[David G4EBT]

I'm very aware that this is a vintage radio forum - not about woodturning, which is well catered for on other forums. However, I hope this isn't too far off track and is sufficiently relevant to the hobby to be worth the space, given that most of us often find cabinetry restoration (veneering, re-finishing etc), and sourcing knobs is often problematical, which is where I came in with this thread.

A question that often crops up in woodturning, circles is ‘what speed should I run the lathe at?’ The answer is that the faster the turning speed, the cleaner will be the cut, and the smaller the diameter of the wood, the faster the lathe speed needs to be as there’s such a small amount of timber passing the tool with each rev when it’s of a small diameter. There’s a loose ‘rule of thumb’ formula for calculating the speed at which a woodturning lathe should ideally be run, and that is: ‘Diameter in inches x RPM = 6,000 to 9,000’. So for example, a 10 inch diameter bowl blank should be run at a speed of 600 – 900 RPM. Most turners will start off at a slower speed when the blank is first mounted as it will be out of balance until trued up with a roughing gouge.

Using this formula, it follows that a 2” diameter spindle would have a turning speed of 3,000 RPM, and a 1” one a theoretical speed of 6,000 RPM -, well beyond the capacity of the lathe, so most turners would run the lathe at perhaps 2,000 - 3,000 RPM.

At 3,000 RPM, a 1” diameter spindle would have about 3” of timber passing the tool for each rev, so that would be 9,000 inches of timber per minute passing the tool (about 750 feet). A 10” diameter bowl with the lathe running at 600 RPM will have 30 inches of timber per rev passing the tool - 18,000 inches (1,500 feet) per minute. Thus, the smaller the diameter of timber, the faster the lathe needs to spin at to get a good clean cut. At too slow a sped, instead of the tool cutting the fibres, it tears them, then it’s necessary to resort to sanding by working through the grits to perhaps 600 grit for a good finish. The sharper the tools, the faster the speed, the cleaner will be the result, and less sanding will be needed.

For sanding, the lathe speed needs to be reduced to 250 RPM or so and the sandpaper not held in one place or it will score rings around the timber.

Just to finish off, here are a few more pics regarding the process of creating a blank to make a knob:

Pic 1 Parting tool ready to part off a knob blank.
Pic2: Blank almost parted off.
Pic 3: 4-prong centre removed from headstock by tapping it out with a bar.
Pic 4: Chuck fitted to the lathe headstock, with blank in place ready to be trued up.
Pic 5: Using a 12mm end mill (not a drill) to mill the hole ready for a brass insert to be glued in.

I hope these notes will give an indication that no exceptional skills are called for.

There are countless woodturning videos on youtube showing spindle turning – some better than others.

Have fun!