Official Luthiers Forum!

There have been several threads in the past on the issue of whether you have to keep the rims centered on the dish. Often the discussion degrades to a bunch of "What?" or "Sure you do" and "No you don't". I think the confusion stems from different definitions of "up". Look at the attached image. On the top drawing the guitar is centered on the dish. If you get off center and still keep the rims pointing in the same direction, you will sand your rims to different heights as shown in the middle drawing.

However, if you let the guitar follow the contour of the dish, (which is what you do naturally when you do this free-hand), you get the situation illustrated in the third drawing. The rims are on a line perpendicular to the tangent of the curve of the dish and this is true regardless of where the guitar is placed on the dish.

Ok, the second drawing is what I'm talking about, if you want your "center" or high point to be in the bridge area, then you have to do it like #2 drawing. For the back I guess it doesn't matter. For the top, you may want the peak of the dish at the bridge and not before it in the center of the guitar.

Also, I have never checked the radius of the dish to see how accurate it was, I am so used to having to sand vertical with braces and keep them over one spot anyway.

So the new question is do you want your bridge at the peak of the dish, or the peak of the dish before the bridge... ???:mrgreen:

_________________
Here is what a Parlor Guitar is for!
http://www.youtube.com/watch?v=oEa8PkjO6_I

I built a simple rig that uses a shaft, assembly mold a simple bushing in the center of my sanding dish (requires a 1" hole to be drilled through the center of the dish) that allows me to keep the body centered and in proper plane to sand the back's spherical radius by hand with no worries of getting out of plane form one side to the other. I just hold the dish sides a rotate the dish around the shaft, the shaft is 3/4" diameter sugar maple dowel and the bushing is made for UHMW. I have several bases that hold the at the proper angle and indexes the mold to proper location. One for dreds, one for SJs and Oms and one for the MJ. They all use the same shaft and bushing.

Oh goody, I get to draw some more pictures. The three pictures show and exaggerated radiused top on a completely flat backed guitar. The first picture the top with a peak in the center of the guitar. The second picture shows the same arch with the neck block smaller. This has the effect of moving the high point of the arch back toward the bridge. If I rotate the guitar so that the arch is in the same position as the first guitar, we see that the rims are not straight up and down but the peak of the arch has moved back to its original position. The effective peak of the arch can be adjusted by the amount of neck set

The profile of the rims should be set during the building process and not when the final sanding is taking place. That means that the guitar can be put anywhere on the disk with the same results. However, the taper for a 25' radius is very small and it would be very easy to get it out of spec just by pushing too hard or soft on the head block.

Many neck setting procedures compensate for errors in this area because they measure the height of a straight edge that is resting on the neck, right above the bridge position and adjust the angle of the neck accordingly.

True dependent on a few issues like the pressure applied by the builder during sanding and the accuracy of the initial profile and the way the builder set the top of the lining. many builders set the linings height above the profiled rim at 1/8" or so proud. Many do this by eye so it is possible to have an inconsistent height above the profiled rim that could lead to lopsided sanding. also the neck block being square bottend. If the approx. taper formed by the dome is not pre cut into the neck or tail block this can cause the dome to contact at the wrong plane in the early stages of sanding that can thow some misalignment into hand sanding the dome. I have dome this unitentionally in my early days of building.

Also some builders rough profile the sides (+or- 1/32 to1/16 to perfect profile) and allow the sanding to square and smooth out all.

Putting the linings a little high wasn't really a problem but when adding the cherry blocking for the armrest and ribrest made it difficult as it was about 1/16" above the side so it would be profiled also for gluing. I had to actually hold the mold up and apply pressure to the side with the rest and try to keep it level. You can imagine how much faster the side without the blocking would sand.

I think if someone wanted to use this new mold from LMI, there might be a way to do it.. Probably not
worth the effort but I think it would allow assembly of the guitar and removal of the supports through
the soundhole with top and back in place. It's a really poor photoshop job, maybe you can get the idea.
Just section the pieces as I've shown, install one more turnbuckle and you should have something
that could be removed through the soundhole.
Or, make your own. I just did for an OM and I think it will work. This LMI version does have some
merits, however.
http://www.redshift.com/~gibson/BENDING ... dMOLD3.jpg

_________________
Bruce Herrmann
"What lies behind us and what lies before us are tiny matters compared to what lies within us."

Chris, in situations like that, I glue a piece of sacrificial wood on the other side, such as a strip of veneer. In your case, a piece of rosewood binding glued on top of the linings would have held that side of the ribs in a good position. If it sands away too quick, just glue another piece on.

Good idea Barry.

Is there another way to do it?

Jim