MOTM rack panel notes

[Paul Schreiber]

After banging my head against the slide rule all day (it's a K&E mahogany w/ivory inlay, c. 1931) here is what I've
come up with. As usual, all comments welcome.

The first order of business is to decide on the pots. I selected the 12mm (this refers to the size of the 'body',
12 x 12 mm) Panasonic pots sold by Digikey. Now, these pots pose several issues:

a) the shafts are 6mm, which is NOT exactly 0.250", the standard. So, the knobs are "special", but they do exist.
b) the fun part is selecting a combination of panel thicknesses and standoff heights to allow the pot shaft
to stick out long enough to push a knob on (whihout hitting the panel) AND to clear the bushing of the pot.

After about 6 hours, I came up with the following:

Panel thickness can be either 0.092 or 0.100".

The standard PEM (will get into PEM in a minute) height is 22mm. Now this is measured from the FRONT of
the panel. So, using a 0.100" panel thickness gives about 19.5mm inside height restriction (surface of PCB
to inside surface of panel). The pot height from pcb to top of bushing is 18mm.

So all that remains is to find a 6mm press fit knob that has a insertion depth of 7.1 to 7.3mm Which is what
I'll be doing next week. Isn't this fun??!?!

And, the screws to secure the PCB will be metric M3 threads, which is the most common. FYI: they are what's used
to hold your hard drive in the bracket (unless you tried to jam in 6-32 screws).

Now comes the issue of other things one may find useful on the front panel: switches. There are several cases
where a rotary switch is VERY useful. However, the pcb mount version I found is too tall: it requires 23mm
from PCB to front panel. SO......if you know of one that will work, let me know. Else, we'll all be doing it the old
fashioned way!!!

I have found rocker switches (like on a Minimoog) that are pc mount and that will align correctly in a proper
hole. This is not an issue as these panels are punched. People that do their own panels will have to probably
substitute panel mounted toggle switches.

Lastly, and something I haven't looked at is to specify the 3.5mm jacks so that the threaded bushings are long
enough to allow mounting hardware to secure to a 0.100" panel. I will probably buy these in bulk and offer them
to those how want to use them.

>>>PANEL SIZE REVISITED<<<<<

Since the case vs rack people (you know who you are!!) are about 50-50 divided, I propose to change the widths to
match 1U/2U (1.75 & 3.5"). The heights remain 5U (8.75"). This is so I can use standard rack rails and the
rack-a-maniacs can use them as well. Well????

Also, the issue of "snugness" arises. If you measure a 1U rack, it really is something like 1.70" This is to allow
"slop" when mounting gear next to each other. I would probably do the same, so that you could wiggle the panels
right next to each other without a gap showing. This is also a big advantage of using thicker panels. The way
Moog did it, using a custom extrusion to overlap the gap, is the prettiest way but is quite expensive.

Well, see all this modular stuff isn't always about circuit design! It's all got to fit together!

Paul Schreiber
Synthesis Technology

>>>>APPENDIX<<<<<

Q: What is a PEM?
A: PEM is a brand name of Penn Engineering in Danboro, Pennsylvania. It is used as a 'generic' name
(much like Coke or Q-Tip or Kleenix) and refers to a type of fastner or in our case, a standoff that is
press-fitted into a panel using a special machine. The PEM allows a flush fit, in sheet metal, that can then
be painted over (note that if you look closely, you can still see the outline of the PEM in the metal). 

Quite simply, a small hole is drilled in the panel. The PEM is placed through the hole. The PEM has a flared
rim that keeps it from falling out the hole. Next a die is placed on the panel and a 10 ton press smashes the
helpless PEM right into the panel. The flared flange is in effect "squished" flush with the panel.

The PEMS are threaded inside. So now we can screw stuff (like our pc board) to the panel from the backside.
No spot welding required.

These things are really cool! No, the hobbiest can't play with them. But they are used everywhere, almost
as popular as a "pop-rivet"