Why?

This project covers making a spring loaded V-bit that can be used on a CNC machine to reliably remove cured solder mask layer. It is a result of not being able to find the right tool for the job I had at hand, and which needed the following:

  • 1/8" shank
  • 0.2mm, 30 deg or smaller tip
  • decently balanced (0.1mm of wobble acceptable)
  • repeatable results

The search for the tool took me both commercial way:

And DIY way:

However, none of these really fit me. They either had too large tip diameter (0.3mm) or worked with ER-11 collet and require special setup, or are completely made of plastic, including the shank which I'm not a big fan of. Because of that, I wanted to take a shot at making another DIY version, slightly better suited to my needs.

How?

For design, I went with the proven one shown in the examples above. With the help of a spring, V-bit is pressed against the hardened solder mask and is able to machine it away, leaving the copper beneath it (mostly) intact. Such tool usually contains a shank that is used by a CNC machine to hold the tool, a body to house the parts, actual cutting tool, and some kind of way of preventing the cutting tool from jumping out of the body.

Body

The body in this case is a 3D printed part, holding the bit together. It's cylindrical, with openings on one side for the shank and on the other side for the V-bit to be inserted. The side where the V-bit comes in also features a side hole, used to lock the bit from falling out, once inserted.

Shank hole is made with press fit tolerance, so the 3mm shank is pressed into position and requires no glue. 

The hole for the V-bit needs to be just the right fit, so the bit doesn't have any free room sideways, but at the same time can freely move up and down. The design file for Fusion 360 is attached to the parts list, but might require some tuning depending on the tolerances of your 3D printer.

Finally, in my design, the end around the shank is a bit thicker than the rest of the body to work with ATC on my machine, otherwise it's not needed.

Spring

Spring is probably the most finicky part of this assembly. It cannot be too long as that extends the overall length of the tool and is bad for its stability, it needs to be soft enough to not dig the tool into the copper, and is probably even more unforgiving on a 0.1mm tool than on a 0.3mm tool due to the pressure tip exerts.

This is some of the materials I tried to use as a spring and how they failed:

  • spring from a standard pen - too hard
  • custom-made spring from a piano wire - too hard
  • random foam - good, soft to start with, but quickly lost springiness

Eventually, I cut a thin strip (around 1mm wide) from the softest metal I could find around me and wrapped that over a 2mm rod into a "spring". It happened to be an aluminum beer can, around 0.1mm thick. Around 4-5 turns is sufficient.

This step can take some trial & error, but there's light on the other side!

Assembly

The assembly looks something like the picture below.

There's one final little caveat when locking the bit in place using the hole on the side. I resolved to hot glue to do so, mostly because it made it easy to prototype. It's a simple way of securing the V-bit, while being easy to remove to test a new spring or bit. 

However, the V-bit would often end up getting stuck to the glue, causing the tool to completely lose springiness. To prevent that, I added a small L-shaped guard shown close to the tip in the picture above. The guard goes through the hole on the side and is meant to be in direct contact with the V-bit (between the bit and the hot glue), preventing it from touching...

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