A headless punch is essentially a plain punch,except for the fact that it does not contain dowels. It does contain mounting screws, which are used to fasten the punch within the confines of the opening provided in the punch plate.  To ensure accuracy and stability the punch must fit snugly in the punch plate opening. The punch is then confined laterally by the punch plate and, thus, prevented from shifting or being displaced sideways.

Fitting for Assembly

A lead or very slight taper is incorporated in the punch, the opening in the punch plate is made to a close push-fit. (insert image fit assembly)

There is a guide to follow for normal punch fit allowances.  For punch plates harder than Rockwell C48, the fit allowance should be only 10 to 20 percent of the allowance indicated above, with a smaller percentage applied to the larger size.

Note: To preclude the possibility of scoring and assembly troubles, a lubricant must be employed when assembling interference fits.  Lubricant should be applied to both the punch and punch opening.

Clearances Facilitate Punch Assembly

To avoid needless fitting for corners, drill clearance holes in the corners and, when providing corner clearance holes in the narrow punch, don’t crowd the holes; rather, move them to the outside of the punch.

Assembling Pressed-in Punches

The lead will simplify assembly of the punch plate and will allow for a minimum of care to be required to assume that the punch is perpendicular to the plane of the punch plate.

Additionally, here are two small but important points which apply to pressing punches into punch plates:

1. Provide a chamfer around the edge of the punch plate opening on the punch entry side.
2. Do not attempt to press a sharp punch into a punch plate.

This applies also to situations where the punch is used as a broach to shear the opening into the punch plate.  The chamfer is an aid to the cutting edge radius because it will tend to prevent the punch from broaching the opening larger than would be desired for the interference fit.

Step-Head Punches

In addition to being positioned by punch plates, punches of this  type are retained by punch plates. A fillet may be made very small if necessary, but do not eliminate it entirely since it is important to punch strength.

Head height requirements depend upon the severity of the work, especially the severity of the stripping effort. In many instances, it is a good idea to allow for grinding stock on the top of the head of the punch so it can be ground flush.

Bevel-Headed Punches

Bevel headed punches require more accuracy in fitting the head and punch plate. A nice bevel angle generally may be made any angle from 30 degrees to 45 degrees, depending upon what is most convenient for the diemaker. Keep in mind that an angle of 30 degrees can give stronger head proportions than a greater angle.

Peened-Headed Punches 

As opposed to other headed punches, a peened-headed punch offers the advantage of permitting straight-through machining of its contour, because the head is peened on after the punch contour has been finished.  This is the sole advantage of peened-headed punches.

General procedure for installing a peened head punch is as follows:

1. Finish the punch to its required contour and size. 
2.  Displace by peening the material necessary to form the head. A bevel angle of 30° is predominantly the optimum angle. 
3.  Finish the head to the desired bevel angle blending smoothly into the punch sidewalls.
4.  Heat-treated head portions should not be hardened more than Rockwell C48.
5. Grind back surface square with punching access; leave grinding stock for grinding flush with punch plate assembly.
6.  Apply lubricant and press punch home in punch plate.
7.  Check for perpendicularity; if the punch is slightly out of perpendicularity, apply localized pressure at the appropriate spot on the back surface of the punch.
8.  Grind back of the punch flush with punch plate mounting surface.

 Multiple Pressed-in Punches

 When possible avoid having more than one pressed-in punch of irregular contour mounted in a single punch plate. This does not include round punches.

Clamped Punches

A suitable open slot is machined in the punch plate to receive the punch. The punch is held in the opening by pressure from the side screws acting on the clamp. For most assemblies of this kind, slot width should provide a tap fit or a light drive fit for the punches. Slot depth should normally be 0.001 to 0.002 of an inch less than the dimensions. To assure clamping pressure to be effective, the clamping screws must be related closely to the punch. Large punches will require one or more additional clamping screws. The additional screws are installed through the punches note that clamp screw heads are shown counterbored into the clamp.

[insert image]

Clamped Punch 

Clamped punch assemblies are sometimes employed in order to make it possible to remove the punch while the assembled die is mounted in the press. There must be either sufficient space to permit complete withdrawal of clamping screws which extend through the punch or some other provision which permits punches removal. For situations where it is not advisable to depend only on side clamping one can use other means to support clamped punches:

• Tapped holes are provided in the punch to receive the mounting screws. 
•  Step head is provided on the punch.
•  The clamp is made so as to key the punch to the punch plate.

Floating Punches

Floating punches are not positioned exactly by the punch plate, rather,  they are guided in the stripper which gives them their exact positioning. In most cases, this requires the use of a spring strip, since such punches should not be completely withdrawn from the stripper during the punching cycle. 

The back of the punch should be smoothly finished. A hardened thrust plate smoothly finished also should be provided behind a punch. It is a good idea to lap the surface to the degree necessary to provide an adequate low-friction bearing surface.

Pyramid-Type Punch Plate

In this instance, the punch plate has been contoured to achieve a pyramid effect in its third dimension; specifically, this is a clamped punch plate which provides extra deep support for punches. 

Material for Punch Plates

Factors to be considered:

1. Production requirements: These are usually tied in with die cost and quality.
2. Complexity: The more complex the punch plate the more stringent the material requirements.
3. The severity of the application: Severe work requirements will necessitate a more rugged punch plate made of better materials. 
4. Precision requirements: Strongly influence the quality of all die components including punch plates.
5.  Maintenance considerations: Fragile punches, punch subjected to unusual wear, interchangeable punches. 

Cold drawn steel is often adequate for simple punch plates, especially those used to retain round perforators; this material is also considered desirable from a cost standpoint.

However, cold rolled steel is notoriously subject to warpage from machining. Where warpage is apt to be a factor, machining steel is preferable to cold rolled steel as a material for making punches. If the plate is to be left soft, either water hardening or oil hardening tool steel will be an appropriate choice of material the prime consideration for soft tool steel punch plate is machinability.

Punch Plate Thickness Requirements

1. Punch height: Punch plate thickness should be in direct proportion to overall punch height. Higher punches will require thicker punch plates.
2.  Punch Strength: Is derived from general punch configuration; however, this is a relative situation often influenced by factors other than punch plate. 
3. The severity of work performed: Which also is a relative consideration associated with punch strength.
4. Unbalanced work: When unequal lateral forces are induced or applied, punches will require more support from the punch plate.
5. Punches guided or supported by other components require less support from the punch plate.
6. Step headed punches commonly require a heavier punch plate than equivalent headless punches.
7. Clamp type punch plate assembly will normally be thicker than equivalent

“solid” punch plates, which are probably the most widely used perforator type.

The typical configuration is as follows:

1. Point diameter must be as required for the application. It is round in this situation, but may be overall triangular, rectangular, or hexagonal.
2.  The lead diameter should be 0.001 of an inch smaller than shank diameter. Eliminate any sharp corners on the shed.
3.  Shank diameter should be a diameter that will permit the use of a standard reamer for sizing the meeting hole in the punch plate.
4.  Head diameter should be 1/8 of an inch larger than the body of the punch.
5.  Point length should be in direct proportion to point size.
6. Lead length, to be effective, must be a minimum of 1/16 of an inch. Permissible lead length is limited by the individual situation.
7.  Fitted length should permit shank diameter to extend through the punch plate at least.
8.  Head thickness should be a minimum of 2 W 1/8 of an inch. This should be increased to 3 W for large perforators.
9.  Overall punch length is directly proportional to the point and shank diameter.
10.  Radius is tangent to the point intersecting the lead diameter.

[insert punch dimensions, typical configurations, image]

Finish

Quality of finish is important to the function and life of a perforator.

Service Requirements

The most prominent difference between a punch for normal service and a heavy-duty punch is the point length. 

Off Standard Shank Diameters

For most perforators with this configuration, it will not be possible to use a standard size reamer for sizing the punch plate opening. It will be necessary to bore a special sized hole derived from the required punch size. In spite of this, these punches are in common use because applications are encountered from time to time where they are desirable.

Pyramid Punch

The pyramid configuration is often handy for supported punches where point guiding is impractical. These punches can be supported on the intermediate shaft diameter, as indicated in the tip diameter. For such applications, the difference between tip diameter and shaft diameter must be small enough to provide adequate stripping. Length proportions are also critical; the punch must be adequately supported before it enters the stock material.

Keyed Perforators

The arrangement is shown where the head of the punch has a flat in order to key the location.

Assembling keyed perforators

The assembly procedure is to start the shank in the opening with the key flat approximately aligned.Then twist the flat into exact alignment checking with the indicator. Press the punch in the opening restoring alignment if necessary.

Bevel Headed Perforated

It is generally agreed that a 30-degree angle provides a stronger head than larger angles do. Bevel heads are not as keyable as stepped heads. If a key is necessary it will be better to switch to a stepped head perforator.

Notched Headless Perforator

Since this perforator is headless the entire perforator may be hard. The back surface should be ground flat in assembly, as indicated by the dotted outline. The whistle notch is essentially the same as that.

Slug Ejection

For air ejection, the punch is provided with a suitable hole and tends to prevent the slug from

pulling out of the die opening. In some cases, a constantly blowing air-jet is satisfactory; in others, the air blast may have to be timed. Largely because of this, a spring pin ejector is the predominantly preferred method.

Quilled Perforator

The purpose of quills is to provide support for relatively long slender punches. The inside diameter of the quill should provide a close slip fit for the perforator. Hardened thrust plates are normally required behind quilled punches because most quilled perforators can be considered heavy duty or severe application punches. 

Purchased perforators:  high-quality economical perforators are commercially available 

Conclusion

As one can see there are a number of ways to support punches in punch plates. The chapters in the textbook have far more detail than I summarized here. This article mainly outlines a general knowledge of how to mount punches in punch plates and some of the variations for study purposes in preparation for my C of Q to complete my tool and die apprenticeship. Feel free to leave a comment or ask any questions and, if you would like to receive updates, please subscribe.