Plain blanking dies as described this far are of the simpler type and are used only where a variation in blanks is permissible, for any die that allows the blanks to pass clear through is given clearance, and each time the die is ground the die becomes larger. With sub-press dies - sometimes called compound dies - the outside diameter or size of blank does not change, as the dies are made without clearance, for the blank only enters the die about half the thickness of the stock being punched, then the blank is forced back into the strip.
Before entering upon the making of a sub-press die, it is well to thoroughly understand the working of this type of die which in some instances is quite complicated. The term sub-press die means that the punch and die are mounted in a sub-press, or, to make it plainer, the punch and die work within a frame which has a babbitted bearing to guide the plunger to which the blanking die and piercing punches are attached, and this frame or sub-press is in turn actuated by a power press.
Fig. 363, Tool-Making, Part III, is an excellent illustration of the working principle of sub-press construction in its simplest form. Bear in mind that this die is a sub-press in principle only. Referring to this illustration it is noted that the blanking die A is mounted on the upper portion, which is characteristic of all sub-press construction. The blanking punch B also contains the piercing die C, and inside the blanking die A and surrounded by the upper stripper E is the piercing punch D. The lower stripper F surrounds the blanking punch.
In operation the stock is placed on top of stripper F, and as the upper portion descends blanking punch B enters blanking die A, causing Stripper E to recede. At the same time that the blanking punch enters the die, piercing punch D enters piercing die C in the blanking punch. In fact, all parts interlock. The blank is forced into die A, and the scrap punching passes down through hole C. As the upper section ascends or separates, strippers E and F move toward their original positions, due to spring pressure, just as fast as the upper section ascends. The result is that the blank is forced back into the strip by both strippers.
While the die shows only the principle and would blank a washer at each stroke, it can be readily seen that the blanking punch and die can be of any shape and that a number of piercing punches may be employed. Any one of the sample punchings shown in Fig. 379, Tool-Making, Part III, is made at one stroke, and the clock plate shown has thirty-four separate piercing punches.
Fig. 15. Blank Produced by Sub-Press Die.
Fig. 16. Sub-Press Base.
To make a sub-press die to produce the blank shown in Fig. 15 - the balance wheel of a clock - looks at first glance to be a difficult job, but in reality it is simple, and the die can be made without touching a file to it except to remove a few burrs. The sub-press base. Fig. 16, is made by first planing the bottom, then, strapping to a lathe faceplate, the top face is turned level and the recesses ab are bored. The recess a, Fig. 1G, is the seat for the blanking punch a, Fig. 17, and the large recess b, Fig. 10, receives the lower stripper 6, Fig. 17, for the blanking punch. The frame a', Fig. 18, is next machined by gripping the end b in a lathe chuck and facing off the bottom, and boring the recess to a good push fit for the outside of flange c. The inside of the frame must be bored tapering, but not while gripped in chuck, for the frame is thin and that portion gripped is slightly distorted, and the inside it would not be round when the chuck pressure is released.
Fig. 17. Sub press parts: a - Blanking punch and piercing dic: b - Lower stripper: c - press Frame Cap: d - plunger: h - punch Holder: j - Blanking Dic:M - Upper stipper.
Fig. 18. Details of Sub-Press Punch and Dic for Fig. 15.
The base, Fig. 16, is now drilled as at dd, the frame is placed on the base, and the holes dd are transferred to the frame. The holes in the frame are tapped, the frame is removed, and the holes are slightly countersunk to remove all burrs to insure the frames resting level on the base. The base is then returned to the faceplate of the lathe, and the flange c indicated to run true - the base must be attached to the faceplate so that both, if possible, or at least one of the screw holes in the base comes opposite a slot in the faceplate. The frame is then pushed on the base and the screws put in from the back, securely holding the frame to the base. In this position the inside taper bore of the frame is bored, and the end is threaded to fit cap c, Fig. 17 - previously threaded - at the same setting, and the end of the frame is faced off.
After the hole is bored a splining tool is placed in the tool post and the spindle locked by means of the back gears, and a groove is splined lengthwise of the bore by sliding the carriage back and forth. Light chips must be taken until a groove about 1/16 inch deep is made. The spindle is rotated one-half or one-third and one or two more grooves are splined. These grooves are simply to prevent the babbitt from turning, and the accuracy of the spacing is immaterial.
The cap c, Fig. 17, is now screwed on the end of the frame, the edges smoothed by turning, and the hole bored to the desired diameter, which should be a sliding fit for the plunger d, Fig. 18. This completes the lathe work on the base, frame, and cap.