Dowels and Joints

[zacjones]

Hello all, and thank you to everyone here for the knowledge you share. I'm going to be building a version of Jed's dynamic 4T design with the help of Jed and a very reasonable local machinist ($35/hr) I've found here in Roseville, CA.

I'm trying to take advantage of any benefits offered by the "Easy Button" of CNC routing, so I've incorporated roundover edges on all the bracing, a little extra bracing, and dadoed grooves in the cabinet for the bracing.

I'm using 1.25" MDF for the front baffle, and 1" for the rest. I'm skimping a little on the baffle by not going 1.5", but I added a little extra bracing, spaced the woofers a little more, and got some extra strength that way. I was thinking of doing rabbeted joints for this project, but it seems to me that butt joints are the way to go here as I'll be using 3/4" hardwood trim on all edges and if I do anything but a butt joint I'll be cutting away at the glue when I go to do the 3/4" rabbett.

I started thinking about dowels to help me line up the panels, then got to thinking about whether they could provide any added joint strength. The little research I did indicated that in a 3/4" MDF cabinet build, 96mm spacing between dowels would be optimal, and to stay away from the corners.

I've attached diagrams of the various joints I'm considering. Advice, anyone? Will the dowels help or hurt my joints given that I'm just going to rabbet half of the dowel out on one side right after the glue has dried? Another option would be to pre-rabbet the panels and use slightly longer dowels to go well into the hardwood edging, but it might be tricky to glue all that stuff into place at once (the bracing and all six panels and 12 trim pieces). If going down this road, I should probably have the hardwood strips machined as well for length and dowel holes.

Another question -- when planning the dado cuts do I make them exactly 3/4" or a little wider to make room for some glue? I'll be using MDF for the bracing that I measured at 3/4" with a caliper.

So what do you think?

option A: Rabbeted joints
option B: Butt joints with a couple dowels to help line up the panels
option C: Dowel butt joints with dowels spaced every 4" and the dowels sawed off
option D: Dowel butt joints with dowels spaced every 4" and the pre-machined hardwood edging to line up with the dowels.
option E: Just go watch some TV and forget about the whole thing

[---k---]

This question comes up all the time. Simple butt joints are more than adequate for speakers. For bigger speakers, some of the fancier joints helps with assembly, but are not needed for strength. Save yourself time, energy, and money and just go with butt joints.

Dado cuts for the bracing also isn't needed. I've used them for several projects, but again it is to help with assembly. The dado router bits I have cut the slot just slightly bigger than the plywood.

Not sure why your skimping on the 1.5" baffle. If it is money. I would rather have a 1.5" baffle than a roundover on the internal braces.

[zacjones]

So butt joints and no need for all the dowels then. Just a couple to line up the panels. As for the baffle I was just trying to avoid sandwiching the MDF. The local lumber yard has the 1.25" in stock, I guess I'll make some calls to see about special ordering some 1.5" MDF. If I can't find any I'll go ahead and make my own then. Thanks for the response.

[---k---]

Wow, 1.25" in stock. I've never heard of that. Very cool.

You could consider making 1.5" thick using the 125" MDF and a piece of 1/4" hardboard.

[BeerParty]

I'm skimping a little on the baffle by not going 1.5", but I added a little extra bracing, spaced the woofers a little more, and got some extra strength that way.

Be careful here - the spacing between the drivers is part of the design for the speaker. Changing the spacing may require a change to the crossover. If you are going to do this I would post a specific question about your proposed change to make sure you are not compromising the design.

[ThomasW]

If the front baffle gets too thick, the rearwave off the mids and or woofers can be masked by the sides of the cutout hole. If that's going to occur those sections of the baffle must be back beveled to allow the rearwave to flow unobstructed....

[zacjones]

Thanks for the responses guys.

Ryan - I'm glad you questioned my going with the 1.25" baffle. I called around and Berry Plywood here in Sacramento can order 1.5" MDF from about a mile away for same day pickup. $74 per sheet instead of the $45 for 1.25", but I'm glad I found some anyway. Thanks for the tips.

BeerParty - Good points. I did talk to Jed about my proposed baffle changes and he cleared them. He said that the differences he found when using a wider baffle were pretty minimal with the 4T, and that I could go no bigger than 10-10.5" with the 4C. There were no issues with lengthening the baffle to 49" and spreading out the woofers a little more evenly. I did this so I could fit full bracing between the drivers and to for looks to go with the wider baffles. I did the wider baffles to incorporate roundovers which were also cleared by Jed, and to increase the cabinet volume to go just a little deeper. Thanks for the considerations.

Thomas - There will be plenty of room to do that and Jed's instructions that come with the kit also incorporate that into the design. Thanks for making sure I'm on the right path though.

I like big speakers and my original idea was to replace the RS180's with RS225's but that WOULD have required a complete crossover redesign. Jed's put so much work into these it would be a shame to let all that go to waste. Besides, the RS180's perform better down to about 50hz anyway. If I cross over to the subs at 80hz Jed's design will actually be better. 60hz could use the bigger woofers, but it seems like a lot of trouble for what would probably be minimal gains or losses.

thanks again!

[Hank]

You'll get good advice here. No nails, no screws. Simple Elmers or basic Titebond glue will result in a butt joint (IF the panel component cuts are 90 degrees) that is stronger than the wood/MDF. For large (tower) speakers, instead of dowels or biscuits, I use a pneumatic brad nailer, NOT for strength, but to hold panels in alignment until I get them all clamped.
Now, go forth and make dust.

[PoorboyMike]

I think that dowels could be quite handy when doing baffles, especially when the finish is different from the rest of the enclosure. You could dry fit the baffle to the box, flush trim with a router, route edge treatment, then remove and apply the finish. Not only would it still fit perfectly when you were done, it also wouldn't slip and slide all over the place when you glued it up.

[bluewizard]

I would think some type of rabbet or compound rabbet would give you more glue surface, less end grain, and a more stable and stronger joint.

I'm sure a simple butt joint is adequate, but it is not without problems. True the glue bond may be stronger than the wood or MDF, but how strong is the MDF?

Without something to re-enforce it, like dowels, biscuits, or a rabbet, I don't think the MDF is that strong on the surface. That is, under stress, you can peel away the surface and break the joint. Yes, the glue will still have held, but the wood or MDF will not have.



Just one man's opinion.

Steve/bluewizard

[Dennis H]

I think the thing that saves speaker boxes is that they are so well braced by the front and rear baffles as well as any internal braces. The edge grain of plywood makes a pretty weak butt joint compared to gluing two pieces of face grain together. But once you get everything assembled, the individual joints don't have to be very strong for the overall structure to be quite strong.

[MT Speakers]

Okay, this may be a little late and off topic. I'm building some SR-71's and didn't trust glued up butt joints either. So I decided to use biscuits! I bought a continer of 100 #20 biscuits for about $7.

[Dryseals]

My favorite.

[---k---]

:roll:

[BigguyZ]

:roll:

I agree.

We're not making precision/ fine furniture here. The stresses involved aren't great enough to warrant a joint that requires more work and/or a complicated setup. As previously noted, a simply butt joint with sufficient glue will be stronger than the MDF itself, so at that point, the panel will fail before the joint does...

[Hank]

Yep, new come on board thinking that they have to use screws, nails, biscuits, dowels, glue joints, etc, but in actuality, butt joints are plenty strong. Just make sure your saw cuts are 90 degrees so the butt surfaces ( :B ) are gap-free. Trust us on this and save yourself some labor.