KIM Uno /PiDP-11 plans...

[Mike Barnes]

On Thu, Oct 22, 2015 at 1:37 AM, Mike Ross <tmfdmike at gmail.com> wrote:

> Well done. Have you looked at 3D printing? Not fast, not especially
> cheap, but potentially the least difficult developmentally. And once
> it's set up you can just let her rip and churn them out.
>
>
A few comments on 3D printing for projects like this... About a year ago I
created a 'blinkenlights' project for an HP Access system running under
SimH (for pic: https://www.flickr.com/photos/mikebikes/22213398140 ). An HP
2000 Access system in real life consists of a CPU and an I/O processor. For
this project, I used a 4-core ODROID-C1 running SimH under Linux-- one core
dedicated to the main processor, one core for the I/O processor, and one
core to manage the lights (and one for Linux to play with). The lights are
connected by SPI to Adafruit 24-channel PWM boards daisy chained together.
The I/O processor box is empty except for a PWM board and some LEDs. The
core running the lights computes the percentage each display bit is '1' for
the HP cpu cycles between SPI updates. This becomes a PWM value which is
gamma corrected so that the dimming looks correct. I forget the display
refresh rate, but with SPI it's in the many hundreds to thousands/sec range.

The boxes are printed an a well-tuned and heavily modified Makerbot
Replicator 2. Probably each box took 48 hours of printing (that doesn't
include equipment problems and design errors!). Because the size of the box
exceeds the dimensions of the build plate, each surface needed to be split
into (usually) two pieces-- the entire project then screws together using
screws and thermoplastic inserts. Not visible in the photo is that there is
a black 3D printed part inside where the LEDs mount, creating light tunnels
preventing light from escaping to an adjacent LED (and also precisely
aligning the LEDs). I was expecting to have to sand and paint the pieces,
but while you can see layer lines, it's really not distracting. It turned
out well, but only for a 'one-of' project. Doing this on any production
scale would not be practical (as others have pointed out). The quality,
time to print, and build volume size all cause issues.

Since this project I have been moving toward using a combination of my CNC
mill and 3D printing. I create positives by 3D modeling (with OnShape or
OpenScad) followed by creating a positive using prototyping boards and CNC
milling. I cast the positive in silicon and create polyurethane parts. If
you haven't seen it yet, an excellent tutorial is the 'Guerrilla guide to
CNC machining, mold making, and resin casting' (
http://lcamtuf.coredump.cx/gcnc/ ). You get much better quality with
molding and can mix your own colors to get exactly what you want. There are
some parts that have very complex geometries where 3D printing is still
useful (some of my light tunnels, or switch interfaces match this
criteria). For some parts (like PDP-8 and PDP-11 switches) you can get a
great result with two part molds-- they look a lot better than a 3D printed
switch.

I do have a vacuum former and have tried using that for creating things
like the front trim for a PDP-8. It's much easier and you get a better
result with a mold. If you were trying to get the details of the embossed
'digital' part of a PDP-11/70 trim, that would be really hard (for me
anyway) to get a good result from vacuum forming, but pretty
straightforward with molding.

One more note for original equipment holders-- I don't have, and in most
cases have never seen, the device I am creating. It is all done with
pictures on the web and whatever technical information I can find. So when
you are taking pictures to share, try to take at least some pictures
absolutely perpendicular to what you are photographing (including a ruler
attached to the object would be an extra bonus!)-- you'll make life easier
for us modelers! Thanks.