Having now described the design (and the mistakes) and the build guide (and the workarounds) this post looks at the mechanical side of things that I had completely neglected when designing the PCB!
- Part 1: Design.
- Part 2: PCB assembly.
- Part 3: Mechanical design and assembly.
Warning! I strongly recommend using old or second hand equipment for your experiments. I am not responsible for any damage to expensive instruments!
Mechanical (Post)-Design
As mentioned previously I was really using this as a "think with objects" design process for the PCB so hadn't included any kind of provision for mechanical fixtures. Of course with hindsight that was a significant oversight 
It just so happens that there are a number of places on the PCBs where there are gaps big enough to support mounting holes, so I've now gone back and added them in and use the positioning as a template for hand-drilling some holes in my actual PCBs.
Note each mounting hole is for M2 sized mounts, nuts, screws, and spacers.
The holes are positioned such that when the OUTPUT PCBs sit across the INPUT PCBs as shown below, the holes will line up.
I used the KiCad printout as a template for the first holes to be drilled. Then once I had a blank OUTPUT PCB with holes I used that to mark and drill the others.
The holes in the INPUT board have to support both "halves" due to the overlapping OUTPUT board, so there are holes that won't be used when they are paired up. Consequently I only drilled the holes I actually needed as can be seen above.
Mechanical Assembly
The holes are 2mm diameter so require M2 standoffs. I've found that M2x20mm standoffs seem to sit just about right to avoid the DIN sockets. In my case I've added an extra nut as an additional spacer too.
I've used 6 standoffs between the two INPUT boards and the first OUTPUT board, then 5 standoffs between each OUTPUT board.
Here are a few photos of my boards fixed together.
My drilling isn't massively accurate, but it is close enough that a little flex in a nylon spacer isn't a problem. With them fixed like this, I think having just the rotary switches panel mounted might work out ok.
Inter-PCB Wiring
Once the mechanical assembly of the boards seems sound, there are two additional sets of wiring required: 5V and GND to all the boards; and the MIDI interconnections between the INPUT and OUTPUT boards.
I initially thought I'd keep the additional power section of the input PCB connected and use it "on board". But after hunting around for something to use to enclose the boards, I opted to remove the power section and will wire it up separately to the main PCBs.
I ran "bus wires" vertically through all output boards for 5V and GND and then connected them to the input boards using jumper wires as shown below. It would have been a lot easier if I'd thought to include 5V/GND on the other side of the INPUT board too, then they would have been right next to each other and directly underneath the power/ground for the OUTPUT boards.
Then I wired up the MIDI ports. Looking from the front I wanted port "1" to be on the left and to correspond to the most anti-clockwise position on the switches. This means that (as the silkscreen labels are backwards on the output PCBs) from the back, the leftmost "THRU2" port needs connecting to the pin headers labelled as "1" and the rightmost "THRU1" port needs connecting to the pin headers labelled as "4". On V2 of the boards, the same holes will be used, but they would be labelled in the opposite manner to correctly reflect the positions of the switch.
Note that it doesn't actually matter which of the 1-10 THRU pins on the INPUT board is connected to which output board/port as they are all active THRU ports all the time, but I've opted to start from the bottom left (from the back) and move right and up. In order to keep track of where I was in the wiring, I've chosen to use four different colours for each of the four input ports.
Power Connections
The power circuit is then added by soldering to the "bus wires" too.
In the final mounting I've fixed the power circuit to the side of the box and added a power switch. I've also added a power indicator LED, but more on that later.
Enclosure and Panel
I have some CD shelves left over from my CD Rack Format synth box, so I've used those to create a box for the patch bay.
I was initially thinking of making a CD Rack Format panel, but I don't have space in my CD Rack right now, so I'm building a temporary enclosure and panel instead with a panel made from some hardboard.
The dimensions of the panel are 160x185mm with the following cutouts (which are either 6mm or 7mm holes).
The panel template has been produced in KiCad so I could add some neat labels and actually get a PCB panel built at some point in the future should I wish to. For now, drilled holes in hardboard is fine.
The only proviso is that as the TRS jack sockets don't protrude enough from the front of the panel, I've drilled out larger holes around them to support a jack plug being inserted without the panel getting in the way.
Here are some photos of it all. It is all very crude but will do the job for me for now.
Power Switch and LED
I've added a power switch and power indicator LED. The LED is soldered to the top of the "bus wires" for 5V and GND via 1KΩ resistor as shown below.
Closing Thoughts
Some of this is still a little crude, and there was quite a bit of "making it up as I go along" whilst putting it together too, but in the end I'm pretty pleased with the results.
One of my cheap switches is a little dodgy. In the first attempt of a build the panel was putting pressure on it slightly which meant it wasn't; quite connecting properly. Making the panel hole slightly bigger seems to have helped but really it ought to replace it with another one, but that would mean dismantling quite a lot of the build again, so I think I can live with that.
I'm particularly pleased I took the time to patch the OUTPUT PCBs to use the switched TRS sockets. It is really quite useful to be able to have fixed MIDI links plugged in the back of the unit, but then override that with a TRS connection in the front.
Whilst on that topic, the MIDI DIN connections for the INPUT boards are quite wobbly - I don''t have a particularly good way of mounting the INPUT boards to the box. I could drill through the bottom and add some mountings for the lowest set of 2mm nylon spaces, so might still do that if it seems necessary. But I don't plan on plugging things in and out of the back of the unit very often, so it will probably be ok.
All that remains is to see how robust the switches are when in general use. I might end up doing a rebuild in a few months time!
These boards have been manufactured using the Seeed Fusion PCB service, which I am happy to continue to recommend. They have been supported with discount vouchers that I've been sent by Seeed for my previous projects.
Kevin
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