Knee-chest-finger-rests, bridge and amplifier for the O‘Cello Remixed from - The O’Cello by cokane http://www.thingiverse.com/thing:1703629 and - AddOns by Zarlor http://www.thingiverse.com/thing:1833270

Breast- and Finger-rests Sometime ago I decided to do an update to the top-rests of the instrument. I was not satisfied with the look and feel – too clumsy in my opinion. So here is the new version.

This is the orientation of the whole rest-system which should fit between part_2 and part_3 of the neck assembly. I would advise to glue it together before mounting into the neck, as it will be much easier to get a good finish while handling a smaller part. Another idea - which came while writing this – would be to print the loop on the right over a preprinted 3-layered sheet, and after finishing it glue some soft and dense foam material to the body-side. This would close the loop and stabilize the instrument – think of violin shoulder pads.

If the middle part is too high for you, then just 3D cut it and slide the halves together. Removal of a piece of up to 36mm / 1.5” length should be possible without problem. I would use FlashPrint by FlashForge.com for this re-modeling – it is free and allows checking and modifying of stl-files. I find it most useful for all of my 3D-needs, without paying for expensive programs which need a lot of time to learn. The picture above was mad by importing all the three files and orienting them to better understand their position.

Knee rests Thinking of ladies’ dresses I tried to improve on the design by removing sharp edges and corners and giving the otherwise brilliant idea a nice soft and flowing look, more in harmony with the sound of the instrument and made the knee rests a bit smaller too… For ease of printing I added two roundels of 0.25mm height at the bottom ends of the knee pads so no further support will be needed. The contact area with the build platform is very small, but more than enough to stabilize the print, and the round pads are easy to remove. I printed it in 0.2mm layers. To print in 0.1mm steps would take too much time. I suppose that even 0.25 or 0.3mm steps (roughly 4 hours at 3 layers wall and 20% infill) would be quite OK. Finish by sanding (and painting if you like to do so).

The 12mm holes may need opening up with a drill or a dowel with sandpaper rolled around. Test fit the Carbon fiber tubes for a good seating. Don’t glue them yet.

I found these 12mm Carbon fiber tubes in 0.5m length for RC-quadcopters quite cheaply ($6 for 2pcs) on the internet. They need shortening to about 370mm, and a bit of sanding with fine sanding paper (250-400 grit) to round off the cut edges. After final mounting them a coat of lacquer will take off the roughness of the fibers.

Amplifying the O’Cello For those who want to play loud(er) you need to insert a preamplifier into the O’Cello (look for Electric Violin Preamplifier on amazon, the price is about $18 now).

This set comes with a piezo pickup, separate output jacks and battery case (for a 9volt battery) and has three separate sliders (for volume, bass and treble). There is NO switch included with this set - you need to buy an on/off switch and have a bit of soldering to do. This fits perfect into the amplifier panel I have provided. This panel has a cutout to route the cable from the battery-case to the amplifier and should be printed with a high infill. I have not included holes for screws (provided) as you might opt to glue it in place after successful testing the setup. I have combined the next panel (with the cutout for the battery case) and the holder for the knee rest tubes into one part. I did print this panel in black PLA (with high infill!!), and then printed the holder in white filament (30% infill – my small printer has no means to change any settings while printing). This holder-part has to be glued to the panel - take care to set it exactly at 90° to the axis of the instrument – I have drawn one layer on the panel to help in orientation (a second file has all in one). Glue the assembly to part_6 after installing the tuners. DON’T forget to put the battery cable in the groove before gluing! A second small cutout might be necessary for the switch – there are many types, so drill, cut and file to the size you need. Again, the 12mm holes may need a bit of opening up with a drill or a dowel with sandpaper rolled around. Test fit the Carbon tubes for a good seating.

Use masking tape around all glue joints to get a nice clean surface. I always use white epoxy resin with 24h curing time as this gives me enough time to correct any misalignment.

A crazy idea came to my mind – add a Bluetooth transmitter from banggood and play without cables – to your computer, or pair it with another one to a big(ger) amplifier …

• but remember – even a small 10Watt training amp on full power will shake the house!

The bridge The bridge I have modified to accept the piezo-pickup. The cutout (3x44mm) is a bit bigger than needed, but as the pickups vary in thickness I found it easier to put a strip of dense polyolefin foam below the pickup thereby pushing it hard up against the bridge. The leads of the pickup I have bent carefully at right angles to point forward, and guided them down through the opening in the O’-panel to the amplifier. Some slight filing with a round file might be necessary here to fit the isolation of the cable above the sharp corner of the triangle of the guide for the bridge.

And this is how it looks now. You can barely see the pickup - just a tiny black shadow.

As an afterthought - I could have printed the O’-panel in white and painted the recess black, now it looks a tad grey because of the split print with low infill.

Sound ? Well, for testing I took the least expensive strings (about $3.30 per set, free shipping too) and my son’s smallest amplifier (Ibanez IBZ3 with 20Watts output). And I am fully satisfied with the sound and volume of the instrument. A great Thank You to Conor O’Cane for the design of the instrument and to ZARLOR for his important add-ons! Printer used: Dremel 3D20 Ideabuilder

Material used for the panels: PLA Layer height: 0.2mm Infill: >50%

Material used for the holder: PLA Layer height: 0.2mm Infill: 30% - might be overkill, but with a small part and high torsion it is better to be safe

Post Printing: Some TLC with a sharp knife and some sanding paper (180 grit and finer) will be needed to finish.

Painting the instrument is up to you, but use some hard drying paint. My instrument is just white and black PLA, black Carbon fiber rod (12mm!!) and black Carbon fiber tubes (12mm), silver anodized tailspike and silver tuning pegs (I forgot to order black ones…). The finger board I have veneered in black walnut, stained dark before gluing, and painted it thrice with thinned polyurethane lacquer – polishing between and after after each coating. The bridge got printed in red – as an optical accent.

Designed with 123D®Design by AUTODESK®

==> Links to the carbon rod and tubes can be found in the accompanying PDF <==