Punching their way through the metal
Classifying Punches
A punch is a male member of a complete die. It complements the female die in order to produce the desired effect upon the material that is being worked.
Punch Categories
- Cutting Punches: In this category are the punches which blank, pierce, notch, trim, shave or otherwise effect cutting of the stock material
- Non-Cutting punches: This category includes punches which bend, swage, draw, extrude, or otherwise act to form or deform the stock material
- Hybrid Punches: Punches in this category combine both cutting and non-cutting functions in the same punch; some typical hybrid punches are shear-form punches, semi-pierce punches, stab-piercing punches, and pinch-trim punches
Punch Groups
Segregated punches: These punches are self-mounted in the sense that they are positioned and retained by means of self-contained screws and dowels. These punches are self-sufficient as to both positioning and retention.
Integrated punches: Punches in this group depend on other components, such as the punch plate, to locate and position them. In most cases, they depend upon these other components to retain them also. Punches belonging to either of these groups may require additional support or guidance. In this event, they may be supported or guided by a stripper or some other appropriate die member.
General Punch Group
Two Basic Punch Types: Plain punches and pedestal punches
Plain Punches
It is mandatory that a die maker keeps in mind that pierced openings are sized by the punch and that the die opening imparts their size to the piece parts produced by blanking. Plain punches are generally rectangular. A plain punch is actually a self-mounting straight-through punch.
Advantages of Plain Punches
Lower die construction costs is a major advantage associated with plain punches.
Plain punches require the minimum amount of material removal. Ease of setup for machining is a generally desirable characteristic of plain punches. The absence of projection flanges or shoulders can, in many instances, simplify setting up the punches for required machining and grinding operations.
The simplicity of mounting plain punches to the die, is due to the fact that the text states that they are self mounting. Plain punches do not need to be fitted to other components, such as punch plates or yokes.
Another advantage is that they are able to be machined and ground straight through. The more complex the cutting edge contour, the greater the comparative savings when punches are machined or ground through the directional effect of the lay. That means there are no shoulders or corners that require directional changes or re-setup for machining and grinding.
The need for polishing the sidewalls of a punch can often be eliminated by grinding to a smooth finish. This is especially true when the lay or grinding finish is parallel to the direction of punch travel.
Proportions of Plain Punches.
The overall cutting height (H) of plain punches can be less than other punches without sacrificing potential punch life. This is a desirable feature when vertical space is restricted. If the punch proportion is not favourable, consideration should be given to the following:
- Assembling the punch into a yoke or a punch plate to provide support.
- Supporting or guiding the punch in the stripper or other suitable die members.
- Using a pedestal-type punch instead of the plain punch.
A plain punch must be large enough to safely contain a sufficient number of suitably located mounting screws and dowels.
Screw and Dowels
If the work is heavy or severe, especially if the cutting forces are unbalanced as well, it may be necessary to provide a hardened thrust plate to back up the punch area. The thrust plate must, of course, be adequately hardened to resist the severe punching forces.
Mounting Plain Punches
A spotting punch, commonly called a “spotter”, is used to transfer the centers of the mounting screw to the punch holder. For this kind of mounting procedure, the spotter diameter is a slip fit to the tap-drill size.
To make the procedure more time-saving, one can secure shims with a rubber band or masking tape or bend the shims in some manner to stabilize them.
Insert the spotter through the screw holes, using a small hammer lightly tap the spotter three to four times.
Do not attempt to create a deep center impression for drilling, enlarge the center impression with a center punch. When using transfer screws, the die block is mounted first with parallel blocks placed in the die opening to support the punches.
A punch, sheared a slight distance into a die opening, is used in order to transfer the die opening contour to the punch.
Dowling Plain Punches
To dowel, position the punch in the die opening using a shim if required. Tighten the screws, not quite to locking tightness, but tight enough for security. Tap the punch till it is in the required location. Tighten the screws all the way, this tightening may disturb the punch position slightly, check and correct. Then drill and ream for the dowel using the punch as a jig.
Visual checking is the best method for final alignment of cutting punches to dwelling position. To check visually, enter the punch a minimum distance into the die opening. Arrange for light to shine into the juncture then do a visual check to see if the light shines evenly through the die opening from the bottom. For dies without clearance opening, the punches can be accurately positioned by making trial cuts on softer materials or some appropriate material. The conditions of the trial blank can be examined and analyzed to determine the existing punch and die edge relationship. Another method, which is sometimes appropriate, is to form a piece of soft wire between the punch and die. The flattened wire section can be measured to determine the existing relative punch and die position.
Pedestal Punches
The flanged portion is an integral part of the punch. A pedestal punch is a solid unit which has a base area larger than its cutting-face area.
Stability of Pedestal Punches
If properly designed and proportioned, pedestal punches are inherently stable. Stability is inherent because of the relatively broad base surface characteristic of the pedestal configuration.
Load Distribution
The load-distributing qualities of pedestal punches are excellent. In all types of punches, the cutting force is concentrated along with the cut-edge profile.
Mounting Advantages
Like plain punches, pedestal punches are self mounting and each punch is mounted individually.
On smaller dies, the flanges provide an ideal ledge for clamping the punch in position, while transferring the screw holes by drilling. A convenient way to transfer mounting holes is by using parallels to raise the punch closer to the punch holder with the mounting screw in place then lowering the die slowly to have the impression. When punch and die clearance is more than 0.002 in. per side, shim should be used between the punch and die walls.
Proportions of a Pedestal Punch
There is no rule for determining flange size. Keep in mind that flanges must be strong enough to resist force and that heavier work will require a stronger flange.
The purpose of a fillet is to blend the flanges into the cutting portion of a punch. If a fillet is too small, there will be an abrupt transition instead of a blend. This causes stress concentration in the area of the flange. An inadequate fillet is, therefore, a potential source of punch failure. Fillets should be reasonably smooth and free of sharp irregularities which are always a source of detrimental head strains.
Fillets are thought of as concave in shape and, generally, the concave shape is structurally strong and desirable. CAUTION! If an angular fillet is used be certain that the flank of the fillet blends smoothly into the punch side walls and flanges.
Strength of Pedestal Punches
Strength is a basic quality of the pedestal configuration, if the punch is properly proportioned.
If a cutting portion of a punch is weak, it can be blended into the flange base. If properly done, the weak section improves the conditions considerably, producing a punch that is practical to heat-treat and that has very good strength characteristics.
Sometimes the cutting portions and the base of a punch can be so large that special consideration must be given to blending out the disparity. For these applications, it is good practice to pyramid or double-step the punch as shown ( insert image)
Offset Pedestal
- Space considerations: It is, on occasion, necessary to eliminate the flanges from one or more sides of a punch, in order to provide a space for other components of the punch assembly.
- Machining and grinding accessibility: At times, the required cutting contour may be such that it becomes desirable to eliminate part of the flange area in order to permit straight-through grinding or machining.
Offset Bases and Heavy Work
When the base is set off to one side, the load distribution is unbalanced and the cutting forces are not dispersed equally throughout the base area. The resulting misalignment can cause shearing and possible damage to the punch and its mating die opening.
This can be prevented by using hardened thrust plates.
Warping Tendency of Offset Punches
A large percentage of L-shaped pieces will tend to warp open or closed.
In the large majority of instances, it is practical to correct this kind of wear plate by grinding the base surface to restore its perpendicularity with the punch sidewalls. Perpendicularity of dowel holes are affected by warpage work. This can be corrected by lapping dowel holes with the punch resting on its base.
Boss Punch
Punches are sometimes made with an integral positioning boss. If the work contour is irregular, the punch must be keyed to prevent rotation. Radial rotation may be secured with a dowel offset. Without a position boss, this would be a plain punch.
For many applications, they are not as simple to make or mount as some other punch types.
Flanged Punches
A flanged punch is essentially a pedestal punch which incorporates an integral position boss. The extended base provides extra space for attachment screws or for stability.
Heeled Punches
The purpose of a heel is to support the punch by resisting lateral displacement. This type of heel is an integral boss extending beyond the working face of the punch. The heeled portions are made to a sliding fit in the die opening on three sides.
Heel Proportions
Width of the heel is made to a sliding fit with the die opening. Its actual size depends upon the required die-opening width. Length is made long enough to provide ample bearing surface along with the die, which will in most cases provide an adequate H/L ratio for heel strength. Quality of surface finish, on both punches and die sidewalls at the heeled periphery, as well as the working periphery, should not be neglected.
Die Opening Fitted to Punch Heels
View A: vertical split line die opening is ground offset on an amount equal to the required cutting clearance to produce a sliding fit to the heels
View B: Horizontal split linework section is made to required die size including cutting clearance, heel section dimensions are made the required amount smaller, thus providing a sliding fit for the punch heel.
Conclusion
Punches are the workhorse of a die; all components are meant to support and facilitate efficient bending, forming, and cutting of material. The text description of mounting and doweling punches was exactly how I learned to do it on my small die in the shop class, so I feel like that will be important on my C of Q test. This article is a summary of the text that I am using to prepare for my test. The book has many further examples of punches and punch applications, so I encourage you to get the textbook if you are in the field or just interested in making some of your own tooling. Thanks for reading and please subscribe to our mailing list for even more valuable articles.