Myth -- Magic -- Mathematics -- The Straight Line Principle

[ken cierp]

Tim,
Martin sound-board braces are contoured to 52' -- the rim is sanded flat then a 1.5 degree slope is sanded on the top edge of the rim from the sound hole to the neck block. They do not sand any radius/bevel on the edge. Please read the subject article again for clarification. The article has video from the Martin factory stating the flat sanding approach and even shows photos of the actual flat sanding machine. For sure the method you are using has been used and is used -- but I can assure that the real time Martin factory process is inspiration for MMM article. The content for the MMM article is based on actual observations at the Martin factory and conversations with Martin factory employees. Perhaps this will help set the record straight -- no pun intended

[Tim Benware]

Tim,
Martin sound-board braces are contoured to 52' -- the rim is sanded flat then a 1.5 degree slope is sanded on the top edge of the rim from the sound hole to the neck block. They do not sand any radius/bevel on the edge. Please read the subject article again for clarification. The article has video from the Martin factory stating the flat sanding approach and even shows photos of the actual flat sanding machine. For sure the method you are using has been used and is used -- but I can assure that the real time Martin factory process is inspiration for MMM article. The content for the MMM article is based on actual observations at the Martin factory and conversations with Martin factory employees. Perhaps this will help set the record straight -- no pun intended

Maybe I said it wrong, I was taught the exact same way except a 25' radius not 52'. Top braces contoured to 25', sand top flat, sand in 1.5 degree slope on top edge of rim (I said neck or neck block) from neck block to sound hole. I do not sand any bevel on the edge. Of course I do it by hand and not machine. My instructor was a long time Martin factory employee and still does occasional work for them.

[ken cierp]

Tim and I have had a couple of PM's so he was able to straighten me out and I think we are on exactly the same page. If I get a chance at some point I'll do the calculations to determine the approximate dimensional changes to be expected when changing brace contour radius. Now I say approximate because one may vary well machine a perfect 28' radius on the brace gluing surface -- but in reality the brace pattern by its very nature is not going to be transferred exactly to the sound-board. The top will most likely be significantly flatter after glue up -- and even further flattened with the bridge plate.

[Dave Bagwill]

"I recommend a fall away angle of at least 1.3 degrees no matter what top radius (which I recommend at 52' true Martin or 40' which allows for a thinner lighter bridge).

Ken, could you explain the 'thinner lighter bridge' comment? I'm not making the connection between top radius and bridge weight.

[ken cierp]

Relative to the neck angle and the string plane, the 40' top radius moves the bridge/saddle closer to that plane then 52' radius --- therefore you can shave a bit of material off the bridge gluing surface and still meet the string plane at the same (height) intersection.

[Dave Bagwill]

Got it. Thx.

[Tim Benware]

Relative to the neck angle and the string plane, the 40' top radius moves the bridge/saddle closer to that plane then 52' radius --- therefore you can shave a bit of material off the bridge gluing surface and still meet the string plane at the same (height) intersection.

Does that continue to work going down to say a 28 or 25' radius? Each lower radius would have a little thinner bridge?