A long time ago, I saw a video or article or something of a guy who was building a solid resonator back by using a router with a compass guide and routing different depths all around to eventually create a semi-smooth surface and then finished with some other tools (I can't remember which). If anyone know what video or article I'm referring to, please let me know. Thanks!
I don't know the video in question but there are several videos on youtube of guys carving Les Paul tops using routers and templates.
You could probably do something similar using diminishing diameter round templates and a pattern bit.
Or easier if you could beg, borrow or buy a lathe. I found a used one a few years ago with a gap bed with 16" swing for $200. You could pay more for a good router.
I bought a lathe many years ago just to make a banjo rim. The next thing I knew, I was making peppermills and pens. Turning can be as habit forming as banjo building!
You have to be careful with solid back resonators unless you live in a climate with no humidity fluctuations from season to season or keep the banjo in a climate controlled environment.
Since wood shrinks and expands across the grain by a measurable percentage, but not along the grain, the resonator will fluctuate between being round and oval seasonally. Because there is no opposing top on a banjo resonator as there is in a guitar, this will cause it to "boat".
The Gibson top tension banjos had solid wood backs, which were flat inside, and do this despite the flange acting as a mitigator to a certain point. Just in the past three years, I have had 2 requests to repair boated or cracked resonators like that.
Edited by - Ken LeVan on 04/01/2020 10:42:11
Years ago in the Les Paul forum they were doing tops on Les Pauls sort of that way. A stack of terraced router templates left a terraced surface they then would plane and or scrape down and smooth out the transition to the edge. Gibson did that with a gang saw on those tops, it rolled over the top and ultimately was finished on a huge belt sander pushed onto the surface. Needless to say they were all different slightly depending on the day the man was having on the sander. I haven't seen it done using any sort of compass arrangement.
All solid banjo resonators do not twist and bow like the one shown above. I built this banjo in October, 2015 and made a lathe turned resonator from curly maple. I like extremely rounded edges on my stuff. I made this resonator and put it on my civil war inlaid, Nichols air banjo. I did not use binding on it but rounded the edges over for comfort. I hate trying to play a banjo with sharp edges on the resonator. This resonator is roughly the size of a Gibson style with similar dimensions. It has remained stable since I built it. Here are some pictures.
'Good Wednesday Morning' 8 hrs
'Poltergeists ?' 9 hrs
'Eye Bleach' 10 hrs
'Goodbye Liza Jane' 11 hrs