Shaping Braces for an OM 28

[AluminumTop]

'Very nice shaping on those braces.
I will try to emulate this as best I can."

The pdf is excellent thanks to the cad creator of it
print it in 1-1 scale, and match brace shape to paper templates.
or spray glue to thicker paper and cut.
or Scan it and convert to digital.

LUMA OM Soundboard Integral Scalloped Bracing.JPG

[tippie53]

The braces are close to what they should be
do not over shape the braces . You do want to round the braces and sand them pretty. If you go too far the top will rotate and sink in at the top in front of the braces.
Think of it this way The upper part of the braces are holding the compressive loads and the lower part are in tension. You take too much off the upper you will have structural issues.
You don't want the braces under a 1/2 in height at the X and no lower the 5/16 at the lower part of the scallop

[scamp]

John
Thanks so much for the heads up on this.
I definitely might have taken too much off of the X brace scallops which would have been bad.

I assume there is less concern with the upper and lower tone bars and treble and bass finger braces.
Correct me if I' wrong.

[MaineGeezer]

Remember - stiffness primarily comes from height, not width, and in shaving down the braces you're trying to reduce mass, not stiffness. At least that's my take on it. So I tend to make braces narrower, not necessarily lower.

Of course you also want to get the stiffness correct, too....

[tippie53]

Main got it right its about mass Physics, heavier it is the harder it is to get it to move. Make it too light and it won't support the load

[scamp]

I tried to take all of your excellent input and here is the result.
Struggled a bit using a chisel as I don't have a lot of experience but it wasn't too bad.

[Kevin Sjostrand]

That looks GREAT. You did a fine job. Nice and neat too.
Well done

[tippie53]

nice work
if you tape on it what note do you get? You want the top and back to be a semi tone apart avoid harmonic notes

[phavriluk]

ASK JOHN! Alterations are permanent.

[Diane Kauffmds]

I tap the top and back to make sure they sound different. You don't want them singing the same note (usually there's no problem). I also listen to the top for resonance and sustain. I don't go below .75" tall at the X. If I don't feel and hear sustain and resonance, I take a little off the lower x's or tone bars, very little.

When you first start building, it's best to stick to tried and true measurements. It's easy to over trim. Tap and listen, also feel the vibration. You'll soon get the feel for how much should come off, some tops more than others.