Me either!BadDog wrote:What you describe is indeed severe wear, but I'm still not seeing where the force moving the cross slide deeper into the cut is coming from. If you approach the cut properly (feeding inward to take up the slack in this case), then the thread should be seated against the far side of the nut, presumably in the wear surface that it has created. Even if there is a ramp ramp that would be traversed thus pushing the nut and attached cross slide toward the axis should the screw be forced sideways in the nut, what force is pushing it sideways hard enough to overcome friction AND the step in order to shift it outward and up onto the step?
It still feels like there must be something else going on here because nobody has explained what force is acting to overcome both the cutting forces that generally push the tool away AND friction in the slide. And apparently doing so simply by starting the spindle before the cut is entered. The only guess I have at this point is that the maybe the excessive wear is somehow causing the screw to bind and "wind up" somewhat, then unwinding when vibration causes the friction to drop? Or maybe the screw has a bind that rides in the wear groove as you wind in the cut, then likewise the vibration lowers the friction allowing the screw to drop down (or up, or sideways) onto that step you describe in the nut?
I suppose another possibility is that the worn shaft at the taper housing allows the screw to be forced offcenter a degree or two, forcing the cross slide to move obliquely forward due to tool pressure. Or some lateral moment drives the outside diameter of the screw threads into the ramp action Spro mentioned. This would be equivalent to pivoting the tool into the work, deepening the cut. But the tool itself isn't moving in the tool post holder, it's the cross slide -driven by whatever mysterious force is set up in the nut's uneven thread shape, that is causing this.
Also, remember, during the cut itself, thetool floats several thou in and out on the work. Again indicating the threads and the nut aren't holding a constant pressure.
I like Harold's early comment that weird, unpredictable reactions occur when the outer diameters of thread and nut come in contact with each other, because of wear. I've done a bunch of other cuts with smaller and larger depth of cuts. The results sometimes are truly bazarre. E.g. Dial .005 on the machine, cut .020, etc.
Thanks !!
Glenn