COURSE IN WOOD TURNING - CHAPTER XI

SPIRAL TURNING

Spiral turning is a subject that has received very little attention by most schools in which wood turning is taught. Spiral work is seen in antique furniture and also in the modern furniture of the present day. It seems that it takes the wheel of fashion about a century to make a complete turn, for what our forefathers neglected and destroyed the people of the present day value and cherish.

Spiral work gives excellent practice in shaping and modelling wood. It brings into play the principle of the helix as used in cutting threads, etc.; and its form, size and shape may be varied according to the taste of the individual. As in threads so in spiral work we have single and double spirals, and their form and proportion depend upon their use and application in furniture making. A variation of the spiral may be made in several ways: First, by changing the number of turns of the spiral on a straight shaft; second, by running a spiral on a tapered shaft; third, by changing the shape or form of the spiral itself; and fourth, by making more than one spiral on a shaft. It is uncommon to see ten or twelve spirals running around a single shaft.

Some of the forms of the above types are fully taken up and explained in the work that is to follow.

PLATES B-V--1-a, B-V--1-a’. SINGLE SPIRAL. STRAIGHT SHAFT

To work out a single spiral for a pedestal proceed as follows:

1. Turn a cylinder 2¼" in diameter. Make the ends slightly larger in order that the design may be turned on each, after the spiral has been worked out.

2. Lay off spaces 2-1/16" apart on the cylinder while the spindle is turning in the lathe and divide each of these into four equal parts. Each one of these large spaces represents one turn of the spiral. A good proportion is slightly less than the diameter of the cylinder; thus the diameter of the cylinder equals 2¼" and the width of the space 2-1/16".

3. On the cylinder parallel to the axis draw lines A-A B-B C-C D-D. These lines should be 90° apart as shown in the top diagram (Plate B-V--1-a’). Line D-D is on the other side of the cylinder as shown in the top and middle diagrams.

Fig. 23.

4. Start on line A-A at point X, circle 1, and draw a line connecting it with line B-B on circle 1’. Then connect B-B on circle 1’ with C-C on circle 2 and so on until a spiral has been drawn the entire length of the cylinder. This line will form the ridge of the spiral as shown in the middle diagram.

5. Next begin on line C-C at circle 1, and draw a line connecting it with D-D on circle 1’ then to line A-A on circle 2, and so on as before. This spiral represents the center of the groove or the portion which is to be cut away. This is not shown in the diagram because more or less confusion would be caused with the line representing the ridge of the spiral.

6. Begin on line C-C at circle 1, and saw to a depth of ¾". Saw the entire length of the cylinder leaving about 1½" at the ends. Do not follow the line here, but switch off gradually and follow circles 1 and 15, so as to allow the spiral to begin and end gradually and not abruptly.

7. Rough out with a knife or chisel by cutting on both sides of the saw cut. Then use a wood rasp to finish shaping out the spiral. When properly shaped out allow the lathe to turn slowly and smooth with sandpaper by following the spiral as the lathe turns.

Fig. 24.

Fig. 24-a.

Fig. 25.

8. Cut the design on both ends of the cylinder and polish.

PLATES B-V--2-a, B-V--2-a’, B-V--2-a’’;. SINGLE SPIRAL. TAPERED SHAFT

To lay off a single spiral for the electric lamp shown in Figs. 24 and 24a proceed as follows:

1. Select your wood and bore a hole through it. Plug the hole and center the piece in the lathe. This insures getting the hole exactly in the center, and it will not be cut into while the cutting of the groove of the spiral proceeds. A groove may also be cut in two pieces of stock and glued together to form a hole through the stock.

2. Turn a cylinder 2½" in diameter, tapering it to 1½" at the one end; this part should be 12⅛" long. Both ends should be left larger than 2½" as the lower and upper designs must be cut here.

3. Let the spindle revolve in the lathe and draw circles as shown in the layout (Plate B-V--2-a’). The number of circles will vary with the taper. Since seven turns are needed in the present spiral, 28 circles will be necessary--four circles for each turn of the spiral as shown in the middle diagram. A good proportion to follow is to measure the diameter of the spindle at circle 2 and lay off this distance from circle 1 to circle 3. Then measure the diameter at circle 4 and lay off this diameter from circle 3 to circle 5 and so on until all circles have been made. Then divide these large divisions into four equal parts.

4. Draw four lines the entire length of the spindle, each 90° apart as shown by the heavy lines in the middle diagram. The heavy circles of the same diagram represent the complete turns of the spiral.

5. Lay out the line representing the ridge of the spiral as shown in the middle diagram. Begin on circle 1, where the straight line crosses it, draw to circle 1’ at the point where the next straight line crosses it, then to 2--2’--3--3’ and so on until the end is reached. This forms the ridge of the spiral as shown in diagram 3. Next it may be more convenient to draw another line representing the groove. In this case begin at point X in the middle diagram, opposite the point where first started, and continue in the preceding manner, making this line parallel to the other line.

6. Saw on the line last made, being careful not to saw too deeply. The depth must be ¼" less than half the diameter of the spindle where the cut is made. This saw cut forms the groove of the spiral. The groove is then cut out by hand with a chisel or knife, by working down the wood on both sides of the saw cut. After the spirals have been roughed out, a rasp is used to finish shaping them. The work is then sandpapered smooth, while the spindle is revolved slowly in the lathe.

7. Cut designs on the ends of the cylinder and polish.

PLATES B-V--2-b, B-V--2-b’. DOUBLE SPIRAL. TAPERED SHAFT

To work out a double spiral for the electric lamp illustrated in Fig. 25 proceed as follows:

1. Turn up the spindle in the usual manner. Since the base of the shaft is larger than the top, the spiral must also be in proportion and lines A-A’, B-B’, C-C’, D-D’, and E-E’, are drawn around the shaft. To get the approximate spacing from circles A-A to B-B measure the diameter at A-A’ plus about 3/16" and lay off from A-A’ to B-B’. Then take the diameter of B-B’ plus about 3/16" and lay off from A-A’ to B-B’. Then take the diameter at B-B’ plus about 3/16" and lay off from circle B-B’ to C-C’ and so on. If the shaft is tapered more, a different proportion must be used. Also if it is desired to have the twist wind around the shaft three times, a variation must be made in the number of circles.

Fig. 27.

2. If it is desired to have the twist wind around the shaft twice, draw circles 1-1’, 2-2’, 3-3’, and 4-4’ and the spaces will grow proportionately smaller at the small end.

3. Draw four lines running lengthwise on the spindle and 90° apart as shown in the midde figure in heavy lines (Plate B-V--2-b’).

4. Begin at A and draw a curved line to where the 90° line crosses circle 1-1’. From there extend the line to where the next 90° line crosses circle B-B’ at point B’. Continue in this manner until the other end of the shaft is reached. Begin at A’ and draw a line on the opposite side of the shaft. These two lines running around and along the shaft form the grooves while the portion in between forms the beads of the double spiral.

5. Saw to the desired depth, being ¼" less than half the diameter at the point where cut. With a chisel or knife form the grooves and beads. It is necessary to be careful about not ending the grooves too abruptly. (See point 6 in Plates B-V--1-a, B-V--1-a’.) Smooth with a rasp and sandpaper while the lathe is revolving slowly.

6. Cut the design on the ends and polish.

PLATES B-V--3-a, B-V--3-a’. DOUBLE GROOVE SPIRAL. STRAIGHT SHAFT

To work out the double groove spiral for the magazine holder illustrated, proceed as follows:

1. Square up the stock to 1⅜". Center carefully and turn the design on both ends as shown, in the upper diagram (Plate B-V--3-a’). Turn the cylinder between the top and bottom, making it 5½" long and 1⅜" in diameter.

Fig. 26.

2. Divide the cylinder into two equal parts. Each part represents one revolution of the spiral.

3. Divide each half into four equal parts as shown in the top and center diagrams (Plate B-V--3-a’), 1-1’, 2-2’, 3-3’ and so on. The proportion of the distance between these circles should be one-half the diameter of the cylinder.

4. Draw lines A-A, B-B, C-C, and D-D, parallel to the axis of the cylinder 90° apart.

5. With a band 3/16" wide of any substantial material (preferably a narrow strip of tin or a watch main spring) begin on the line A-A at circle 1, and connect circle 1’ at line B-B, and then connect circle 2 at C-C, and so on until the spiral is made the entire length. Mark on both sides of the 3/16" band so as to keep the spiral parallel.

6. Next begin at the line C-C where circle 1 crosses it and connect from here to 1’ at B-B. Proceed as in Step 5, as shown in the center diagram.

7. Now erase the extreme ends of the spiral near circles 1 and 5, and deviate from the original spiral and follow the circles in a more parallel direction so as to allow the spiral to begin and end gradually and not too abruptly. Refer to the lower diagram for this.

8. Cut out portions of wood between the bands previously marked around, as shown in the lower figure. The wood should be cut out with a knife so as to leave the corners sharp on the narrow bands. The portion cut out should be a semi-circle and can be sanded by making a spindle a little smaller than the distance between the bands and fastening sandpaper on the spindle. Place in the lathe and hold the spiral on the sandpaper cylinder at an angle so that the spiral will fit. Turn gradually and the sandpaper will smooth up the portion between the bands and true it up. At the ends where the grooves are smaller, use a smaller stick around which sandpaper has been wound and work out by hand.

9. It is well to cut straight down, about 1/32" deep, along the lines marking out the narrow bands. Then the wood will not be so likely to split while removing the stock which forms the grooves between the bands.

10. Cut out the mortises in the square portions which have been left at both ends. Make the frame work for the sides and cane. Glue together and polish.

Note:--By making the posts smaller and using the same construction for a side a nice looking book stall may be made. The proportions for the posts are the same as mentioned in Step 3.