I'll have to look up the article but in one of my old router books it has plans to build one using a bicycle chain and gears. It's more like a Sears router crafter. It's not a lathe. I was stayed with a turner at my last demo in Kingsport TN and he was building one that fits on a lathe. He got a video from someone online that had all the plans. It was complicated but looked like it would be easy to use once you had the instructions for actually setting it up.
I'll try to remember to look up which router book it's in. I have several.
Here is one of the lamps am working on having not done any spiral turning in many years. Also completed potpourri vase.
I get pitch through layout lines on a piece of wood before doing anything else. Also make mental note of waste allowance, coves or beads before and after spiral work. I turn beads or coves before cutting spirals
Some people use graft paper (1/4” squares) to draw lines and wrap around whatever they are making. Some folks just use masking or painters tape. Diameter and length go into factoring pitch whether doing spirals by hand or with other tools.
Photo one is example of layout lines:
Red lines are do not cut lines
Blue lines pitch lines
Black lines my start and stop lines
Green lines, cut lines for valleys
Orange (looks red) top of my bines
Layout is the same whether going to open up spirals or leave them closed. You do have to drill center for open spirals.
Learned from Stuart Mortimer’s book Techniques of Spiral Work, pretty expensive if can find it. Less expensive book by Bill Bowers, The basics of Turning Spirals, does not interest me.
Lot of spiral turners do the math in head and do not draw layout lines on their work.
Look at the Legacy Ornamental Mill. They do what you are talking about....and much more. I have a model 900. It will do rope twists, barley twists, and open barley twists easily. There is a five part video on YouTube on how to use the legacy to make an open barley twist candle holder. Do a search for Legacy Ornamental Mill and you will find them.
Really like legacy Mills 900 & 1200 watched a few You-tube and training videos at this site machine is pretty impressive. Guess apprehensive learning set up, using machine, and different router bits. Think will stick with what I know.