Methods For Making An Acoustic Radius Plate

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I am in need of several radius plates for an acoustic I intend to build in the near future. Instead of spending \$100+ dollars on a set does anyone have a method for making these? I have every tool possible and enough MDF to do the Job many times over just not a viable method that doesn't involve vaulted ceilings and a large pendulum.

Any techniques and methods that you have seen or used that really work would be appreciated. No I dont have a CNC router (yet).

Woodenspoke

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Here is a site:

I built mine using his method.

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Not that hard; there are charts to lay out the arches in the MIMF.com library (if, like me, your cord geometry is much disused and mostly forgotten). I did it the 'sevy way', basically, but without the long compass to draw out the arc. Make an arching template, accurately sanding to the line, copy route to make a matching pair, then make a very simple cutting setup with the scraps you have from cutting your MDF into a circular shape:

Router rides on the arched rails, temporarily affixed with doublsided-tape, and held in place laterally by guiding it along a straighedge I clamped in place temporarily.

However, the niftiest/laziest way I saw of doing a similar setup recently (now expired discussion on the MIMF, not yet archived) was using the long compass itself.

explanation of long compass:

In essence, the guy made a sled for the router that WAS a long compass, the two outer points being supports at the edge of router setup, and the router bit being the point where they come together. Basically, a big V-shaped sled, correct amount of drop/rise for the arch required, and then slide back and forth. The router bit then follows essentially a cricular arc (I think it's mathematically ellipsoid, but someone did the math, and the difference is tiny fractions of mm). Disadvantage is that you need a lot of space to do this (longer sled), but it's much easier to make two straight lines than it is to make precise arching templates. Of course, you can also just buy pre-made templates from a number of supply houses if you don't want to sit around measuring and cutting, but it increases costs, of course.

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What about the flat spot created between the outer edges of your router base and the jig doesn't this effect the final radius of the board? Second how are you centering the dish? It looks like you are using a form which is screwed to the bottom of the radius jig made from the cutoffs as you mentioned. Is that correct? I was also wondering if you pinned the center of the form as to be absolute sure its aligned with the router.

I realized I could create a radius template in a cad program and transfer the printer output to the wooden pieces; then bandsaw them to rough shape and sand. How many forms have you made with this method and was there any overlap from the router bit on the plate and what size /type of bit did you use?

Woodenspoke

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I used a 3/4" straight bit for these, bowl and tray would work as well, might leave slighly fewer ridges. I routed an arch, rotated so I got about 1/2 overlap, routed the next one, so on, so forth. You'll want to give the whole thing a light sanding (orbital sander, fine grit paper) to knock back any small ridges left, and seal it with some cheap varnish or shellac or similar, or you risk ripping chunks out of the surface when removing glued-on sandpaper.

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I used a 3/4" straight bit for these, bowl and tray would work as well, might leave slighly fewer ridges. I routed an arch, rotated so I got about 1/2 overlap, routed the next one, so on, so forth. You'll want to give the whole thing a light sanding (orbital sander, fine grit paper) to knock back any small ridges left, and seal it with some cheap varnish or shellac or similar, or you risk ripping chunks out of the surface when removing glued-on sandpaper.

Thanks thats what I wanted to know. How long did the routing take?

Woodenspoke

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The actual routing? Dunno, maybe 15 minutes? Depends how fast you can move the router back and forth over the entire dish. Single pass, mind you, since you're hardly taking off anything.

Oh, and do it OUTSIDE, and WEAR BREATHING PROTECTION. MDF dust is nasty, and will go everywhere. I even make an 'engineers raincoat' (ie, garbage bag with holes for head and arm) when routing this much MDF.

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