This instructable shows how we built a spiral staircase using CAD, CNC, and hand sculpting techniques.

Step 1: Design

A friend of mine was building a custom home and wanted a one of a kind spiral staircase. He had determined the rise and run and how each piece would bolt together forming a helix with a 10” hole in the middle. Each tread would bolt to the one before it using long bolts and threaded inserts. Before we started the building project, we took the CAD model and added curves to every surface that wasn’t touched by feet. What we came up with was very organic looking and became the sculptural element to unify the three floors.

Step 2: Efficient Use of Material

Once we had a model finished, we had to figure out how we were actually going to build it. Luckily, he owned a machine shop with a CNC machine we could use. All of the wood we used came from a 150-year-old matchstick factory in New England. After the wood was delivered it was measured and put into the computer model to show the most efficient use of materials.

We were then able to cut the reclaimed lumber into the sizes we needed and glue the blocks together to load into the CNC machine. Since the reclaimed lumber had bolt holes and blemishes all over it, we either hid or accentuated them adding to the stair's character.

Step 3: CNC Milling

Now that we had the blanks glued up we could program the machine to cut them out with robotic precision. Using the model of the final stair tread combined with the model of the wood blank, we were able to use the CAM software to write tool paths to remove excess material leaving only our curved stair treads.

Step 4: Hand Sanding

After the stair treads got out of the machine they were still a little rough. Since this project was in Maine during the winter, the sanding would have to be done inside. We set up a little pop-up and put up some plastic walls. Sanding and grinding is a messy job and dust got everywhere, but the Dexter-style kill room helped contain the dust quite a bit.

Each tread came out of the machine looking like a topographical map. All curves were actually made of different layers with a bunch of "steps" that needed to be smoothed out. Using black spray paint, we could see exactly how much we had sanded so we could sand everything evenly. Very aggressive sanding discs were used to get the black lines to where they were close to disappearing. Then the medium grit was used to make the lines almost disappear followed by the fine grit until the black paint was all gone.

Step 5: Final Glue-Up

After all the treads were sanded they were lined up and organized by color and grain pattern. Then it was time for a dry fit and assembly. Each tread was pinned into the concrete with a stainless steel rod. The rod was epoxied into the stair tread, then epoxied into the concrete, as well as epoxied to the tread before it before tightening the bolts. It was a pretty complicated glue-up but once we got the process down it flowed smoothly. One issue we had was the concrete epoxy going off too fast in the mixing nozzle. We mitigated that a bit by putting it in a bucket of snow between glue-ups, slowing down the chemical reaction.

Step 6: Finishing Touches

After the treads and landings were all in place it was time to have some more fun. We weren't sure what we wanted the transitions to look like and to model something in the computer seemed counter productive. For transitions, we left plenty of wood to then be sculpted by hand. CAD and CNC are great for so many things, but certain things are better if done by feel. We decided that the most simple curve would be the most elegant. One transition was done in a matter of hours one day, the second on the following day, then both were touched up at the end of the second day. It would have taken longer to model what I did in the computer than it was to make it in the physical world. Plus, when I tried to come up with something in CAD there were too many options. When presented with the raw form in real life, it was clear what needed to happen.

When it was all sanded and felt great to the touch it was time to hang plastic on the walls and oil the stairs. We used a product called Waterlox that worked great. If you've ever oiled a nice piece of wood, you know how satisfying it is to completely transform a wood project. It brings out all of the grain, deepens all of the colors, and lets the true character of the wood shine through.

In the end, this project was a huge success. The rest of the home is unique and stylish. There's a strong industrial feel with a lot of exposed concrete and steel, and this wooden spiral staircase with a TON of curves stands as a great contrast.

Many hands and minds made this project so successful. Huge thanks to Tom West, Tim Bickford, all the guys at Limerick Machine Co., and Doug Ruuska.