Five DFM Tips for Punch Press Operations in Precision Fabrication

November 22, 2023

A CNC punch press is the most economical machine used in high volume sheet metal part production.  The press is used in fabrication for “punching” shapes into a flat metal sheet. The press operates using paired sets of punches and dies. 

The punch is a solid piece of metal that pushes through the sheet metal and into the matching die. The opposing force from the punch and die, when employed correctly, create a clean-cut hole or indentation without deforming the surrounding sheet metal. 

At All Metals Fabricating, we use these versatile precision fabrication machines for knockouts,  embossing features, lances, countersinks, extruded holes,  threaded holes, and much more. 

However, we often come across designs that could be improved with simple Design for Manufacturability (DFM) best practices.

Here, we outline a few issues that can arise during production, and provide guidelines on how to avoid them using DFM tips to ensure parts of the highest quality are produced using punching technology. 

Avoid These Potential Issues When Designing for CNC Punching

  • Material Orientation - When cutting the blanks for forming, designers should consider how many parts will nest in a full large sheet, and how the grain of the material aligns with the planned bends. Parts that nest efficiently can save material cost by reducing scrap volume. The material grain may also dictate the orientation of the parts on the nested sheet. Using heat treated or less ductile materials can cause cracking and weakness at bends. So, when using these materials, it is crucial to pay attention to part orientation on the sheet and part nesting.
  • Bend Radius - Bending operations cause material to permanently deform. This process induces stress into the part. The key is to do it without compromising the functionality of the part. For easy to form, (highly ductile) materials, it is best to keep the bend radius greater than, or equal to the material thickness. However, when harder materials are specified, the bend radius will also have to increase, and it may be significant.
  • Bend Relief and Material Tearing - Many designs require bends to be located adjacent to unbent material. In an effort to prevent cracks from propagating from the stress of the bend, it is best to include a bend relief. A bend relief consists of removing a small amount of material where the bent and unbent metal interface. This relief will allow for controlled deformation of the bend and will prevent stress concentration at the bend interface. The rule of thumb is to strive for a bend relief with a width greater than or equal to half the material thickness.
  • Holes - When punching metal, you want to consider the size of the hole relative to the material thickness. You want to avoid very small holes, (diameters less than the material thickness), as they may not punch properly. Additionally, holes should not be located within 1.5 times the material thickness away from the edge, and about 2 times the material thickness apart. The risk when violating these guidelines is that the edge of the material can deform, or when considering adjacent holes, the second hole punched can deform the profile of the previous hole.

Five Additional DFM Tips for Punch Press Operations

For your next part requiring CNC punching, reference these DFM tips to reduce costs, save time, and ensure the quality of your parts.

1.  Choose from the fabricator's tooling library if possible

Custom tooling extends lead times and increases costs.  Instead, consider designing your part using standard punches the shop has on hand.  Custom tooling, if necessary, is always doable, it just takes some time to procure.  

Request the tool library from the fabrication shop you’re working with to see your options.   

You can find AMF’s tooling library here for Amada and Trumpf

2.  Consider material thickness

When designing parts for metal stamping or forming, always consider the volume of parts being produced for the entire assembly / sub-assembly.

For smaller production runs (e.g., less than 1,000 parts), using a consistent material thickness may be more cost-effective and efficient, even if some parts could be made with a thinner material. This uniformity simplifies the manufacturing process by eliminating material handling for multiple material types.  It allows all parts to be nested and fabricated together.  In doing so, downstream operations can see all the parts and easily ensure that all parts fit together while running.

For larger production runs, the cost savings from using optimal material thicknesses can be substantial. Always balance the upfront effort of process adjustments against the long-term savings in material costs.

3.  Pay attention to tolerancing

Parts with tighter tolerances require more inspection time, skilled labor, and often may require custom tooling to manufacture. Consider loosening tolerances where possible. 

4.  Differentiate between hardware and clearance holes

When designing parts with both press-in hardware and clearance holes, try using visually obvious different hole sizes to prevent downstream operations from trying to install press-in hardware in clearance holes.

Intentionally adding a tooling change might seem counterintuitive. However, it can prevent common, time-consuming human mistakes later in production.

5.   Pay attention to part symmetry

It is often overlooked, but parts that are “almost” symmetrical can easily be inverted without recognizing it.  And more often than not, it is discovered at the final assembly of the product.  And by then, the discovery of a nearly symmetrical part being inverted will cause a schedule delay.  Nobody wants this.  Review all your parts for symmetry, and if they are truly symmetric…awesome!  If it is not quite….consider adding a feature like a cutout on one side to make it blatantly obvious to all processes of fabrication which side is inside, and which is outside.

Choose AMF for CNC Punching 

Following our tips for DFM will reduce costs, expedite production, and result in high-quality parts. AMF has state-of-the-art machinery and highly skilled fabricators to take on your next project and exceed your expectations.   And don’t ever be afraid to call and talk to any of the account managers, engineers, or programmers about design questions.  We are here to help you exceed your expectations every step of the way.

Entrust your next project to AMF’s experienced team. Request a quote.