Wednesday, June 23, 2010
Wood turning - The Marvellous Skew
The skew is a marvellous tool. I was determined to get to grips with it when I first started turning, possibly because of its reputation. Of course I’ve had dig-ins and catches a-plenty; it's all part of the learning curve. Call them what you will, if you aren’t prepared to risk anything, you’re not going to learn. So I suggest you get some scrap wood and go to the lathe. Some people ask where to get wood for practice work. I’ll scavenge from skips, local tree surgeons, my own garden, and any other place that has a branch, trunk, or waste wood. Avoid wood with nails, screws, or cracks. I’ve done a lot of practice on branches and firewood. If you want to use branches for skew chisel practice, I suggest you choose a piece with no knots or branches, as these are a source of complications.
To understand my instructions you must be familiar with the features of the skew. The skew is usually a flat section of steel. I prefer round bar for skews as they provide a continuous support point when rolling over a bead. The shaft of the tool ends in a double-bevelled grind so that the cutting edge is in the middle of the blade when looking at the narrower edge of a rectangular section blade. The bevel is the part of the blade that is ground to sharpen the tool. When looking at the wider face of the blade, the cutting edge can be anything from 90 degrees across the blade. By definition, the skew has a skew grind, but it can also be used when ground to 90 degrees - the tool is simply held further around. More on this later. If it is ground at an angle, the longer (more acute) point is known as the toe, and the shorter (more obtuse) point is known as the heel. Many turners like to radius the cutting edge slightly. I prefer mine dead straight - I find it easier to grind and keep a keen edge on.
Although the skew chisel is used almost exclusively in spindle work, it is still a very versatile tool. There are four principle cuts that the tool can be used for.
One of the most popular and common uses of the skew is to plane a flat surface such as a rolling pin. This is a good cut to start with.
Peeling cuts are used to reduce the diameter of a section of spindle work very quickly. The skew is horizontal on the tool rest with the cutting edge of the tool facing the very top of the wood as it rotates over the cutting edge. It removes wood very quickly, but can be difficult to control. Whilst using the skew in a scraping manner gives more control, the peeling cut will give a better finish. Peeling can also refer to cuts where the toe or heel is underneath the fibres being cut. This results in a feathering of the fibres in front of the cut because they are being lifted from the spindle rather than cut off.
V-grooves and facing off end grain are examples of situations where the skew is being used to cut directly across the fibres of the wood. The skew is used vertically and as little as possible of the point is used, using the blade in preference.
The only time I would recommend using a skew in a non-spindle situation is when it is used to scrape. Its keen edge can be used as an excellent scraper, even if only for the dovetail recess in the bottom of a bowl or platter.
Now that we're familiar with the situations and uses of the skew, let's get to grips with how it works and why people struggle with it. For learning, I suggest you get the smallest skew you can. A beading or parting tool is good, as is a length of 8mm rod that's been sharpened like a skew and fitted in a short handle. Remember, spindle tools don't need long handles - they just get in the way.
Understanding why the skew Digs-in or Spirals
If used incorrectly, the skew will dig-in or spiral. These are the two most common mistakes. Understanding why they happen will help you look for the tell tale signs.
A dig-in occurs when the toe or heel of the skew comes into contact with the spinning wood. To make my point clear, the toe and heel are indeed used in a number of cuts such as when cleaning the end grain or on a bead, but in these cuts, the support from the toolrest is underneath the tip. The crucial aspect here is whether or not the tool has support under the cutting edge. Quite often a dig-in starts with the blade and bevel in contact with the wood. The cut is allowed to travel up the blade, away from the point of support. The tool is twisted in your hand by the pressure of the spinning wood against the blade. The twist allows the cut to travel still further and further up the blade until it reaches the heel or toe of the blade, which is when the dig-in occurs. This all happens so quickly that you barely have time to counter it. The only way to prevent it is to avoid it altogether.
Things that help prevent dig-ins
Slow the lathe speed. If pole lathes can produce perfectly good spindles at such low revolution speeds, why do we need 2000rpm? The slower the speed, the more likely you are to understand what is going on. I would however recommend a minimum of 50rpm, but having someone else turn the wood very slowly by hand will give you immense insight into how the tool is making the cut.
Keep the bevel in contact with the wood.
Angle the blade 30 degrees from vertical. When perfectly vertical, no cut occurs. As you rotate the blade, it cuts more and more until you are peeling rather than planing.
Use only the bottom 1/3 of the blade to cut. The closer the cut is to the bottom of the blade, the better it's support on the toolrest is.
Use a round bar skew. The continuous point of support is less likely to catch you off guard than the two corners of a flat steel blade. And even if you do have a dig in, the bar will roll on the rest rather than slam down on the wide flat blade.
Use a skew with a blade as wide as the piece of wood you are turning. The further the distance from the cut to the tip of the blade, the better. The counter to this is that the wider the skew chisel, the more monumental the dig in is because the point of support is that much further from the toe or heel when the dig-in occurs. Use a 1-inch blade on a 1 to 2-inch diameter spindle.
Use the skew heel-up. It may not reduce the chance of a dig-in, but it will reduce the severity of the dig-in.
A spiral occurs when the bevel is no longer in contact with the wood and the blade is not vertical. To produce a spiral, use a perfectly good planing cut, and try bead the wood in one single cut. The likelihood is that as you roll the bevel over the non-existent bead, it will leave the straight surface you planned, and the only part of the tool in contact with the wood is the cutting edge. The cutting edge is over at an angle (30 degrees?) because this is ideal for planning. However, because the bevel is now no longer in contact with the wood, you have no leverage to lift the cutting edge out of the cut. The skew makes a remarkable spiral on the wood, proudly displaying it's cutting efficiency.
Things that help prevent spirals
Keep the bevel on the wood.
Use the bottom point of the skew. Whether this is the heel or toe doesn't matter. It is the point nearest the toolrest.
When entering a cut, keep the blade vertical and start with the point, moving the cut to the blade once you have produced support for the bevel.
Hold the tool firmly on the rest. This helps when beading. The tool almost has a natural tendency to spiral back up the bead and over all your hard work. Firm pressure on the blade can help prevent it from running away.
Learning to plane wood
Rough down a piece of wood with a bowl gouge, roughing gouge, or large spindle gouge so that it is fairly cylindrical. Stop the lathe. Take your skew in your preferred hand by the handle and hold the shaft of the blade with your other. Place it on the toolrest, perpendicular to the wood, with the blade vertical and the toe down. Many turners, including myself, usually plane wood with the toe uppermost, but this toe-down method is less likely to produce monumental dig-ins. Turn the handle away from your body and lie the bevel against the wood. With the cutting edge still just in contact with the wood, push the skew along the wood. If you have the angle correct, the piece will either rotate backwards or plane a fine shaving from the surface.This is the angle you want to use when planning. Try this a few times until you've taught your hands the correct angle. Then power up the lathe and have a go. When you dig in, for you almost certainly will, just try again.
Learning to cut V-grooves
Use the cutting edge pointed directly at the wood, with the blade perfectly vertical and the toe at the bottom - this helps you see what you're doing. Enter the cut with the toe. Don't push too hard, just enough to cut a shallow groove. Remove the skew and do the same cut on either side of the scratch you just made. The fibres should break away, making the groove wider than before. Continue to cut in further and further from the centre line. You may have to angle the handle by moving it further over from the centre line than the cutting edge, but always enter the cut with the toe or you will develop a spiral. As the groove widens, you will find it also becomes deeper and deeper in the middle where the two cuts meet. Once you have a deep enough groove, use the walls inside the groove to support the bevel as you move the cut from the very toe to the cutting edge. Possibly try this on one end only, producing finger tops is an excellent way to learn.
Learning to cut beads
One way of teaching yourself to cut beads is to prepare them with your tool of choice and then to just rub the skew's bevel over the bead. Don't try cut the bead, just rub the bevel of the skew on the bead. This helps you realise how to roll and turn the skew. And at hte same time you'll be polishing the beads.
Another way to learn is to use your parting tool. I started using the parting tool to cut beads. I soon realised that the short cutting edge of the parting tool was just like a very small skew chisel. I watched the cutting edge as I rolled the parting tool over the bead. I pictured the bevel underneath, rubbing against the bead. I started with a small skew, just trying to do exactly what the parting tool had done. Oh the joy to see a shiny bead produced directly from the tool!
Learning to peel
A peeling cut can end in disaster if the bevel is allowed to move too far from the wood. Start learning to peel with a very narrow skew, maybe even a parting tool. The secret is being able to stop the tool from burying itself in the wood. The way to do this is to provide enough forward pressure. You will not be able to lift the tool from the wood if it buries itself. Keep plenty of downward pressure on the handle to hold the blade up, and enough forward pressure to keep it at the very top-most cutting position. A nudge from behind should raise the cutting edge over the work. The cut will pull the tool into the wood. You must resist this with forward and down pressure on the handle. Peeling is easier if the cutting edge can be held slightly off the horizontal.
Get out there! Someone once asked a demonstrating turner where the best place to learn to turn was. The witty professional simply said "In front of the lathe". So gather a load of sticks of scrap wood, plane each down, cut some V-grooves, convert these to beads and, if possible, repeat the process. Turn at least 50 beads - why not turn small eggs for children to paint at Easter time. You get the practice, and your local pre-school thanks you for the gesture.
I hope this has been of some use to someone out there.