Radius Dishes Continued

I have pretty much followed through on my plan for the radius dishes.  I had enough scrap ply left from the shop cabinets to build the jig that I used to cut the plates.  There are a couple of things that I should mention up front.  It makes an unholy mess of MDF dust.  Wear a respirator.  If I were making these dishes for a living, I would have figured out some method of dust collection, but for the two-off that that I'm making, I assumed clean up would be easier than rigging some elaborate method of dust collection.

Pictured, you can see the Xmas Tree Stand that serves as the base, the two end pieces to the right, the two rails for the router on the left, and the two sides on the foreground.  The 1/4 inch MDF attached to the one side is a template for the radius that will be cut into the side piece.  I have a nice one inch pattern bit that cuts such things like a champ and the template was an excuse to use it.  Since we're not talking a ton of material that has to be removed, you could join the two side pieces together, scribe the arch, then remove the material with a belt or bench sander.  Either way, it would work, but I recommend the investment in the bit.

Pictured, you can see the dish blank mounted on the Xmas Tree Stand.  It's a rather simple affair.  Use a wooden dowel.  I used a 1/4 inch dowel.  Your router bit is going to hit it, and you don't want to waste a router bit on this project.   To make the blank, I did as described.  I cut the MDF to size into two 22 inches squares.  I then used a very simple circle jig.  In a scrap of 1/4 MDF, I cut a hole large enough to accept a brass bearing on my router.  The measurement is to the outside edge of the bit -- in this case a 1/4 inch spiral upcut -- and I marked the location on the MDF for a 21 inch circle (10 and 1/2 inches from the edge of the bit).  I drove a finish nail through the MDF at the radius mark into the center of the dish, and viola!  A circle cutting jig.  It's pictured in my previous post.  I cut the circle in the MDF to a depth of about 1/4 inch, then used my jig saw to cut the majority of the scrap away.  On the router table, I finished the edges of the blank using my pattern bit running the bearing against the circle cut with the  router.  Since MDF chips easily, I rounded over the bottom of the dish with a 1/4 round over bit.

Pictured, you can see the router channel assembled.  It didn't work exactly as I had imagined.  With classic engineering oversight, on the length of the box, I had taken into account the width of the of the router base in determining the long end of the box, and so cut it only 3 and 3/4  inches longer than the 11 inch radius.  I didn't take into account the handles on my router base.  Also, I profiled the radius only on the long end of the box to the center marker.  In another well-duh! moment, I messed up the measurements on the first go round and didn't take into account the off-set of the long end.  I cut the radius from the center line of the side, not the center-line of the Xmas Tree Stand.  I disassembled, trimmed it off and started over, but you can easily see that you would want the center line over the center of the dish not the side and the 1/4 deflection (or 1/8th) should be at the edge of the dish not the end of the side.

Here you can see the whole set up put together and ready to rout.  Again, it makes an unholy mess of MDF dust, so use a respirator or something to protect your lungs.  On the router, I used a 3/4 inch straight bit.  The router wants to throw and spin the dish for you, but I could easily hold the router, hold and control the dish, and slowly advance the router from the outside to the center while turning the dish.  The rubber pads on the clamps helped with that.  I lightly pushed them against the dish and that too helped against the tendency to spin.  I had thought of taking the handles off the router to make it fit the channel better, but thought better of it, and was glad I didn't.

When the dish comes out of the jig, you'll want to sand down the ridge on the edge and take away the router marks.  But it worked, and I now have two radius dishes, one for the back and the other for the soundboard.  The stuff in the jar is a mixture of titebond glue and water at about 50/50 mix.  It's mostly just glue from nearly empty glue bottles.  I rinse out the bottles and save the remnants.  For lack of a better term, I call it sizing and I use it to harden MDF for sanding and just generally.  I'll also put a coat of poly on them.        

 

 

Shop Cabinets & Radius Dish

I did dig through my available plates, and unfortunately I didn't have one that would allow me to get a start on an actual guitar -- mostly scraps saved for various purposes, but mostly because I couldn't bear to throw out exotics, no matter how badly stained by the flood water.  So, I am still awaiting parts for my band saw.  I'll be able to re-saw some redwood for a soundboard, and I will probably buy the back and sides from LMI.

In the meantime, I have decided to go ahead and build a couple more shop cabinets.  I had gone to the Depot to get what I needed to make my radius dishes, but they had cabinet grade ply on sale for a little over twenty bucks, so I took the opportunity.  I will have scraps sufficient to make the router jig, and I did buy the 3/4 MDF that will be the dishes.   The shop cabinets are relatively simple affairs.  They are basically plywood boxes, joined with a tongue and groove -- that is, I cut a 1/4 groove on the sides of the cabinet, and a tongue on the top and bottom ends to fit the groove.  One will be used for my routers, and the other will be used for miscellaneous power tools.

As I was building the lattice jig, I did get a start on the jig to cut the radius dishes.  The plan is this:  I built what I am calling a Xmas Tree Stand, which is basically just a plywood X with each arm 22 inches long.  The MDF dishes will spin on this base.  The router jig will be similar to the one used to shape the drum for the thickness sander with a couple of exceptions.  My router base happens to be 6 inches wide, so two end pieces will be cut at 6 inches wide by about 8 inches tall.  The sides will about 6 inches wide as well (conservation of scrap) and about 25 inches long.  One end of the jig will be attached to the Xmas Tree Stand and the other will overhang about three inches or the radius of my router plate.


I plan to profile the sides with the arch on one side for the back using my router and a flush cut bit.  The first step is the creation of a template for the arch.  I use my metal shop ruler to create the arch.  I c-clamp a small block of scrap to the back-side of the rule, then clamp that down at the center of the arch.  I then deflect the ends up to create the arch, scribe it, then sand to the line on 1/4 inch MDF.    Remember, the low point of the arch should be over the center of the Xmas Tree Stand, and should allow enough clearance to attach the sides of the jig to the ends.  To accommodate that, I measure in 3/4 inches from one end of the side (where it will attach to the fixed side) then 11 and 22 inches from that mark.  The 11 inch mark is the center, and I deflect the rule up 1/4 inch on each of the other marks.  I then double stick the template to the side, and cut with the pattern bit.

The amount of deflection at either end can be easily calculated (though I had to be reminded of high school geometry and algebra to do it).  Take the radius of the dish (in my case, 11 inches) and square it (121 inches).  Take the radius of the back and square it (in my case, 20' or 240 inches, squared is 57,600).  Imagine a line tangent to circle 20' in diameter.  Now, imagine a point 11 inches out on that line from the point where it intersects the circle.  Using the Pythagorean theorem (a squared + b squared = c squared), you can calculate the distance between the point 11 inches out and the center of the circle (in my case the square root of (121 + 57,600) or the square root of 57,721, or 240.25195, which I rounded to 240.25).  The difference between the radius of the back (or the distance to the point where the circle intersects the tangent) and the distance to the point 11 inches out on the tangent line is .25 or 1/4 inch.  So each end needs to be deflected up 1/4 inch.  For the soundboard, I'm going with 1/8th inch.

When I have the arches correct, I will attach a 7 and 1/2 inch wide strip of thin ply to the arch with screws.  I'll clamp the ends to the sides so I can get the width exactly right, and then I will attach it to the Xmas Tree Stand so there is sufficient clearance for the MDF dish.  Before I cut the dish, I will cut the slot in the 1/4 ply for the router bit, put the dish in place, and cut up and back to the center mark while spinning the dish slowly. 

I finished the shop cabinets, but didn't get as far beyond that as I had imagined -- focus on one thing at a time.  So the dishes are a project for next weekend.  I will let you know how it really went.