Top Slope Math Recheck

[tippie53]

Martin always used a few bridge thicknesses. they use a gauge with a go no go setting and as long as it is within the go , your good. The perfect set up is at point of set up the height should be about .375 that allows a .350 bridge .150 saddle and your are perfect. 7/16 is as low as I want and I don't like more than .550 . Over stressing the top can be of more harm than good.

[allenb]

Ben-Had wrote: "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."

This makes perfect since we're forcing the top to conform to a new shape. I totally agree. No matter what anyone does to try and produce a given outcome, a particular set of wooden parts cut to the same dimensions as the last build will have a different degree of elasticity.

So, in noodling the issue some more of sloping the first 4" or so of the neck end of an acoustic guitar top rim, I pondered what would happen if we took a 12" diameter flat wooden disk, say around 1" thick and mounted it in a large vice at a 1 1/2 degree angle. Then take a flat sanding board that can be mechanically fixed in a perfectly horizontal position and can slide forward and backward over our angled 12" disk and is able to raise and lower to allow dropping as material is removed.

As we start removing material from the disk's edge and the sanding board drops little by little, we stop when we see the sanded slope distance is around 1 1/2" in from the edge of the disk and covers a width of maybe 8" or so. At this point, we've sanded a bevel looking like a frowney face. Now let's say we want to continue the bevel another 3" toward the center of the disk but only take material off of a 1/4 to 5/16" band around the edge of the disk and not take any from the area in between. How would we accomplish this? One way would be to stop using the sanding board and switch to a rotating sanding beam on a shaft and skip the areas inside of the 1/4" perimeter band so we're only taking wood off of this area and leaving the rest untouched.

We now have the area we first beveled at a higher elevation than the area of the 1/4" band extending to the 4 1/2" point.

Allen

[Ben-Had]

You lost me.

[allenb]

Sorry for the odd post but I wanted to describe an analogous process that showed when we only sand down to the point where we stop at the inside edge of the neck block and then continue the sanding only on the rim up to the soundhole location of the rim which requires slight downward flexing of the abrasive bar to accomplish, that this will produce other than a continuous plane within the sloped area.

The 12" disk in the analogy is our guitar rim in our mold, the sanding board is our abrasive bar if it couldn't be flexed.

It's obviously such a small error that it doesn't cause any problems with accomplishing a good solid glue up but I like to have clarity when discussing a method of construction.

Allen

[allenb]

I successfully tapered the top 4 1/2" of my OM top to a 1 degree angle since the last post. I started by setting the shaft stop collar to the point where it would end the sanding when reaching the inner edge of the neck block (as directed). This allowed rim and neckblock to be tapered to this point which was a total distance of 1 1/2". Any further sanding on the rim, (if stop collar was left at that depth) would only be possible if flexing the abrasive bar downward. At this point the total height reduction of the outer edge of the neckblock came out to .026".

I then reset the stop collar to a depth allowing further sanding the rim to a distance of 4 1/2" from the outer edge of the neckblock which coincides with the top of the soundhole. Total height reduction at outer edge of neckblock came to .079" which is close to the depth mentioned by Ken.

It seems that the directions to stop sanding when reaching the inside edge of the neckblock is only advisable if one is to stop the tapering operation at the distance from edge of neckblock to inside edge of neckblock. Has this been the goal all along? And if it was the goal, I'm not understanding how this would allow attaining a continuous angled surface for the fingerboard extension to have a place to land.

Allen