If your first finishing pass is pretty aggressive then it may be worth it to run it twice. If you do a clearance pass with a bit not too much larger in diameter than the ball end bit you finish with, then I wouldn't both doing the finish pass twice.
I would not make that assumption. Any moves that cause rapid direction changes can also cause flex in a structure, even if the cutting loads are low, the inertia of the moving parts is the main culprit (blame that Newton guy”s first law :smile. You can lower your acceleration rates and speeds, but that has a adverse impact on cutting times, and will not “fix” a non-rigid machine. It also means you are not cutting fast enough for the chips to carry away heat, causing burning, overheating and reduced bit life.Can I assume that errors produced by flexing of the machine's structure during a roughing cut can be effectively removed by using a finishing cut (or even two) designed to put much less stress on the framework.
Just to add, the primary reason for me going with the SBR20 on my build is the added rigidity that they add with their support. If you are looking for savings, both ebay and aliexpress have bundles for all 3 axis that are fairly cost effective. I got a bundle deal on ebay, with one caveat. I decided that the rails that came for the X axis weren't long enough, so I did replace those with longer ones to get a few more inches of travel.If you have not purchased the unsupported round rails for your future upgrade, please look at SBR type supported rails. They are relatively inexpensive (compared to profile rails at least). Look at the build @MikeMa is doing. Any type of unsupported rail will always have more flex than a supported one.
Has not been an issue on mine.I'm also concerned about differential expansion of the steel rail v the aluminium support. Temperatures in my shed vary between about 10C to 50C so this may be an issue.