Cello Cross Arch Design

I've been avoiding the basement; it's just too cold down there right now. So I planned out my arching. I did have it somewhat figured out. I knew my long arch, and I knew how the inside long arch would go. I roughed out the inside catenary, leaving it some ways in from the edge, so I could always widen it up if I needed to. Now I needed to see where I need to be.

I drew out the f holes on the inside of the belly. This way I can see how the f hole sits on the arch.

The area of the lower wing looks like it should work into a nice shape. I make that area outside of the f holes as their own shape. It fits into the rest, and starts as a continuation from above the f hole; but it terminates as it flows into the lower wing by the eye. The arch below the eye sweeps into the edge in a big curve. At least that is the way I do it. That's what I like. I don't know if it is anyones style, but it makes sense to me, and it works well with my method of making. This is how it looks on the viola. First the upper arch flowing into the lower wing.

And this is the treatment at the bottome end:

It's nice and sunny today. Everything is SOOOO much better when it is nice ans sunny. I drew up some archings the other day at work. It's really easy to do. I had the measurements of the width and height, and just went form there. I started 40 mm up from each block, and made them 30 mm apart. I ended up with 21 of them. The most interesting ones are the middle ones.

I stay quite a ways away from the edge. If you get too close to the edge, your arching will be more bolbous looking. I'm not trying to get that. I want it looking right. You can see how the catenary defines the center portion of the arch, but the recurve is a differnt thing altogether. It seems to me the recurve is where the aching templates would be the most useful.

Make some purling

I decided to make some purfling. I read all the people who wrote about making theirs. A big peice of stock, a big plane, and a big workout. I can deal with the first two, but I'm not interested in a big workout. I decided to do it differently.

I found a piece of maple in the wood bin at work. planed the side to see what I had, and sliced off two 1/2" wide pieces. I have enough left to get three more pieces. I sawed two strips to make three strips about 3-4 mm or so wide. That will be the height of the purfling. Then I started planing. I have no idea where the purple came from. Very strange.

It worked pretty good. The strips were are about .7 mm thick or so. I stopped for lunch. After lunch I read Michael Darnton's way he makes purling again, and noticed that he brushe the wood with hot water, and lets it set 20 seconds, and then planes it. I gave it a try, and it works great. Dry, it cut fine, but a little chatter; but that will just show up as handmade. The biggest difference is the curl. The first ones curled a lot, and with water they only have a gentle curl. I found a piece of wood to use as a brace to keep the bench from moving.

I kept planing over and over, and sawing the slots down when I got to where they disappeared. Next time I will saw them deeper, and stop sooner. When the wood is wet it is harder to saw; you end up with fiber, and it is harder to keep straight. If the groove was still 1/8" deep, it would be easier.

So now I have two piles: one to be the center whites, and one to get dyed black. I'll go through the whites to see if any are too thin. I need to get some chemicals to dye the blacks. Roger Hargrave uses ferrus sulfate, Pot. Hydroxide, and logwood chips. Darnton uses ferrus sulfate and tannic acid. Roger's method needs boiling, but appears to be a one shot deal. Darnton's method seems to be a repeat as necessary thing.

Does anyone have a preference? Have you tried either method?

Cross Archings

I had a question about re-curves and cross arches. My outside arches are very much like a curdate cycloid. I don't make templates for them, and I don't have to get them perfect. The arching is composed on the inside.

With at cyloid, the point where it switches from convex on the top to concave on the edges is mathematically given. It is 1/2 the width, plus on half of the arching height. For a c bout that might be 50/2=25, plus half the arch height so 16-3(edge thickness)/2=6 1/2. So, the point of inflection is 31.5 mm from the centerline. 38% is concave! Much more than you would expect. In the bouts it is even a higher percentage because the arch height is lower, and the width is wider. The one constant is that the inflection point is always one half of the arch height.

If you have a cross arch from a poster, it is a simple matter to draw the inside arch in. Then draw a line for the bottom of the plate, and hold the poster upside down, and use your chain to see where the width comes on the bottom. If you do this for all the arches given, you have a good idea of where the original catenary cross arches can go to, if you want to end up with a similar arch. If you have done this for a few, you can come up with your own discoveries.

Here are a couple of sketches that show some of my ideas on cross arches. It is easier to draw them, then to put it in words.

What do you think?

Roughed out Cello Belly

I have the belly roughed out; really rough. It is 8+ mm everywhere, and way big on the outside, but I may have to add a peice at the lower bouts as wide as the purfling, maybe. I don't know what I did, but I must have either measured wrong, traced wrong, or the joint was way off when I planed it on the lower half. I don't remember any of that, but we'll get to that when we have to.

It is surprising how fast it went. About 5 hours total after gluing it up. It is about double the weight at 880 grams. (I used the kitchen scale) and the tap tones are viola like at 360/180hrz. I don't know if th etones will drop in half when the weights drop in halve; but it does seem to me that at least the wood is in the ballpark.

Here it is this morning before cutting it out with a coping saw. It is much easier to saw spruce than maple. But, I bet you knew that.

I can refine it some before I have to build the ribs so I can finalize the outline.

First off, I will have to be sure that the bottom is still flat. It doesn't seem to have moved.

Then I will cut the eges down to about 6 mm. That should be plenty.

On the inside I can take the long arch out to the blocks, or close to them, and re-do the cross arches to match.

Then I can carve the ceter area of the front down to a smooth circle, and even out the thicknesses.

Then I can blend the cross arches into the edges. They will still be high.

That is about it. If I do the same with the back, I can arrainge things like arch height (I have some room on the belly, but not the back) and thickening, to get the plates to be close in pitch. I don't really know if it matters, but it is easy enough to do. THat is about all that can be done until it gets down to finishing.

Drill press work

I worked on getting the cross arches roughed in, and the long arch refined. Before I start drilling hole in the other side as a guide, I want to make sure that the inside arch is halfway decent. It took about an hour and a half.

Now it's time to drill the holes. I use a 1/4" or so drill in my drill press. It is nothing fancy. The bearings aren't great, and there is up and down slop, and it isn't exactly square, (I don't think) but it was $40, and it drills holes. I made up a little post that the plates sit on from stuff I had sitting around, and I set the drill above that. I use 5 mm for violins and violas; I'm using 8 mm for the cello.

It took longer than I thought to drill the holes. The drill press won't reach the middle, but there isn't a whole lot of stock there, and my thickness punch will reach the middle of the plate.

After that it is just hog away at it with planes until the holes dissappear. I still have work to do on it, but there is a good hard hour worth of work there. An hours worth of good work is enough at one time, We watched a goofy movie that we had never seen, "Princess Bride. What a corny movie! Well, we've seen it now! And we had fun watching it, and maybe my joints won't be as sore tomorrow.