ENTASIS IS A SUBTLE THING - HOW TO TURN WOOD COLUMNS
Wood columns are frequently made to simulate the ancient Greek and Roman stone columns. These architectural forms were perfected thousands of years ago. They fall into four basic styles or "orders": Tuscan, Doric, Ionic, and Corinthian, but all designs consist of three main parts: a shaft (sometimes fluted), a capital, and a base. The shaft is tapered, but the line is not straight. The taper follows a convex curved shape known as entasis.
The diameter of a column at the top is usually 5/6 the diameter at the bottom. Blanks for columns are glued up as tubes either tapered or straight. Usually 8/4 lumber is used. For most columns of 10" to 14" diameter, I use 10 or 12 pieces or staves (a term borrowed from barrel makers). It may be tempting to use fewer, but this results in less wall thickness, (or requires the use of thicker lumber), because more wood is removed in roughing out.
Just below the capital is a small decorative ring or collar called the astragal. The protrusion of the astragal is just enough to fit into a straight tube, that is, its diameter is slightly smaller than the bottom of the shaft. This is why most tube blanks for columns are made straight. In this case the finished wall is thinner at the top. Some wood columns do not have the astragal, and while this many not be strictly to form, it is an effective simplification. Much saving of lumber and turning can be achieved by making tubes for these columns tapered at the outset. In any case, when ripping the boards, turn the board over and end-for-end after each cut to make the bevel on each side. Do not rip the boards square first and add the bevel in a second operation, as this is a waste of time and lumber.
It is not necessary to involve any spline, tongue, biscuit etc. when gluing for the following three reasons: 1. The machining of a tongue-and-groove glue joint at a bevel angle involves the waste of a huge amount of material (not to mention time). 2. Added surface area for glue is not needed with modern adhesives, 3. Alignment at the time of gluing is achieved by the application of chain clamps which pull the pieces into a circle.
The rough blank should be made oversize by an amount equal to the total of circularity error and centering error. Calculate carefully to minimize roughing out on the lathe and maximize wall thickness of the finished piece. On a 12 inch column, for example, I allow about 1/4 inch for circularity error and 1/8 inch for centering error, so I would make the blank about 3/8 inch larger than the finished size, or 12 - 3/8".
RIPPING
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The boards are bevel ripped in the rough form, and dressed individually later. This maximizes the thickness, especially when rough boards are cupped or twisted. (It is important that all staves are planed to the same thickness, because the centering is done from the inside.) When staves are to be ripped to a taper, it is best to do this in two steps. A wood wedge is used against the fence to produce a tapered piece at the outset. After each cut, the board is flipped upside down and end for end. This allows for maximum use of the lumber as the taper of each piece is nested in the last. In fact you can get two 3 inch pieces out of a 5 inch board this way. After the pieces are jointed and planed on their faces, the bevel is finished on one side on the jointer with the fence angled. The final sizing is done with a second table saw taper jig. Here the work rests on a plywood base and is held down to it while being ripped. Although it is best to finish again on the jointer, this second sawing step is for sizing each piece so that they are all the same width.
CLAMPS
Many
types of band and strap clamps have been devised for woodworking. I prefer chains
because they are stronger and it is easier to chip the glue off of them. Indentations
on the wood left by the chains are removed in the turning process. The system
which I devised uses Vise Grips which have the jaws modified into a hook shape.
This is done with a round-nosed grinding wheel which makes a groove in the jaw
to receive the chain. These are used to tighten the chains around the column.
Use about one clamp per foot of length. THE GLUING PROCESS
At
the stage of the final fitting, tubes should be assembled dry and checked for
correct angular fit and correct diameter. When pieces are assembled dry, they
must be bumped into circularity with a mallet (while tightening the clamps).
In the presence of glue they will do this by themselves, because the glue is
slippery.
Aliphatic resin glue (the waterproof version of yellow glue, such as Titebond II) is convenient to use, but it requires rapid assembly. To use it you must be prepared, rehearsed, and with an assistant. Slow setting glues (such as two-part glues) are probably best for beginners.
Pieces are weighed and placed so that the blank will be balanced when run on the lathe. They are paired according to weight and placed opposite each other in the blank. For example, in a 12 segment column, the two heaviest pieces would become #1 and #7, the next two become #2 and #8, then #3 and #9 etc. After arrangement each segment is numbered for final assembly. Failure to weigh the pieces could result in a blank which is so out of balance that it will shake the lathe while turning.
The following method allows the staves to be taped together and rolled up like a tambour desk top, then stood up on end for clamping. Strips of heavy tape (such as duct tape) are placed sticky side up on the bench or horses. The staves are arranged, small side up, and pressed down to the tape. The tape must extend a foot or so beyond the last piece on each end, so cut the tape two feet longer than the circumference of the column.
Glue is dispensed into each v-groove and one outside edge, and spread with a pointed stick. By pulling together the ends of the tape, the tube can be rolled up and held in place by the tape. Now the tube is stood up on end and the chain clamps applied, starting in the middle. While the glue is still slippery it is critical to tighten the clamps gradually and wiggle the tube so the pieces settle into a perfect circle.
After all the clamps are tightened, it's best to pick up the whole column and place it on a couple of sticks (Still standing up). This allows for better air circulation and will get the tube out of the puddle of glue which forms on the floor.
Often I am asked why I assemble the columns standing up vertically. There is some inconvenience having to climb on a ladder to place the upper clamps, but in my experience this method is the best way to achieve circularity. It is most efficient to spread the glue with the wood lying down flat, but to stand the tube vertically for clamping.
MOUNTING THE WORKPIECE ON THE LATHE
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The blank is made 1/2 to 3/4 inch longer than the finished size. This leaves about 3/8 to be machined off each end, which is enough to clean up any errors of alignment. The ends must be plugged from the inside one or two inches back from the end to allow for machining. [see diagram] This will yield better results than placing caps over the ends, or using tapered plugs, and if you follow this procedure, it is less work than other methods.
Use plywood for end plugs. Bandsaw discs and mount them on a face plate. Turn the disc to diameter, and make a groove for the screw heads to seat, and a center hole. [see diagram]
When mounting the end plugs, remember that the depth of penetration of the screws into the tube is somewhat critical, as you do not want to hit any screw points with your chisel. Washers can be used to control this during the installation. The exact squareness of the plugs to the axis is not too important, as this will not affect the centering much.
With the end plugs in place, and center holes already made in them, there is not much problem putting the column on the lathe. Use pairs of opposing wedges (at each end) to support the workpiece at the exact height of the centers, then apply the tailstock center and remove the supports.
If you have done your job of weighing each piece and placing them appropriately, the work should now spin with little vibration or shaking of the lathe. I cannot stress too strongly, however, that to do this kind of work you must have a lathe which can be made to run at very low speeds. For 12 inch columns, use about 300 to 500 rpm. Turning heavy workpieces can be dangerous. If your glue job is not good, or if you use too high a speed, your workpiece could come apart violently while turning.
TURNING
After roughing out, check the end diameters.
Next, face each end. [see photo] When facing the ends, the tool rest is set perpendicular to the axis, and an ordinary bowl gouge is used.This important step makes the column the correct length, and assures that the ends are perfectly flat, square to the axis, and therefore parallel to each other.
Face one end and measure to the opposite end and draw a mark. Face the other end up to the mark. Check the flatness of the facing often with a straight edge.
If there is an astragal or collar, do this first in order to establish the diameter below it, which is usually 5/6 of the diameter at the bottom. Follow the rules for entasis: the bottom 1/3 of the column is straight (use calipers), then curve gradually to the top. This can be checked with a long straight-edge made from 3/8" plywood. It is easy to see high spots and work them down. If the plywood rocks smoothly along the side, then you're done. If you make a template of the entasis it only has to be as long as the upper 2/3 of the column. Often I use a plywood template which has entasis on one side and is straight on the other side. The template may be used while the work is rotating, but for the final check it is best to have the lathe stopped. Allow for sanding. The best kind of sandpaper is floor sanding paper (8" wide, made for drum machines) in sheets from the local hardware store, or more economically in rolls if you can make the investment.
CAPITALS AND BASES
Caps and bases are glued up as necessary, band sawn, and turned on a faceplate. Bowl gouges are called for here, although small details may have to be scraped. Be sure that the details come together with the diameter of the tube so the assembly will look like one piece when painted. I should add that there have been many variations of these details in an attempt to adapt column design to wood instead of stone. Clearly the grain of the shaft is vertical but the grain of the base and cap are horizontal, and because of this, certain design considerations must be made. For example, to avoid fragile end grain, shafts often are not flared at the bottom but set upon a cove which is part of the base. (See figure) Finally, caps and bases must be drilled to provide air circulation into the shaft.
ASSEMBLY
Do not make the mistake of cutting a recess in the base to receive the shaft.
This only provides a place for water to accumulate, and it looks unsightly
after the wood has shifted from weather changes. The base and cap should be
flat where they meet the shaft. They need to be attached only loosely, as gravity
holds them together. Shifting of the pieces will occur from seasonal expansion
and contraction, and fastening them together too firmly could cause the parts
to crack.
CONCLUSIONS
Thousands of years of column design have brought us to the point where the
various styles have been mixed and matched in nearly infinite ways. Yet the
original classical columns in Greece and Rome have endured the ages and can
be measured and marveled at even today. Why has this single architectural form
been used over and over throughout the history of western civilization in spite
of the fact that architecture and materials have changed greatly? I think it
is because the column is a form of functional sculpture which provides many
things at once: Loftiness and formality; smooth continuity combined with fine
detail; columns in rows define the architecture; and of course we need some
way to hold up the roof.
BIBLIOGRAPHY
Benjamin, Asher. The American Builder's Companion   1827, reprinted by Dover Publications
Vitruvius. Books on Architecture   c.100 BC, First published translation in 1914, Harvard University Press, Reprinted by Dover Publications, 1960
Ware, William R. The American Vignola   originally 1865,
reprinted by Dover Publications