Around about the time I bought my first lathe as a boy in the mid 70s (a Portass Model S), I found a book about lathe operation in the local library that wasn't written or published by the South Bend Lathe Co but had many photos that were attributed to them. A constant theme that struck me was the continuous, spiralled swarf that came off the tool, something you don't see these days. There was clearly nothing unique about their machines that would cause this effect so it was evidently down to common practice at the time. Recently I bought a copy of South Bend's "How To Run A Lathe" which is still readily available to buy or download off the internet. Most of the content dates from the war years give or take a couple of decades. What are often described these days as "clever" techniques like the use of the cross slide to draw the tool in and out while screwcutting with the topslide at 29 degrees are simply stated as the way it should be done – and this before most of us were born.

In the photos, that evocative swarf is there in abundance. It got me thinking…and experimenting.

With some consideration, a few things seemed clear:

· Cutting speeds must have been pretty modest, otherwise these spirals would have been lethal. Also, coolant was generally applied by brush (lard oil etc!), so heat generation must have been low. It all points to slow RPMs.

· Feed per turn must have been fairly modest too, otherwise the power and rigidity required would have been significant.

· The angle and offset of the cutting edge relative to the work piece are critical. It's got to be presented at a significant angle to the radial direction to generate a continuous spiralled swarf. This is achieved by angling the toolbit at about 30 degrees to the horizontal and where possible gaining another 5 degrees by deliberately raising the tip of the toolbit above the centreline. It’s rather like planing a piece of wood with the plane at an angle to the direction of travel.

I managed to buy an Indian or Chinese copy of one of those "American" toolbit holders after a bit of searching (they are out of fashion these days), cut and welded it to better follow the angles shown in the SB book and ground up a couple of 5/16" HSS bits as recommended therein.

I did some trials on my Bantam using a piece of scrap 1" dia mild steel. With 120RPM, 2.4''' per rev and 6mm width of cut, I was soon making some fine spirals. Lowering the tool tip reduces the pitch of the spiral (bunching it up), while raising it increases the pitch as you might expect. Raising the speed to 180rpm caused chattering despite tailstock support and coolant. I tried a second bit with a longer cutting edge and managed to get a ~10mm width of cut but of course I couldn't generate a decent vertical offset with this tool turning a 1" dia workpiece, so the swarf had a very short pitch – I'd drilled a 1/4" dia pilot hole for it to work into. I'd need a 2" or so diameter piece to allow me to raise the tip of tool enough to lengthen the spiral pitch to the point where it is flexible enough to clear the tool holder and flow from the work cleanly.

That's it. Probably nothing of any great practical value but an interesting experiment to me at least. The feed rates are a lot lower than we tend to use these days with the prevalence of carbide tools, pumped coolant and high power high speed machinery. Note that I had to fit a larger 60t gear on my Bantam geabox to get the feedrate low enough.

Hope you find this interesting!

Muzzer

John H: google "wewilliams.net" The current edition (1966) is well worth buying if you can find it, as it is more comprehensive. It's freely available near me but then again I am currently living in Canada…..

Note the article on screwcutting too. Just good, common sense stuff, albeit with a distinct US flavour!

Muzzer