Really exciting day – I just collected the wood from David Dyke down in Sussex to start on cello number 2.
Procuring the cello wood
Front and back plate preparation
And here are the belly and back book matched pairs getting glued up after spending hours getting a perfect joint – lovely flame on the maple back.
Managed to do the back without clamps but the spruce always seems to move a bit when heated up for gluing so I just needed a slight clamp pressure to get it tight.
The belly wood has a wonderful ring to it and great grain even and straight structure.
I also started to thin the maple ribs down to size on the drum sander but not to finished size so that I could scrape a finish on what will be the outside.
The cello ribs
Will have to scrape them next because I don’t want to see any sanding marks on the finished instrument.
And here are the blocks cut ready to to glue into the form. The end blocks are in spruce, corner blocks in willow as it is more forgiving to carve.
Note I have added some removable sections to the central form at top and bottom to make removal easier after gluing the linings.
Just need to carve the blocks to shape and chamfer the linings.
Carving the cello back plate
I built a simple jig to use with my small router so that I could cut the edges to the right thickness (5.5mm all around, 5mm in the C-bouts and 6mm at corners) with the top firmly clamped down onto the workbench.
This is an enormous improvement to the approach used on my first cello and as well giving a really consistent edge thickness, it gave me something to aim at when rough chiselling the back profile.
Edges cleaned up on the drum sander and slots for purfling marked up.
I used my new pillar drill to good purpose to drill out depth guide holes across the whole back (the old drill didn’t have a wide enough throat to reach the centre of the instrument).
Now to start the long process of thumb planing the inside to give the correct thickness profile and plate tuning to get the right mode shapes and frequencies.
Carving the cello front plate
Now to start on the spruce top. I got to this point in just a day.
Came across a resin inclusion when carving out the top. On scouring the internet this is apparently quite common in good tops (1 in 10 maybe) and no problem – just have to to shape a plug to fit the cleaned out hole.
Then a final bout of scraping to get a perfect profile before marking out the purfling channels and deepening with a scalpel.
Next step is to cut the channel and fair everything in before hogging out the back side of the plate.
Cello plate tuning
I thinned the plate until I got good clean mode 2 and 5 shapes but before going further on the top I have to cut the f-holes and fit the bass bar.
Tuning the bass bar
Started with a bass bar 11mm wide and initial depths set according to the following table:
|Proportion of length||0||1/8||1/4||3/8||1/2|
Then progressively reduced the depth to get the changes of mode shapes shown in the gallery below.
Reducing the ends helped to close the ring-mode shape but also reduced the x-mode frequency which I was generally trying to maintain. Reducing the centre of the bar predominately reduces the ring-mode frequency but I also had to thin the edges of the top and the centre of the upper bout between corners to try and get the ring-mode down to an octave above the x-mode.
A final tap tuning allowed to identify and remove any thick or high stiffness spots both on the top and the bass bar.
The final bass bar ended up only 18 mm high in the centre with X-mode of 58.2 Hz and ring-mode of 121.5 Hz. I then thinned the back to match those frequencies – much easier – and got 59.2 Hz and 120 Hz.
Making the box
Started by gluing the sides onto the back after first re-enforcing the sides with strips of linen.
The Cello Neck
The neck blank I cut back in March had settled a bit so I squared it up with the plane and then marked up and cut it out on the band-saw.
Next step is fitting the neck to the body.
I did a drawing of the fingerboard profile to give an even break over the strings at the bridge of 21 degrees and then an action over the end of the fingerboard ranging from 4 mm at the A-string to 6.5 mm at the C-string. This showed I needed a 2.4 degree slope on the neck towards the bass side. I have heard of 1.8 degrees being used so ended up going for 2 which still looks quite a slant!
Marked up the heel with this 2 degree slant narrowing the heel down to 27 mm at the bottom with enough material to give me a 22 mm overstand on the A-string side.
Before fitting the neck I will cut and shape the fingerboard and stick it in place temporarily to aid in getting the set of the neck right.
Finishing the cello
I started by making up a varnish ground coat from a recipe I found on the internet in an article by Harris, Sheldon and Johnston.
This consisted of cooking up Boiled linseed oil and rosin at 180C for a few hours in the ratio of 1 part linseed oil to 1.5 parts rosin by weight until the mixture achieved a sticky toffee like consistency when cooled. I used my trusty mini fat fryer to do this safely in a double boiler arrangement using cooking oil instead of water. I then added turpentine so that the resultant cooled varnish had a golden syrup like consistency.
Then I ground in 10g of Kaolin (china clay) to 30ml varnish with 7ml of rabbit skin glue and added water until I had a nice creamy consistency that could easily be applied with the fingers and then ragged off to give an even finish.
Then into the UV cabinet for a day.
I added a few coats of shellac to make sure it was all sealed properly, cut back with 600 grade wet and dry and then a final wiped on coat of shellac before applying the first colour coat and drying in the UV cabinet.
Next I mixed up some oil pigments – 1 squirt of burnt umber, 1 squirt of burnt sienna and, a bit less than half a squirt of alizarin crimson and a little boiled linseed oil and then smeared on by hand until the colour was the right depth and fairly even and then back into the UV cabinet.
Wasn’t entirely happy with that – a bit streaky on close up – so rubbed most of it off with a turps soaked cloth and put it back on with several much thinner layers so as to leave no marks.
Result has a nice warm glow and enough variation to look interesting!
It will darken up a bit more after another 3 or so colour coats.
One more colour coat and then a couple of clear coats before cutting back and polishing.
Curing outside in the sun on one of the few sunny weekend days this year! – Just like the old Cremona makers used to do.
It all looks a bit shiny at the moment but that will go once the finish it has been cut back – aim to get a nice satin sheen.
Clear coats next.
Applied 2 top coats and then cut the varnishing back with micromesh down to 4000 grit and then finished with rotten stone and linseed oil. The cello has a wonderfully rich chestnut sheen to it.
Glued the neck on after removing the keeper, and then fitted the pegs, saddle and endpin.
Just finished the instrument and delivered to a very happy customer (George) who christened it “Beryl” in time for our weekend concert with the Stratford Symphony Orchestra!