Having secured the block to the planer, a cut is taken across the top, and, chalking or coppering the top, the width of the dovetail is scribed on the face. The stock is then planed away and the angle forming the dovetail is planed. Should the angle be 30 degrees from the vertical the head of the planer is Bet at 30 degrees by means of the graduated dial, and the flop which carries the tool should also be set way over in the same direction that the head is swung, to prevent the tool gouging in on the back stroke. Some tool-makers lock the tool so that on the back stroke the tool drags in exactly the same line as on its forward or cutting stroke. This is not good practice, however, for the back drag causes more wear on the tool than the cutting or forward stroke.
The dovetail shank is planed to fit a templet, and the angular sides must be smooth and straight, for any irregularity of surface prevents the long taper key from properly holding the block. For instance, should several ridges stand out on the angular sides of the shank due to uneven planing, the key would bear only on these ridges, and after a few blows when in the drop press the ridges would be likely to flatten, causing the key to become loose. The shoulders aa, Fig. 67, must be on a line; in order to obtain the best results, both sides are roughed nearly to the line, and the last chip is taken first on one side allowing the tool to start as close to the shank as possible and to feed out, then, without changing the elevation or position of tool, the head is moved over so that the tool is on the opposite side of the block, and a finish chip is taken. The angle sides are then roughed out, and the last chip should be light - a rigid keen cutting tool being used.
Fig. 68. Cutting Tools for Machining Die.
For roughing out any heavy work the roughing tool a, Fig. 68, is best adapted. The diamond point should not be used for heavy cuts unless the cutting point is on a line with the tool face that the tool-post screw bears against. By referring to sketch b, Fig. 68, which is a diamond-point tool, the cutting point is seen to be so far advanced from the line of tool support that any springing of the tool would cause it to dig in, as will be understood by noting the line showing the radius traveled by the tool point in springing. It will be noted that the lowest point of the radial line is below the line of the cutting surface. By using a roughing tool, as at c, Fig. 68, the springing tends to force the tool away or above the cutting line.
Having fitted the shank to the templet, the straps can be removed and the die block fastened to the bed, shank down, and the edge of the shank brought to bear against the parallel strip. The top surface is machined sufficiently to obtain a true clean surface, and the final cut should be taken with a spiral finish tool, d9 Fig. 68, for the top must have the die outline laid out on its surface, and a rough surface makes it difficult to see the scribed lines. The finishing tool when working is shown at e, Fig. 68.
One top edge of the block is machined by using the down feed to a distance of, say, 1½ inch on one side only. The block is then turned crosswise of the planer, and the top edge of one end is planed down the same distance. The machined edges of the end and side must be at right angles, for from these two machined surfaces - on both blocks - the dies are laid out, and the machined surfaces or edges are also used to set up the dies in the press by bringing the edges and ends in line with each other.
One way to square up the block so that the machined edge will be at right angles is the old-fashioned cut-and-try method - that is, to set the block as nearly as possible by using two squares, one against the cross-head of the planer, and the other along the machined edge of the block. After the first cut is taken down, a square is tryed on the block and the block is shifted a trifle, etc., until the end and the side are square - the shop term for being at right angles. This is a slow method, and a more workmanlike one is to clamp an indicator to the planer tool held in the tool post, allowing the pointer of the indicator to travel along the edge of the square blade while the base of the square is held against the side of the block. The pointer of the indicator will remain at the same point when the block is exactly square.
After finishing the tops and the edges of both blocks the top surfaces of each are smoothed with emery cloth on a file. A center line is drawn lengthwise of each block by using a sliding-blade square as at Fig. 69 and a cross-center line is scribed also. Scribing the line on each block with the same respective settings of the square - working from the end and side of each block, and scribing along the end of the blade of the square - insures the center of the cross-lines being the same on both blocks.
Fig. 69. Sliding-Blade Square Used for Laying Out.
Assuming that the die is to be laid out to produce the forging of the sprocket wheel shown in Fig. 66, the first move is to place a fine prickpunch mark at the intersection of the lines on both blocks. As this particular forging is round, both blocks may be laid out exactly alike, but in the laying-out of forgings, such as Figs. 64 or 65, the outline of the forging must be laid out right and left, so that the outlines will match when the faces of the blocks are together. The center circle for the hub is scribed with dividers, as are also the circles for the rim, the inner diameter of rim, the circle for the diameter at the bottom of the teeth, and the outside diameter. The spokes are now laid out at right angles with each other, using a sliding-blade square.
If there is a dividing head on the milling machine or die-sinking machine, it is unnecessary to lay out the teeth, as a cutter of the right shape may be used and a tooth can be cut into the outside line, then the index shifted for the next tooth, etc. If it is necessary to cut the teeth by hand, then of course each tooth must be carefully laid out.