Tapering MDF Panel

[Paul Canaris]

What approach would you use to tapering an MDF panel, if you did not have something exotic like a CNC.

The panel is going to be about seven foot long, and about two foot wide. It will need to end tapering from its 1.25” center thickness to about .75” at the edge. It needs to end up as a smooth ark.

I need to make four of these panels.

How would you approach doing this?

[Rob Wright]

Paul - is the taper over the long 7-ft length or the 2 ft width? If it was over the 2-ft width - a sled/jig setup for a router to rest in with that profile moved along that profile, just move it down over the length a half a bit width at a time? I have seen jigs like this for carving out seats.

- R

[Dale Lesak]

Is this going to be a curved panel the length of the panel?

[Tyler Purcell]

Perhaps a long "ramp" of sorts for a router to slide on. The ramp could be adjustable, and sit on the sides of the strips. On one end, the sled for the router would be propped up to 1.25" and the other end would slope to .75".


I drew an extremely bad sketch for you to explain what I mean. The MDF panel is the orange-brown, the jig is the grey matter. A router would slide along this jig to surface with a large upspiral bit, for example. The left side is .75" tall, the right 1.25".

[Jamie Buxton]

It sounds like your idea is to start with a thick blank and remove a bunch of it. How 'bout instead building it up with two sheets of MDF and wedges between them? For instance, start with two sheets of 3/8" thick MDF. Cut wedges of anything convenient which are zero thickness at one end and 1/2" thick at the other. Insert the wedges between the two sheets of MDF, and glue it all together. You get an object which is 3/4" thick at one end and 1 1/4" thick at the other. You also get the nice smooth factory faces on the MDF.

[Tim Sproul]

How would you approach doing this?

I'd bend a 1/4 inch board. Use a 1/2 inch board as the substrate and lay a stip down the midde to get 1 1/2 thickness overall....and lay subsequently thinner strips as you get to the end to get 3/4 thickness eventually.

Use a vacuum bag to get good clamping over the entire piece.

[Paul Canaris]

Paul - is the taper over the long 7-ft length or the 2 ft width? If it was over the 2-ft width - a sled/jig setup for a router to rest in with that profile moved along that profile, just move it down over the length a half a bit width at a time? I have seen jigs like this for carving out seats.

- R
It's over the two foot dimension

[Paul Canaris]

I'd bend a 1/4 inch board. Use a 1/2 inch board as the substrate and lay a stip down the midde to get 1 1/2 thickness overall....and lay subsequently thinner strips as you get to the end to get 3/4 thickness eventually.

Use a vacuum bag to get good clamping over the entire piece.

Would work normally, but this is for a speaker enclosure so the sides need to be very solid, so it will all need to be solid.

[Jamie Buxton]

Would work normally, but this is for a speaker enclosure so the sides need to be very solid, so it will all need to be solid.

Folks used to build speaker enclosures with hollow walls and then pour sand in them. Now that is high-density, non-reverberant stuff.

[Jamie Buxton]

Paul, I'm getting the feeling that we don't really understand what you're building. Folks are proposing all sorts of tapering schemes that don't do the same thing. Could you post a sketch of what you're trying to do?

[Paul Canaris]

This will be a seven foot tall, two foot dep rectangle, the sides will be curved from front to back. So the sides will have a rounded appearance.

[Jamie Buxton]

This will be a seven foot tall, two foot dep rectangle, the sides will be curved from front to back. So the sides will have a rounded appearance.

Something like this?

[Paul Canaris]

Something like this?

You got it buddy!!!!

[Jim Kountz]

How bout just building the box square, then make some curved cleats and space them out evenly down the entire edge. Then lay on a nice easy to bend 1/4" sheet of ply. Rout or sand the edges flush and you have your desired shape without any tapering to do at all. Kind of a box in a box but not if that makes any sense.

[Paul Canaris]

How bout just building the box square, then make some curved cleats and space them out evenly down the entire edge. Then lay on a nice easy to bend 1/4" sheet of ply. Rout or sand the edges flush and you have your desired shape without any tapering to do at all. Kind of a box in a box but not if that makes any sense.

Resonance would be a serious issue, otherwise that would be a great approach.

[Jim Kountz]

I dont see resonance being an issue at all, the box when done would essentially be a solid unit. If the cleats and sheet of ply are properly done it would be rock solid. Besided you could always fill the voids if you just want to make absolute sure.

[Todd Hoppe]

Build a bending form from whatever is handy, and laminate three pieces of 1/4 mdf so that the panel is a consistent 3/4 in thick. The bending form shouldn't be too hard to make. A vacuum bag would be perfect for the large glue up

["Jason Belous"]

would bendable mdf work for you in this case. The stuff I use is 3/4" thick mdf with a hardboard surface for laminating or painting. This inside is obviously ridged. It come in burch and other materials as well. It will be solid and will give you the arch you need. Plus you can just go buy it.

[Jamie Buxton]

You got it buddy!!!!

Whew! That's one big speaker enclosure. You gonna put wheels on it? No -- even better, put it up on skids a bit so you can slide a pallet jack underneath! :)

[Tim Sproul]

OK.

You need essentially the same thing that is used by some folks to flatten their workbench or other large surface....except you need to make the horizontal guides match the curve you want. Use a router on a moving sled, essentially. The guides along the long axis will be straight and flat and the guides along the short axis will be crowned. I'd do this by making a subbase for the router that you attached runners that matched the curve of the short axis guides. This only works if the curve is a single radius curve....if the radius changes, like a french curve, you need to switch the axes. The router would travel along the long axis on straight, flat runners and these runners would move incrementally along the short axis and the short axis runners would be shaped to the curve rather than being flat. EDIT: Forgot to mention - you'll be doing a lot of sanding to get a nice and smooth surface and probably want to use a core box bit, not a straight bit. I hope you've got a good respirator! /

Another possibility if this assumption is correct - the curved part has no effect on the sound. You need the interior certain dimensions for resonance and the box needs to be a certain density so that you don't get echoing or other distortion. The curved part can't be hollow because the hollowness will distort sound? If so, can you just make the curved part hollow but apply it to a very thick, flat substrate, effectively isolating the hollow away from the resonance box? Perhaps fill the hollow with expanding polyurethane or other insulation to help prevent echoes....as I don't think sand would be a realistic option for something on this scale. Where I earlier said to use 1/2 inch thickness mdf, use 1 1/2 inch thickness MDF or 1 inch or whatever is appropriate. This extra thickness could be accomodated by changing the exterior dimensions of the speaker box, if possible. I don't know if the extra thickness will distort the sound too. Or perhaps the changing thickness of the front is part of the speaker enclosure design to help produce good sound quality?

[Mike Marcade]

You don't happen to have an 84" bandsaw do you? :D

[Paul Canaris]

[quote=Tim Sproul;715656]OK.

You need essentially the same thing that is used by some folks to flatten their workbench or other large surface....except you need to make the horizontal guides match the curve you want. Use a router on a moving sled, essentially. The guides along the long axis will be straight and flat and the guides along the short axis will be crowned. I'd do this by making a subbase for the router that you attached runners that matched the curve of the short axis guides. This only works if the curve is a single radius curve....if the radius changes, like a french curve, you need to switch the axes. The router would travel along the long axis on straight, flat runners and these runners would move incrementally along the short axis and the short axis runners would be shaped to the curve rather than being flat. EDIT: Forgot to mention - you'll be doing a lot of sanding to get a nice and smooth surface and probably want to use a core box bit, not a straight bit. I hope you've got a good respirator! /

Tim, you hit the nail on the head with approach #1. :) I can’t vary the internal volume of the cabinet, and to prevent driver diffraction effects I can't widen the front baffle to accommodate a thicker side panel material. So I will have to start with a 1.25" blank, and taper to 3/4" which will put the sides flush with the front baffle. The router jig you describe should do the trick, but you're coreect, it will be a bit tricky to execute, but certainly do-able.

The only other thought I had that would simplify all of this would be to make the sides of the cabinet the nominal 3/4" thickness, but make them of a denser material than the rest of the cabinet (effectively increasing rigidity). Not sure what that might be though...any suggestions.

[Alan Turner]

Paul,
Interesting discussion. If you could use a 3/4" thick, curved front, for the speaker box, with the top and bottom front edges milled to this same slight curve, then one solution which would be quick and leave you with a smooth surface would be kerfed mdf, at a 3/8" thickness. Build a form which has the inside curve formed on ribs about every 6 - 9", apply a good adhesive such as Unibond 800 to both kerfed surfaces, and use curved clamping cauls. This would give you a curved sheet of much rigidity in the shape you are seeking. Sections of the curves used for the bending form could then be used to give you a flat surface on which to mount the speakers.

I should add that I know nothing about building speaker boxes, as is probably apparent from the above.

[Walt Nicholson]

I used to build a lot of speaker enclosures, mostly for cars for competition and resonance was always a problem, like three 18" subwoofers in a Honda. The market is full of dampening products like B-Quiet, Dead Zone, Dynamat. If you google sound deadener you will find dozens of things that will work. Build your box rock solid, seal the inside of the box joints with silicone or similar, add the 1/4 curved outside and pack it with material and resonance won't exist. Good old R19 fiberglass insulation packed in tight will do wonders in stopping resonance also.

[Cliff Rohrabacher]

Without a slider I'd set up a circular saw and a guide.
Make your guide nice and straight. clamp and cut.