die maintenance handbook chapter 5

Solving Punch Head Breakage Problems

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5

Solving Punch HeadBreakage Problems

Punch head breakage can be a serious problem, especially inheavy punching and high-speed work. Punch compression occursduring cutting operations (see Figure 2-1b). Rapid strain ratesthat occur in high-speed stamping may increase the punch-cut-ting load from the normal shear strength to a value approachingthe ultimate tensile strength.

PUNCH HEAD BREAKAGE

Contributing Factors

High stripping loads often contribute to head breakage. How-ever, extremely high stripping loads alone cannot pull the head offthe punch. The amount of stripping force depends on several fac-tors. These include:

• type and thickness of stock;• lubrication used;• any galling or metal pickup on the punch; and• sharpness of the punch and die.

Depending on the speed and severity of the work, another headfailure mode is breakage due to extreme compressive forces andload reversal. This is especially a problem in high-speed operations

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Die Maintenance Handbook

and when cutting heavy stock. Figure 5-1 illustrates the part ofthe punch where the failure starts with a view of a punch head ina retainer having a hardened tool-steel backing plate. Typically,the backing plate is heat-treated for toughness to 38–45 RC. SeeFigure 5-2 for a close-up view. In good severe work designs, thepunch and retainer should have a small radius or relief to accom-modate an unavoidable radius in the corner where the punch headjoins the shank body.

Figure 5-1. Head breakage problems usually start at the step where thehead and punch body join.

Figure 5-2. A close-up view of Figure 5-1 shows where the head and punchbody join.


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Punch Head Flexure and Crack Formation

As the punch cuts through heavy metal, compressive strain oc-curs in the punch body, backing plate, and die shoe. This resultsin deflection or bending of the punch head as shown in Figure 5-3.

At break through, there is a recoil action resulting in a tensilestrain concentrating on the punch head as shown in Figure 5-4.Repeated flexure of the punch head can result in crack formationat the juncture of the punch head and body.

Figure 5-3. Deflected punch head is deformed and compressed into thebacking plate.

Figure 5-4. Compressive strains release as the punch cuts through the stockand stripping occurs, resulting in tensile strain on the punch head.

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Once a crack forms, continued operation under high cyclicalloading is almost certain to cause the crack to grow larger (seeFigures 5-5 and 5-6). The ultimate result often is complete sepa-ration of the punch head from the body.

Addressing the Root Cause

Designs are available that resist head breakage. Good designsare usually effective in eliminating the head breakage problem.The punch head should have a slight interference fit in both the

Figure 5-5. Stress concentration at the juncture of the punch head and bodyunder heavy cyclical loading can result in crack formation.

Figure 5-6. Stress concentration at the juncture of the punch head and bodyhas caused crack formation. Continued operation results in crack propagation.


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retainer hole and counterbore. The head thickness also should beslightly thicker than the counterbore depth. Keeping these partstight will prevent breathing and movement.

Figure 5-7 illustrates two simple solutions that will eliminateall but the most serious head breakage problems. The first modi-fication is to grind a slight back angle on the head to permit thepunch to compress into the backing plate without flexing the punchhead.

The second improvement is to provide a generous radius at thehead-to-body juncture. This will reduce the stress concentrationin this area when punch recoil occurs at break through.

Elastomer Washers

Figure 5-8 shows an elastomer washer placed under or aroundthe punch head as a means to reduce shock. This method has beentried a number of times with very limited success. The elastomermaterial tends to break down and extrude around the punch shank.

Figure 5-7. A back angle on the punch head and a large radius at thejuncture of the punch head and body reduce stresses. This solution will solvemost head-breakage problems.

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Head Modifications

If head separation as shown in Figures 5-5, 5-6 and 5-7 is not afrequent occurrence, grinding an angular taper around the headas illustrated in Figure 5-7 is simple to accomplish for round shankpunches. The angular relief recommended is approximately 3–6°.It leaves a flat surface in the middle the same size as the shankdiameter. Grinding the angle is a simple procedure. The methodsinclude:

• the use of a cylindrical grinder;• using a lathe with a tool-post grinding attachment;• grinding the tapered land with a simple “whirligig” hand-

actuated spinning fixture on a surface grinder; and• the use of universal cutter grinders that can hold punch

shanks and grind the taper easily and quickly.

Applying a Stress-reducing Head Radius

A new punch must be made if the radius shown in Figure 5-7 isrequired. The retainer must have clearance for the radius. If thepunch retainer is heat-treated to no more than 48 RC, it should bepossible to machine the radius in the retainer with a high-speedsteel or counterbore.

Figure 5-8. An elastomer washer is placed under the punch head as a meansto reduce shock.


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A radius is easy for a skilled cutter grinder or toolmaker tomachine from drills, milling cutters, and existing counterbores.Carbide counterbores are excellent for this work. However, a jiggrinder, inside diameter (ID) grinder, or conventional electricaldischarge machine are other options.

Increasing the Ratio of the Head to the Point

Another solution to the head-breakage problem is to make thebody and head diameter much larger than the point. This effec-tive solution usually will work if other methods fail. However, thereare two drawbacks to this solution. A large-diameter, expensivetool-steel blank is required. Secondly, the large head diameter limitshow closely together punches can be spaced (see Figure 5-9).

SAFETY OF PERSONNEL

If the head separates from the punch body, it can fall out of theretainer while the press is being cycled. Figure 5-10 illustratescomplete separation of the punch body from the head. If this oc-curs, severe die damage can result. In addition to the pressworkingequipment damage, there is a great possibility of serious injury to

Figure 5-9. A large body-to-point diameter ratio is an effective, but expen-sive, solution. It also limits how closely punches can be spaced together.

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pressroom personnel. The punch and other tool-steel die compo-nents may shatter, causing sharp fragments to become airborne athigh velocity. Serious or fatal injury to pressroom personnel hasoccurred in this way.

The main emphasis of pressworking safety regulations involveskeeping body parts out of the point of operation during presswork-ing operations. Safety distance, guard-opening sizes, and othersafeguarding measures address the safety of personnel.

All pressroom injuries are avoidable by proper process planning,process control, employee training, and good pressworking prac-tices. Good safety practice in the case of dies that can shatter andeject objects requires a physical barrier guard. This is clearly statedin all government and insurance safety regulations.

If there is any danger of a die shattering in a pressworking op-eration, a physical barrier guard is necessary. Clear plastic of suffi-cient thickness and strength to arrest any airborne shattered diecomponents is a solution that may work in some cases. If this isinsufficient, solid steel guards of adequate strength may be required.

FUNCTION OF THE PUNCH BACKING PLATE

The purpose of the backing plate is to provide a hard surface toback-up the punch. A frequently used backing plate material is oil-hardening gage stock. The usual practice is to heat-treat the back-

Figure 5-10. Complete separation of the punch head can result in thepunch falling out of the holder. This can result in die damage and injury topersonnel.


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ing plate to approximately 38–45 RC to provide for toughness andwear resistance. This hardness is in the spring-temper range giv-ing long flexure life. Some experts consider this hardness range toprovide the best vibration damping capacity. The damping capac-ity of steel is low in both the hard and soft state. In any case, thebacking plate, properly heat-treated, serves to spread the load overthe underlying plate without cracking.

As shown in Figure 5-3, the punch head will compress into thebacking plate. The backing plate in turn will compress into the dieshoe. This compression occurs each time the punch penetrates thestock. Recoil as shown in Figure 5-4 occurs as the punch breaksthrough the stock.

IRONWORKER- ANDBULLDOZER-STYLE PUNCHES

The ironworker-style punch, shown in Figure 5-11, has been inuse for many years in portable equipment used to punch rivet andbolt holes in heavy plate and structural members. A modern iron-worker is a special type of steel fabricating machine capable ofcutting steel bars, punching holes, cutting steel angles, and bend-

Figure 5-11. An “ironworker”-style punch is an old design that avoids stressconcentrations in any area of the head.

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ing operations. Mechanical bulldozers have also made use of thistype of punch.

The mechanical bulldozer originated in the middle of the 19th

Century as essentially a powerful C-frame press placed on its back.The early mechanically driven machines were typically double- ortriple-back-geared and, hence capable of exerting great forces. Thedesign found widespread application in all sorts of heavy-platefabrication such as punching rivet holes and bending operations.Metalworking bulldozers were in use before the development ofthe familiar earth-moving bulldozer.

A derivation of the 19th Century design was adopted for punchhead types used for boiler shell and other fabricated wrought iron-work. The design probably evolved at the hands of clever black-smiths who produced simple tooling for this type of work. Theadvantage of this style of punch is the highest possible reductionof stress concentration in the head configuration.

The design is both economical to produce and seldom fails un-der heavy punching loads. A disadvantage when applying the de-sign to pressworking tooling is the need to use a punch retainerthat may not be readily available.

Many good punch designs for modern ironworker punches sim-ply use a generous angle rather than a sweeping radius. Thesehave proven to avoid the punch-head breakage problem in heavy-steel fabrication work.

die maintenance handbook chapter 5

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