The live page lists M2, M35 and M42. M2 is a common general-purpose grade offering a good balance of wear resistance and toughness. M35 and M42 add cobalt for higher hot hardness and abrasive wear resistance, but trade off some toughness. Select the grade based on sheet material, volume and shock loading.
The page lists 64 ± 2 HRC, which provides high wear resistance for the cutting profile. However, because square punches experience high stress at the transition to the round shank and impact at the head, the head and shank often require a softer, tougher condition (annealed or tempered back) to prevent shattering.
The step where a square profile meets a round shank creates a severe stress concentration. Breakage occurs if the blend radius is too small, machining marks are present, the punch is subjected to lateral feed-error loads, or stripping friction is excessively high. A generous, polished blend radius is critical.
Corner chipping results from perfectly sharp (zero radius) corners, tight die clearance causing corner interference, misalignment between punch and die, brittle heat treatment, or hard inclusions in the sheet metal. Specifying a small corner radius and ensuring accurate orientation prevents most chipping.
Galling is the adhesive transfer of sheet material onto the punch. The broad flat sides of a square punch create high sliding friction during stripping. Galling is promoted by tight clearance, poor lubrication, rough finish, and soft or sticky materials like aluminium or copper.
A smooth, longitudinally ground or polished finish on the square profile significantly reduces stripping friction, heat, and galling risk. The page lists 0.3 to 0.04 micron, but the drawing must specify the required parameter (e.g. Ra) and the direction of the finish for the active cutting length.
Yes. Coatings such as TiN, TiCN, or TiAlN provide high surface hardness and low friction, which helps reduce galling on the flat sides and protects the vulnerable corners from abrasive wear. The coating must match the HSS base, heat treatment, and sheet material.
Head failure occurs when the stripping force is too high (due to galling or tight clearance), the backing plate indents (allowing axial movement and hammering), the retainer counterbore is uneven, or the head is too brittle. An annealed (toughened) head helps prevent fracture.
The square cutting profile must be perfectly centred and aligned with the round mounting shank and the locating flat. If the profile is eccentric or rotated slightly relative to the key, the punch will enter the die opening crookedly, causing immediate corner collision, severe uneven wear, and broken tools.
It uses an anti-rotation feature, usually a precision machined flat on the head or shank, or a dowel pin slot. This feature aligns with a corresponding key or dowel in the punch retainer plate to ensure the square profile matches the angle of the die opening exactly.
A suitable stamping lubricant dissipates heat, reduces cutting friction, prevents galling on the broad flat sides, and lowers the stripping force. This is vital for maintaining edge quality and preventing the punch from snapping under high withdrawal tension.
Sharpen the punch when the edges or corners show rounding, burr height on the part increases, hole dimensions drift, or press tonnage rises. Do not wait for deep chips to form. Light, regular regrinding restores the sharp cutting action and extends overall tool life.
Grind the face evenly using a precision setup referenced to the shank, removing only enough material to restore a clean edge. Use a sharp wheel and heavy coolant to avoid overheating and softening the HSS. If a shear angle was present, it must be recreated exactly.
Replace it when it is cracked, bent, heavily galled, chipped too deeply to regrind economically, worn below the required square tolerance, or when the overall length drops below the minimum required for stripper engagement. Check the die opening before installing a new punch.
