

I actually prefer not to use MDF and tend to use chipboard insted. Over here, Plywood runs at about double the price of chipboard.

What is chipboard, mayhem? Is that like masonite here? We tend to use luan- its very light weight, but it does require either filling or muslin for a smooth finish. Of course once you paint and repaint it a few times the pores are filled.
Hum - how do I explain? It is similar to MDF but instead of being made of pressed dust, it is made from pressed chips of wood. The surface is very smooth but the ends are porus and as you say, require filling or coating.

I think that what we call Masonite is the brown board that is smooth and shiny on one side and rough and almost fury on the other.

Sorry that these descriptions suck but it is late and my brain is about to dial out.

Any other Ausies out there that can help here??
Will/Mayhem.... glad I’m out of this debate, but glad to see people butting heads about things again. What you read in such debates while it might get harsh at times expands the subject so much more than nicy nice postings as long as such debates are kept civil. Kudos to the both of you for keeping it civil also. PS, think of chip board as Wafer Board here. Larger chips than partical board. Orientated Strand Board it can also be called even if those orientated strands are more random than orientated. Hate the stuff unless I need a sponge, but after that, I’m staying out of your debate but watching with interest. MDF is aptly described as dust as opposed to partical board that is made of saw dust being larger in fiber for all otherwise not so far following this discussion. Masonite/Hardboard - tempered or not is very different than MDF, MDF is pressed to a much lower pressure and has less glue content. It will also finish much more soft in both being routered as per a soft wood, and cut the same as a soft wood. Hard Board frequently is also more sedimentary in having layers to it you can notice when you break it. MDF is not orientated in a sedimentary layering type of way unless it’s possibly more of a partical board in grain/fiber size. MDF say you would want to use for very thin laminates, Partical Board for countertop type more solid laminates, wafer board for garage roofs. Any help? Luan Mahogany is a plywood more akan to the MDO or our BCX except that a economy grade of lumber similar to Mahogany but not that lumber is used instead of sanded and patched southern yellow pine for the face. MDO being fir plywood with a thick layer of craft paper applied over it on one or both sides. Good stuff, much stronger dimensionally than normal plywood of the same thickness. Also given the paper face, it paints up really well and is exterior grade glue automatically.

Thanks, on the MDF conformation. T hought I had caught a problem in the difference between MDO and MDF. MDO would work great for this scenic item. Thought for a moment I was confused.
If it helps, the both of you have good points. Keep it to the tossed scenic item however as a focus if you can however. Speaker construction and finish carpentry are different subjects after all. At least to some degree, but all than is common to all also.

By the way, it's been a while now, wonder how it all worked out???

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