If you’ve never heard or seen a turret punch running at full speed you’ve missed out. Watching the machine maneuver a sheet of steel while the punch hammers through at ten strokes a second never gets old. It’s one of the most impressive sights you’ll ever see in a sheet metal fabrication shop.
Tell Me About the Process
Punching is the process that puts holes in sheet metal. It’s done with three pieces of tooling: the punch, a die and a stripper. The other side of the sheet from the punch is the die. This has a cavity or pocket that accepts the punch. The punch is brought down against the metal and pushed through, shearing the metal as it goes into the die. As the punch pulls back it tends to lift the sheet. The stripper holds down the sheet, “stripping” it off the punch.
2 Aspects of High Quality Metal Punching
There are two aspects to doing high quality metal punching: the finish of the punched edge and putting the hole in the right place. Edge finish is determined by the clearance between punch and die, although minimizing the clearance doesn’t lead to a better edge. The keys are matching clearance to sheet metal thickness, and keeping it constant around the punch.
The reason is that metal punching is part deformation and part fracturing. As the punch pushes into the sheet the metal bends then cracks. Clearance allows both the deformation and fracturing to happen. When there’s not enough clearance the sheet gets drawn into the die, causing stretching and wrinkling.
In a turret punch the other aspect of producing quality work, the hole position, is taken care of by the machine. Our turrets – we have three – are CNC machines that grip each sheet tight and move it in X and Y axes between punch and die. Positional accuracy is very good, typically better than 0.2mm (0.008”).
The other point to make is that turret punches can nibble. Nibbling means punching one hole, moving the sheet and punching a second overlapping hole. This can continue all around the sheet, letting us cut out any shape that’s needed.
Manual Punching Machines
Turret punches aren’t the only way to make holes: we also have Minster and Bliss presses that will do it. These are manual machines with higher tonnage. While they’re not as fast as the turrets they can punch larger holes through thicker material.
Making the high quality holes needed in a sheet metal fabrication is harder than it might seem. It’s also a fascinating and impressive process, especially when done on turret machines. If you’re interested in learning more, call or email. We love talking about this stuff.