Wednesday, January 20, 2010

HP Lathe - using it

I'm using conventional modern lathe tools - just a basic set until I get a feel for what works well for this style.

It is not the same as a powered lathe, and particular techniques are different. The available power at the spindle is no more than 100W, and the useful speed range is 400-600 rpm. Deep or heavy cuts are not possible; fine cuts are the order of the day. The low power sets limits to the practical size (diameter) of work.

Tools need to be very sharp, especially for seasoned wood. Making sawdust is inefficient except for fine finishing - making long shavings is much better. Someday,  I want to try hook or ring chisels as an alternative to “ordinary” turning chisels. A hook chisel has a loop bent in the end and a bevel ground on the inside of the loop – a bit of a trick to make (see A ring chisel could be easier to make - a short length of steel tube brazed on the end of a shaft mounted in a handle and put a bevel on the inside of one end of the tube for the cutting edge.

There are other techniques to be learned as well: how to operate the treadle for extended periods without falling over, and while keeping the tool steady .

I have been changing the spindle pulley to find a good drive ratio. I found that I didn't need a groove for the drive belt in the flywheel; it sits there without any trouble, as long as the pulley is in the right place.

The original spindle pulley shown here was 50mm diameter, hence giving a drive ratio of 12:1 speed up off the flywheel. This didn't give much torque, meaning lots of belt slippage and stalls - very annoying.
My current pulley is 100mm diameter, hence only 6:1 speed up. The lower spindle speed does not seem to be a big problem, but more torque is good. Now I can easily turn material to 100mm diameter, and can handle 200mm diameter work with some care.

As well changing the pulley size, I can tinker with the travel of the treadle. The initial travel of the crank arm was about 100mm. Increasing this to 150mm travel along with the larger pulley makes for a better feel.

Future plans
Rebuild the bearings on the crank arm for better lubrication and sealing against dirt. The 8mm bolt which is the bearing shaft on the treadle is wearing out rather faster than I expected, although not so fast now that it is mostly in my relatively clean workshop.

Change the drive belt from nylon cord to a more authentic leather belt. This is not so old fashioned as you might think. During my apprenticeship, I have used commercial milling machines dating from well into the 20th Century which had leather drive belts, and which worked quite well. This will probably need a tensioning idler wheel to the drive belt, which should allow better tension control and less belt slippage.

Make another driven centre for the headstock. The first one is a bit inconvenient to fit material onto, sometimes, and the points are not as sharp as they could be.
Make a hollow centre for the tailstock. I will need this for long length-wise drilling of the bores of musical wind instruments. (as you may have noticed from the earlier posts about shawm making, this has already happened).

Do more work with green wood, i.e., fairly unseasoned wood.

Is it a good lathe? Well, sort of, within its inherent limitations.

What would I do differently if I made another one? Make the flywheel heavier by using more layers of lamination. Take more care with the spindle trueness.

Is it fun to use? Oh yes! It's more approachable than a power lathe, because it doesn't spin fast, and it's pretty quiet, and because all the action is at a human scale. And it keeps you warm in a cold winter workshop.

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