Bending occurs when the piercing load exceeds the critical buckling limit of the slender column. Causes include point length being too long, excessive stock thickness or hardness, tight die clearance, uneven stripping, poor guidance or misalignment. Use stepped shanks and quill support to reduce unsupported length.
It should be supported as close to the cutting point as possible. A guided stripper plate with closely fitted bushings, or a telescoping quill sleeve, provides lateral support and prevents buckling during penetration and withdrawal.
Tight clearance increases the cutting force and severely increases stripping friction on the slender point. This leads to overheating, abrasive wear, galling, edge chipping and a high risk of the punch point snapping off during withdrawal.
Excessive clearance can cause the sheet material to bend into the die opening before shearing, creating a large burr, poor hole quality, and edge rollover. It can also increase slug-pulling issues, which are particularly troublesome for small punches.
Slug pulling occurs when the punched slug sticks to the punch face due to vacuum, lubrication tension, magnetism or incorrect clearance, and is drawn upward instead of falling through the die. A pulled slug can jam the strip or smash the punch on the next stroke.
Because spring-loaded ejector pins rarely fit inside very small point diameters, solutions include adding shear angles (if size permits), using anti-slug die buttons, controlling clearance tightly, venting the punch if possible, and minimising heavy oil on the strip.
Slugs jam in the die button when die taper (relief) is insufficient, the parallel land is too long, slugs expand after shearing, or lubrication is excessive. A jammed column of slugs creates massive back pressure that will instantly snap a slender punch.
Breakage at the transition between the point and the shank often results from a sharp internal corner acting as a stress riser, bending under side load, stripping friction, misalignment, or brittle heat treatment. A smooth, generous blend radius must be machined at the transition.
Galling is the adhesive transfer of work material to the punch surface. It is promoted by friction, heat, poor lubrication, rough finish, tight clearance, and soft or sticky sheet materials (like aluminium, copper or soft stainless steel). The buildup changes the hole size and increases stripping load until the punch breaks.
A suitable stamping lubricant reduces cutting friction, dissipates heat, prevents galling and significantly lowers stripping force—all of which are critical for protecting a slender point from breaking. However, excessive oil can worsen slug pulling.
The page lists this range without specifying the parameter (e.g. Ra) or the location. A very smooth, longitudinally polished finish on the point reduces stripping friction and galling. A drawing should specify the required finish for the point, shank, and transition radii.
The slender point, step transition, shank and head must share the exact same axis. Runout causes the point to enter the die off-centre, leading to uneven clearance, hole marking, severe side loading, accelerated wear, and punch breakage.
They typically use standard headed designs (cylindrical or countersunk), ball-lock retainers, or headless press-fit shanks. The mounting method must hold the punch securely against withdrawal forces without crushing the head or allowing the point to wander.
Head breakage can occur if the stripping force is exceptionally high, the backing plate is soft and indents, the retainer counterbore is uneven, or the head is too brittle. Some punches require the head to be annealed (softened) while the point remains fully hard.
Chipping results from tight die clearance, shock vibration at breakthrough, lateral punch deflection, hard inclusions in the sheet metal, abusive regrinding, or selecting an overly brittle condition. The die condition and alignment should be checked.
Sharpen the punch when edge wear becomes measurable, burr height increases, hole quality declines, or press tonnage rises. Do not wait until the point chips deeply or snaps. Regular light regrinding removes micro-cracks and restores the clean cutting action.
Use a sharp wheel, light cuts, and copious flood coolant to avoid overheating and burning the delicate tip, which would destroy its hardness. Ensure the cutting face remains square to the punch axis. Re-check the die clearance after maintenance.
Replace it when it is bent, cracked, heavily galled, chipped beyond the regrind allowance, below the minimum working length, or when the point diameter has worn out of tolerance. A bent pin cannot be reliably straightened and will break.
While the dimensions might fit the hole, the required die clearance, coating, tool material, face shear and lubrication differ significantly for each metal. Attempting to pierce thick stainless steel with clearance set for thin aluminium will quickly break a slender punch.
