Fig. 589.
In [Fig. 589] is shown a star feed applied to a slide rest. a is the slide rest, upon the end of the feed screw of which the star, b, is fitted. c is a pin attached to the face plate of the lathe, which, as it revolves, strikes one of the star wings, causing it to partly rotate, and thus move the feed screw. The amount of rotation of the feed screw will depend upon the size of the star and how far the circle described by the pin c intersects the circle described by the extreme points of the star wings. Thus the circles denoted by d e show the path of the pin c; the circle f h the path of the star points, and the distance from f to g the amount which one intersects the other. It follows that at each revolution of c an arm or wing of the star will be carried from the point g to point f, which, in this case, is a sixth of a revolution. If more feed is required, we may move the pin c, so that it may describe a smaller circle than d e, and cause it to intersect f h to a greater extent, in which case it will move the star through a greater portion of its revolution, striking every other wing and doubling the amount of feed.
It will be observed that the points f and g are both below the horizontal level of the slide rest’s feed screw, and therefore that the sliding motion of the pin c upon the face of the star wings will be from the centre towards the points. This is better, because the motion is easier and involves less friction than would be the case if the pin contact first approached and then receded from the centre, a remark which applies equally to all forms of gearing, for a star feed is only a form of gearing in which the star represents a tooth wheel, and the pin a tooth in a wheel or a rack, according to whether its line of motion is a circle or a straight line.
It is obvious that in designing a star feed, the pitch of the feed screw is of primary importance. Suppose, for example, that the pitch of a slide rest feed screw is 4 to an inch, and we require to feed the tool an inch to every 24 lathe revolutions; then the star must have 6 wings, because each revolution of the screw will move the rest 1⁄6 in., while each revolution of the pin c will move the star 1⁄6 of a revolution, and 4 × 6 = 24. To obtain a very coarse feed the star attachment would require to have two multiplying cogs placed between it and the feed screw, the smaller of the cogs being placed upon the feed screw.
Fig. 590.
In many lathes of European design, the feeds or some of them, are actuated by ratchet handles, operated by an overhead shaft, having arms which rock back and forth. Thus in [Fig. 590] is a lathe on which there is provided at a crank disc, carrying in a dovetail slot a pin p, for rocking the overhead shaft from whose arms a chain is attached which may be connected to the ratchet handle shown on the cross-feed screw, the weight being for the purpose of carrying that handle down while the chain pulls it up. To regulate the amount of feed the pin p is adjusted in the slot in a, or the chain may be attached in different positions along the length of the ratchet arm, the weight being provided with a set screw so that it may be set in any required position along the ratchet arm.