Monday, January 4, 2010

HP Lathe - background and design

This is based on an article I wrote for a local human-powered vehicle newsletter (Kiwi HPV).  It's not about any sort of vehicle but it does involve human power. It's about a human-powered wood lathe that I built in 2008.

I had been sort of interested in a wood lathe for some years, but I did not have any specific need, and it was not a burning desire, so nothing had happened. However, I started thinking about making woodwind instruments, which really does require a lathe.  Even so, I was still pretty lukewarm about having another large piece of power machinery in my workshop, taking up space.

Then I was persuaded to make a lathe that didn't have this problem, and one that connected with my interest in early tools and techniques.

Some history
Lathes are old machines. They predate Roman times. I read of person's one categorisation of early lathes into ones that you sit down to use and ones that you stand up to use. The “sit down” type were usually bow lathes where the shaft is directly driven by a bow-string pushed back and forward by the apprentice or other source of cheap labour. The “stand up” type were generally pole lathes where the shaft was directly driven by a cord pulled down by the operator's foot and pulled up by a springy pole acting as a return spring.

The form of a lathe has been well established for many centuries. There is a drawing in the “Mediaeval Housebook” (c.1475) of a screw cutting lathe with all the basic forms: horizontal bed with two rails, and two movable vertical supports for the work.

One of Leonardo da Vinci's notebooks from about 1505 shows a sketch of a lathe with a large flywheel driven directly by a treadle and crank. The shaft speed would have been very slow, and the ~2 meter diameter flywheel would have presented some practical difficulties in stopping and starting quickly. The key aspect of this lathe is that has continuous rotation rather than the alternating forward and backward action of a bow or pole lathe. This makes it useful for more detailed engineering work, turning or swaging or polishing of small metal items, especially if the shaft speed can be increased. Using a belt drive from a treadle-driven or cranked flywheel to a smaller spindle pulley is a way to do this.

There are plenty of designs around for basic powered wood lathes. Woodworking magazines through to at least the 1960s regularly ran articles on DIY lathes. If you go looking on the web, you can probably find copies of these.

I came across a web-published book by Steve Schmeck on how to build a treadle lathe ( (he also does recumbents; surprise!).  “That doesn't look so difficult”, I thought. However, there were a couple of details in the structure I was not so keen on.
Then I came across Roy Underhill's book “The Woodwright's Shop”, where he describes a smallish treadle powered lathe still in current use in his workshop. This had a structure that looked stiffer than Steve Schmeck's design, and I liked it a lot. As well as the look, the material cost was not not very much, so if it turned out to be a dud, I would not have invested much.

I used the dimensions from Steve Schmeck's design, and incorporated the triangulated supports for the flywheel from Roy Underhill's example.

So, my lathe consists of a triangular subframe supporting the flywheel and spindle. The subframe is supported on the left end of a footplate which also mounts the righthand support for the bed. The bed is two horizontal beams fastened to front and rear of the three uprights. A moveable upright supporting the tail stock sits on the beams of the bed and can be clamped in place with a wedge. In principle, this structure should be collapsable for transport or storage.

My intent with this lathe was not particularly to produce an historically accurate example of a lathe from the 16th Century or earlier. I wanted something that captures the much of the "feel" and appearance from that time, and could be shown off at mediaeval re-enactment events, but was also practical and useful in the here and now for doing real work. I also wanted to be able to make it using hand tools as much as possible.

What I have ended with is a representation of a style of lathe that could have been extant in the 16th Century but certainly was in use in the 17th Century. At the same time it has some modern components that are not too obtrusive.

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