Well, I made the short arm (which has nothing whatsoever to do with the old military "short arm inspection", heh, heh…) and worked on the guide block.
GUIDE BLOCK
The block needs to have a close-fitting ID but only a small contact surface with the linkage rod itself. It needs to minimize any shifting of the rod along its path while allowing for the small bit of deflection I expect such a long, curved rod will have when the lever is pressed. I imagine this rod will want to bow slightly right at that "corner". The purpose of the block is to force the rod to stay within its imaginary track or pathway, but a small bowing at that location would cause the rod to bind up.
(And no one likes it when their rod binds up.)
So my solution is to minimize the actual surface that the rod will touch, even if it goes slightly out of alignment.
I tried one of my Noga deburring tools, but — alas — it was one that I have never used before. And that sucker is SHARP! So when I started turning it within the guide block's hole it bit in and started corkscrewing into the brass! This is a very bad thing!!!
So I got the faces of the hole opened up, but too much and very unevenly. I made the cuts in a few seconds and then spent quite a bit of time with that tool, several grits of sandpaper, and finally a Swiss file in order to salvage the block.
(For various reasons I have to make another one. This one was made using existing cast-off parts that were chopped up to suit my needs. But more space between the two cap-head bolts is needed, and I want to use new material that is not so scarred up. My trial-and-error work today does not really affect anything; it just wastes more time.)
Anyway, I was able to open up the guide block's hole to fit the larger 7/32" diameter rod. Nothing exploded. Nobody died. Fair enough.
SHORT ARM
The long rod is like a convoluted extension of the lever and not the linkage itself. At the end, where it pops up between the two slide tubes is, in effect, the end of the lever. So, between that and the stop arm, a complete linkage arm is still needed. To avoid the dreaded "link flop" I pinned one end to a simple swing arm and moves only on one axis, unlike a Heim joint like the Minibal. Any inaccuracy along the plane the stop arm travels will be very small, and the Miibal will eat that up. Having the other end fixed rigidly will prevent the long arm from being able to flop around, too. Having a T hinge on the lever end will make things rigid like an S arm. (Remember that I made a T hinge between the 5th lever and the Z rod for the same purpose: to prevent the long, wide arm from flopping around during use, causing all sorts of noise, and probably some sort of interesting freak show for the audience. A typical Heim joint is not very good in this regard.)
I reused one of the *many* prototype levers that I made (and quickly cast aside) while sorting the issues in the 5th thumb lever geometry. I cut it to length, end-drilled it, used an M3x0.5 tap, installed some all-thread, then bent the lever arm to help it make the curve around the stop arm. This effectively captured the all-thread, so that sucker cannot back out. I installed a small "jam" nut and the Minibal, and it looks pretty decent. I need to clean it up, some, though. Right now it is held in place using a brass shoulder screw with a shoulder that happens to be the exact, same length as the hinge tube, so it can be tightened down and does not bind. I want to replace the screw with some drill rod and then peen that over a small rivet, like on S arm linkages, but that can happen later.
I also made a completely different short arm. It was a challenge because it was made from flat bar stock, and I had to hand shave the end into a rod of the right diameter to use an M3x0.5 die. I have never cut threads into a rod I eyeballed from bar stock. I would have used it, too, but despite my trying to make the transition from the round threaded end back to its flat bar profile at the hinge end, well, it looked pretty fugly. However, it is strong and of the correct dimensions. If I need it. the thing is in a box ready to go. I will show it in the pic, too, but none of this stuff was cleaned up after having been sanded and filed on for hours. *ALL* of it is pretty darn fugly.
It turns out that I *do* have the needed 7/32" stainless rod for this. I will grit my teeth and try to make this horrible shape with it tomorrow. I haven't been a drinking man for many years now, but this one might merit a cold beer if I can get it to work.
I made a video. It is the typical craptacular affair that all my videos are. I am not posting it, I need to make it again, and this time use notecards or a script. It was truly a cinematic disaster. Here, instead, is the single photo I took.
Fun stuff to know: Mrs. E put my iPhone Xr through a steam/sanitize wash cycle after my having worked out in the poison ivy. I missed that my phone was in my pocket, and so did she. Whoopsie!!! Anyway, after turning it off, removing the SIM tray (to allow more airflow), and leaving it TF alone for four days, it works fine. The screen is screwed up but it still works great — including the camera. I am amazed that this phone is so tough.
EDIT: I added in two vidcaps from that deleted video. The focus is not fantastic on the photo of the short arm as it was moving rather quickly from right to left in the video. And yes, I burned TF out of my fingertip today, and you can see the blister in one of the pics. I also broke three drill bits, two brass rods, and one tap. It was a pretty bad day, all told, so I am that much more pleased with what I managed to get done despite this.
Starting at the top:
• Schaublin "Unibal" link (came with the tuba) It has too wide of a head for this project.
• A lever hinge rod of the needed diameter to replace the brass shoulder screw.
• The guide block's bore has been opened-up to prevent binding but is incompletely rounded.
• The winner of the Short Arm Competition, with its nice bend, fixed swing arc, and Minibal on the end.
• The runner-up in the Short Arm Competition, it's fugly because I shaped it using a cutoff wheel.
D'OH!
