Should my rotary 3D finish passes run along the length or around the cylinder?

What the Raster Angle means on a cylinder

In the 3D Finish Toolpath, a Raster Angle of 0 gives passes mainly parallel to X and 90 gives passes mainly parallel to Y. In a rotary job one of those axes is the length and the other is rotation, so:

How to choose

Around the cylinder (the part spins during each pass, the gantry steps along the length):

• One machine maker recommends this for finishing (they quote 90 or 270 on an Along X machine) because the passes always cut across the surface and the seam is less pronounced.
• The stepover is a real linear distance, so it is the same everywhere regardless of diameter.
• The cutting motion is all rotary. If your rotary axis is slow, the job is slow.

Along the length (the gantry travels, the part turns a small step between passes):

• The cutting motion uses the fast linear axis. The official guide reminds you to consider which axis rotates, because the rotary axis may be slower than the linear ones.
• The stepover is now a small rotation. On thin sections those steps land closer together than on thick sections, because wrapped paths compress toward the axis.
• Each pass ends near the chuck and tailstock, so keep the boundary clear of them.

If you have no other reason to pick, start with passes around the cylinder for finishing and judge the result.

Steps

1. Open the 3D Finishing Toolpath and choose Raster under the area machining strategy.
2. Enter the Raster Angle from the table.
3. Pick the Machining Limit Boundary (Model Boundary, or selected vectors to stay off the tabs).
4. Preview, and compare the estimated time for both directions before you commit.

Pro tips

• Sources do not agree on a single best direction. A machine maker favors passes around the part for finish, and the official guide only notes that the rotary axis may be slower. Your slow axis decides.
• For stepover values see the entry on rotary 3D finishing setup.