Here at Tightline Productions, in the past 3 or so years, we’ve generated more than 100 fly tying videos. Almost since the beginning, the most commonly asked question we get is “what are you using to cut the thread at the end of the whip finish?”. Although I’ve tried to explain it more times than I can count, I think a video might really help to best answer the question, so here goes.
What I use is simply the butt end of my Matarelli whip finish tool that I’ve ground to a sharp chisel-like edge. I’ve had this same Matarelli for 30 years now and it still keeps a fine edge. Matarellis and their imitations come out of the package looking something like this and grinding an edge on them is a fairly simple task. A bench grinder gets rid of metal at a rapid rate but be careful not to heat the tip up too much or it'll turn blue and lose it’s temper. The idea is to make equal bevels on both sides that join to form a knife edge. This one’s pretty close but could benefit from a little more grinding. A belt sander with a medium grit belt also works very well and won’t overheat the metal as easily as a bench grinder will. An edge about like this cuts thread just fine but using a series of oil or water sharpening stones, going from coarse to fine, will give you a much more refined edge that’ll stay razor sharp longer. Be sure to maintain the same angle on both sides of the chisel point.
If you really want to go the extra mile, charge a felt polishing wheel with jeweler’s rouge and give both sides a thorough polishing.
An edge like this isn’t necessary but it looks good and will cut thread at the slightest touch. If the chisel sticks on your fingernail or scrapes up nail, it’s plenty sharp.
So, let’s just say you formed a thread head on your fly and you pick up the whip finish tool and do a whip finish. Rather than put the tool down and pick up scissors to cut your tying thread, flip the tool around and, while keeping tension on the thread, touch the tool to it, close to the wraps. This is what it looks like up close and slowed way down.
To keep the edge ultra sharp, I’ll occasionally run it over a piece of belt leather just like a barber stropping a razor. Although it doesn’t happen very often, do be careful when handling the tool so as not to cut or stab yourself with the sharp edge.
I hope this has answered your questions. Happy tying!