Andrea Bustreo is a product engineer in the R&D department at Gasparini Industries. Click image to enlargeby Andrea Bustreo

How to avoid scratches and dents when forming stainless steel

Stainless steel is a wonderful material. Shiny, resistant, durable. But it’s also expensive, difficult to bend, and easy to scratch. When working with pieces that will be used as cladding, home appliances, and furniture, even a small scratch becomes a big problem. How can you cut and bend stainless steel without damage?

Shearing is often the first critical point. A bench equipped with plastic ball transfers, that let sheet metal to slide freely, is highly recommended. When the blank is cut, it has to be pressed against the bench to hold it in place. This system has a downside: pressure against balls leaves dents. Top manufacturers provide an anti-scratch system, with ball transfers mounted on a pneumatic support, that lets them disappear below the resting surface.

PTools must always be oiled to reduce friction and kept clean from materials that can scratch the surface.Click image to enlargeressure of blank holders must also be adjustable. Smaller and thinner pieces require a lower force, as cylinders will leave marks on the surface. Quality machines are equipped with a CNC able to vary the applied force through an independent hydraulic circuit. Also, cylinder feet must be capped with anti-scratch plastic material. Nylon is typically used because of its low coefficient of friction. Pistons must also be absolutely tight to avoid oil or grease leaks that might stain the sheet.

Hold-down cylinders. Cylinder feet must be capped with anti-scratch plastic material.Click image to enlargeAfter shearing, forming comes into play. Most frequent cosmetic damages in this phase are due to sheet metal scraping against the die edges. To limit this problem, you have to use special dies, with a bigger edge radius. Another more expensive solution is to use dies with rollers. Small cylinders are embedded at the edges of the die opening. These pins can rotate, reducing friction and scraping. This type of tools must be kept particularly clean so that dirt does not block the rollers. Specific protective plastic films can be applied to the die to limit scratching, but they may also lead to lower accuracy.

It’s also important to choose a punch/die pair suitable to the sheet metal, its thickness and the desired angle. High strength steels have a bigger bending radius and need a bigger die opening, otherwise it could crack on the outer edge, damaging both the aspect and the resistance. Tools must always be oiled to reduce friction, and kept clean from debris, dust, dirt, rust, chips and other material that can scratch the surface.

Front supports equipped with ball transfers, seen here, support metal sheets to prevent them from curving due to their weight.Click image to enlargeStainless steel sheets are often large and thin. If they are not properly supported during forming, they curve under their own weight making the so-called “counterbend” around the die. Sheet followers solve this problem. They are front supports equipped with ball transfers that, during the bending phase and when the ram is raised, support the sheet. As well as rotating, they have to shift because the centre of rotation is not fixed. Top quality press brakes are equipped with retractable supports next to back gauges. In certain cases, sheet followers can be mounted also in the inner side of the pressbrake. These accessories support the sheet while it’s resting against the back gauges, avoiding deformation. SMT

Andrea Bustreo is a product engineer in the R&D department at Gasparini Industries.


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