For milling OTHER than cutting straight through, how do you avoid getting a staircase effect ? e.g. a surface that has a slope of 1:8 would have 1/8 inch difference in X/Y position for each 1/64 inch change in Z axis. These "steps" may or may not matter in most applications, but if you wanted to avoid/remove them how would you do that ? Hand sanding seems too laborious, is there something like a sanding flap wheel for routers/dremels that could be used for a sanding/polishing pass ?
I'm not totally sure about your question. But I think the answer to it is that there are multiple types of tool paths. Not everything is cut out z level by z level. For things that are not flat you would usually start with a roughing cut done in flat planes stepping down in z. You would then follow that with another type of tool path depending on the shape and what's available in your CAM software. The most common would be parallel finishing. This runs the tool over the part in a series of parallel lines. This will still leave small ridges on the part but you can specify the "step over" or the distance between the parallel lines. The quality of the finish will be affected by the tool being used and the amount of step over. Smaller step over usually results in a better final finish at the cost of time. A side note. A ball end mill is not always better for curved parts.
Thanks to both of you for replying. I thought I had convinced myself that a ball or other rounded end bit would do it, but later thought that a large movement in X and/or Y between layers would cause the Z step to show whatever shape the cutting tool. I can see that running up and own a slope with step over could smooth things and it shouldn't be too hard for tool path software to do that as some function of slope angle - given that it "understands" the 3D model. For anything I'm likely to do this really doesn't matter, I was just curious about it. BTW, it really SHOWS on most 3D printers and so far the only solution I have found is to use thinner layers, but of course that means extending the print times and those are already horribly long anyway. Another trick with ABS plastic is to use an acetone wash after the piece is finished, analogous to hand sanding, but I digress.
I do quite a bit of 3D model cutting, I get the best cuts using a 1/8 or 1/6 tapered ball nose bit with a very small step over 10% or less.
