The scratcher tool is used to scratch a pcb that has been covered by a thin layer etch resist solution ( using a etch resist marker). It could also be used to ensure precise delimitations between the traces that have been drawn by the pen tool. The precision here is higher than the pen tool, but it is critical to have this tool slide gently within the tool holder, but without any gap in x and Y directions. 

I am using a sewing machine needle specifically designed for Jeans. ( strong needle) for this purpose. I had it already, so I cannot tell you how much such things cost.

To build the scratcher tool  :

Print the scratcher holder, then fix the needle on it :

That’s it.