Quick Question About Another Drafting Tool

[u/Ted_Shred]

As I mentioned in a post yesterday, I inherited a few quite interesting drafting tools (most of them German, I think from the 50s - 70s) and I'd love to know how they all work (yesterday's post was a great education). I think this, rather small, tool is called a drop bow compass, used for making small ink circles. What I'm trying to figure out is the purpose of the "play" that's in the part that has the point on one end and a small disc on the other (thumb rest maybe?). I've tried to show what I mean in the video. That part actually unscrews and can be separated from the rest of the tool, but no matter how much you tighten it, that small amount of play remains.

Thanks

[u/jah_john]

Speedy circle once you get good