Today I was playing with some techniques for making a square blank into a round blank on the rotary. I know that if you move primarily in the X axis (mine is set X+), rotating a bit between passes, you end up with an N-gon instead of a cylinder (i.e. the result is faceted).
So I tried moving primarily in the A axis - spiralling from X0A0 to X5A12960 (or the equivalent). After tweaking that a bit, and pondering the not-flatness, it occurred to me that router bits are not flat on the bottom - the cutting edges are angled a bit so that the center doesn’t rub/burn. But that means if you run the bit right at Y=0 you actually mill a triangular spiral, like a very flat thread, because the center of the path is slightly higher.
I ended up offsetting the bit in the Y direction a little, and using smaller passes, so that I’m milling away from the center of the bit, but it’s not perfect.
I suspect there’s no intersection of “normal router bit” and “desired cylinder” that’s straight… so is this a fool’s errand? Or is there some magical bit or technique that will do this job correctly?
You can get flat bottom tools.
The best way I’ve seen this done is using the spiral toolpath you mentioned, but with a fairly large flattening bit. With a small stepover, it presents a large flat cutting edge that peels away the outside layer of the desired cylinder.
Vectric has a couple of gadgets that do a nice job with a rounding toolpath on a rectangle. The first couple of cuts take off the corners and then it does a spiral toolpath for rounding. Certainly, a piece of cake for Fusion too.
I found the best method for creating rounds it to use a pocket based on the corner to corner of the square dimension, cutting to final round diameter in 3 passes. The first pass will cut some air as it cuts off the corners. The final cut is smoother than using a spiral cuts or offset cuts with a planer bit.
The nice thing about a pocket is you don’t have to allow for the bit offset.
I use a 1/2" 3-flute spiral bit.
Most of my rounds are used in making two piece walking sticks (see Vectric’s Aspire Gallery). I started making sticks in 2000, these were profile cuts and full length.
The last 15+ years they are 2-piece with 3d art carvings. I have completed hundreds of them, gifts to people with medical needs without charge.