Wednesday, January 20, 2010

HP Lathe - construction

I needed fairly dense stable timber for the major structural items. I used recycled rimu framing timber – well seasoned, stable, reasonably hard, cheaper than new wood. I cut all the timber and cut the tenons and mortices by hand, but I cheated and dressed the large items with my planer, which also made it easier getting the beams nice and square. I drilled large holes with brace and bit, but used a battery drill for small holes (there were not actually very many of these).

The spindle is made from a 16mm steel bolt running in sealed ball bearings. In period, this would probably have been forged nearly to shape and dressed with a file to final size. The bearings might have been brass or bronze sleeves, or more likely a hard wood such as lignum vitae, greased with tallow. These would have been lossy bearings.

The threaded end of the bolt allows different headstocks to be mounted and unmounted quickly, but is not any sort of period method that I am aware of. Morse tapers are distinctly too modern and too hard to make. A friend Bruce helped me by turning the spindle down on his metalworking lathe, fined down to a firm but not tight fit in the ball races. I tinned the shaft with solder where bearings would be and drifted them into place.

The flywheel shaft is a 12mm bolt also running in ball bearings for convenience. The bearings are inset into the wooden structure and are not very obvious to a casual glance. The flywheel and shaft assembly can be readily removed from the frame for transport or storage. The mounting bush for the flywheel was also turned on Bruce's lathe, and I arc-welded a fastening flange on. My smithing skill is not up to forge welding.

The flywheel consists of three layers of plywood glued together. Plywood was convenient to use and is conceptually sympathetic with period construction in that a period flywheel would have been made by laminating several layers of planks. Smaller planks are more available than large flat sheet, and cross plying improves the strength and stiffness of the flywheel. I took some pains to balance the flywheel, but because of its slow speed, the balance does not need to be particularly good.

I had some doubts about using wood screws to fasten the metal cover plates. However, I found a reference to drawings of screws in a late 15th C manuscript, showing detail of them being used to fasten plates in a matchlock mechanism, so that was OK (see “One Good Turn”, by Witold Rybczynnski – this is a good read; recommended). However, machine screws were not in period use, so I did not use grub screws to attach bushes to shafts - these were locked in place with tapered pins through the shaft.

The threaded end of the spindle was not wonderfully true. I could have gone back and made another spindle being more selective about the parent bolt, or cut threads into a trued shaft, but I was running out of time. My workaround has been to make the driving centres with a compensating offset, noting the position where these run true and locking them in place with a locknut. It's workable enough .

I fastened the beams of the bed to the three uprights with coach bolts. In period, bolts might have been used, although the particular types of bolt would have been different (square head, not hex). However, bolts would have been expensive, and fastening would most likely have been with trenails (wooden pegs or dowels) or snug fitting mortice and tenon joints. I chose to use bolts since this allowed the lathe to be collapsed for storage or transport. All other major joints were inset and mostly dowelled, and fastened with modern adhesives.

The moveable poppet locks in place on the rails with a wedge through it as shown here.

The leather strap through the wedge is a labour saving device to prevent the wedge from popping out onto the floor when tapped on the narrow to free it (which happens often enough to be a nuisance).

I was aiming to show the lathe at a mediaeval fair in early February 2008. My original timeline had some spare time in January for me to get some practice in using it. However, the reality was that I was adding finishing parts and adjusting the running gear right up to the last week, and had virtually no time for practice. So I did my practice during my demonstrations at the fair. No-one seemed to mind - there was lots of interest from many people, and a small queue of other woodworkers keen to have a go. To see some images, go to, then find your way to page 9 of this album.

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