construction continues week ending Aug 18, 2013


One of the push-buttons on the 1130 is a 'short' one, whose face is flush with the top of the black plate rather than standing proud like the other buttons. This is the IMM STOP button, which halts the processor in a way that abandons input-output activity and the remaining cycles of the current instruction. As such, it is not considered safe to push the START button to resume processing if this button has been used.

Other buttons that have un-reversible effects, for example PROG LOAD or RESET, are interlocked so they only operate when the processor is in the stopped state. The immediate stop function is recessed flush with the plate to make its accidental activation less likely.

I have a 'short' button, but it is green rather than the red color of the IMM STOP button and further it has different text etched into the face. Thus, I will endeavor to manufacture a short button with a blank face, one that I can color red during casting and then label with my dry rub transfers.

To do this, I took two superfluous buttons I had, one a full size button with a clean blank face and another where the condition of the lower half was good. I sawed the face of the blank button very near the top, so it was much more shallow than other buttons. I also had sawed the other button in half, leaving me the bottom part that slides in the holder and activates the switch mechanism. Some epoxy between the short face and the bottom section gave me a button model that is approximately the right shape and height to serve as my 'short' IMM STOP button. That is, it will serve as a model to make a silicone mold, with some glue to seal any openings at the glued junction.

With the 'short' button model put into the mold container, I mixed and poured the mold materials and let it gel. The result looked good, so I did a test casting of the short button as well as a test cast of the full sized button. I found that a mix of 1 drop red to 1/2 drop yellow pigment got me close to the right color, but still a bit dark. I might add in some white on the next trial.

The main remaining problem is the relatively slow curing rate, along with a thin wet film on the outside of the solid button shape. I am already up to about 14 drops of catalyst per ounce of resin, but for the next trial I will kick it up to about 18 or 20 to check out the results. If it gets too hot, with too much catalyst, it could crack.

I can wipe the wet excess resin off the outside by using acetone on a kimwipe but the surface becomes dull/matte instead of glossy. I have to find out how to correct this surface effect, otherwise the buttons won't be satisfactory. The brochure for clear casting resin suggests three things - heat the mold in the microwave before casting, warm the resin to about 90F before mixing, and leave the casting over night as it takes long for the resin to harden at the surface with platinum silicone molds, but it supposedly will if I am patient.

I also built a mold for keycaps, to let me cast some blue and gray keycaps for the keyboard, for those keys whose captioning was different than the 1130. I used a blank faced keycap as the model and produced a nice mold. There will be a complication with these, as they need a thin slot in the bottom into which the metal tang of the key mechanism fits. I will figure out a method to cut these later.

The keycap result met all my expectations - dried hard overnight but didn't crack so the extra MEKP catalyst was the right change. The color is great and these are ready to add text, cut the bottom slot and seal with gloss paint.

Authentic keycap has lettering 'P', the rest are my castings

My trial with the buttons was mixed, as I used the smaller amount of catalyst and still had the wet surface issues. The red color is good and I have an acceptable quality red button which I will label and send to Bob Rosenbloom for his 360/30 panel project. The short button was structurally good, although a bulge formed on one of the sides due to a mold defect. I can cut off the side bulge without compromising the look or operation of the button. A more serious problem arose. There was a fleck of yellow pigment on the face - apparently it didn't mix as thoroughly as I had thought. When I tried to brush it off, I put a fingerprint on the face.

There is a spray that is claimed to help remove fingerprints and other defects from these cast objects. I will pick it up and see if I can rescue the short button. If it worked easily and well, I could speed up the production of buttons, otherwise I need to allow 12-24 hours of cure time before removing each of them from the mold.

After testing the spray, I found it to not be exactly what I expected, but has a bit of value. It just wets the surface, but not sure how to even a face up with the spray. Any wipe I used smears the surface and often sticks onto the wetted resin. There might be a method with plastic or metal tools, but it isn't obvious.

A run with 14 drops of catalyst and 24 hours before demolding gave excellent results - my green 'program start' button is fully satisfactory. My green color is a bit brighter than IBMs but acceptably close, my red is getting very close to a match, and I am working on the blue as I prepare to cast my first blue button.

my green upper left, ibm button bottom left. ibm button middle bottom, my red upper right
I refabricated the decals to adjust the font size and thickness, making them match the 1130 better.  Once I produce the decals, probably later today, I will be able to title the buttons I have already finished. Since each casting run requires 24 hours in the mold to get a good hard outer surface, my rate of production of buttons and lights would have been limited to 1 per day with the single mold on hand. To speed things along, I made two more molds of the blank faced button/lamp kind, which will allow me to pour three buttons or light cubes or combination per day.

To date, I have cast the following buttons and keycaps but not yet applied the titling to them:
  • Int Req keycap     (blue)
  • EOF keycap        (blue)
  • KB Rest keycap     (blue)
  • Erase Field keycap  (blue)
  • Back (arrow) keycap   (tan-gray)
  • (blank) keycap 1  (tan/gray)
  • Program Start button (green)
  • IMM Stop button (short red)
  • Program Stop button (red)
  • Load IAR button (blue)
The remaining casting needs include three tan-gray blank keycaps, two blue buttons for the black plate area, three blue buttons and one special blue handle for the typewriter, one red light cube, one yellow light cube, two green light cubes, and four white light cubes of which two will be untitled. The buttons and light cubes will require a minimum of five days with three molds in use, the keycaps will take four of those five days. and I will need to make a model and mold then cast the special button sometime in the interval.

I made an experimental green light cube casting, using a mold whose face is a bit rough and will likely produce an unacceptable casting, but it will prove out the coloring and other aspects. It will be curing until Monday evening and the outcome will be documented in next week's blog entry.

The special blue handle is a rectangular solid that fits over the bat handle of a toggle switch, and two flat wings at the rear that fill the slot in which the solid is pushed up or down to the momentary on positions for setting or clearing tabs.

Special blue handle on typewriter faceplate

I have to find a solid chunk the right size with smooth sides for part that sticks out.

The flat wings can be a flat plate with a hole for the bat handle, held by rails on the back side of the typewriter panel. The flag plate will slide up and down in its channel as the switch is pivoted up or down. This keeps the flat blue at a constant depth in the slot for all positions of the switch.

This has to be molded as two pieces, one built from a flat plate of suitable size to fill the slot, the other made from the rectangular solid. The alternative is to build a curving backplate which is glued to the rectangular solid, then the combined piece used to create a mold. There will be some challenges in the mold design since the back plate curves up away from the face of the handle, thus up and out of the top of the mold. It may require a two-piece mold, something I have not yet attempted.


I spent more time working with Autodesk Inventor to design the mold for an injection molding machine. Due to the complexity of interfacing the mold to the Morgan G-100T press at Techshop, I decided to take a class they offer on using Inventor to design molds. Amazingly for such a limited-appeal class, one was scheduled for next week and already has more than the minimum complement of students registered. I will put my design on the back burner until then, when I have an experienced instructor to ask all the detailed questions I need to get a successful mold.

I found some blue ABS plastic pellets on ebay that are a reasonable color, albeit darker than the IBM classic blue I would have preferred. Since plastic is normally sold in hundreds of pounds to shipping container lots, the only way to get a reasonable amount, such as the 10 pounds I bought, is through an individual selling off excess from a larger purchase they had made. White or black, easy to find in any given week, but specific colors like classic blue are pretty challenging to find in small sizes.

Five pounds of blue ABS pellets - knob probably requires 3-4 ounces each

Once the mold is designed, I move on to the CAM tools in Inventor where I will design the set of milling operations for the Tormach mill that will produce a mold from blank aluminum stock. The integrated CAM tool needs the driver for the Tormach installed in order to understand its milling capabilities, but no driver is yet available for the latest versions of Inventor and the CAM tool - those are what I have installed at home. This means that I have to take the mold design down to Techshop and do all the milling design work on their workstations.

I will have to buy the aluminum from a local recycler and may need to buy a few end mills to ensure high quality cuts since the mills used by other members tend to be beaten up, dulled and low end quality even when new. I anticipate molding the knobs in about two weeks.


I trimmed the openings for the push-buttons, adjusted the fit of the button holders and plate holder brackets and had everything fitting together. The light cubes and the push button switches are wired to cables with plugs on the end. Once I have the final keycaps titled and the replacement buttons and light cubes in place I can install this on the frame. The buttons and light cubes all need to be cast in the right colors and then have the appropriate titling installed on the faces.

Experimentally, I determined that I could use a 1/16" drill bit in my drill press, making a close sequence of holes in the shape of the desired slot. This slot will be cut into a toggle switch handle or into the bottom of a keycap, allowing a 1/16" thick tang to wedge into the slot to secure the resin cast object to the metal lever.  I have settled on this method to prepare the POWER and KEYBOARD toggle switch handles, and the nine keycaps that I had to manufacture.

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