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I had some speaker baffles cnc'ed and it looks like there is a mistake in either the drawing or the actual machining.

I need to increase the diameter of the outside step by about 1/8" not the through hole.

What would be the best method to accomplish this. The material is MDF.

Below is an image of the speaker and the hole.enter image description here

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  • Have you thought of turning it on a lathe? :) – dfife Mar 24 '15 at 18:16
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    I should mention I didn't trial fit the parts before and the towers themselves are 43" tall no way could I get that in a lathe. – draksia Mar 24 '15 at 18:19
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    I was kidding about the lathe. – dfife Mar 24 '15 at 18:20
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There are a few different options, but I would use a router with a pattern bit:

  1. Use a router, circle-cutting jig, and straight bit to cut a template out of hardboard, plywood, or MDF with a hole of the exact size you want the outer circle to be
  2. Center the template around the existing hole and tape, clamp, or screw it in place
  3. Use a pattern bit set to the same depth as the existing outer circle to trim away the extra 1/8". The pattern bit's bearing runs along the template while the cutting edge trims the speaker box's outer circle flush with the template.

You can also do the same thing with a guide bushing and straight bit, but this is slightly more complicated because you need to make your template slightly oversized to account for the gap between the bit and the guide bushing's outside edge.

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  • I was thinking this is probably the only way I could reasonably do it, I was hoping there was something I wasn't thinking. – draksia Mar 24 '15 at 18:20
  • I just looking if I found a rabbeting bit that had right length I could use that without creating a template that I have to clamp to the piece and that makes centering not an issue. – draksia Mar 24 '15 at 18:39
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Your best bet is a straight cutting router bit with a bearing on the bottom. If you're lucky, the distance from the inside hole to where you want it to be will be of a size that there's a bit for. (Bad grammar aside.)

If not, you could cut a template in hardboard (we used to call it masonite, but I'm not sure what the commercial name is) and use a bushing collar on a router with the corresponding straight-cutting bit. The masonite acts as a template for the collar to ride around.

If you do this second method, then centering the template becomes critical.

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    I'm having trouble visualizing your first suggestion. If the bearing is on the bottom, how is the straight cutting bit going to make an end cut? Or are you referring to a rabbeting bit, while I'm envisioning a flush trim bit? – Doresoom Mar 25 '15 at 14:42

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