Determining punch problems What the slug can tell you By Ted Brolund October 25, 2001 This brief article provides hands-on, helpful info on how to analyze a slug's appearance to determine punching problems related to punch shape, material type, die clearance, punch and die misalignment, and punch and die wear. Excessive punching machine downtime or poor part quality often can be traced to tooling problems or the machine operator's lack of understanding of good punch and die tooling conditions.

A quick, effective way to find solutions to tooling problems simply is for the operator to study the appearance of the punched slugs found in the slug pan and take corrective action. This article includes actual photographs of typical punched parts and slugs that can be used for comparison purposes to troubleshoot problems easily.

Seaberg Industries, Inc., Rock Island, Ill., is a company that successfully implemented the practice of examining slugs to identify problems. Seaberg is a major parts supplier to Caterpillar®, where just-in-time (JIT) daily deliveries and near-perfect part quality are musts. Two large W.A. Whitney combination punch/plasma machines are first-operation machines and are critical to their company performance levels.

Excessive machine downtime and high maintenance costs had become major problems for Seaberg, but by training machine operators and maintenance personnel to analyze punched slugs, the company has experienced major improvements. To help make these improvements, they actually attached to each machine a laminated set of the captioned photos used in this article.

The edge condition of a punched hole (see Figure 1A) usually is very similar to that of the slug (see Figure 1B). Therefore, operators usually can use the slug, which is viewed easily, to find the cause of poor-quality parts or substandard press performance.

Operators and maintenance personnel first must understand how a normal slug should appear under ideal operating conditions to determine if a tooling problem exists. A normal appearance depends on the type of punch being used (such as shear on the punch face to reduce tonnage requirements) and the type and hardness of the material being punched. The slug's appearance can give the operator clues regarding:

Figure 1: The edge condition of a punched hole (A) usually is similar to that of the slug (B). In this example, the material is 0.375-in. mild steel with a punch diameter of 1.000 in., an overall die clearance of 0.060 in., and a clearance (percentage of thickness) of 16 percent.

• Punch shape.

• Material type.

• Die clearance.

• Punch and die misalignment.

• Punch and die wear.

Punch Shape

Punched slugs take the shape of the punch face, reflecting such punch characteristics as flat face, centerpoint, concave shear, and housetop shear (see Figure 2).

Material Type

Type and hardness of the material affect the cut edge condition. Punching of soft material such as aluminum (see Figure 3) results in a deep cut band about one-half of the material thickness, as well as a shallow fracture band.

On medium material such as mild steel (see Figure 4), punching causes a moderately deep cut band about one-third of the material thickness, as well as a deep fracture band.

Punching hard material such as abrasion-resistant (AR) plate (see Figure 5) results in a very shallow cut band only one-eighth of the material thickness, as well as a very deep fracture band.

Die Clearance

Machine operators generally have control over three variables that can either give excellent results or cause problems: amount of die clearance, punch and die alignment, and punch and die wear. How these variables are controlled produces either excellent results or causes problems.

Figure 2:

Punched slugs take the shape of the punch face.

Figure 3:

Punching of soft aluminum results in a deep cut band about half of the material thickness (1) and a shallow

fracture band (2). This aluminum is 0.187 in. thick with a 1.000-in. punch diameter, 0.024-in. overall die

clearance, and 13 percent clearance.

In most cases when a problem exists, incorrect die clearance is the cause. Seaberg, like most companies, has several dies with different clearances for each punch, and it is up to the operator to select the right clearance that will yield the best results.

The quality of the punched hole is greatly dependent on the amount of clearance between the two opposed cutting edges (punch and dies). The appearance of the slug tells the operator if the clearance needs to be adjusted.

On the slug, the upper and lower fracture lines should meet; if they do, a clean hole is produced with a minimum power requirement. Recommended clearances for short-run punching are shown in Figure 6.

With insufficient clearance, a characteristic known as secondary shear is produced. The two fracture lines are unable to meet, leaving a ring of material that must be stressed again to its point of fracture with a further expenditure of energy.

Excessive clearance between the mating edges causes extreme plastic deformation, a large burr, and a high angle of fracture.

Punch and Die Misalignment and Wear

When the punch and die are misaligned, various edge condition problems can result from side to side. If the problem is severe enough, tooling is damaged.

Worn punches and dies produce large burrs on the slug and the punched part.

Figure 4:

On mild steel, punching causes a moderately deep cut band about one-third of the material thickness (1) and a deep fracture band (2). This mild steel is 0.187

in. thick with a 1.000-in. punch diameter, 0.030-in. overall die clearance, and 16 percent clearance.

Figure 5:

Punching of AR plate results in a very shallow cut band only one-eighth of the material thickness (1) and a very deep fracture band (2). This AR plate is 0.250 in. thick with a punch diameter of 1.000 in., an overall

die clearance of 0.040 in., and a clearance of 16 percent.

Figure 6:

Recommended clearances for short-run punching increase with the hardness of the

material.