Repairing Cases for Calculators and Similar Products

Graphing calculators are amazing tools. They are handy and perform complex operations in just a few milliseconds. However they are also quite expensive : a typical Texas Instruments or Casio graphing calculator costs anywhere from 100€ to 200€ (about 100$ to 200$) so breaking one can be disheartening and can cost you a lot. Some calculator manufacturers skimp out on the structural integrity and material choice for their products which make them prone to breaking. That was the case of this particular Texas Instruments Ti-83 Premium CE calculator which broke from a single fall. In the second photo you can see the damage on the front casing of the calculator: it has been chipped on the top and most of the screw holes have been stripped and ripped, making it hard to reassemble because of the lack of screws. Some snap fits haven't broken and support the case when assembled however it is easily disassembled. A solution to this issue is to recreate the original case with 3d printing. We will add a bit of thickness to places that need it to improve its durability. Additionally this guide may be used to fix other cases for similar products.

For this project you will need :

• An advanced 3d modelling software such as Fusion 360
• Callipers
• A 3d printer
• Epoxy glue or superglue
• Files and sandpaper
• A vise
• Patience

The Texas Instruments Ti-83 Premium CE calculator is quite complex. Being non-metric, it often follows non standard measurements, which makes it harder to reproduce and repair. The upper outer case that broke is made out of two parts: a large part that contains all the buttons (the white part in the first picture) and a clear plastic with a black bezel (the upper part in the first picture) through which you can see the screen that sits behind. The latter was able to withstand the fall so it will be repurposed. The former part features many rectangular holes through which to place the buttons and follows a pattern. It also has a few screw holes and a support so that it can be glued to the clear and black part. It also has curved features on the corners and a bezel near the edges so that it slides into the black back plate (seen in the second picture) of the case assembly. Since it features some organic shapes that are hard to measure many iterations are needed to successfully reproduce the part.

The design process of this part has to be taken by parts because of its complexity. First we need to make a rough estimate and then refine the part until it works. Being as precise as possible in this stage is incredibly important so the use of callipers and other precision instrumentation is unavoidable. Take global measurements and create the outline of the body such that, when extruded you can already recognise its silouhette, such as in the second pitcure. Then, take measurements of the position and size of the other prominent features such as the rectangular holes and the screen hole (see picture 3). Create the outer bezel which will allow you to successfully slot the back plate to the front plate. Adding webs (picture 5) is necessary in this design: it not only adds stiffness but acts as a barrier for the buttons such that they do not escape their intended position. Add screw holes to allow a successful assembly to the back plate. Finally, refine your design: add fillets, chamfers and the organic shapes in the corners. The model should be ready to 3d print!

If only repair would be that easy: you cannot expect everything to work on the first try, especially when repairing something you are not sure what the measurements of it are. This project took about 5 iterations until the fished product actually worked, which is to be expected for this type of part. The best that can be done is to analyse why something failed such that it can be fixed by another iteration. An example of a failure analysis is the following : in the picture (don't mind to position of the buttons, they are there for illustration purposes) the calculator seems to be able to assemble. However, taking a closer look we can see that the buttons are extremely crooked. This is due to them being pushed by the electronics behind them too much, which means that they do not have enough space to be where they should be. Some room needs to be added for a correct assembly, which means we need to go back to the drawing board!

As mentioned before, this part failed on the first few tries, so what needed to be redesigned and taken into consideration? The first typical failure is poor measurements and not allowing for good tolerances : for example, I created my bezels too wide in the beggining which didn't let me assemble the calculator with the backplate. My curves in the corner were also wrong since their radius of curvature was too small, so I needed to make that radius bigger. Furthermore, we need to take into account the limitations of a 3d printer: although amazing tools they often lack structural integrity in the vertical axis when printing, especially in thinner areas. As you can see the screw holes are thin and printed vertically so when they are screwed into they often snap or strip and need to be reinforced by making them wider. The final design looks like picture 1 and can be compared to the first design in picture 2, they look quite similar but function extremely differently!

The are mainly two ways to print this: the easy way and the messy but more accurate way. The easy way means putting the front of the case face down, touching the build plate. This means that, although some supports will be needed on the sides of the case to support its curvature, these will be easy to remove. However, the supported parts will have a very bad surface finish, which will be noticeable every time the calculator is used. Printing the part with the front facing up will need a LOT of supports but will leave a great surface finish on face that will be seen by the user. This will demand a lot of post-processing and depends on personal preference if this is something that you want to invest time and energy into. Furthermore, decreasing layer height will make the layers in the print less visible. Finally, turning on the ironing setting in your preferred slicer will leave a smooth surface on the top face.

Some post processing will be needed to ensure that all the parts fit correctly. The first step is to remove support if any was added as mentioned in the last step. If the dimensions of your print are almost correct, instead of redesigning the part, you can use a file or sandpaper to fix small issues with the print.

First of all, we need to glue the clear and black part to the 3d print. To do so, you can use many sorts of glues, although I would recommend a two part liquid Epoxy since it will last for much longer and will be stronger than a superglue bond. Place small dabs of the glue on the black panel making sure to avoid the clear plastic and press it onto the 3d printed part. You can tape the clear plastic so that it will not be filled by glue. Place the assembly in a vise to ensure the two parts are in contact when the glue is drying. You can let your friends add an embarrassing message in the vise to ensure that no one will touch the assembly when drying. After the flu has hardened, you may take the assembly out. Clean the parts thoroughly with compressed air to get rid of any dust and use isopropyl alcohol to clean up any smudges. Then add the buttons in the button holes, place the membrane pad on top of the buttons and the LCD-PCB assembly. Check that everything fits correctly. If the fit is too tight disassemble it and use a file or sandpaper to make small corrections to the dimensions of the part and try again after recleaning everything. Once everything fits correctly you may add the black back plate and insert the screws. Add the battery and you're done!

Overall this was a fun project. I helped a friend fix her calculator so she didn't have to get a new one and was able to give it a unique look compared to the rest of the calculators out there. I'm satisfied with the outcome and I think I can apply what I've learned to some of my other projects.

Furthermore, the great thing about repairing a product is that it can be customised: boldly coloured prints, multicoloured prints, texturing the outer surface... The possibilities are endless! The only limit is what you can imagine.