They say you can’t fit a square peg in a round hole. Well you can of course, if you wallop it hard enough, but you shouldn’t. Hammer on that peg and you’ll damage both it and the hole while the hammer will spring back at you with surprising force.
Metal punching involves pushing pegs, (the punches,) into holes, (the dies.) Between the two there’s a sheet of metal that the punch makes a hole through. If punch and die aren’t aligned you are doing that square-peg, round-hole thing and tool life and machine life will suffer.
The Punching Process
Most sheet metal fabrications pass through a punching process. This adds the holes needed to make the fabrication useful. (Cable entry points, for example, or locations for push buttons.) Simple punch press machines make one or more holes per stroke, (depending on how many punches are in the tool.) A turret punch on the other hand can “nibble” to create complex curves.
We run three turret punches. Two are capable of 600 “hits” per minute (hpm), the third will run at up to 1,100 hpm. (that’s 18 hits per second!) It probably goes without saying, but we couldn’t achieve this kind of performance without taking care of both machines and tooling.
The physics of punching metal is complicated. Basically, the punch deforms the metal until it cracks or shears. The goal is to get a smooth edge with no burr.
Just as with our peg-and-hole analogy, how the punch and die fit together affects edge quality. It also influences the loads on the punch press, which affects how long it will last.
8 Tips for Maximizing Punch Tool and Machine Life
1. Maintain perfect alignment
Punch and tool must be lined up in x and y as well as rotation. Ideally the tool should be guided over its full stroke to prevent deflection. A good stripper system helps a lot in this regard.
2. Use correct clearances
Clearance between punch and die must suit the material being worked. Stainless needs a clearance of up to 20% of material thickness while the figure for copper is 12- 16%. Mild steel and aluminum are between these two.
3. Keep tools sharp
Repeated use rounds the edges of the punch. This increases the force needed to push through and creates larger burrs. Best practice is to regrind punch tools at frequent intervals, taking off the minimum necessary each time. (Edge radii should never exceed 0.004”.)
As the punch moves through the material friction generates heat. That can lead to galling (a build-up on the punch,) which reduces clearance and tool sharpness. A punch lubrication system is the best answer, but lubricating the sheet is a close second.
5. Use coated tools
A thin layer of gold-colored titanium nitride (TiN) or blue-gray titanium carbonitride (TiCN) reduces friction and wear. That keeps temperature down which, like lubrication, reduces galling.
6. Match the tool to the material
Think beyond A2 tool steel, especially when punching the new high strength alloys. A tough sheet metal needs a tough punch but for an abrasive material a wear-resistant steel may perform better. The modern M2 and M4 materials may be good choices.
7. Consider tool design
Grinding shear angles onto punch faces reduces punching forces and the load on the machine, (although it complicates sharpening.) Likewise, a back taper along the flanks can reduce friction. In general, make punches as thick as possible with the largest feasible radii under the head.
8. Practice good tool storage
Careless storage results in damaged tools. Always ensure tools are put away in a manner that protects them from damage. Clear labeling is good practice too.
Looking after the Tools
Punching is integral for many fabricated metal parts. Taking good care of the tools keeps them cutting cleanly and smoothly. That results in better edges and extends the life of punches, dies and the machines themselves. It’s attention to details like these that keeps us at the top of our game.