Introduction: Nested Helix
In this Instructable I’ll explain how to make nested helices (spirals) from wooden dowels or sticks. A few years ago I came across this article in Make Magazine that describes a spiral cutting technique first published by Steve Garrison. I was very intrigued, but other projects kept me busy. Finally, I decided to design and build my own jig to cut these spirals.
(Edit March 2021) Here is the latest version of the spiral cutting jig:
Step 1: Basic Concept
The basic idea is to use a scroll saw to cut a dowel lengthwise. If you turn the dowel while cutting it, you will end up with a spiral cut. Some people might be good enough to do this by hand, but I am too klutzy to do that. You have to be very steady and accurate, otherwise the two parts will not separate (unscrew) after the cut, or they’ll break apart while cutting. Also, if you want to do several consecutive cuts on the same dowel you need to be very precise.
I decided to build an Arduino controlled jig that uses two stepper motors: one to advance the dowel into the blade, and a second one to turn the dowel around it’s long axis. By adjusting the speeds of the two motors independently, you can make a spiral screw of any pitch. I define pitch as inches advanced per revolution. A 1/4-20 standard machine screw takes 20 turns to advance one inch; so per my definition it has a pitch of 1/20 inch per revolution. You will need a spiral blade for the scroll saw; i.e. a blade that cuts in every direction. This is very important! I use Olson #0 spiral blades. Since this is a very fine (46 TPI, 0.032 kerf) blade; and since I am cutting very, very slowly (20 to 30 minutes for one 10” long spiral cut), the resulting cut is very smooth and requires no sanding.
This technique WILL NOT WORK ON A BANDSAW as there are no spiral blades available for the bandsaw. Repeat: it will not work on a bandsaw! You might be able to set up a jig with a Dremel motor or router and a small cutting bit; but your kerf will be much larger.
Step 2: Jig
I will not go into great detail about the jig; that may be a future Instructable if there is sufficient interest. The pictures and movies should be helpful. A brief description of the jig is as follows.
The central part is a 26” long sled that is driven towards the scroll saw blade by a stepper motor and a 1/4-20 threaded rod. As the stepper motor turns the threaded rod, the sled is pulled forward. Inside the sled, the dowel is mounted similar to a lathe. A second stepper motor turns the dowel around its center axis at a speed controlled by the Arduino program. The sled is mounted into a 30" long outer frame that clamps securely to the scroll saw table. When the system runs, the inner sled advances the dowel into the scroll saw blade while the dowel is being turned. Choosing the advance speed and the turning speed gives you a great deal of flexibility over the shape of the spiral.
The stepper motors are controlled by an Arduino micro-controller. Programming is done in the Arduino IDE environment.
To use the jig, you mount the dowel into the sled, install the spiral blade, and clamp the jig to the scroll saw table. Tilt the table if desired. Position the jig so that the blade enters the dowel at your chosen offset from the centerline. Then you start the scroll saw and initiate your program on the computer that is connected to the Arduino. The program contains your desired parameters of pitch, cutting speed, and length of the cut.
Step 3: Centerline Cut
If you cut the dowel along its center line, you get two pieces which thread around each other beautifully. To get a center cut started drill a pilot hole through the dowel about 1/2" from the starting edge, then position the jig and insert the scroll saw blade through this hole.
Use hardwood like walnut, cherry, maple, etc. to create a strong spiral set. Poplar dowels are cheap and will work, but they can be more fragile. I have used dowels from 1/2" to 1.5" in diameter.
The dowel may be round, square, rectangular, hexagonal, or octagonal. A square-dowel center-cut spiral will be very confusing to people, because it looks like ‘it should not unscrew’. But it will! It’s a pretty cool toy to play with.
Step 4: Offset Cuts
By making cuts that are offset from the dowel center axis you can ‘slice off’ several spirals that wrap around a center spiral ’screw’. My jig can handle dowels up to 1.5 inches in diameter and 10 inches long. I have been able to cut four outside spirals, one inside spiral, and a center spiral screw. That is six separate pieces wrapped around each other! The resulting wood spirals are a little fragile and should be handled carefully.
Step 5: Angled Cuts
You can tilt the scroll saw table to get even more variations on the shape and look of the spirals.
A special case occurs if you tilt the table so that the blade enters and exits the same cut line. That tilt angle depends on the offset from the dowel centerline, the dowel diameter, and the pitch. The math gets complicated but I worked it out. For example, a 5/16" offset on a 1-1/4" dowel with a 0.8 inch per revolution pitch needs a 25.3 degree table tilt to meet this requirement. The offset parameter is difficult to set accurately, and the results are very sensitive to the offset; so some trial and error is advised. The result is a beautiful set of nested spirals like these.
Step 6: Dowels
You can make your own dowels from hardwood boards.
My dowels are 12 inch long, resulting in 10 inch long spirals. Some wood species cut smoother than others. Poplar, maple, cherry, and walnut cut really well. Pine and oak do not work well with the #0 blades; they leave a jagged/uneven edge and you may not be able to separate the spiral pieces. In that case use #2 or #4 blades for a more aggressive cut and larger kerf. Avoid wood pieces with knots.
You can add interesting designs by laminating different colored woods. Cut your dowel blanks on the table saw to make a square, rectangular, hexagonal, or octagonal cross section; or turn round on a lathe. Then cut your spirals; the results will look amazing!