The choice of tooling for a punch press is important as it can affect the quality of the holes, the life of the tooling and the potential for rapid failure if poor choices are made.
Round Versus Shaped Tools
Round punches are by far the most common type of punch and are generally the easiest to find in stock and maintain. For making slots, squares, D-shapes and other profiles a shaped punch is generally the better option as it will produce a clean cut in a single hit as opposed to a number of overlapping round holes.
Carbide or Tool Steel
Tool steel is generally best for mild steel, and aluminium work carried out in moderate to high volumes. It is, however, not suitable for punching stainless steel or other abrasive coated materials. Carbide punches last significantly longer for these types of applications but are more expensive as a first purchase. They should be chosen based on the sheet material and the job run length to minimise regrinding costs.
Getting the Clearance Right
The clearance of a punch and die is critical to getting good quality holes and not breaking the press. For mild steel, 10% to 15% of the material thickness per side is typical. For harder materials like abrasives or stainless steel, slightly more clearance is typically required. It is better to refer to the manufacturer’s clearance chart rather than try and work it out for yourself.
Multi-Tool Clusters and Special Forms
CNC Punching Machines that are fitted with a turret can hold multi-tool clusters. These are groups of different size punches that can be mounted within the turret to save time when indexing from one hole to another. A special-form tool is also a cost effective way of producing holes of an oblong shape or even complex embossed shapes rather than using a series of smaller hits with conventional shaped tools.
Spotting Worn Tooling
A worn punch will produce poor quality of hole, with rough burred edges. The marks from the punch edge will also start to appear on the surface of the material. It is always best to regrind or replace a worn punch before it cracks. For more information on the inspection and guarding of worn power press tools see the HSE’s safe use of power presses publication. A useful reference for CNC Punching Machines is https://www.cotswold-machinery-sales.co.uk/euromac-bending-machines/punching-machines
Good quality tooling at the start will save you lots of time in the long run.