March 12, 2025
Description
EDIT: older Master 3 version bottom plate (not tested !) added in files
MX master 3S for lefties
This guide will show you how to convert an existing MX Master 3S to a left handed version.
The conversion is reversible, however you will need to file down some plastic parts in order to have a good fit in the new body
What you need:
Dismantling
First we will open up the original mouse. Lots of tutorials for that online so go ahead and watch those if you’re not comfortable enough for the disassembly.
Once we get all of the parts disassembled, we will keep:
Original component preparation
We will have to file down some areas of the components:
The scroll wheel toggle button
Needs to be filed down to a more pleasant shape to touch. The front lip needs to be completely removed or it will touch the casing. I advise to file down while fitting often enough. You’ll feel it when enough material has been removed.
The thumb scroll wheel assembly
Needs to be filed down in 4 spots.
Remove a good 5mm from this side
File down this part until it sits flush with the pcb
File away this whole corner (or it will touch the motherboard when installed)
File away as much material as possible, but leave the mounting hole
And cut away this bracket
That’s it, no more filing needed.
Printing the left handed parts
There are 3 parts. MX top, MX bottom and MX button.
I've used Impact PLA for the top, it as very good layer adhesion when printed hot and very good overhangs with a lot of cooling.
The bottom part is easy to print. I opted for PCTG because it has a softer ring to it when bumped into something. It also takes screws a little better than PLA.
You can use a little bit of support on the usb port but no other support is needed. Just print it solid and hot (for strong layer adhesion on the screw hole towers)
The button can be printed with normal supports. I am not 100 % happy about this design because it’s still quite flimsy. It works for now but I plan to change that design. Use 0,1mm layers for fine detail
The top part is the most challenging print. It will require a lot of support and that support can be difficult to remove. The settings that worked for me:
Once printed most supports will be easily removed. The supports underneath the mouse button flex mechanism however are difficult and require some precision slicing (both Bambu slicer and a slicing knife afterwards)
Play with the supports so that you have everything well supported, BUT make sure no huge blobs of support are located underneath the flex mechanism. I had to insert a support blocker on the right mouse button to avoid too much supports there.
UPDATE
I managed a good print with a lot less supports by checking “only support critical regions” and painting some extra on. You can download the 3MF file for bambu printers here with the other files
Once cleaned up, we can start drilling all of the holes.
100% of the holes in the top part can be drilled immediately with a 1,2mm drill.
7 of the 10 of the holes in the bottom part must be centerpunched before drilling (I use a metal scribe for this) because I did not want to weaken those holes too much with a printed hole. Those are the mounts that go through the PCB so they cannot be meatier.
Make sure you do not pierce the body of the mouse when drilling (I did this often)
Assembly bottom
The light passthrough and the PC switch button can be put in place just like that. They will stay in place (press fit for the light passthrough and the motherboard keeps the button down.
The motherboard can be slid in place with the usb port first. It is then friction fit with the small pin on the button for a low tolerance fit. Screw in all of the screws (5 for the motherboard, 2 for the left click button and 3 for the battery).
You’ll have to make a hole in the battery mount for the 3rd screw but I just left it out and all is fine.
Assembly top (order matters)
We start with the scroll wheel toggle button. It should sit perfectly in place since you filed it down as needed in one of the previous steps.
Then we install the side scroll button. Make sure you route the cable for the single thumb button underneath the mounting point. There is some play in the mounting holes before you screw them down so you can position it just right.
Put the single thumb button (printed part) in the body of the mouse, and screw down the pcb. Test if the button works correctly
Lastly, we install the pcb for the scroll wheel toggle button. First connect the cables (!) and only then install the pcb. I wanted to use longer FFC cables but managed to get the original fit, but there’s a little bit of strain however.
You can order longer FFC cables (10 pin 0,5mm pitch and 14 pin 0,5mm pitch) to be sure that the connection is solid.
I'm sorry for not having more pictures of the assembly but it should be quite straight forward.
Connect all of the cables, turn the mouse on and start mousing.
I did plan to stick some glide pads underneath (sensor distance has been set for this) but for now it works just fine without.
I have printed exactly one 100% working model after many many iterations. It can be very difficult to dial in the click sensitivity and I have noticed that it's not always perfect even with the exact same print settings and exact same model so you might keep a file at hand to dial in when fitting both parts together. (you can do this best without the scroll wheel present)
If you print this and try the mod, please comment on how I can make the model better.
License:
Creative Commons — Attribution — Noncommercial
9
Matthias
