Punching Holes with Optimal Die Clearance - Mate Precision Technologies

PUNCHING THE PERFECT HOLE
Fabricators frequently ask how to punch perfect holes in sheet metal. The answer is actually quite simple: proper die clearance, which also gives you the added benefit of extended tool life.
Die clearance is equal to the space between punch and die as the punch enters the die opening. Since the slug is essentially a mirror image of the hole in reverse order, you can check your die clearance by examining your slugs. To provide you with a better visualization, this animation shows a hole being punched with correct, tight and excessive die clearances
Correct die clearance creates an ideal slug; the fracture planes coming from the top and bottom of the material have the same angle and form in alignment with each other. This keeps punching force to a minimum and forms a clean hole with little burr. When clearance is proper, tool life is extended.
If die clearance is tight, the slug shows a fracture plane with little angle and a large burnish zone. Inadequate clearance makes a hole with small rollover and steep fracture so that the profile is more or less perpendicular to the surface of the material. When clearance is tight, tool life is reduced.
If the clearance is too excessive, the slug shows a rough fracture plane and a small burnish zone; the larger the clearance, the greater the angle between the fracture plane and the burnish zone. Excessive clearance makes a hole with large rollover and fracture so that the profile is somewhat pointed with a thin burr. When clearance is too excessive, tool life is reduced.