Abraham Bagherjeiran
StackCalc32 is a tactile RPN calculator being developed as one connected system. Next milestone: the fabricated RP2350 boards are expected in the next two weeks. The next log will document the first physical board fit, display alignment, matrix check, and bring-up results against this prototype.
What is on the desk now
The current physical prototype is a four-part mechanical assembly: a tapered chassis with internal rails, a unified faceplate, a TPU keypad membrane with a wide ENTER key, and a snap-retained top cap. The parts slide and close without screws, glue, or a separate fastener kit. We printed earlier button fixtures with print-in-place springs, then moved to the TPU membrane because it gives us one continuous alignment layer and a faster way to change key geometry.
The board is not a generic placeholder inside the case. Its KiCad layout fixes the display opening, controller and connector locations, switch layer, and clearances that drive the chassis geometry. The fabricated board is the next physical integration milestone: seat it in the printed enclosure, connect the display and matrix, then compare the same calculator sequences now running in emulation.
What already runs
The companion app is live on iPhone, iPad, and Apple Watch. It shares an RPN calculation model with the project’s other surfaces: values enter onto a four-level stack, ENTER controls the stack transition, and operations consume the visible values. The firmware path compiles the embedded calculator image, feeds it row-and-column matrix contacts, and reads the 132 × 65 framebuffer back from an RP2350 emulator. That lets us exercise key routing, display rendering, state persistence, and idle-loop behavior before board bring-up.
Why the physical design matters
We are not making a retro shell around a phone app. The handheld has to teach the same stack model with its layout, key spacing, screen hierarchy, and physical assembly. That is also why we build the Learning Lab: printable stack tiles, fraction pieces, expression-tree tokens, cards, and teacher materials let someone handle the same sequence they later enter on the app or calculator.
The packaging work belongs to the same problem. We began with a scored-cardboard origami support and evolved it into dovetail-connected printed pieces that protect the kit, form a 78° shelf stand, or become a low 11.6° desk wedge. That gives the calculator a place to live after unboxing instead of leaving the packaging as dead weight.
Build materials and the current record
Current prototype stage
The photographs show unlabelled fit prototypes. Button legends are a separate engraving step after the mechanical iteration, so the blank key faces in these images are intentional. The KiCad RP2350 board design is complete and in fabrication; it is not installed in the pictured mechanical assembly. Watch the 24-second proof reel for the prototype, app, and simulation sequence.
The matching four-part calculator STL set is now available from the StackCalc hardware guide; teacher PDFs and the printable Learning Lab STL pack are also canonical on StackCalc.io. The project Files area carries the matching PrusaSlicer 3MF reference projects, membrane model images, and app walkthrough clips. We keep source code private; the public material focuses on printable parts, teacher resources, visual models, and the design decisions they support.
The first integrated board will answer the physical questions that code and CAD cannot: connector clearance, display alignment, switch reach, matrix behavior, power, and how the printed membrane feels over real hardware. Until then, this project is the build...
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