I figured a steampunk calculator would be made using a number of modules that are handcrafted from smaller components–so I just designed a circuit board which can drive 3 7-segment LEDs using a shift register circuit, and ordered 12 of the circuit boards from Pad2Pad. They’re 1.5 inches wide by 2.5 inches tall, and hold 3 LED segments; my calculator will then contain 3 of these for a 9 digit display: 6 digits to the left of the decimal point and 3 digits to the right, but with only 2 on by default. The beauty of the design is that I only need 4 control pins total: clear, shift, load and data, and I can drive the entire 9 digit display.
I also ordered a SP4T switch which I plan to use to switch between the four operators (+/-/×/÷), and a key switch to turn the calculator on and off. I also ordered C-cell holders for the 3v power supply. And a whole bunch of buttons for the calculator face.
Once I have the boards I can start testing the calculator code. In the meantime, when I have free time I intend to get the Java emulator of the HC08 working.