Tearout that shouldn't be there
When you work on a CNC mill, it's all about feeds and speeds. For a particular material, the questions are: how fast do you spin a given milling cutter, how fast can you move the tool across the work, and how deep a cut can you take? Do it right, you get a nice smooth finish and fast cutting. Do it wrong and you waste time, break the tool or damage the work.
With a wood lathe, when it's just you holding the tool in your hand against a toolrest, the same actual criteria apply - but we don't actually calculate them. Instead, you go by feel. When making a cut, how much pressure do you put on the tool so it feeds into the wood and how fast you move the tool across the work? A treadle lathe has totally different feeds and speeds than a regular powered wood lathe has.
For example, I've been playing around turning a piece of cherry wood. Cherry is dead easy. The wood turns easily, but because everything is coming out well, I have noticed my tendency to do what I do with powered equipment: lean into it. That is, subconsciously my brain is thinking that everything is cutting nicely, and the lathe seems to have plenty of power, so why not push a little deeper?
If I am lucky, when I stop the lathe I just see more tearout than I would want; if I am unlucky, this tendency is rewarded with a rough cut - chatter. Chatter in spindle turning occurs when the interplay between the sharpness of the tool, the speed of the lathe, and the depth of cut exceeds the force of the tool needed to shave the wood easily. So you have the tool unable to cut cleanly, and one of three things will happen.
1 - You get a shaving - but probably not since the force on the wood isn't enough to remove a thick shaving.
2 - The tool is so jammed into the cut that everything comes to a screaming, dangerous stop. The tool could break or get jerked out of your hands; the work could shatter; or the lathe could stall.
3 - The most likely outcome on a treadle lathe is that the wood just pushes the blade out of the cut and keeps turning. That doesn't sound too bad, but the wood is rotating while you are still trying to take that heavy cut, and as the wood revolves the tool is sneaking back for another try, going for a heavier and heavier shaving until the tool gets pushed out of the cut again. This is how you get chatter.
Treadle lathes run slower than powered lathes - under 1000 RPM vs 2,500 to 4,000 RPM on a 2" spindle. With the same pressure on a tool, the tool will advance into the wood 2-3 times the feed rate per revolution. Less power and a potentially higher feed is why a treadle lathe is more prone to chatter than a powered lathe.
What I just described really applies to spindle turning where the grain of the wood is perpendicular to the cutting action. In bowl turning where the grain of the wood is parallel to the tool, the problems are worse. You get tearout (above) or chunks of wood flying out from the work. Half the time when turning a bowl you are cutting against the grain of the wood. Bowl turners have strategies for combating the grain problem with tool selection and cutting approach, but unless you are controlling your feed, you will have this problem to various degrees.
The picture shows the tearout in my cherry bowl that occurred when I went against the grain - my tool wasn't very sharp and I was holding it wrong. (Since I took that picture, the bowl has progressed and the problems were fixed.). On the other side of the bowl blank (below), I am cutting with the grain, and in spite of my dull tool I am getting a clean surface.
The other problem is when starting a project, the wood isn't round yet, you are cutting air a percentage of the time, and there is nothing for the bevel to ride on. When that happens, the tool wants to lurch forward into the cut, and suddenly you are taking way too thick a cut (and bad things will happen - see above).
The first step towards a solution is sharp tools. While sharp tools are critical on any lathe, it's even more important when turning with a treadle lathe. A sharp tool lessens the force needed for a clean cut so you can take heavier cuts before the lathe rebels. With a sharp tool, the shaving is cut away cleanly, and not just ripped off the wood. I have all HSS tools and I keep an extra fine diamond stone next to the lathe and routinely touch up the edge. A strop with green honing compound helps too.
The second step is to understand how to keep the tool steady so that you can control the depth of cut no matter if you are cutting wood or air. I have learned enough to now know that have been doing it all wrong. I am used to resting the tool on the toolrest and the bevel on the wood and then raising the handle so the tool cuts. I maintain pressure, relying on that pressure on the wood to keep my cut consistent. This doesn't work at all when cutting air on an interrupted cut. Resting the bevel on the wood and raising the handle is how you set the depth of cut, and riding the bevel against the wood keeps the tool from shifting about. But keeping the handle in a consistent position is what keeps the feed rate constant. You do that by holding the the tool at the end of the handle. (The other hand is at the rest moving the tool back and forth.) Any hand movement is reduced proportionally by the ratio of distance toolrest to work versus toolrest to where you hold the handle. You can also brace the tool against your side. Long- handled tools allow for a steadier cut. But long handles do get in the way on a treadle lathe. Increasingly I find myself automatically changing my position and alternating feet as I turn to get out of the way - and the long handle is becoming less and less of an issue. On a bowl I do a lot of turning standing facing the headstock, treadling with either foot from the side. This is a big learning curve for new treadle turners, myself definitely included.
Some turners will say that this idea is obvious - but obviously for me it wasn't. Short handled tools are easier to use on a treadle lathe because they get in the way less, but they need more gentle, steadier holding at the handle end. Long handled tools make for a steadier turning session, once you learn to get out of their way. We know that the lathe has attracted attention from folks who are new to turning and - as someone myself learning new tricks - I want to acknowledge the learning curve (and also say developing the skill has been a lot of fun).
No tearout when the grain direction cooperates on the opposite side of the bowl
Joel's Blog
Built-It Blog
Video Roundup
Classes & Events
Work Magazine