10. Flat Polishing
Flat or black polish is a finish applied to high end hand-made watches. It showcases the skill and attention to detail put into the watch by the craftsman.
The term flat or black polish is descriptive of what the polish achieves. The polish is applied to flat surfaces so that when light is not directly shining on the surface, it appears black. When light is hitting the surface, the surface looks like it glows.
After some trial and error, I was able to produce the flat polish on the M1.2 screw head (1.5mm diameter).
The screw’s head had a pretty bad surface finish before polishing. The image below was taken before I started polishing the screw head.
To achieve the mirror finish, the first thing is to make sure the head of the screw is held completely flat and rigid against the surface that the head will rub against for polishing.
The video below shows the tool and method watch makers use to achieve the flat polish.
I designed a simplified version of the ‘tripod’ tool which I 3D printed. I also designed the screw / work piece holder to be easily swappable. I also put a level on the body of the tool so I can tell when it the work piece is flat.
Flat polishing tool that I designed and 3D printed.
The working principle of the tool is very simple. It uses the two thumb screws on the back to adjust the work piece so it is flat against the plate that it is resting on. The level helps fine tune the flatness - with the obvious assumption that the polishing plate is flat too.
The work piece holder on this tool can be swapped easily. That way, I can design the work piece holder custom to the part that needs the flat polish and 3D print it.
Swappable work piece holder that slides into the tripod
Based on my research and reading of how watch makers achieve the mirror finish, I bought a zinc plate which is what the work piece rubs against. This makes sense for steel or stainless steel parts since zinc is much softer and will not scratch the steel while the polishing compound can act on the steel. I used 0.5 micron diamond polishing paste from McMaster.
The process is pretty straightforward:
Setup the workpiece and level the tripod right next to where I intend to polish it on the zinc plate
Smear on a thin layer of diamond paste
Place the workpiece on the diamond paste and make small circular motions
I don’t apply a lot of pressure, just enough to keep the tripod contacting on all three points at all times
As the work piece starts becoming smooth, you can actually feel the friction reduce
For my screw head, I also put a polished chamfer. I did this by setting it up on a mill, using a jewelers cup-bur tool and running the mill at a relatively slow RPM (~140) and going 0.5mm into the screw head. In haute horology vocabulary, this is called “anglage”.
Putting a polished chamfer on the screw head