An In-depth Guide to CNC Punching: Everything you need to know

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An In-depth Guide to CNC Punching: Everything you need to know

 

Origins of CNC Punching

The process we now know as CNC Punching has a long history. Metalworking as a skill dates back thousands of years when the early practitioners worked out how to hammer malleable metals like gold and silver into thin sheets, which were then easy to pierce.

Although the first applications were focused on jewellery and ornamental objects, the early metalworkers realised they could strengthen shields or weapons by edging them with these thin metals.

However, it wasn’t until the early 1800s that the first rolling presses were developed, which allowed thin steel sheet to be mass-produced. As metals became thinner, the opportunities to work them increased.

 

Early Applications of Metal Punching

By the mid-1800s, bridge builders needed a supply of materials with holes already created for fixing the various structures together. Moving along the timeline, the advent of the automotive industry and a greater drive towards automation increased the need for punching and shearing metal sheets.

 

What is CNC Punching?

The “CNC” part stands for Computer Numerical Control. The Grandfather of NC was John T. Parsons, who, in the 1940s, worked as a machinist in his father’s company. Along with a colleague, they created a system where one read out X and Y coordinates to operators who made the cuts in the materials. They teamed up with MIT (Massachusetts Institute of Technology) to create punch cards programmed to provide the first automation.

Having seen the massive advances that could be made, MIT ran with the idea further and, in 1959, combined the fledgeling CAD (Computer-Aided Design) technology with NC controls to produce the first automated CNC programme for making aluminium ashtrays.

Allowing designers to draw the parts on screen was a revolutionary idea, and the modern-day CAD packages like SolidWorks and Inventor owe their existence to those early origins.

Modern applications for CNC technology are widespread, with metals, textiles and even house builders using the same concept.

The “Punching” part refers to the use of a punch and die to create a shape. Hardened tooling is made with the punch being the desired shape and size and the die being the same but slightly larger to allow clearance. Very simply, the punch is driven through the material into the die, leaving the required shape in the material. The scrap material (slug) is ejected through the die into a collection bin then sent for recycling.

 

Which materials can be punched?

Although metals were (and still are) the primary use for CNC punching, nowadays, there are many other applications for CNC punching. Materials such as felt, rubber, card, paper, and plastics can all be punched as long as the sheet material has suitable properties and the punch and dies are appropriate for the application.

 

The Science of CNC Punching

Punching is one of the oldest forms of metalworking process but is often the least understood. It sounds like it should be simple, but there are plenty of ways it can go wrong.

One area to understand is the size of hole that can be punched. It isn’t possible to punch any sized hole in any material as the gauge (thickness) of the material has to be factored into the calculation. This is a common issue that designers face if they aren’t familiar with sheet metal working techniques.

The main rule to bear in mind is that you can’t punch a hole that is a smaller diameter than the gauge of material—for example, a 2mm diameter hole in 3mm thick steel.

Hole positioning has to be considered in the same way, as the hole must be greater than a diameter away from the edge of the material being punched. For example, there must be at least 2mm of material between the edge of a 2mm hole and the edge of the sheet.

Another factor to consider is that a punched hole is not perfectly uniform throughout the material. A hole punched in steel will be the correct diameter on the top face but slightly larger on the bottom face of the material, as the die will always have a larger clearance. If a true hole of equal diameter is required, it needs to be drilled, which is a far slower and more costly process.

Keeping this taper to a minimum is part of the skill of the manufacturer. The die clearance has to be large enough to allow the material to be punched but not too large that the material shears and leaves a large burr on the underside.

As you can see, CNC punching is another sheet metal working process that requires great skill and experience. At Pegasus Precision, we have many years of knowledge around punching, which we regularly share with our customers.

If you have any design queries, we’re happy to advise on 01233 801649 or email us at sales@pegasusprecision.co.uk.

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