Saturday, November 10, 2012

Chasing the elusive thread

I knew I had a label for it. Screw chasing, that's what it was. Took forever to find it though. So today's post returns us to March and April of last year. When summer comes I move outdoors so nothing got done since then. Well, I watched  lot of thread-chasing videos on YouTube and I must say I admire the dexterity of these people, not to mention the fact that they put their knowledge out on the net for all to see. Generous to a fault. Anyway, you will recall that this label deals with cutting threads in wood. The methods range from ancient Greek techniques (cut thread with chisel) to wood taps and dies and finally chasing on a lathe. Since I want to use my Polecat this is not a trivial undertaking.

You will also recall I am cutting a 4mm pitch thread. This is very large. Most of the people on You Tube are (a) using powered lathes and (b) cutting something like 16 TPI or 1.6mm pitches. So you might say I am behind the power curve, since the smaller the pitch the easier to chase. However, we press on. We have evolved an intermediate technique. It is somewhere between the Greeks and the modern chasers. So we start out in Greek style, wrapping a piece of paper marked off in 4mm lines. It must be offset when you wrap it by one interval or you will cut circles.

Tape the paper down to your cylinder really well. else it will slip and make hash out of your efforts. Now, just like the Greeks, take your mini-dozuki saw and saw out the lines, rotating the lathe by hand. Do not try to treadle.

You do not have to saw very deep but you have to saw very carefully indeed. You must hold the saw along the lines of the spiral. Now we want to deepen the groove. For this I find the skew chisel is just the ticket, held vertically for a change.
It is important that the tool be at center height. Nice thing about having the work between centers is that you can rotate it at will. Thus we clean up small mistakes in the sawing. We get the groove as deep as we think it needs to be. So that the next tool has a path to follow. Next we really start chasing the thread.
Here is my one-point chasing tool which I cobbled up from an old piece of steel. It has a 60 deg. point. Modeled on a machine lathe thread-cutting tool. It also has some side relief. Probably not enough. Still experimenting. But we still do not treadle. We apply the tool and start forming the thread, also deepening the groove. Turn by hand.

When you get the groove deep enough you will find you can actually treadle; the tool will follow the groove all by itself. Not only that, the multi-tooth chase actually works! See previous posts. But you must treadle very slowly. This is hard to do. The temptation to bash the treadle down is overwhelming. Resist it. Any machinist knows that to cut a thread you put the lathe in backgear and take the slowest speed you have. I have no backgear, it's all my leg. It is all too easy to have the tool jump out of the groove, or dig in.

The above shots were staged for the blog. When I was working this out I had no thoughts of taking pictures. But at the end of the day I had cut a quite respectable thread on the left end of the turning.
There are problems. For one thing the cylinder has a big crack in it, making things difficult. For another it chips. I think this is a problem with the paper guideliner. Lines not exacty spaced. I can fix this. It is also a problem with my one-point chaser, I think. Need more side relief, or is that side rake? But I can now say I chased a thread on a pole lathe. Stay tuned, as usual.

And at last we have snow on the ground. Not much, about 2cm. Enough to ski on! I was so tired of walking.

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