Before I continue with finishing the neck and scroll I decided to do the contours of the cello back. As I have a whole day working I want to get this job done. Its quite hard work and you need to pace yourself. A few weeks ago I joined the two halves of the back using hot hide glue. I didn’t have my camera so I borrowed this photo to show what the outcome is. The link to follow for the image is http://www.jasminedavis.com/cello/gallery/albums.php its an excellent site for the complete process.
The two halves are book matched. This selected tone wood is purchased as radialy split or sawn wedges. They have been almost completely sawn through but still connected at one end to retain the matched halves until you are ready to use them when they are then seperated. the following picture shows some viola wedges of figured maple.
The next photo shows the tools and how the wood is held during the joint prep. The joint surfaces are all done by hand although I use a surface planer to get the initial square edge. You can’t afford machine planer ripple marks and it has to have perfect surface contact the full length and width of the joint. If not the joint will be painfully obvious, possibly have gaps and potentially weak. Ideally contact should be so good that a rubbed glue joint is sufficient without clamping.
Since it is such long a joint in a cello and the wood is heavy I tend to clamp it lightly until hard.
Once this is completely dry its time to level the back which is used as a reference surface for arching heights. The wedges glued on to the back in the earlier photo are for steadying it on the work bench. Remember each side is tapered and it would rock from side to side.
Its at this point the rib assembly is used for marking out the shape of the cello on the flat reference surface. I usually level the edges of the ribs so that they sit flat against the back. The glue joint is the line of symetry. So the rib assembly needs to be lined up centrally. Its held in place and a line drawn to give the overlap of 4mm. An excellent device to use for a consistent overlap is a suitably sized washer that rolls around the rib with your pencil in the washer hole. It is important to shape the corners of the ribs properly before marking them on to the back.
This cello profile is then cut out on the band saw allowing about 1.5mm for finishing.
Next I mark out the thickness of the edges using a marking gauge. This is referenced from the flat side of the back. I usually leave about 2 to 3mm extra above the finished size at this stage.
Next is cutting down the edges. I use a forstner bit with the centre pin ground off and the bench drill. It can be a pain getting the cutter holes initiated without the centre pin but it is quick, painless and effective. The depth stop is used to maintain the edge thickness at a constant size.
In an earlier blog I prepared the templates for the shape of the back from the cello drawings.
Its time to use these to mark out the contours of the arching.
Using the glue joint as the line of symetry the contour lines are marked out on each side through the small holes. the nails are used to located the template in the same alignment once it is flipped.
Each contour has a specific height from the flat underside of the back. These are cut out on a block and used to set the depth stop of the bench drill.
The contours are all drilled out to the specific depth. These are used as markers for carving later in the process.
I like to draw in the lines before drilling to avoid drilling holes on the wrong contour. Just follow the line for each height.
When all the holes are cut its time for removal of the waste and carving out the contours. I use a number of methods for this, drilling, electric plane and hand plane what ever is easiest and fastest with results.
Its now easy to see how the contour holes work. When you go down far enough they disappear. i always allow a couple of mm extra height so that I will be clear of the holes when I finish carving. Next bit is hard work planing and gouging the contours of the arching.
Arching templates are used to check the shape. First thing to do is establish the long arch from top to bottom. After that its the cross arching to be shaped.
Its important to constantly check as you work. I rub pencil into the edge of the arching templates so that it is deposited on the high points of the surface. Then I know where to remove wood.
I use spokeshaves, specialised planes and gouges to remove the wood. Later when the final shaping takes place I will use cabinet scrapers. For now the following pic shows how far I take the shaping.
The next step is to finish the thickness of the edges to 1mm over size and shape of the edges to give the exact 4mm overlap around the rib assembly. This needs to be done before the purfling is put in. I make my own purfling so check out this link to see what I do.
Bye for now
-- Dave D