Fig. 3172.
The rolls 5 and 6 are idle rolls, and are set to just relieve the work from undue pressure on the work table.
By this construction of feed mechanism the following ends are attained. First, sufficient feed power for heavy cuts is obtained without driving the lower rolls. Second the work is held to the table on both sides of the cutter head, hence there will not be left on the end of the work the step that is left when but two upper and two lower rolls are used, and which occurs because the work falls after leaving the feed rolls, whereas, in this machine the work is held to the table by rolls 2 and 3.
The cutter head h, [Fig. 3170], has in front of it the pressure bar p, whose lever is shown at l and the weight at w. On the delivery side of the cutter head is a pressure bar r, which is acted upon by a spiral spring in the box c. In the engraving to the right of [Fig. 3170] the knife k is shown in action on a piece of work, and it is seen that the end of the pressure bar p coming close to the edge of the knife prevents the pressure of the cut from splitting or splintering off the end of the work at a, and therefore acts as what is termed a chip break. Furthermore, the sides of the cutter head between the knives being hollowed out gives the shavings s room to curl in and prevent the work from splintering at the end when the cut is terminating.
Balancing Cutter Heads and Knives.—Planer knives must be balanced as accurately as possible, in order that they may run steadily and smoothly, and therefore produce smooth work.
The first requisite for proper balancing is that the cutter head itself be properly balanced, and in order that this may be the case the faces forming the knife seats must be equidistant from the axis of the cutter head, and the journals must run true, being best tested on dead centres. The holes for the cutter bolts should all be drilled to the same depth, and tapped equally deep. The faces or seats for the knives should be parallel one to the other, and this may be tested by a pair of straight edges, one pressed to each face and the width between them measured at each end, or if a long surface plate is at hand, one face of the head may be rested on the surface plate, and the straight edge ruled on the other face, and its distance measured from the surface plate at each end, with a pair of inside callipers delicately adjusted.
A straight edge rested lengthways along the knife seat of the head and projecting over the journal will show whether each knife seat is equidistant from the journal as it should be, the measurement being taken with a pair of inside callipers adjusted to just sensibly touch the journal and the straight edge. This measurement should be taken at each end of the head.
In all tests made with straight edges, the straight edge should be turned end for end and each measurement repeated, because, if the straight edge is true, turning it end for end will make no difference to the measurement, while if the straight edge is not true the measurement will vary when the straight edge is reversed.