3/4" mahogany plywood - is it possible to screw into the edge of this stuff without splitting it (such as a butt joint), or is there a better way? Maybe a cleat?
I would use MDF with a mahogany veneer, but weight is an issue for this project.
You generally do not want to do that. Describe the project in more detail and maybe we can help
Butt joints are crappy looking, in addition to being weak. A rabbet with biscuits would be both strong and better looking. For projects where I don’t want to go through messing with biscuits, just polyurethane glue it, shoot in a few finishing nails to maintain alignment, and bar clamp the assembly until the glue sets.
The project is a speaker cabinet - an imperial folded horn containing two 15" drivers, with a bent wood HF horn on top. The cabinet (minus HF horn) is large - roughly 25.5"x37.5"x63.5" completed dimensions. Anecdotal reports from others who have built from this design put the completed weight at between 300-350 pounds, and that is using plywood construction. If I built out of MDF it would be entirely unreasonable. The problem is that apart from building several simpler cabinets (MDF boxes with internal baffles, etc.), I don’t have a great deal of woodworking experience. I know that the vibrations that this will be subjected to probably preclude using glue only. On previous cabinets I have built I have always used wood glue in screwed-together butt joints - easy with MDF, maybe not so much so with the plywood. This will also be my first cabinet which is intended to look like a piece of furniture (nicely finished, stained, etc.) as opposed to a big box that just gets painted flat black and hidden in the back of a room. Unlike previous projects, I can’t really damage the workpiece and then fill with epoxy and paint over it, so I’m trying to take a bit more care in planning this one.
So, I need to know if there is an easy way to secure edge-on joints with the mahogany plywood, or if I can teach myself new woodworking skills given my limited experience and limited access to specialty tools. I have a table saw, jigsaw, circular saw, router, drills and sander.
Thanks for the project idea. To keep things relatively simple, provided you can set a perfect 45° bevel on the table saw, you can mitre the edges of the top, bottom, and sides, and assemble the box with square cleats ripped from dimensional lumber, and glued/screwed to the plywood on the inside, hiding all hardware. Using a router table or dado, I’d cut a dado at appropriate setback from the front plane of the box on which to install the front panel on which you’ll mount speakers and also attach either grille cloth or removable foam. Also rabbet the rear to a sufficient depth for setting the rear panel flush, to which you can attach your crossover assembly and terminal spring lugs or binding posts/banana jacks. Run the cleat material around all four sides and attach the back panel with either flathead screws that you’ve countersunk or use oval head screws and cup washers. The only part you will have to fuss with regarding appearance prior to staining will be the front where edge band veneer would be applied to hide the ply core.
The crossover is in my rack, and the system will be tri-amped from three QSC amplifiers. I don’t even need so much as an L-pad in the cabinet. The only components that need to be mounted in the cabinet are either binding posts or Neutronik Speakon receptacles. I’m pretty sure that the rear panel can stay as is - I may route a slight recess for the receptacles, but they really don’t matter, since they are on the back.
As for the baffle (on the front, where the drivers are mounted) - could you explain a bit further what you are getting at here? My first thought was simply to leave the drivers exposed, and varnish the baffle as with the rest of the cabinet. I suppose it might make sense to cover them for protection - what about a low profile expanded metal grille, painted black, stood off by about 1/2" over top of the varnished baffle? I’ve never been a fan of cloth, since it does little to protect the drivers from accidental physical damage, collects dust, and is seldom sonically transparent. This is not solely a low-end cabinet - one of the 15" drivers is a mid-range coaxial.
I’m not sure I understand your construction advice here - the baffle isn’t really readily removable, as it is part of a larger internal assembly which forms the compression chamber at the horn throat - including internal baffles and panels at odd angles. The front baffle, however, is set back from the outside edges of the cabinet by 1.5 inches, though.
I like the cleating idea to hide all of the hardware - I’ll have to calculate the volume of all of those cleats to see if it changes anything. In that case, the only exposed ply edges would be the outer walls of the cabinet which face forward. Incidentally, how bad does exposed ply core (on panel edges) look if it were to be finished? I hadn’t really given any thought to veneering this - would it be easy to find a veneer that wouldn’t be obvious compared to the panel sides? I was thinking of doing this in some combination of thinned ebony stain (exterior panels) and a varnish slightly redder than the natural wood (baffle, horn mouth). Comments?
A quick internet search uncovered the following bit of advice:
To avoid splitting plywood when screwing into the edge, drill a 3/8" hole at each screw location, then drive a glue-covered 3/8" dowel into each hole, trimming the dowel to be flush with the edge. Wait for glue to cure, then drill a pilot hole in the center of the dowel and assemble the parts, driving the screw into that.
This struck me as odd. If you do this, are you not simply relying on the glue strength to hold the parts together? Doesn’t seem much better than simply gluing the parts.
If the cleat volume is going to cause problems, you can go to metal L-brackets as an alternative to the cleats.
Exposed ply core can actually be used as a design feature, but you need a more modern look for it to fly; plywood doesn’t really go well with Louis IV. If you leave it exposed, you may need to use a sealer to keep the end grain from fluffing up when you stain it.
If you go the veneer route, you can buy rolls of mahogany veneer about 1" wide that are made for finishing plywood edges. If your panel sides are also mahogany, this should blend in perfectly. The tape is available in “iron on” and “glue on” preparations. After applying, you trim it very carefully to the plywood width, and the result will look like solid wood.
Alternatively, go for something that looks completely different. Try using a contrasting wood like ebony or birch for the veneer and make the contrasting wood colours part of your design.
When you say BUTT joints are you talking about 90 degree corners, or edge to edge. I think your talking 90 degree here so I’ll suggest “pocket holes”.
There’s a special jig, a Kreig Jig or a clone, that makes shallow angled holes in the surface, inside of course. These holes receive screws that exit the middle of the edge and enter the adjacent piece of wood.
With a 90 degree joint, the screw would be going across the grain of the plys and should be plenty strong. This technique is used for making cabinet “face frames” where the holes are not visable. Of course you can glue a dowel into the hole and sand it off, but it makes it VERY difficult to remove the screws if required later on.
Here’s the mini http://www.kregtool.com/catalogbyprodid.asp?prodid=MKJKIT that goes for $20.
A properly glued joint is pretty darn strong; in my experience when making butt joints (I’ve only built a couple of speaker cabinets though) the screws are pretty much acting as clamps to hold everything together until the glue dries. Screws alone just don’t have a lot of holding power in this case. A properly braced cabinet will be incredibly solid once glued joints dry. My last one was a subwoofer cabinet with 1.25" thick walls (1/2 birch ply over 3/4" MDF); I put it together from the inside with pocket screws but the joints are held by glue. I could park a car on top of the thing.
The reason to drill holes and glue dowels in is that even with pilot holes, driving screws parallel to the plies (that is, into the edge of the plywood) tends to split the plies apart. If you have solid wood to go into the screws will hold much better and won’t split the plies. The other option is to rout a groove into the edge of the plywood and then glue a strip of hardwood into the groove. Drill pilot holes and sink screws into that. It’s like the dowel idea only you’re doing the entire edge, not just a couple of spots. If you go with the dowels, do some test holes in scrap, including gluing and hammering the dowels into place. You may find that you’ll need to sand the dowels down a hair to get them all the way in with clearance for the glue (I just found this out with a bench I built using pegged joints).
Some bare plywood edges can look nice (high-quality birch or appleply drawers for example) but you need plywood with no voids or anything so that you can finish it absolutely smooth. You might have better luck with either veneer or a solid edge.
By solid edge I mean a thicker strip of hardwood glued onto the edge. You have to make allowances for this in your plans (since it’s not .01" thick or whatever the veneer is) but it gives you some additional options; if the plywood edge is covered by a 3/4" strip of matching hardwood then you can round over the edge with your router (so you have a radiused corner). This will also make your corners and edges more bump/bang resistant than a plywood edge.
Different folks, different strokes.
Let me echo GaryM. With due respect to the other folks–and I mean that–biscuits can’t compare to pocket screws in terms of speed. That’s what the cabinet industry typically uses and they use what works well. I’ve used both systems. The speed of pocket screws is astonishing. In many apps, you don’t even have to clamp. In terms of joint strength, I’d say the two systems are equal.
Sorry to be a contrarian, but there’s nothing inherently wrong with butt joints, depending on the application. You can cover with trim or use decorative brass screws to create a nice effect. (Not for a speaker, perhaps, but a bookcase can look nice, this way.) Sure, if you’re crafting an heirloom piece or a perfectionist, you’ll want a more refined method. Moreover, while butt joints aren’t as stronger as other joints, they are perfectly fine in many apps and fast. You can use cleats or brackets to strengthen.
I have both in my shop and use them for different purposes. I find that biscuits are more for keeping parts aligned and adding a bit of strength to the joint whereas pocket screws take the place of clamps (and if you’re assembling something with multiple parts they are super-handy, screw the joint together and you can handle it immediately and move on to the next stage while the glue is still drying).
When it comes to light duty, you need not worry about pocket hole joinery versus biscuits, because the joint will be plenty strong either way.
If you are not worried about technique and you won’t lose sleep knowing you did a butt joint with some brackets…go for it. If you look at the project you are working on, it isn’t a big deal.