THe last entry saw the contour shapes made. These were chacked against the cross section templates.
I need to check the form is accurate against the contour plan. Remember the plan shown below. It is used to make comparisons with the existing contours on the cello back.
THe contours are drawn on to the back using a simplistic type of surface gauge aka a bit of wood with a pencil held on to a flat surface. The pencil point is set at the height of the contour and this is marked onto the cello using the flat base as a reference surface. See below.
THe lines can be checked against the plan by lining up the drawing on top of the cello back and marking through the pin holes with a pencil. From this you can tell how close you are to the contour form.
The lines which bulge out from the expected contour shape indicate wood that needs to be lowered in height. The areas that bulge towards the centre indicate areas that are too low. Luckily I didn’t have any as the solution is to radically alter the original form.
I worked under a low angle spot light as this help to identify fluctuations in the flow of the surface.
THe next day I prepared the bed for hollowing out the back. See below. I had kept the off cuts from making the internal mold back in October. These are used to make the raised parts of the bed that the cello back sits on. The hollow in the centre is to accomodate the arching as the back lies upside down in the bed. Before I drill the cello back plate I test the set up using a scrap and gauge it to make sure the set up is ok.
First step is to mark out graduation pattern. Generally the thickest part of the back in all these types of instruments i.e. violin, viola, cello is just slightly higher than centre. I marked the areas out and have indicated their respective thicknesses.
Next is to set the depth stop on the drill to stop leaving the thickness required under the drill. I have a simple jig that is a 1/2” dowel with a domed top end stuck in a peice of MDF. THis is clamped under the drill bit and the cello plate sits on this while it is being drilled out.
Eventually all the holes are drilled to the appropriate depth.
The triangle in the middle is the bit I can’t reach with the drill. I have to remove that the old fashioned way. The good bit about drilling is that it gives you a marker for stopping when you are hollowing out and if you drill the holes close together removes 80% of the wood at the same time. The next pic shows the removal of the triangle bit using a spoke shave followed by the removal of the other waste using a gouge.
Once the bulk of the waste is removed then the surface needs to be leveled down to the points of the drille holes. I use the curved sole miniplanes and gouges to do this. The next pic shows this done on half the cello plate.
Once again the low angle light is useful for seeing the pin points of the drilled holes.
The thickness of the plate is very close to the graduation pattern that was marked out. These are not the final thicknesses.
So here we are at another key point in the cello. The back is contoured and hollowed out. That was quite a physical activity. The next bit is to smooth down the inside of the back plate, tune it and finalise its thickness.
I like the challenge of plate tuning but I shall need to do some research on this before doing it. I’m looking forward to next weekend already.
Bye for now.
-- Dave D