Avid Rollamajig Repair

Avid Rollamajig with new ball socket
Avid Rollamajig with new ball socket

Mary’s shifter cable broke at the rear derailleur, causing the Avid Rollamajig to undergo spontaneous auto-disassembly. The only part we couldn’t find was the socket between the ball and the derailleur’s adjusting thimble.

Good news: my parts heap had the Rollamajig from my bike, which I’d replaced because the most recent derailleur has an integrated gadget that serves much the same purpose.

Bad news: the socket had a chunk broken out of it and I didn’t want to put a broken part on Mary’s bike.

Good news: at least I could measure the dimensions to build a new socket.

Bad news: it needs a spherical socket for what measures out to be a 6.8 mm (0.268 inch) plastic ball and that’s not one of the three ball-end mills I have in the tooling cabinet.

Good news: this isn’t a really critical high-speed / high-stress rotating joint. Pretty good will be close enough.

Sherline chuck in lathe chuck
Sherline chuck in lathe chuck

Turning the part was a quick lathe job on a random hunk of what’s probably nylon.

Bad news: the nylon was a rectangular cutoff from a slab and the three-jaw chuck on my lathe has been firmly stuck for the last year. It’s resisted all the non-Armageddeon-scale techniques; I fear I must machine the damn thing off.

So I…

  • mounted the nylon in the Sherline 4-jaw chuck
  • grabbed that teeny little chuck in the lathe’s much bigger 3-jaw
  • converted one end of the square hunk into a cylinder
  • removed the small chuck
  • mounted the cylinder end in the 3-jaw
  • completed the mission
Offset roughing mill
Offset roughing mill

Lacking the appropriate ball-end mill, I offset a ball-end roughing mill in the tailstock chuck so the near side was at the right radius from the lathe axis, then poked it into the end of the socket-to-be.

Which, of course, produced a not-quite-spherical dent that was a bit too shallow, so I chucked up a too-small ball mill (on the centerline) and carved out the bottom of the socket. The result was a more-or-less spherical socket of about the right depth, pretty much.

The right way to do this, and what I was going to do before I came to my senses, was turn the part on the lathe, drill the axial cable hole, then chuck it up on the Sherline CNC mill. Getting a spherical socket of exactly the right radius and depth using a too-small ball-end mill is then a simple matter of G-Code. Maybe I should write that up for my Digital Machinist column…

Yeah, you could use a ball-turning attachment, if you should happen to have one. Sue me.

Broken and new sockets
Broken and new sockets

Anyhow, it all worked out OK. The new socket is slightly longer than the old one, as it’s made to fit the derailleur thimble at hand. The end around the socket is slightly thicker, too, as it seemed more meat would add more durability where it was most needed.

The Rollamajig seems to be discontinued, although some of the smaller online sources still offer it. Building one looks like a straightforward shop project to me.

Ball socket dimensions
Ball socket dimensions

The sketch has dimensions in inches, because I was doing this on the lathe. Our daughter measured it in metric and came out with much the same answers, so it’s all good.