I've been salvaging old douglas fir beams from building demolitions for carving. Douglas fir takes on
this wonderful texture when it gets old: it becomes more consistent,
less difference in hardness between the rings, and it's easier to carve
across the grain than new fir. With a sharp chisel, you can make big,
bold cuts. Very satisfying. These two mobius carvings were early projects from reclaimed fir.
First attempt -- about 6"
It was a bit tough to visualize how the surfaces wrapped around the shape, so I started with models. First, I made a donut from modeling clay, drew the corners as they wrapped around the shape, and then scraped a flat plane between the lines. Next, I built a series of models in Sketchup -- a bit laborious, but it helped a lot to see what needed to be done.
To make the small mobius, I started with a donut with a round cross-section, traced the corners as they wrapped around the part and carved a flat surface between the lines, being careful to measure the flats radially. The larger one was harder: I made it in two halves, doweled together. The majority of the shaping was performed on each half separately, then the finish work was done with the part glued together.
I always knew that the big challenge would be cutting
internal threads. What cutter? How much extension? Speed? How to manage chatter
and deflection? The
criteria was that I wanted to cut threads a half inch deep, at least 3 inches
into a bore. For this I needed a 60 degree side cutting bit that I could extend
3 inches past the end of the router. Good luck finding that router bit…
My solution was to create an offset spindle with a half-inch
shaft that holds a fly cutter. The spindle is made up of a couple of bearings and a .515" ground stainless steel shaft, turned down to a half inch on the ends to fit the bearings. The spindle frame and router clamp is made of birch ply. The spindle is hinged so it can be swung free to check the fit of the bore. The spindle is driven by a vacuum cleaner belt off the router with a 4:1 speed reduction. I bought a cheap Harbor Fright trim router to run it, just in case this is hard on router bearings. (better to trash a $30 tool than a $130 tool). I also got one of their router speed controls to slow it down. (I seem to be buying a lot of cra stuff from those guys lately)
speed control
spindle
.
In order to keep the rotating weight down, I
used 1/4" drill rod for the fly cutter, inserted at an angle through the mandrel, and ground so that
each end only cuts one side of the thread groove.
cutter layout
Grinding the tool bit on my other wood lathe. It was actually much
easier than expected to get the angles and correct extension on the
cutter. The only problem was the drill rod I was using turned out to be
stainless, not heat-treatable tool steel. It holds an edge - kinda.
Grinding the cutter.
To grind the cutter, I slowly spun the lathe spindle by hand while cranking the cross-feed vise back and forth. The cross-feed vise is set at 30 degrees to the cutter. It took a different setup for each end of the cutter. This fixture only ground the angle. All back relief and rake on the cutter were ground by hand & eyeball.
You can hear the harmonic vibration from the stretchy belt, right before the cutter exits the end of the bore. This is a problem I'll have to confront before I can increase depth of cut and cutting speed.
Here's a better video made after the outside of the nut was formed.
Lessons learned:
Needs mass. There's a fair amount of vibration that would probably be dampened by more mass in the router carriage and spindle. I was planning to rebuild anyway after this initial test, I'll just beef things up a bit more.
Stretchy belt -- not so good. I'm getting a significant harmonic vibration, especially when cutting across end-grain that I think comes from belt stretch. (you can hear it in the video, right before the cutter exits the end of the bore) I can dampen it somewhat by putting my finger on the belt. I'll look into a better drive belt to replace the $2.99 Hoover belt I used. More mass in the router carriage may help this as well.
Cutter needs rake. I just ground a flat cutting face on these cutters, so there's zero rake on the cutting edge. I think if they had a little rake ground in they would cut better -- slice the wood rather than scrape.
Here's the Sketchup model I used to work out the mechanics. The lathe bed is the angled piece with the t-slot on the right. The two rails on the left are 1.5" x 3" aluminum rails from a discarded telecom equipment rack. The big angle pieces on the bottom are the base of the same rack.
The assembly in the upper left is the feed carriage. The whole unit runs on skateboard bearings by a 1/2"-13 feed screw (not shown in the model, but visible below). On top of that is a z-axis-like router carriage that slides on 1/2" rails, driven by a 1/4"-20 screw to move the router in and out. Stepper motors will rotate the lathe spindle (not shown), and the 1/2"-13 feed screws. The router's depth of cut will be manual, for now at least.
After I got it all assembled, I discovered the angle I used for the base wasn't square, so I
have to shim everything that attaches. Re-using the aluminum network
rack seemed like such a good idea, but I've made a series of compromises in strength, wear and accuracy, in order to use it. Once I get everything figured out, I may rebuild the rails using something better suited.
