Got A Brand New Banjo

[Jim Frye]

My latest turning project of the bowl exposed a bit of a design error on my part. Back in 2000, when I designed the lathe, I had no cheap access to metal machining, so most everything was constructed from wood. Well, I AM a woodworker. I constructed the banjos from plywood, glue, screws, and a lead shot/epoxy matrix. They have held up well until the stress loads of bowl turning combined with the 12" tool rest, and my learning curve catches, overcame the design. The threaded insert used to lock the tool post in place pulled out under the load from the clamping bolt and split the plywood. I glued the plywood back together, but the threaded insert remains displaced. While I was showing the lathe to my Son, he offered up a replacement for my wooden banjo. He does CMM (Coordinate Measuring Machine, CMM is a device that measures the physical geometry of objects with high accuracy and precision), 3D CAD, and 3D printing in metal and plastic at his employer. He also has outside 3D printer suppliers to work with. He took my damaged banjo, scanned it with the CMM to produce a 3D model, and rendered it into a 3D drawing with some revisions. The part was solid printed in a high strength ABS plastic with cavities to allow for the lead shot/epoxy infill. The cavities lowered the printing cost due to using less material, and allowed for a heavier part to absorb vibration. This combination resulted in a vastly improved lathe part.

These are the original plywood banjos I made with the damaged larger one, which weighs 29.6 ounces. These two also have lead shot/epoxy infills in them.


These are the 3D rendered banjo drawings with an improved post locking bolt and threaded insert mount.





This is the 3D printed banjo with the recesses for the lead/epoxy infills. It weighs 15.9 ounces. I used my Dremel (Ryobi) hand tool with a dental burr to carve small keying reliefs into the sides of the recesses and etched the cavities by swabbing them with acetone.


This is the printed banjo with the #7 1/2 lead shot fill.


This is the printed banjo with the MAS Easy Cast epoxy infills in place. It tips the scale at 55.3 ounces.

[d_meister]

If you need even more rigidity, you might look into something like PCBWAY. It is a one-off maker in Asia that will machine or fabricate almost anything inexpensively. Having an .stl file would be enough of a CAD source for them to duplicate it from metal . I hear the prices are quite reasonable.

[Jim Frye]

I realize a metal banjo would be stronger and as I stated, my Son works in plastic and metal 3D printing and has outside suppliers that he has used who are here in the US. Given how long the assembled plywood banjos have held up, I'm pretty sure this new single piece one will out perform them. It has twice the mass of the original and an improved post locking mechanism. If it doesn't hold up, we'll just send his digital drawings out to a printer who does metal printing. The lathe already has die cast metal parts that have been lead shot/epoxy filled and they are vastly improved.