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Strength Of Design


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I have designed another guitar in Rhino, again with the intent of it being a CNC-Made guitar.

http://xs307.xs.to/xs307/06415/Idea.jpg

The idea (I'm sure it's been done before) is to hollow out a blank (In this case, I was thinking Mahogany or Koa) to give it a sort of acoustic property, leaving a center chunk for pickups and bridge. This particular design actually uses the cavity to support a soundboard inside of it (Spruce or Cedar in this case). My biggest concern is, will a CNC tear out the wood when it pockets the cavities, when the sides become thinner and thinner? Or is it a workable design?

Sorry for more of these fantasy design questions.

The sides are actually .15" thick, and the bottom is much thicker (Not a uniform thickness, it is a bit of a bowl design with swelling towards the outside)

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Don't be surprised if those .15" thick sides blow out when you try to mill them. I'd make it about .5". This will allow a bit of a roundover on the back as well. That center block design is very, very common. Don't expect it to sound a whole lot different than a solidbody. It will probably sound a bit different, just not nearly like an actual hollowbody or acoustic.

peace,

russ

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Don't be surprised if those .15" thick sides blow out when you try to mill them. I'd make it about .5". This will allow a bit of a roundover on the back as well. That center block design is very, very common. Don't expect it to sound a whole lot different than a solidbody. It will probably sound a bit different, just not nearly like an actual hollowbody or acoustic.

peace,

russ

I wanted to change the vibration of the guitar for use with an L.R. Baggs T-Bridge, without losing the solidbody sound when switching over to the magnetic pickups. Thanks for the advice!

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Not that there is anything wrong with the design concept, but those sides will be risky. Remeber bent sides do not have that much exposed end grain. Another thought from an acoustic guitar design point of view. Stiff sides are a plus as sides that are overly flexable tend to dampen the action of the soundboard. Another thought about soundboards. This design will not have a sounboard that acts at all like an acoustic. You have a center block that will freeze the top for the most part. You really wouldn't want it to act like an acoustic because that would not improve piezo or magnetic pickups performance. I would personally opt for thicker sides maybe 1/2-5/8". Top and back thickness 3/16"-1/4". If you want a more active body refine your center block(minimise it- but maintain enough material to keep the structure and components in tact). If you are really wanting sides that thin. Maybe bent sides and braced top is more in line with your expectations (more of a full blown jazz box). Take the suggestions for what they are worth. You may be testing a theory that you are basing your design on and I wouldn't want to discourage experimentation.

Peace,Rich

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Not that there is anything wrong with the design concept, but those sides will be risky. Remeber bent sides do not have that much exposed end grain. Another thought from an acoustic guitar design point of view. Stiff sides are a plus as sides that are overly flexable tend to dampen the action of the soundboard. Another thought about soundboards. This design will not have a sounboard that acts at all like an acoustic. You have a center block that will freeze the top for the most part. You really wouldn't want it to act like an acoustic because that would not improve piezo or magnetic pickups performance. I would personally opt for thicker sides maybe 1/2-5/8". Top and back thickness 3/16"-1/4". If you want a more active body refine your center block(minimise it- but maintain enough material to keep the structure and components in tact). If you are really wanting sides that thin. Maybe bent sides and braced top is more in line with your expectations (more of a full blown jazz box). Take the suggestions for what they are worth. You may be testing a theory that you are basing your design on and I wouldn't want to discourage experimentation.

Peace,Rich

Wow, very informative, thanks!

I wanted to avoid something that would cause feedback problems that a lot of archtops give when overdriven, but use the piezo with the neck to get a jazz tone when I do want that sound, but have enough body to switch to the bridge and player overdriven rock/blues, all with a lightweight guitar that has that "alive" feeling.

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The wood should not tear out if you use good machining practices: take shallow passes with some breathing room at first to take out most of the material, follow with a finishing pass of ~0.008" Set your feeds and speeds well and you can cut anything on a CNC machine, but this is where experience comes in. Frankly if you know what you're doing you could make the walls as thin as you please, to thousands of an inch if you fixture your parts well, use incredibly sharp tools, run insanely fast cutting speeds, and push a nice and slow feed rate. I'm not sure I would recommend leaving the walls 0.020" thick for structural reasons, but you certainly could fabricate it if you were careful.

Also one thing to note: on your model the center block goes into hard corners at the equivalent of the neck block and tailblock, which while aethetically pleasing is impossible to machine. Any internal corners need to be radiused to be slightly larger than wahtever cutter you're going to use. Using at 3/8" end mill or router? Make the internal radii in the corners about .400" (NOT 0.375" ).

A nit pick at this stage, but somthing you will need to drill into your head over time is to model only manufacturable things. The bane of the machinist is the designer who does not understand processes or design for manufacturability (DFM in the industry). Learn how it all works and both of your lives will run smoother when it's time to make chips.

Oh, and just to clarify, I don't know jack about what wall thickness to actually use in a solidbody, the one I'm building will have 3/8" walls at the thinnest. I'm merely noting that it's phsyically possible to do whatever you want if you really really want to. Listen to the guitar gurus for guitar advice, I'll stick to non-specific DFM work.

-Dave

Edited by davee5
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Sorry but I would be amazed if you can rout out to such thin walls without it all ripping/cracking. I tried it on a small box I was making from some left-over pieces of mahogany. I would not attempt anything thinner than 6 to 8mm, even then you have to be s-o-o careful.

Do they really need to be that thin? Why risk it? Are the potential returns worth risking a nice body blank for?

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To quickly add one more comment on those sides. One issue (constructability aside) is that end grain and cross grain patterns that you are setting up. Will both accept moisture faster and less evenly across the sides surface(creating a possible problem given the sides are very thin), and the cross grain sections will be weaker. These are not issues with thin bent sides because we have exposed side grain only. Another note about thinner sides. You will need to reinforce the area where the side meets the top and back with some form of kerfing or blocking. Even if it held up structurally you need additional surface area to make the glue joint work.

Peace,Rich

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