Question

Explain following constructional details of CNC machines with neat sketches. (i) Hydrostatic slideways (ii) Recirculating ball screw and nut

Answer 1

(i) Hydrostatic slideways:

In hydrostatic slideways, air or oil is pumped into small pockets or cavities into the carrieage or slides which are in contact with the slide ways. The pressure of the fluid gradually reduces to atmospheric pressure as it seeps out from the pockets, through the gap between the slide and the slideways. The hydrostatic slideways almost a frictionless condition, it is very important that the fluid and slideways are kept clean. Also a hydrostatic slideways need a large surface area to provide adequate support.

(ii) Recirculating ball screw and nut:

Ball screw is also called as ball bearing screw or recirculating ballscrew. It consists of a screw spindle, a nut, balls and integrated ball return mechanism a shown in Figure. The flanged nut is attached to the moving part of CNC machine tool. As the screw rotates, the nut translates the moving part along the guide ways. However, since the groove in the ball screw is helical, its steel balls roll along the helical groove, and, then, they may go out of the ball nut unless they are arrested at a certain spot. Thus, it is necessary to change their path after they have reached a certain spot by guiding them, one after another, back to their “starting point” (formation of a recirculation path). The recirculation parts play that role. When the screw shaft is rotating, as shown in Figure, a steel ball at point (A) travels 3 turns of screw groove, rolling along the grooves of the screw shaft and the ball nut, and eventually reaches point (B). Then, the ball is forced to change its pathway at the tip of the tube, passing back through the tube, until it finally returns to point (A). Whenever the nut strokes on the screw shaft, the balls repeat the same recirculation inside the return tube.