Casio F-91W Aluminum Case MOD

This Mod converts the humble Casio F-91W into a more robust and classier watch. This iteration was manufactured to be made with aluminum. Theoretically if the dimensions of the button holes and the tapped threads in the back of the case were to remain at 3 mm the CAD file and the Drawing file could be used to make this watch case in most materials that can be put under a CNC mill. The watch module inside both of the watches shown in this instructions is the sensor watch module. The sensor watch was developed by Joey Castillo (Oddly Specific Objects) and his team.

The sensor watch page is linked here : https://www.sensorwatch.net/.

The link to the CAD and where to to order the case from is here : https://www.pcbway.com/project/shareproject/C_CN9W799384A_CNC_Case_3_LITE_361cc50d.html

This link from PCBway is sponsored.

The link for the black silicone band is here : https://a.co/d/6ut82cN

In order to successfully install the Casio 593 into the new housing and have it function properly along with retaining up to a tested water resistance of 5 min at a 12 ft depth you need the following tools.

Note: I anticipate the water resistance to be greater than this record but currently do not have any means to test it further. After I tested the for the water resistance I noticed no failure in the watch at this depth. For those outside of the US this depth is a little under 4 meters of water depth.

The First Image : the first image shows the double sided tape I used to secure and retain the water resistance of the transplanted crystal from the F-91W. I noticed that the F-91W does not use a double sided tape to secure the crystal but other Casio watches notably the A158 does and has a similar water resistance rating.

The Second Image : the second image shows the eBay listing I used to obtain the button gaskets required for this case mod. There are countless of these listings but just make sure to get the DW5600 button replacements. The color does not matter as these buttons are not needed for this mod only the gaskets are required.

The Third Image : the third image shows the amazon listing I used to obtain the M1.4 x 3 mm screws used to secure the backplate to the back of the new case.

The Forth Image : the Forth image shows the hand tools required for this modification. The first is a Dremel, the second is a pair of pliers and the third is a small Philips head screwdriver. Not shown in the image but also required for the deconstruction of the Casio F-91W and reassembly of the new module are a hairdryer, an exacto-knife, and a sheet of 350 grit sandpaper.

The first step is to unscrew the back for screws in the Casio F-91W and remove the backplate, gasket, and the 593 module from the Casio F-91W case.

The First Image : the first image shows a hairdryer heating the back of the crystal from the rear of the watch. To easily remove the watch crystal the crystal must be heated as shown for a minimum of 10 min on HIGH. Be aware that extra heating may warp the acrylic so carefully monitor the heating.

The Second Image : the second image shows the process for removing the crystal. After heating for 10 min the watch will be hot, grab a thin piece of cloth or use gloves as an insulator and press on the back of the crystal around the edges.

Note : It is common to not apply proper pressure and have the crystal partially chip on the edges. It takes practice and a bit of luck to get a clean crystal.

To remove the buttons simply use the tip of the pliers and press on the back of the button through the case.

The First Image : the first image shows how to push the button.

The Second Image : the second image shows the how the button should look like after it is out.

The Third Image : the third image shows how the button just falls out if you tilt the case in that direction.

The First Image : the first image shows the button being attached to the Dremel for sanding. My Dremel collar (that secures the collet) was too big for the button to be sanded. The solution to secure the collet to the Dremel is to add a piece of electrical tape to act as a buffer. This will widen the diameter of the collet enough for it to be snugly attached to the Dremel.

The Second Image : the second image shows how to sand the button. Use the 350 grit sand paper to sand the button down to 2.47 mm diameter. This gives a clearance of 0.015 mm between the hole of the aluminum case (2.5 mm).

Note : To sand the button down accurately, measure the button every couple of sanding passes. The more times you check, the more accurate. I usually wait 10 - 15 seconds before checking and reduce my checks or increase my checks based on the changes in the recorded measurements. If the diameter decreased more than expected reduce the time between checks. If the diameter did not decrease as expected or at all increase the time between checks.

The Third Image : the third image shows how to measure the sanded button with a pair of calipers. I use the thick part of the left side of the calipers to measure the dimeter more accurately as using the tips does not work well with a diameter measurement as there is less surface area to clamp on the part. It is reasonable to not have the whole diameter be 2.47 mm variance is normal. Check the variance of diameter by rotating the button and checking the diameter at that new rotation. If the lowest value deviates by 0.01 mm the button will still function so long as the highest value does not pass 2.47 mm. If the diameter is greater than 2.47 mm the buttons will not get stuck when pressed

The Fourth Image : the fourth image shows the sanded buttons and the new gaskets from the DW5600 listing.

The Fifth Image : the fifth image shows the new gaskets on the buttons. It is essential that the old gasket remains on the buttons. Although this gasket does not aid in the water resistance it does make it easier to insert the new gaskets onto the button.

The First Image : the first image shows how to apply the 2 mm double sided tape onto the front of the aluminum case. To have the water resistance be the best it can be the tape must cover the most area it can. This is achieved by lining up the edges of the tape with the sides of the face. In the image the middle portion of the tape has been applied to the face. Then use the exacto-knife to cut the excess off at an angle near the side of the wall. Repeat for the rest of the watch face to look like the second image.

The Third Image : the third image shows how the tape should look when properly applied. It is normal to have some gaps in the tape so long as the gaps are not breaking an inner perimeter. If the inner perimeter is not fully covered the water resistance will not be maintained.

Although not shows the next step is to simply place the crystal into place using pressure. To do this insert the watch crystal on top of the new case in the cavity. One side of the crystal will remain elevated. To press the crystal into place simply flip the watch case upside down on a cloth over a flat surface. Then press the back of the case until you hear a click.

The First Image : the first image shows the first step in reassembly of the watch. Once the crystal is secured simply input the 593 module (In my case a Sensor Watch), and place the rear gasket.

The Second Image : the second image shows the assembled case with the backplate. To secure the backplate to the case simply use four M1.4 x 3 mm screws and screw the backplate on.

The Third Image : the third image shows the final reassembled watch. The last step after the backplate has been attached is to add the buttons with the new gaskets into the case.

Note : This means that the buttons are not retained in the case, but this has not been an issue with daily use or water resistance testing. The buttons are in fact retained by the friction applied from the gaskets to the walls of the case. The buttons can be removed by pinching them and pulling them out.

I did not take pictures of the water resistance testing, but I did take pictures of the theory for why one watch survived and the other failed. The testing was conducted in a dive pool that has a depth of 12 ft or just under 4 meter. The duration for both of the tests was 5 minutes. The test inducted was to lay the watch flat on the bottom of the pool for the given time duration and inspect the watch every minute to see if any water got inside the watch.

The First Image : the first image shows the two watches tested. The watch on the right is my original prototype that was fabricated using steel. This watch had a considerable amount of assembly and design complications. One of these complications was the reason that I suspect it failed.

The Second Image : the top diagram shows the gasket interaction with the aluminum watch. This gasket has more contact and remains in contact with the outer wall of the case hole as the button is displaced by a distance (delta). The bottom diagram shows the other gasket has one point of contact and as the button is displaced by a distance (delta) the seal is broken. This is why I assume the previous version failed where the aluminum watch did not.

Update : The button gaskets (from the 5600) are rated for 100m of water resistance. The seal between the crystal and the case is similar to that of the Casio A158 which should function the same. For the Aluminum one the case back seal uses the same gasket as the original and has a better fixture as the body is aluminum and not plastic (less prone to shearing). I fixed the button gaskets on the steel one (gaskets are the same as the aluminum one) and have conducted an analysis for the TPU gasket. The TPU gasket was printed on a flat surface and the top surface was ironed with minimal step over. Furthermore the water resistance of this gasket was theoretically calculated for using some assumed characteristics that the gasket acts as a sort of beam. The other assumptions are also stated in the calculations. This gives the rating of the watch to be theoretically around 40 meters of water depth. I have yet to confirm this value as I need to test it. I will update on testing data.