INTRODUCTION TO WOODTURNING
Chapter 11: Spindle turning - the planing cut
Many novice turners seem to think that the packaging on a new skew chisel should carry a Government Health Warning. Perhaps, because they have adopted the wrong approach, or because they have used an unsuitable tool, or because someone else has told them worrying stories, there are those who pick up the skew chisel in trepidation of something unpleasant occurring. This is very unfortunate and totally unnecessary. There is nothing to be afraid of. If the instructions given below are followed closely then nothing untoward should occur.
11.2 The chisel
It is customary to use a skew chisel to make the planing cut but a square end chisel can also be used. The skew angle does make it easier to hold the chisel in the correct cutting position. When first attempting the planing cut the novice will need a wide chisel; it is recommended very strongly that a 1¼ in. chisel should be used. This width makes it much easier for the turner to keep the tool cutting in the safe part of the edge. This will be discussed further below.
In addition the chisel must be sharpened correctly. This is very important. Many of the problems which people experience with the skew chisel are probably due to a badly ground tool. As a consequence I must repeat some of the points I made when discussing sharpening in Chapter 4. It is essential that the bevel of a chisel is ground either concave or, at worst, flat. I believe it will also help if the bevel angle is made fairly large; I would, in fact, recommend a bevel angle approaching 45° to start with.
Another point worth making, although it does not affect safety, is that the chisel should slide smoothly along the tool rest. It will slide more readily if the corners of the blade, which come into contact with the rest, are rounded off slightly. If it has not already been done by the manufacturer, the arris should be rounded over with a stone.
11.3 The attitude of the chisel
As a preliminary to the planing operation it is necessary to consider the attitude in which the chisel is used. This is illustrated in Diagram 11.1: the chisel is inclined at an angle so that the cutting edge is at about 45° to the axis of the lathe when seen from the view point of the turner (we can call this the angle of attack). This is the position for a right handed person who is making the cut by moving the tool from right to left, that is, towards the headstock. Note that the long point of a skew should be at the top.
Diagram 11.1 The attitude of the chisel
Note: a square end chisel is shown in these diagrams.
11.4 The cutting point
Ideally, the position on the edge, at which cutting takes place, should be about one third of the width of the edge above the short (or lower) point. For convenience, we can call the position at which cutting takes place the 'cutting point' (see Diagram 11.2). In practice it will be difficult to maintain this position precisely but every effort should be made to prevent the contact point rising above the middle point of the edge.
Diagram 11.1 The cutting point
What has to be emphasised is that the upper corner of the cutting edge must never be allowed to come into contact with the work-piece; if it is allowed to do so the tool will dig in and, at best, will ruin the job. Fortunately, by following the procedures described below this is easy to avoid and there will be no danger.
11.5 Preliminary exercises
With the lathe switched off procedures similar to those which I described as a preliminary exercise for the roughing down operation should be carried out. The correct stance should be taken up and the height of the tool rest adjusted so that a comfortable position can be maintained. The chisel is laid across the tool rest and the work-piece and then slid rearwards, along its own axis, until the heel of bevel is in contact with the wood. It can then be withdrawn a little further until the bevel itself is in contact with the work-piece.
At this point, with the lathe still switched off, some practice can be had in manipulating the attitude of the chisel. By making appropriate movements of the handle the turner should be to be able to do two things:
The necessary movements should be repeated a number of times. As they are made the bevel should be held flat on the work-piece. When the lathe is running this will be when the bevel is rubbing (see Chapter 6 Section 6). Also, of course, the edge of the tool must maintain contact with the rest.
To control the tool whilst a cut is in progress the turner must be able to combine these movements. This necessitates quite subtle adjustments of the tool which on paper may seem very difficult. After a little practice, however, they are performed without conscious thought on the part of the turner. It is a little like learning to ride a bicycle: one moment the learner is wobbling all over the road and then, suddenly he has control of balance and direction.
The movements described are actually easier to make when the cut is in progress than they are with the work-piece stationary. The small amount of vibration created by the cutting action makes it easier to slide the tool into the required positions. In addition, the resistance of the wood against the cutting edge provides some resistance, which the turner can use, to help to provide the control required. With the some practice the planing cut can readily be made with just one hand, holding the skew by the handle.
11.6 Trying out the cut
It is now time to try out the planing cut for real (ie, with the work-piece in motion) starting from the situation where roughing down has just been completed. For the time being the speed is left unchanged , ie about 2000 rpm. It may be necessary to reduce the speed later but the reasons for this will be explained in due course. The following instructions are for a right handed person.
Before the lathe is switched on think about how much of the work-piece should be planed in the first stage. It not advisable to attempt to plane the whole length in one go. In any case the tool rest may well be too short too permit that. It best to do about 6 ins at a time, starting that distance in from the left-hand end. Having chosen this position take up the recommended stance and make sure that the feet are placed in a suitable position.
The procedure to be followed in making the first cut is as follows:
Care should be taken when the end is reached that the tool does not dip into the drive spur. Throughout this procedure the contact between the tool and the tool rest must be maintained.
This procedure is repeated until the work-piece has been reduced to the required diameter over this first section. When this has been achieved the adjacent section is tackled, and so on, until the right hand end of the work-piece is approached. When working to the left the last section should be started an inch or more to the left of the right hand end. This leaves a small section at the right hand end which must be tackled in a different way.
The planing cut should not be attempted with the chisel off the end of the work-piece. This means that the cut must now be made in the opposite direction, that is, moving from left to right. The cut can now be made either right handed or left handed. Either way the tool must point to the right in the same attitude as before. Performed right handed this is a little awkward: the body must be twisted round so that it is between the handle of the tool and the lathe.
When the cut is performed left handed everything must be a mirror image of the attitude described above. The right hand holds the tool on the rest and the left hand holds the handle. This may seem a terrifying idea but in practice most novices do not find it too difficult. The key to success is to take it slowly and carefully. The chisel is applied to the wood so that the heel of the bevel is rubbing, slowly manipulated until the edge begins to take a fine shaving, and the cut is made to the end of the work-piece.
11.7 Possible problems
I have taken pains to stress in the foregoing the importance of keeping the bevel rubbing. Allowing the bevel to come off the work is the primary cause of all problems in turning. Whenever something seems to be going wrong the turner should check whether or not the bevel is rubbing.
The turner may find that although he can start the planing cut correctly, and maintain it for a short distance, he has difficulty in keeping the bevel rubbing. A common reason for this is that the upper body is being allowed to twist as the hands are being moved to the left. When this happens there is a tendency for the left hand to be pulled away.
The correct way to perform the cut is to keep the hands as still as possible with the elbows tucked into the sides of the body. The attitude of the tool is controlled by the hands but it is moved sideways by moving the whole of the trunk. The trunk is moved by transferring the weight of the body from the right leg to the left and allowing the pelvis to shift from above the right foot to above the left foot. As this happens the trunk, arms and hands are moved sideways en bloc, without changing their relative positions.
Another problem can be caused by the tool lifting slightly off of the rest. Sometimes this is revealed by a slight chattering of the tool and/or a slightly irregular cut. It possibly happens because the turner is concentrating so hard on keeping the bevel rubbing that he is not paying sufficient attention to other things. So, the bevel should be kept rubbing and the tool kept in contact with the rest.
11.8 Spiral ribbing
There is one problem which can occur with the planing cut even by experienced turners who, it seems, are doing every thing right. The most common manifestation of this is that a series of spiral ribs are formed on the work-piece. This can be difficult to eliminate. I have come across a number of suggestions for alleviating this problem but have neither seen nor heard an explanation of what causes it.
I have my own theory about this. On many timbers there is a difference in the hardness of the early (or spring) wood, which is laid down when the sap is rising strongly, and the late (or autumn wood) when growth is slow. In ring porous timbers, such as ash and oak there is also a difference in the cellular structure of early and late wood. It is these differences which give rise to the appearance of the annual rings. In most cases the darker wood in the annual ring is harder, or cuts less readily, than the lighter areas.
When a work-piece is held between centres with the grain running axially then the annual rings will tend to run from one end to the other. When the wood is being cut a thicker shaving is taken out of the less dense wood and the work-piece goes slightly out of round. The denser, darker, wood is usually much narrower than the rest and tend to stand up as a series of ridges around the work-piece. When this effect is pronounced the ridges cause the tool to bounce slightly which in turn leads to variations in the depth of cut to form a spiral pattern.
To overcome this the natural tendency of the turner is to speed the lathe up. Certainly this was my reaction when I first encountered the problem. In fact increasing the speed of the lathe exacerbates the problem. What happens is that the increased kinetic energy in the work-piece is transferred to the tool making it bounce even more. To put this another way: because the ridges on the work-piece are moving faster they give the tool a harder kick. As a consequence one way of dealing with this problem is to slow the lathe down.
After reducing the speed of the lathe it may be necessary to remove the bumps and hollows previously created. In this case the roughing gouge should be passed over the work again to return it to the round state. Then the planing cut can be tried again.
In some timbers, where the difference in density between the early and late wood is very pronounced, the tendency to make spiral ribs may still be evident after the speed is reduced. In this case the turner should try the following suggestions: set the rest as close to the work-piece as possible; hold the chisel more firmly than usual and, whilst making sure the bevel continues to maintain contact (ie to rub), allow it to just brush the surface.
If, after trying all these things there is still some ribbing, I have no further suggestions to make as far as the the chisel is concerned. When the exercises being described here are being followed a perfect surface is not necessary. If, when the turner has progressed and is attempting to make a specific object, it proves impossible to plane a satisfactory surface it may ultimately be necessary to resort to some very light scraping and sanding.
Some turners may find it difficult to accept that the answer to the problem of ribbing is to slow the lathe down. But remember this: no-one has to take my word for it. People can try it out for themselves and draw their own conclusions.