Sunday, July 19, 2015

Pin Point Capos?

In a previous post about the mandocello, I mentioned pin point capos. This is a custom feature that Jonathan specified; the idea being that individual “capos” can be screwed directly into the fretboard, stopping the pairs of strings at the required fret to allow for a variety of open tunings.
It appears that there are very few instruments around with these devices, so as far as I 'm concerned there is no right, wrong or should I say established way!
So, from first principles…..
The fretboard has to have a threaded hole, in order to take the screws, but even ebony would not be resilient enough, so a nut has to be inserted into the ‘board. In order to get the nut in the correct position, it can only be fitted once the mandocello has been set-up.
The nut also needs to be as large as possible, so that there is the maximum possible depth to the thread. This, however, is governed by the space between the strings- it so happens that on this instrument, a M2.5 screw fits with minimal disturbance to the alignment of the strings.
So after a trial............

...................... the fretboard had to be drilled, a nut inserted and then filled over. 

After looking at many different types of fixings, I decided to use button head socket screws. They do not have any sharp edges and sit quite low, so they should have minimal interference with Jonathan's playing and as importantly not inflict any injury, if accidentally caught by a stray finger! They’re also made from stainless steel (as are the nuts) so that they will not corrode in contact with sweaty fingers.
The next choice was for the material which comes into contact with the strings and presses them down on to the frets; essentially large, soft washers. Either felt or rubber seemed to be appropriate choices but after testing, the rubber was far superior, the felt just fell apart.
Also I had to perfect a method of making the washers. I made up a mild steel punch which worked very well on thin rubber.

However, it soon became clear that a thicker rubber washer was required, so again a bit more testing.
3mm thickness worked the best but using the punch distorted the rubber, so I improvised this “press” and then made a simple jig to drill a hole in the washer’s centre. Rubber, being flexible, isn’t easy to work accurately and to get 10 good ones, I made 27!

And at last a pin point capo!
One other advantage of the rubber is that, once the capo is tightened down (so that the string is firmly pressed onto the fret) you can still tune the strings and you may need to, because if you overtighten the capo, the pitch of the string will sharpen.Now of course it took many hours to get to this stage, not only experimenting in the workshop, but also on the internet trying to source components, buying them and later rejecting them!
The next question is where do you keep those tiny capos when not in use?
Both Jonathan and I wanted them kept on the instrument and the most unobtrusive position, I feel, is the back of the head. So I designed and made this Swiss army knife inspired gismo.

 The Allen key slips into a slot and is held in place by two tiny rare earth magnets and there are embedded nuts to take the capos. Being fixed between the tuners, also helps to protect the capos from accidently getting knocked.

And there we are; you would not believe how long all that took! Now I'm ready to strip down the 'cello and start polishing.

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