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What and why

This is a complete 3D printable replacement clamshell case for the HP 28S calculator. I made this model because after 40+ years, these old calculators are becoming non-functional due to failures in the plastic case; they’re held together with dozens of plastic "heat stakes" (sort of like long plastic rivets) that become brittle over time and eventually break. This causes the keypads to lose their tactile feel and the keypad membranes lose contact with the main PC board, making the calculator useless. Also, the original design has a weak battery compartment door that eventually breaks, so my version fixes the problem by putting the battery door on the back of the case. If you want to open one of these calculators to repair it, you have to drill out all of these stakes and there’s really no satisfactory way of getting it back together again. Some people have succeeded in doing so with great effort and care, but generally speaking, opening an HP 28S is a one-way trip.

Today, standalone calculators don’t hold the same status as an essential tool compared to how they were regarded in the ’80s or ’90s, so this project is primarily aimed at collectors and enthusiasts who enjoy keeping these fascinating devices alive. Also, I feel like the HP 28S is somewhat underappreciated due to the well-known shortcomings of its physical design and construction, so I’m hoping an inexpensive 3D printable replacement case will allow us to reappraise it.

Who might want to try this

If you’ve got an HP 28S with a badly broken battery compartment door, or your keypad no longer works and you would rather have a functional calculator than a display piece, then this project might be worthwhile for you.

What to expect

Obviously, the end result will have a 3D printed look and there’s a chance the keypad will lose some of its tactile feel after the switchover. I’ve tried to make the 3D print match the original as closely as possible, but I had to make some subtle changes to account for the strengths and weaknesses of FDM printing. Also, because the original calculator is held together by dozens of heat stakes under tension, this is a somewhat challenging project to assemble and it will take some patience and planning. You might want to practice melting the heat stakes on a test print before taking apart your beloved (or hated in some cases?) calculator. I made myself an improvised heat stake melter from a drill press, a soldering gun and a foot pedal which made the process much easier for me, but you can probably get by with just a soldering iron if you’re careful.

Printing tips

The .STL files are already in the correct orientation for printing and in most cases, you’ll need to use support structures. The parts that contain heat stakes are the hinge, the two keypad bottoms and the "case right top" which encloses the display, PC board and batteries. These parts must be printed with the heat stakes in a horizontal orientation for strength and you need to enable tree supports. Once printed, be careful when removing the supports because the stakes are thin and delicate. To prevent drooping layers on the underside of the heat stakes, I’ve added small prism shaped supports which must be carefully snapped off with a pair of needlenose pliers. To do this, you must grip the prism part and gently twist it to snap it off between layers at the point where it meets the heat stake. Also, the keypad tops have little plugs in the countersinks for the stake holes which you must pop out with a stiff piece of wire like a bent-open paperclip. I did this to support the underside of the countersink while it was being printed.

Preparation and assembly

To open and disassemble your HP 28S, follow the instructions here

Once all the parts are printed and you’ve carefully removed all the supports, begin by assembling the keypads. The left and right keypad bottoms have all the stakes that protrude through holes in the keypad membranes. The topmost membrane will be a black rubber sheet with some holes that are a little smaller and hence a tight fit around the stakes. This is a good thing because it will help to keep the membranes in place when you invert the keypad bottoms as you’re mating them with the keypad tops. Arrange the key caps into the keypad tops and press them together against the keypad membranes with the stakes poking through. The stakes should emerge about 2mm from the top surface of the keypads. Once you’ve got the two halves of the keypads fitted together, you need to apply pressure around each stake while melting the tip down into its countersunk hole. Keep maintaining pressure until the plastic has cooled down, which allows the stake to go under tension when you release pressure. You can use the tip of a soldering iron for this, or you might want to try making a drill-press-soldering-gun contraption like the one pictured here:

Use double-sided tape to secure the piezo speaker into the circular ridge on the right case bottom. Then use more double-sided tape to secure a foam strip right under the place where the keypad connector goes. There’s another little ridge in the right case to help you align the foam strip.

Once you’ve assembled the two keypads and stuck down the piezo speaker and foam strip, you can glue the keypads into their respective case bottoms. Be careful not to use too much glue, otherwise it might ooze onto the keypad membranes as you’re pressing the keypads in place. After that, you can glue the U-shaped ridge onto the left keypad and then glue on the hinge anchor covers.

Now, the hinge can be installed which will connect the left and right halves of the case together. One end of the hinge has a slot that the keypad flex cable slides into. As you slide the hinge over this flex cable, it will eventually meet with the lower hinge pivot cones on either case half. Gently rock the hinge back and forth to seat it over those pivots. Once the hinge is fully seated over the lower pivots, you’ll have just enough room to manoeuvre the upper pivots in place, being careful not to bend the three heat stakes in that location. It’s a bit of a 3D puzzle at this point, so if you’re having trouble, it might help to swear a little. Once the hinge is in place, slide the hinge cover over the three heat stakes and melt them down.

Finally, you can assemble the right case top shell that holds the display and battery springs. The springs go in the same way as they were in the original case. There’s a tiny wedge-shaped catch that holds them down and you may need to use a jeweler's screwdriver or something similar to hook the spring over it. Then you need to make a rectangular metal strip to take the place of the metal piece in the original battery compartment door. I cut mine from the metal lid of a pasta sauce jar, scraped off the paint and made a small dimple to make contact with the negative battery terminal. There’s a narrow gap in one of the battery compartment support walls that allows you to slide this strip into place. Once the case halves are together, this strip won’t fall out. Once the battery springs and metal strip are in place, lay in the clear plastic LCD protector, followed by the LCD display & circuit board. Make sure the IR LED aligns with the opening in the top of the case and make sure the circuit board is firmly seated so there are no gaps between the screen bezel and the screen.

Once all the circuitry is in place, clean the keyboard contacts with alcohol and fit the right case top into the right case bottom. At this point, you might want to put in batteries and make sure the calculator works before melting it together. There should be 15 stakes that protrude out the bottom of the case which you’ll have to melt down. The piezo speaker springs and the foam strip will be trying to force the two halves apart, so you once again need to be applying pressure while you’re melting those heat stakes.

Once the case right top & bottoms are together, you can apply your keypad overlays and the HP logo if you wish.

Downloads

The .STL and model files can be downloaded from printables.com