Andy Pugh wrote: > 2009/6/9 Frank Tkalcevic <[email protected]>: > >> How much "backlash" would the rubber teeth on the belt contribute? >> > > "Some" I guess. Though it might matter less than you think. In fact, > that thought has been occurring to me throughout this thread. There > has been a lot of concern over absolute accuracy, when repeatability > is probably more important in the real world. I can't think of many > things where dimensions over a couple of feet have been important to a > few thousandths, and in cases where they are, if the two parts are > made on the same machine then they will fit anyway. (And it has been > a source of some frustration to me watching machinists fretting over > the last n-th of accuracy on a part I wanted yesterday, and where I > have specified +/- 5mm "I don't care" accuracy. ) > > Similarly with a resilient belt or "springy" ballnut, it will always > sit in the same place for the same cutting force, so if you have a > light finishing cut you will probably get better accuracy than the > ballscrew spec suggests. (or even if you have a consistent roughing > cut) > > You will, however, probably get more chatter and vibration, but then a > visco-elastic element might damp vibration that would be there in a > fully rigid system. > > (Note, I am not a machinist, I am not even really an engineer, but I > play one on the internet) > > I seems to me that there is little or no pressure on the screw or belt when roughing because you are generally climb milling. If there is to be a load on the belt or screw it will be when finishing when you are usually conventional milling or when excellerating and decelating. Any load there will be on screw or belt is when the cutter tries to pull itself into the work. I think this may be where a screw has an advantage. A good machinist climbemills as much as he can to take the load off the gearing. This assumes a climb milling attachment on a manual machine. Doug
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