Punching is one of the oldest metal fabrication technologies out there, though it’s also one of the most misunderstood. When done right, it can accomplish a lot – forming a diverse range of shapes, forms, and even bending flanges. Following some fundamental guidelines can help your shop ensure punching success, producing better parts and achieving greater throughput.
1. Understand All Forming Variables
It’s important to take all possible variables into account when it comes time to transform a drawing into an actual part. These include:
- Bend Radii: While most operators intrinsically know that bend radii matters, the same can’t be said for all designers. This is especially true if they aren’t too familiar with working with sheet metal.
- Material Thickness: Operators need to have a firm understanding of the material thickness before they begin forming the part. Sometimes this means interpreting a designer’s instructions. Designers will often specify “top of sheet to top of form” as the critical dimension. Operators, however, need the overall dimension, since problems can crop up if the material thickness is not consistent.
- The Limits of the Punch: Unlike stamping presses, not all punching presses are able to clamp large areas in place while forming parts. Although a simple louver might not need a lot of pressure, a larger emboss in a thick material might. Technological advancements in recent years, however, mean that some presses are able to apply pressure before the form is created, while others can hold the sheet in place with the force supplied by the upper ram.
2. Manage Slugs
Suction created during the deformation of the slug – the scrap produced while the hole is being pierced – often causes it to stick to the punch’s face. To prevent this from happening, operators need to break the suction by:
- Cutting slots into the punch’s face to mitigate the stickiness.
- Using a punch with a hole drilled in the centre and a spring-loaded steel or urethane pin inserted into it. When the ejector pins fail, pull them out to create a “ported” punch and let the hole break the suction.
- Choosing a punch with a “rooftop” or high point on its face. When the slug comes into contact with the rooftop, it will be forced to spring back naturally.
- Using a die with a slug retention system, including negative-positive geometry. A narrower diameter at the top means these systems hold onto the slug once the punch ascends on the return.
3. Discourage Galling
Galling – when pieces of the material being worked with stick to the punch – happens when the pressure or heat being exerted on them gets too high. Preventing it means following one fundamental rule of engineering: never let two pieces of the same grade of material rub against each other. Using D2 to punch stainless steel, for example, would probably result in galling as both contain chromium.
To prevent galling, punch operators should:
- Consider back-tapering the punch, ensuring it will only make brief contact with the material. Tapering the die is a less viable solution, as the material can more easily become trapped and create heat and friction.
- Pay attention to die clearance. Use the tooling charts provided by manufacturers and your own in-house tests on the material you’re working with to assess die clearances.
- Make sure punches aren’t overheating. This can be a problem especially if you’re punching in quick succession, which is why you should consider having double or triple the tooling on hand to cycle them out and let them cool down.
Today’s tools often have special coatings to reduce galling, though you should consult with your supplier to learn about the ones you’re using.
4. Prevent Pinch Points
Pinch points become a problem when rectangular punches nibble at larger cuts, causing the hard corner of the punch to carve significant overlap marks and create pinch points. Obround punches eliminate marks, while pincher-roller wheel tools can coin a grove before the punch penetrates the sheet. The latter requires some extra cycle time, but it’s worth it.
5. Reduce Sheet Distortion
The punch will naturally draw some material inward as it penetrates and shears it. Minimizing distortion will mitigate this effect:
- Ensure the die clearance accounts for material draw. Too little will prevent the material from fracturing cleanly, again producing galling and distortion.
- Provide clamping pressure before punching starts. Be careful using a smaller stripper plate or surface area on the die, as that will cut down on the overall pressure but boost the pounds of pressure per square inch. That makes distortion a more likely outcome, especially with softer metals like aluminum.
- Set your machine to punch holes randomly, especially if you need to punch a lot in a small space. This helps to disperse the pressure.
- Experiment with hole shaving, using two punch sizes for the same hole. Punch the smaller first and the larger second to shave off the stressed area around the hole’s perimeter.
6. Minimize Sheet Shake
To manage the number of parts cut from a single sheet, operators often program microtabs (or “shaker tabs”) in nests. Workers will then shake parts out of the sheet to sort them after they emerge from the punch. But this can cause “sheet shake,” when parts tabbed in the nest shake and become unstable, resulting in errors.
Switching up the punching sequence can help reduce sheet shake, ensuring the web has enough strength throughout the punching cycle and hanging parts by the microtabs for as little time as possible.
7. Keep Track of Your Progress
When not caused by technical difficulties, challenges are often the result of poor recordkeeping and communication.
Operators should remember to:
- Use the right tools for the right job, switching them out in between. Rushed orders can sometimes cause operators to act too quickly and overlook this critical step.
- Take notes of all issues and how you resolved them. If the same problem happens again in the future, you’ll be ready to solve it.
Understanding and executing fundamentals like these serve as a solid starting point, giving you a foundation to improve in the future. More experience using your punch is the key to becoming a seasoned, skilled punch operator.
Westway Machinery offers metal punching machinery to meet your needs from trusted suppliers like Fabmaster. Contact us today to find out more about our products and services!
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