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Avoiding Kick Back
Thursday 29 July 2010
Michael O'Donnell discusses the term kick back, which many turners will be familiar with, and looks at why it happens and how you can avoid experiencing it with your turning
Just as you come to make the last cut, the tool touches the wood and immediately, without warning, skids along the surface creating a spiral cut and ruining the piece. It's a disaster and it can happen at any time in the turning process, but on the last cut there is no opportunity for recovery and the piece is lost. And it does happen to me occasionally, when I lose concentration. But it shouldn't and it can be avoided by understanding what causes it.
What I am going to discuss is what happens at the tip of the tool to cause 'kick back,' and how to present the tip of the tool to the wood to avoid the tool kicking back as it contacts the workpiece. But that is only part of the story. There are other considerations before you get to the point of contact between the tool and the wood.
While this may sound like stating the obvious, these things will contribute to the 'kick back' problem.
1. Make sure the tool is sharp. Using extra force, trying to push a blunt tool into the wood will not give a 'clean' contact and will therefore make the tool unstable.
2. Make sure there is a decent gap between the wood and the toolrest so that you can make a controlled entry in your own time.
3. Stand comfortably and relax; tension and rigidity can lead to a loss of control.
4. Hold the end of the tool handle; this will give you much more flexibility and control rather than holding the tool near the ferrule and against your hip.
5. And finally, concentrate on each and every cut you make, building up a rhythm. Soon you will be in auto mode, making excellent progress. If you get the first contact right, the rest is sure to follow.
Creating 'kick back'
The best way to understand 'kick back' is by being able to reproduce it easily in a controlled manner, and to do this I tried a little test. I turned a reasonably smooth 200mm (10in) - just a little shorter than my toolrest - long, 63mm (2 1/2in) diameter cylinder in soft wood between centres. The next step was to set the lathe at a slow speed - about 100rpm - and I set the toolrest below centre, about half way, and at least 12mm (1/2in) away from the wood.
Taking a 32mm (1 1/2in) skew chisel, just holding the handle, I placed it vertically - a sheer angle of 90 degrees - on the rest rotating it just a few degrees clockwise. With a positive action, I pushed the cutting edge onto the wood. Now it shouldn't come as a surprise that the tool kicks back to the right.
I repeated the process at the tailstock end only this time, rotating the tool a few degrees anti-clockwise. Pushing the cutting edge onto the wood, and this time - again, no surprise - it will kick to the left. No matter how many times I tried this, the result was always the same. But when working at normal turning speeds the 'kick back' will be much more violent. Don't try this at home!
Analysing 'kick back'
Now that we can create 'kick back,' what does this tell us?
Firstly, this is a 'direct entry' cut, i.e. the cutting edge is the first part of the tool to touch the wood and there is no bevel support at the start of the cut - the tool is therefore vulnerable to forces from the wood.
Next, the direction of 'kick back' depends on the angle at which the cutting edge contacts the wood. When the cutting edge is at an angle, there is a force on the upward face - a bevel in the case of the skew chisel - which pushes it sideways.
OK, you might say that was bound to happen because we were only holding the handle of the tool, if we had the support hand on the toolrest holding the stock of the tool then we could have resisted the forces and thus prevented the 'kick back'.
That may well be true for some situations, but you will often be caught unawares, just when you were taking that final delicate cut and the piece is lost. Best not to be rigid and fighting the tool on every entry, particularly when you need to be relaxed. There are better solutions available to you. Having created 'kick back' let's try entering the wood without it and see what happens now.
Avoiding 'kick back'
This time, I sat the tool on the middle of the toolrest, square to the wood with the cutting edge vertical. As before, just holding the handle, I firmly pushed the cutting edge onto the wood. And voila! The cutting edge enters the wood without 'kick back' and is in a position with bevel support, ready to continue the cut into the timber.
So what happened here and why did the tool not kick back this time? With the presentation of the cutting edge vertical at the point of contact, there is no bevel facing the rotating wood, and therefore there is no force to push it either way. Explained in technical terms, this means that when the cutting edge is presented with a sheer angle of 90 degrees and there is no force on the tool to create the 'kick back' that we experienced previously.
In fact, with the skew chisel, the best way to avoid 'kick back' on 'direct entry' into the wood is by making the entry with one of the points of the skew chisel, establish bevel support, then bring the long cutting edge into play. This is particularly relevant when the start of the cut is not at 90 degrees to the surface of the piece you are working on, i.e. in the case of making chamfers and 'V' cuts etc. Which point you use depends on the angle of the cut you are making, and which way up you have the chisel depends on the cut you are making.
Gouges and 'kick back'
The problem of 'kick back' isn't confined to the skew chisel; in fact it probably happens more often with a gouge, for exactly the same reasons, just when things seem to be going smoothly. For example, when hollowing out a bowl, coming to the final cut, and the tool skids across the finished rim.
The same basic rule applies: the first contact between the cutting edge and the wood should be at a point on the cutting edge which is vertical, with a sheer angle of 90 degrees or, in line with the rotation of the wood. Now, of course, this all depends on just how your gouge is sharpened. Personally, I work with a 'fingernail grind' on a shallow fluted gouge, and use a swept-back grind - O'Donnell grind - on the deep fluted gouge.
To make a 'direct entry' cut without 'kick back,' sit the tool on its side - with the edge of the flute vertical - and point the tool at the lathe axis and push it into the wood.
This will make a 'clean entry' without 'kick back' and will establish bevel support, but it is unlikely that tip is at the optimum 'sheer angle' to make the rest of the cut. So, once entry is made, twist - rotate - the tool to about 30-45 degrees and continue the cut.
The sheer angle of entry isn't quite as critical when using a deep fluted gouge with a swept-back grind as compared with the skew chisel, as it is a very forgiving tool. But don't be lulled into a false sense of security - it can still kick back.
Yes, of course, when you are making a direct entry cut, always have the support hand against the toolrest, supporting the tool as the entry cut is made.
Supported entry
The other type of entry is a 'supported entry' i.e. the bevel is the first part of the tool to touch the wood, support for the tool is established, then the cutting edge is brought into contact with the wood to start the cut in a cutting - rather than entry - attitude and continues cutting in the same attitude. With this entry there should not be a 'kick back' problem. But if any problem arises, look closely at the presentation of the tool to the wood to discover the cause then make adjustments to rectify it. You can learn a lot about turning by analysing what is happening at the tool tip.
