Saturday, April 23, 2011

Chasing rainbows, er, threads

And so, driveway out of the way, I can return to chasing threads in wood. I am making progress, I am glad to say. It may not look like it:
But we have a distinct thread to the left of the picture. The right side looks like a porcupine. So here is what I have learned. One, you need to turn down the right side of the cylinder, maybe 2-5mm. The shoulder gives you a reference point to start. Two, take your trusty lead pencil turning tool and use it to establish a passable pencil thread. If you mess it up, just sand it off and try again. Three, you need a tool rest with a square top. The rounded things supplied for wood turning work very badly, if at all, and this is because the chase must be horizontal. Four, any hesitation is fatal. If you stop you will make the chase into a form tool. Pretty sometimes, but useless. Five, when you have successfully done a pencil thread you have a very good idea of how fast to go. For my 2mm pitch, the whole thing takes one second to do a few cm of thread. Six, again a virtue of the pencil, just follow the pencil for the first try -- striking the thread, the chasers say. Seven, on subsequent passes it is easier, but you have to catch the thread just right. The turned-down section is a great help. Otherwise you will cut another thread and that is not easy to fix. Once the thread is deep enough the chase pulls itself along.

On my next try I munged up the start, but the last bit came out very nicely.
The pencil strikes again. I have a highly improvised rest -- a hex key (Allen) wrench clamped in the (metal turning) tool post. But it has a flat top. Mandatory, I think. Got that trick from the late Tubal Cain, Simple Workshop Devices. Mr Cain (T.D. Walshaw) did chasing on his trusty Myford screwcutting lathe, and started the thread by the simple expedient of setting the Myford up to screwcut. All he had to do was hold the chase! But, he said, it is much faster than doing it in the standard way. On subsequent passes he would chase in the usual manner, and the whole thing was done in a few minutes.

I am simultaneously debugging both my technique and the chase. Not advisable, but then nothing ventured, nothing cut. Also the material is far from satisfactory. It is lilac branchwood and far too green. So it chips out. I am also certain now that this can be done on the pole lathe. In fact it may be easier because the pole lathe does not rotate quite so fast. I have the Taig slowed down to the max. Or to the min, to be more exact. Another lesson is that the smaller the pitch, the easier it is to do the thread. That is because on a 2mm pitch the whole thing is done (or munged up) in one second, two at most. Slow speed good, small pitch better than coarse.

I need to try the pole lathe. This summer I will consider uprights so I can bring the lathe in come Fall. Meantime... My shop is a mess; all the outdoor tools are in it and I really have no room. But any day now the snow will melt. There are now bare ground patches on my driveway.

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