Its been a couple of week since I last got a chance to do some work on the cello. I had a stand at an open day at a friends furniture making workshop last weekend. That was a good experience even though it was not well attended by visitors.
The last job on the cello was scraping down the ribs. These were supplied as three 126mm wide by 2mm thick sheets. They needed to be 1.5mm thick at the most. I later discovered that a better thickness was 1.35 – 1.4mm. It helps to make the bending work more effectively especially in the C bouts. First job was to scrape them down on one side to remove any machining marks or scratches from sanding. Then the second side was scraped down to achieve the thickness. I gauged this as I went starting at one end and working back.
You have to be careful and measure frequently writing the thicknesses on the wood as you go. I used a builders glove to stop my thumb blistering. I then found an old patent leather shoe and cut off a peice of the leather to use as a cushion behind the scraper. The leather pad worked best as the builders glove masked the feel of the scraper.
The next job was to match the grain pattern so that the figuring reflects light in a consistent way along the sides of the instrument on both sides then mark out the parts used for lower bouts, C bouts and upper bouts. The individual bits were cut up and marked with “inside top edge lower bout” etc. according to which was which. Then it was bending time. I made my own iron at work where we have casting facilities for aluminium but I never at the time intended doing cellos so it was slightly short of the full height. It still worked ok. Incidently it only cost me £20 a few years ago by buying the heating element 115mm long x 14mm diameter and 300W heating capacity and a 40W rating dimmer switch. Works quite well.
back to the rib bending. I chose to put a damp cloth between the rib and element initially to allow the steam to transfer the heat but gave up on this. Dry heat works best as it sets the wood permanently wet steaming the wood has a tendency to spring back. It needs to be held in position until it is dry. It also raises the grain and negates all your effort to scrape a good surface. The pressure against the iron with dry heat actually compresses the surface fibre and makes it more sheer. You do have to be carefull not to scorch the wood having the temp too high. I wasn’t in a placid state of mind when I did this so the centre bouts got some scorching. They scrapped back ok and I was able to remove the dark brown marks.
The metal sheet is an old aluminium camping pot cut and spread out. Its great as a support for the ribs when bending them. You can buy custom made stainless steel tools for this job but I decided to make do with this. I did the C bouts first and glued them in. Once dry these were trimmed back and the blocks were prepared for the upper and lower bouts. The picture shows the gluing of the upper bouts. You can see how the blocks are prepared to accept the bent ribs.
Also prepared were clamping blocks to match the shape of the corners in the C bout blocks. These are used in clamping to make sure there is good contact and pressure between the rib and block.
The glue I use is animal hide glue, essentially gelatine which goes liquid when heated. Its the only kind of glue that should be used for this as instruments can be easily dismantled for repair etc. Done properly the glue joint is incredibly strong and lasts for decades without deterioration. The only problem is it can’t cope with heat and high humidity. These glue joints don’t survive long in the jungle.
I use a steel sugar bowl to hold the water bath and an oven proof ramekin dish to hold the glue. Its heated on a small heating ring that I bought out the nickle and dime shop.
The glue comes in granule form and needs to be soaked in water for at least an hour before heating. It will swell up and soak up the water. You should have at least 100% more water by volume and you may have to add some during the heating. Never let the water boil just a slight simmer about 60 degrees cent is max. When the glue runs smoothly and continuously off the brush and and starts to form skin it is about ready for making a strong bond. If you want a weak bond then it should be thinner, add some warm water.
The last picture shows the finished rib assembly. I did it in steps mainly to make the job easier in a small space. The C bouts first, the lower bouts second and the upper bouts last.
The ribs are also higher then the blocks. These will be planed down near to their finished height and the linings glued in. Only then will the edges of ribs and linings be finished to their final height simultaneously.
Next job is the linings.
-- Dave D