SPINDLE TURNING FOR FURNITURE; ESSENTIAL TIPS,
ADDRESSING COMMON CONCERNS
Published in American Woodturner September 1997
Although there are many types of furniture which do not employ turnings, the development of furniture design and the art of spindle turning are inextricably linked. Historically, before the use of rotating head planing machines, or even table saws, turning was perhaps a quicker way to produce a finished part. Some styles of furniture, such as Windsor chairs, do not require the stock to be squared, but I will primarily discuss the turning of squares in this article.
While the subject of spindle turning for furniture cannot be thoroughly covered in a single article of this size, the topics I have included here have been chosen for one or more of the following reasons: 1. the most frequently asked questions by my students, 2. inadequately covered in other works which I have read, or 3. attempt to correct common errors.
PREPARING THE SQUARE
Most turnings for furniture have some part (called a pommel) that is left square in the finished piece for the attachment of rails, and these flat surfaces should be dealt with first. It is important that squares of wood for turning be straight, parallel, and truly 90 degrees (not a rhombus). This is usually accomplished by use of a jointer with an accurate fence to surface two ADJACENT sides to a right angle. After two adjacent sides are planed on the jointer, the remaining two sides of the square are surfaced in a planer, keeping the previously finished sides face down on the bed. This method ensures that opposite sides are equal and parallel, and since one right angle has been established on the jointer previously, the result will be a square.
If you do not have a planer, then parallelism can be established with a table saw. The sawn surface can be cleaned up by some hand work, or taking one light cut on the jointer.
Lastly the part is cut to length. Do not cut the stock longer than the FINISHED SIZE, as this will only make the turning more difficult.
After initial preparation, the flat surfaces of the squares will have either planer marks on them or marks from the hand plane, and it is best to sand or scrape these off now. It is difficult to do this after the turning without doing some damage to nearby turned details.
MARKING THE SQUARE'S CENTER
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Many books advise finding the center by drawing two lines from corner to corner. It is much better to use a method which locates the point which is equidistant from the SIDES. Use a marking gage (mortise gage) [illustrated] set to approximately one-half the diameter, and make FOUR marks. This will yield a tiny square, the center of which is punched with an awl.
For production work, a centering jig, [illustrated], is used. The work is slid along the fence and into the pin FOUR TIMES, while being rotated each time. This easily made jig is based on a wedge which is clamped under the block. The pin is actually a woodscrew with its head sawn off and filed or ground to a point.
MOUNTING THE WORK
Some authors advocate cutting grooves in the end of the work to receive the spurs of the driving center. (If you need to cut grooves in order to keep your spur center from slipping, there is something radically wrong with your spur center.) Some say to drill a hole for the center point, or remove the spur center from the lathe and drive it into the end of the work with a mallet. All of these procedures are not only a waste of time, but they prohibit you from MAKING ADJUSTMENTS to the center location on the lathe, which is necessary for accurate work.
Why would you need to make adjustments to the location of the centers
after the wood is on the lathe? There are four reasons.
1. There may be errors in your centering jig or punch mark.
2. Certain woods, such as oak or ash, have hard and soft layers. When the
lathe centers are pressed into the ends of the work, they will drift into
the nearest soft layer, causing inaccuracy. To some degree this can happen
with any type of wood.
3. The wood may have warped between the time it was straightened on the
jointer and the time you are turning it. In order to achieve full diameter
or concentricity in the middle (or some other chosen point) the center
locations at the ends must in fact be eccentric.
4. The wood warps while it is being turned. This is due to the release
of internal stresses.
What is needed is a way of mounting the work from POINT TO POINT and setting it into rotation without the spurs engaged. Then it is possible to achieve perfect centering through a process of TESTING AND CORRECTION.
With the work rotating, the centering is observed. It is easy to "nick" the corners with a pointed chisel and stop the work for inspection. Whichever side has the deepest nick needs to be moved toward the axis. This is done with a mallet, the tailstock tightened, and the centering tested again. This process is repeated until the desired degree of accuracy is obtained.
In order for this operation to proceed smoothly, your lathe centers must be well tuned. Sixty degree points are ideal because they allow the most sideways movement as the point is driven deeper. The total amount of adjustment which is possible by this method depends on factors of hardness and weight of the work, and in rare cases you may have to withdraw the point completely and reset it.
All of the above pertains to furniture (or architectural) parts which have a square portion. If you are making Windsor chairs, you can ignore this. For work that is turned all over, rough saw the stock 1/16 to 1/8 over the largest finished diameter, unless you are riving the stock out of logs as chair makers do. (Riven wood may be centered on the lathe by a method of measuring "negative radius" from an accurately positioned tool rest. It consists of adjusting the centers of a mounted piece of wood until no part is more than a given distance from the edge of the tool rest. I learned this method from Vermont chairmaker David Sawyer.)
CUTTING POMMELS
The first step in nearly all furniture work is making the transition cut from the square to the round. I know this process gives many beginners trouble, so I will say a few words about it.
A pencil line is drawn around the work where the transition is to be made. It does not need to be drawn on all four sides, but it helps. Use a soft pencil so the lines are dark and can be seen with the work rotating.
Cutting a vee into the square is really the same as cutting into a solid round, except that it is harder to see where to start each cut. When making this kind of intermittent cut, it helps to have good light and a black background behind, such as black sandpaper. Use the point of the skew chisel rather than the heel, because it is easier to see what you are doing and the point cuts more freely. Cut alternately from the left and the right, each time aiming at your pencil line. When the pencil line is gone, and you feel the chisel cutting smoothly into the solid wood, stop and check. You may need more depth if for example you want to start a bead at that point.
The transition cut should be made carefully so that it does not require any sanding which would destroy the crisp intersection lines.
It is more difficult to make a transition which is square rather than at an angle. If the cut is not exactly at 90 degrees, then the resulting intersection will not be a straight line as it should be. I discourage this type of design which is intended to mimic architectural stone work, and I do not think it is appropriate for wood.
THE LONG AND THE THIN OF IT
Length is the enemy in spindle turning. Vibration of long, thin workpieces
is a problem in most furniture work, but with the right approach, and a
few special tools, these problems can be overcome. Many factors contribute
to success, and one should try to apply as many of the following rules
as possible whenever workpiece vibration is a problem.
1. Do not leave extra length of stock to be cut off later. Allow only a
minimum (about 1/2 diameter) for cutting finials off.
2. Use a minimum of tailstock pressure. (If the spur center slips, maybe
the center point is too long, or the spurs are not sharp.)
3. Use slow speed.
4. Start in the middle and work toward each end.
5. Use even-textured wood if possible, not layered or ring porous.
6. Use a gouge with a small nose radius, especially for figured grain.
7. Use your hand to steady the work, or if that fails,
8. Use a steady rest.
The use of the hand pressing against the work to dampen vibration is often necessary in long spindle turning, and it gives the turner a needed tactile connection with the workpiece. The hand is able to detect surface errors which whiz by too quickly for the eye to see. The hand is also better able to locate the "lumpiness" in a long straight section than the eye. There are certain dangers involved of course: brushing against the square portion is the most common. Accidents can occur when fingers are caught between the work and the tool rest. But even without accidents, at the end of the day there is only so much skin which you can afford to wear away from the palm of your hand. Eventually a steady rest is needed.
I suspect that the reason most types of steady rests fail is because they
were designed by engineers who had only a metalworking background, so they
were modeled after machine lathe types. These makers forgot to take into
account that when pressure is put on a wood surface, the amount it indents
varies over the hard and soft layers of the wood. Once the surface upon
which the steady rest is bearing becomes rough, it causes vibrations instead
of stopping it.
My own frustrations with this problem led me back to Frank Pain's The Practical Woodturner (first published in 1957 by Evans Bros.; then Drake, 1976; revised by Sterling, 1990) from which I had learned so much in the early years. In this book are pictures of four steady rests, all made entirely of wood. Why are these rests successful? I realized that the AREA where the wood contacts the workpiece is wider than the width of one layer (growth ring), so the workpiece rotates smoothly against it. Eventually the mother of invention led me to the type of steady rest illustrated here, which I have used successfully for many years.
REPETITION AND DUPLICATION
It is amazing how often I am asked the question, "How do you make turnings which are all the same?" I respond, "They are not the same, they just look the same." In order for parts to look alike, they must posses two characteristics: The measurements must be alike, and the curves must have the same shape.
The measurements are taken from critical points and measured in an x - y fashion: a certain distance from the end or some other feature (axial), and a certain diameter (diametral, which is what we measure with a caliper). Please note that very few critical points actually need to be measured in order to successfully duplicate a design.
I have heard some turners brag about not having to use calipers, because they use only their eye. While I wish to be the first to recommend that you develop your "eye", I do not think there is a quicker way to successfully duplicate a turning than to use calipers in a FEW critical places such as at the narrow parts of the turning. Consistent diameter is much more important in duplication than axial measurements, and calipers help you achieve this quickly and without guesswork.
Marking sticks are made from 1/4" plywood (use endgrain of top veneer). The marks are made off the drawing, and extended with a square. File the grooves with a triangular file until you have cut half way through. Hold the stick against the rotating workpiece, and rest a pencil in each of the grooves in succession. Marking sticks [illustrated]
Using a parting tool for initial layout is tedious, but for duplication there is no way more direct. This is because it is easy to cut straight-in with the tool and, working with a caliper, locate a point in both its axial and diametral components. Use this method sparingly however. It is not profitable to use a parting tool in many situations, such as to set the diameter at the bottom of a cove, although it is tempting. The presence of the cut makes turning the cove much harder, since the gouge falls with a crash at the bottom of each stroke. Do not use parting tool cuts to "lay out" long curves (you are not building a boat hull). Also the location of V-cuts cannot be set with a parting tool since the tool is square on the end. Make V-cuts with the skew chisel, alternating equal cuts from the left and right, and hopefully when you reach the bottom, you will still be at the correct axial location. The depth of V-cuts can be measured with calipers which have had the contact points modified to a thin edge. It is risky to use calipers in a vee while the work is rotating.
It is harder to make two turnings alike than it is to make twenty alike. In the second case each turning only needs to be similar to the AVERAGE of the other nineteen. Anyway, making two requires a different approach than making twenty. To make a pair of candle sticks for example, simply make one as you like, and copy it. In production, however it is more efficient to make each one match some pattern. Before a production run, several steps should be taken to clarify the variables. Drawing helps. Making a prototype helps. But eventually each of the turnings must be made to the pattern which you have memorized and is IN YOUR HEAD, that is your mind's eye.
HOW TO VISUALIZE CURVES
Often it is useful to think of curves as parts of geometric figures which are familiar: circles, ellipses, parabolas, etc. These forms tend to make our designs somewhat rigid however. The use of straight lines and true circles should be used with caution in the design of turnings because they tend to look mechanical and artificial; that is, not organic or graceful. There are exceptions to this, such as the ball finial, which has a simple elegance which overcomes its mechanical character. Of course cylinders and tapers have their place as well. Tapers underlie the forms of table legs and bedposts, but they usually need other details to balance them. For these reasons, most turnings involve curves predominantly in their design, and attention to their shape is obviously a big part of woodturning for furniture.
Use calipers and rulers as described above to determine the end points of the curve, now consider its shape.
The most important features which describe the shape of a curve are: the HIGH AND LOW POINTS, the END ANGLES and the INFLECTION POINTS. [illustration-Fig. 2]
The line of a cove has a low point, and it is usually in the center, but not always.[Fig 1] If the end points of the line forming the cove are not the same diameter, then the low point would be off-center. Thus it divides the axial length of the cove into two slightly unequal parts. Figure 1 shows a symmetrical bead and cove [A and B] and an asymmetrical bead and cove [C and D].
All curves begin and end somewhere, and usually this is at a shoulder or corner of some kind. A line which is tangent to the curve at the end point makes an angle to the axis called the end angle.[Fig 2]
Simple curves such as the shapes of beads or coves continue their curvature in one direction only, but S-curves must reverse. The point at which the curve changes from convex to concave is called the inflection point.[Fig.2] An S-curve has both a high and a low point, so its length is divided into three parts. The inflection point divides the curve into two parts, which may or may not be equal. The proportions of these parts play a major role in defining the curve.
The illustration [Figs. 2 and 3] shows how variation in the location of the inflection point results in turnings which do not look alike, even if the end points are the same. The change in the location of the inflection point has an inevitable effect on the locations of the high and low points as well as the end angles.
So the execution of a curve depends on attention paid to the proportions of its length as divided by the locations of the high and low points, and perhaps more importantly, the inflection point. Finally determine what the right end angles should be, and stick to them even if the diameter may go off slightly.
WORKING IN STEPS
When parts are made in quantity, the work proceeds in steps or stages.
An average furniture piece goes on the lathe four or five times before
it's done. Working in steps has many advantages. It allows for better momentum
of work and fewer sequences to memorize. It allows for fewer different
tools (chisels, calipers) to be up an any one time, and it drastically
reduces the time spent in moving tool rests, steady rests, dust collectors,
etc. Spring loaded spur centers facilitate rapid loading and unloading
of work without stopping the lathe. Frank Pain describes in his book how
he manages to accomplish this without springs. For a review of modern spring
loaded spur centers, see American Woodturner, June 1997, pages 42
and 44.
TEMPLATES
HOW TO MAKE DRAWINGS
A simple method of achieving symmetry in drawings is to fold a sheet of paper vertically down the center, and then open it like a book. The fold becomes the center-line of the drawing. Now it is only necessary to draw ONE SIDE of the turning. Use a soft pencil. (Do not draw the lines across yet.) When one side is done, fold the paper back again (drawing on the inside). Use a stylus or some very smooth object to rub the pencil marks onto the other side of the drawing. Open the paper, darken the lines, and draw the lines across. The drawings for this article were made this way. The same method can be used to create bilateral symmetry when designs repeat in reverse (such as the "double baluster" pattern). In this case the paper is folded twice to form a cross. Draw full size whenever possible, then the drawing can be folded inside out and used to mark the turning itself.
SANDING - LESS IS MORE
Many good turnings have been ruined by improper sanding. Generally this
means that the sharp corners have been rounded, and the turning loses its "crisp" look
and appears like something mass produced and sanded on a brush machine.
The way to preserve the sharp details in a turning is NOT to sand them.
Only the broad areas and large features need to be sanded to remove any
tool marks or lumpiness. The following suggestion should be helpful: 1.
Do not attempt to sand inside corners, even with a square stick.
2. Reverse direction after each grit, and a few times at the end. You do
not need a reversing motor for this, simply turn the workpiece end-for-end.
3. For straight parts, rip open a 3" or 4" sanding belt.
4. For very large cylinders such as columns, floor sanding paper is excellent.
5. For smoothing the bottom of small coves, a slightly tapered dowel or "sanding
stick" will press the paper into the cove evenly.