Contour dishes and templates

[johnnyg]

A few months ago I purchased 15' & 28' contour dishes with the matching templates. I am just now using the 15' to sand the contour on the back braces. I noticed a significant difference in the contour between the dish and the accompanying template. The dish appears to have a much smaller radius than the template although both are marked 15'. Intuitively it seems these should match. I checked the 28' set and found the same condition. I was going to use the template to rout the braces close and fine tune them with the dishes but they are so different it's not really feasible.

I am not sure what the template is for if it is so different than the dish. Can someone explain all this for me?

Thanks in advance for the help!

[ken cierp]

No matter where you purchase an item its always best to contact the seller and or manufacturer to get clarification --- who could possibly have better insight?

Since KMG is the only supplier that includes a template with each dish, my guess, that is where you purchased them. If that is the case, I can assure you that the dishes and gauges are accurately CNC machined and will match perfectly just like the many hundreds of each size KMG has produced and sold, -- provided they are set-up and prepared for use properly. Sounds to me as though you do not have the dishes secured to a flat sub-straight as indicated in the product description. If you have high humidity conditions in your shop not following recommendations could most certainly be problematic.

Per listing info:

The dish is left in the square shape to allow the user more options relative to fastening and stabilizing the dish in a go-bar clamp etc. Some choose to make it circular with handles. Do not get ripped off -- all dishes --- no matter what they are made from will have a tendency to warp, plywood being the worst, there is no magic special material. Routing a fancy pattern on the back of a dish can leave areas that are paper thin and only serves to diminish durablity and quality. We recommend screwing the dish to an additional piece of MDF. The machining operation leaves the surface with a velvet like texture. We also recommend a few coats of a solvent based finish.

[ken cierp]

As an important side bar --- shaping braces on a radius dish is not recommended, the problem being that the bottom of the brace becomes distorted width-wise rather than perfectly flat on this glue surface --- not good. Its best to refine the contour so that its perpendicular to the brace height -- here's a simple fixture that can be used to accomplish that task.

[johnnyg]

The main problem here is the dishes were not set up correctly according to the instructions. After some guidance from Ken on how to properly set up the dishes by attaching them to a substrate, everything is correct and the template matches the dish. Thanks again Ken for the help!

[Hans Mattes]

Ken -- I understand your concern about sanding braces on a radius dish if the guitar back is going to be cylindrical. But if the back side of the guitar rim (including head and tail blocks) is sanded on a radius dish, won't it take a spherical shape? If so, wouldn't a brace sanded on the same radius dish be more appropriate?

[dave d]

Hello Hans,
What you may not be seeing in the jig Ken posted, is that it has a radius on the horizontal surface that the sandpaper is glued to. So when using this jig, with one side of the brace pressed up against the fence, you are ensuring that the bottom of the brace is perpendicular to the side, but has a radius on it longitudinally.

As Ken said, the danger of sanding a brace on a radius dish is that it is easy to round-over the bottom (width-wise) and you want to avoid that at all costs because you are reducing the gluing area as well as not having a flat stable base.

Edit: this picture (source: LMI) illustrates the radius that I am talking about.

brace sander.jpg (11.25 KiB) Viewed 8300 times

[Hans Mattes]

Thanks, Dave. I did see the jig Ken posted, and, used properly, it will assure that the bottom of the brace will be flat in the "across the brace" direction. If the back is spherical, that won't yield a perfect fit, but, given the narrow width of the brace, that likely doesn't matter, and it will certainly be better than if the brace gets tilted while being sanded!