Before milling the spaces on the stripper and the blanking punch, it is necessary to make a number of broaches, c, Fig. 19. The number of broaches being governed by the depth of the projections or teeth. The broaches are turned on centers and the end pilot of each broach is made the same diameter on all broaches, and the pilot must be a good fit in the hole of the blanking die to be broached. The broaches are made in steps as shown, each step increasing in diameter by .005 inch. Chip clearance is provided at r. The broaches are numbered 1, 2, 3, etc., and the first step on broach No. 2 is made .005 inch larger in diameter than the last step on No. 1, and so on. The last step of the last broach has the same outside diameter as at c, Fig. 15.
Fig. 19 Brosches and Cutters Used in Making Fig, 15.
A formed milling cutter 8, Fig. 19, is now used to mill the spaces on the broaches, upper stripper, and the blanking punch. The milling cutter is set as nearly central with the center of the dividing head of the miller as possible. Then a test piece, which may be any scrap piece of steel or brass, is gripped in the chuck of the dividing head and milled as at t, Fig. 19. The dividing head is then rotated one-half of a complete turn, which brings the milled portion of the rod on the bottom side. Without disturbing the cross-movement of the table the milling machine knee is raised so that the milling cutter can be matched with the milled shape in the rod. If the formed cutter is not centrally located, it is readily noted, for when the milled rod is turned one-half revolution, the milled surface when matched with the cutter shows just double what the error is, as at u, Fig. 19, and by moving the table one-half the space shown between the milled form and the cutter and then milling a new place in the rod and repeating the operation of revolving the dividing head one-half revolution, the cutter can be very accurately set.
Having set the milling cutter central with the head or centers, the broaches are milled, and, without disturbing the setting of the milling cutter, the blanking punch is also milled, as is likewise the stripper M, Fig. 17. The blanking punch is held while milling on a special arbor having a shoulder as at v, Fig. 19, to grip the flanges of the punch and the stripper, as the small bearing surface in the straight hole in the blanking punch or the stripper would not be sufficient to prevent turning on an ordinary arbor when milling. The broaches are hardened, and then ground on the face by revolving them between centers and using a saucer or cup form of emery wheel.
The die j, Fig. 17, is now placed on a level surface in a power press which will permit the broach passing through the die. Entering the pilot of No. 1 broach in the hole in the die, the surface of the die is flooded with heavy oil and the broach is forced through the die. The succeeding broaches in their order are forced through in the same manner. The screw and dowel holes are put in the die, which completes it except for hardening.
The inside piece o, Fig. 17, of the blanking punch is milled with a formed cutter as at w, Fig. 19. At the same setting of the miller the crossbar for the inside of the stripper is milled, but this bar is made two or three thousandths thinner than the crossbar for the blanking punch. The blanking punch a, Fig. 17, is now clamped, face out, to the faceplate of the dividing head, a splining tool is secured to the arbor of the miller, the spindle of the miller is locked, and the two small recesses are splined the entire length of the straight portion of the hole in the punch. The recesses are to receive the ends of the crossbar punch o. Care must be exercised in setting the splining tool absolutely central with the center of the dividing head, and the blanking punch must be indicated by the hole so that it is central with the head of the miller.
It must be remembered that sub-press punch and dies do not enter when in use, therefore all parts can be made straight except the scrap punching dies. Clearance must be given to that part of the die where the scrap pieces or punches pass through, and when the pierced hole is irregular in shape, it of course is filed out. The piercing dies, however, as a rule are too small to permit the use of a die square, and for straightness of side of filed openings or piercing dies the die-maker must depend upon skillful filing. In order to determine whether or not the piercing die has clearance on its entire length, babbitt is employed. The die is warmed slightly and laid face down on a piece of paper which in turn is on a flat surface, and the opening is filled with molten babbitt. When the babbitt is cool it should drop freely from the die if the die has proper clearance. When forced out, there will be polished streaks on the babbitt indicating just where the die has insufficient clearance.
As to fitting the punches of a sub-press die the same method is employed as in connection with the gang die shown in Fig. 37 - that is, making and inserting the round piercing punches first and using them as guides, then as each irregularly shaped piercing punch is fitted it is left in the punch plate to act as an additional guide.
In making the piercing punches, they are attached to the punch holder and the holder plunger, the frame is assembled and babbitted, and the piercing punches are sheared while the plunger is guided by the babbitt bearing. The upper stripper, that works inside the blanking die, is blued on its face, and, removing the piercing punches from the plunger, the stripper is placed in the die, the plunger is replaced in the babbitt bearing, the stripper brought in contact with the blanking punch, and the outlines of the piercing dies are scribed on the face of the stripper. The stripper is then drilled out and filed to the lines. The punch plate containing the piercing punches may be tried from the back of the stripper, and the openings in the stripper filed until the piercing punches pass through to the face of the stripper without being forced.
For transferring any round hole not centrally located from the templet to the punch holder or to the die, a small prickpunch, Fig. 20, is turned up, one for each hole, and the prickpunch must fit the hole. At the same time that the body of the prickpunch is turned to fit the hole the small point b is turned. The prickpunches are hardened, and the templet is clamped or held to the punch plate by a few small drops of solder, the prickpunch is entered in the corresponding hole in the templet and lightly tapped on the end with a hammer. After the centers of all holes have been prickpunched the punch plate is strapped to the lathe faceplate, the prickpunch mark indicated, as shown in Fig. 21, and the holes carefully spotted and drilled.
Fig. 20. Prick-punoh.
The holes should be bored if possible. It is not safe to trust a drill starting centrally in a carefully made spot. Neither is it safe to trust a reamer to size a hole, even if the hole has been bored nearly to size, for a dull tooth on the reamer, or a soft tooth, or a hard spot in the steel at the edge of the hole, or a burr may cause the reamer to deviate slightly. The only dependable method is to bore the hole with a single-pointed cutting tool - it does not matter whether the tool is stationary and the work revolves, or the tool revolves and the work is stationary.
Fig. 21. Method of Locating and Indicating Prickpunch Mark.
If the die being made is for a product that will be used year after year, or if the quantity of the product is such that new dies must be frequently made, it is good practice to make a master plate. This is done by machining a ½-inch plate of soft steel or cast iron perfectly parallel, and of the same size as the die, then, when making the die, the plate is fastened to the face of the die and the holes are put in as above described or by the button method through the master plate and die, the holes in the master plate being bored one size.
When making the next die the master plate is attached to the back of the die, a soft-steel center in the lathe is turned to fit the hole in the master plate, the master plate to which the die is attached is wrung on this soft center, and the die is clamped to the faceplate, and spotted, drilled, and bored. As the soft center in the lathe was turned, it therefore is central with the lathe spindle, and, as the center fits the hole in the master plate, it is obvious that a hole drilled and bored in the die will be directly in line with the hole in the master plate. This is the most accurate method of transferring holes. Great care must be exercised when transferring holes from a templet or a master plate to have the proper side of the plate against the die. For instance, in transferring from a templet to a die, one face is against the die, but, when using the same templet to transfer to the punch plate, the opposite face of the templet must be against the punch plate. This is caused by the face of the punch plate being up when transferring the holes, but down when the punch plate is in use.
The blanking punch and die, and the crossbar punch o, Fig. 17, for the blanking punch, are hardened, the bar punch is inserted in the blanking punch, and both punches are screwed to the base. The piercing punch R, Fig. 17, which is evenly coated on the face with solder is attached to the punch holder and the punch holder is attached to the end of the plunger. The blanking die is attached to the plunger by screws and dowels, but a thin parallel washer, or ring, a trifle smaller in diameter than the outside diameter of the die and about 1/32 inch thick is placed between the blanking die and the plunger to cause blanking die j to protrude 1/32 inch beyond the face of punch R so that blanking punch a can be entered in the blanking die. The frame is now fastened to base, care being exercised that there is no grit between the bottom of the frame and the top of the base.
The plunger is placed inside the frame, and the punch and die entered. The frame cap c, Fig. 17, which fits the plunger is screwed on the end of the frame, which locates the plunger centrally in the frame. The entire mass is now inverted, and, resting the cap on parallels the frame is slightly heated with a gas torch, and the molten babbitt is poured in the frame. Putty is used at the die end of the plunger to prevent the babbitt leaking. The whole mass is allowed to become thoroughly cooled before disturbing. When cool, the inside outline of the blanking punch is forced into the soft solder on the face of the piercing punch.
The plunger is now removed from the frame, piercing punch R, Fig. 17, is removed from the plunger, and slot S is milled or shaped to the line on the solder. After milling very close to the line the parts are reassembled and the punch is sheared far enough to obtain a good impression on the steel punch. The punch is now carefully milled or filed to remove all marks of shearing.