An In-Depth Review of CNC Punch Presses

A CNC punch press can run at 300 hits per minute (hpm) or more. That quickly puts a lot of holes in the sheet being moved underneath by the CNC control system. Those holes can be separate, as when making access holes in what will be control cabinets, or they can join together in a process called nibbling.

Nibbling lets a punch press cut contoured shapes as quickly as a laser. Punch presses can also do a limited amount of forming: louvers in metal panels are the classic example. If your shop could benefit from capabilities like these, here’s what you should know.

Punching Basics

Punching is, in principle anyway, a simple process. Tooling consists of a male punch positioned above a female die. A metal sheet is placed between the two and the punch pushed through the sheet and into the die. Clearance of a few thousandths of an inch between the two lets the metal shear around the edges of the punch.

CNC Punch Press Basics

Punch presses have been around a long time. CNC added the ability to move the sheet material under the punch on a table. Subsequently, the machines gained tool-holding turrets so they could swap out the punches while in-cycle. This let them punch slots, squares, circles and any other shapes needed.

Punch presses are primarily defined by:

• Table size
• Tonnage
• Construction

Table Size

This refers to the largest sheet the machine can hold. Many modern punch presses are designed around 4’x6’ or 5’x10’ sheets. This provides high productivity when the whole sheet can be handled easily and fully utilized, but results in a machine that needs a lot of space.

For some shops it may make more sense to shear a sheet to a smaller size first, then run it through a punch press with a small table. Machines are available with tables as small as 4’x4’.

Tonnage

This is the force the machine can apply to push the punch through the sheet. The tonnage needed is determined by the material type and thickness.

Most modern CNC punch presses have around 20 tons. This is enough to punch 0.250” steel, and thicker if the punch has a small cross-section. (Force is a function of tonnage divided by punch area.) Some machines offer as much as 30 tons, sufficient for 0.375” sheet.

Modern machines generate the punching force with a servo-electric mechanism. Older machines used hydraulics and before that punch presses were mechanical, using large flywheels to generate the tonnage needed.

Construction

Most CNC punch presses have a “C” frame or gap frame structure. Under high punching loads this opens up slightly, reducing accuracy. The alternative is the “O” frame or straight-sided design where the punch is supported at both sides. These machines are more rigid, more expensive, and less common.

Other CNC Punch Press Considerations

In addition to the points above, determine your needs with respect to the following:

• Turret capacity – typically between 21 and 48 tools
• Tool change speed
• Programmable clamp movement – raises sheet utilization by allowing regripping
• Nesting software – to maximize sheet utilization

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