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Sheet metal fabrication process is not like any other fabrication. But why!
Firstly, sheet metal products usually have low prototype volume and high production runs, which makes the fabrication very cost effective because of large initial set-up as well as material costs. These parts are known for their durability because of which they are exceedingly suitable for the high-end application like automotive chassis, brackets or enclosures.
Components like chassis are made from a single sheet with uniform thickness, and so final CAD designs and fabrication drafts need to be close to design intent. Some fundamental rules and definitions turn out to be a great savior during designs, and in some cases, sole reasons to success, are often overlooked.
For instance, bending the most common operation happening in sheet metal fabrication is not a process that removes metal from the base material. In fact, it elongates the metal and hence that deduction/allowance needs to be calculated well in advance.
This is a fact that has been used by the sheet metal fabricator day in and day out but is often considered “to be an operation that will generate waste.” The reasons could be several. One can be the preconceived notion because of the environment in which it is carried out – a typical shop floor setup.
However, a fabricator closely associated with the CAD designing process can never be fooled by the process masquerades and cannot be kept from what is actually happening. We bring you a walkthrough for CAD designs for sheet metal fabrication process to help you eliminate pain points for the fabrication processes.
The two major accountable factors for metal selection are the metal type and its thickness or gauge. Different sheet metal fabricators possess different capabilities and compatibility of using the type of sheet metal based on their shop floor setup, machine availability and tooling capacity.
Ideally, cold rolled steel, stainless steel, galvanized aluminum etc. are some of the commonly used metals used for the high-end application because of their inherent physical properties. For instance, automotive fabricators prefer material such as steel, magnesium, aluminum or some other composite materials that possess properties such as lightweight, safety and cost-effectiveness. But choosing one is a matter of concern. A metal that meets all your requirements for final products and suits your tooling capability is the ideal one for you.
As far as fabrication process is concerned, bending is the most common and inevitable; welding comes second. But when bending and welding are combined, bends are so selected that the welder is not obstructed or the flame can move freely without hindrance from the bends. Else it reduces the efficiency of the process and the fabricator both.
Fabricator needs to be even wiser while selecting the process because they are costly and if things go south, they pay heavy. Alternatively they chose to rely on CAD drafts and fabrication drawings that aid them all the way through. But challenges appear when there are differences in environment where designs are prepared and products are fabricated.
One universal truth is that when sheet metal is bent under press brakes, it is never possible to get an accurate 90° angle. This is the perfect example of how things vary as designs move from CAD shop to machine shop. In CAD environment everything is perfect; while for machine shop it’s real and hence things and operations deviate from ‘perfect’.
When the CAD designer or drafter knows the fabrication process, the order of fabrication process, exact specifications of the material and approach of the fabricator, he is more likely to prepare a successful fabrication draft and model. Some common thumb rules or design for manufacturing guidelines are enlisted here.
To avoid material failure and waste of time as well as make the process cost-effective, it is advised that the bends in the same plane should be in the same direction. This is because when the bends are in the same direction, the part/job doesn’t need to be reoriented during the operation and it will save the time of the bender, and also make the process effective.
Furthermore, the inner radius of the bend should be at least equal to the thickness of the sheet metal. It will prevent the material from being fractured or distorted. When the metal is bent under press brake [even if it is air bending] the force applied is beyond the yield strength of the material. Hence, the material elongates which itself brings the strength of the material a little lower than the original value.
Another form of bend, curls, too needs to be designed and formed properly. It is recommended that the outside radius of the curls should be at least twice them material thickness. Secondly, if there are any holes to be punched near the curls, they should be punched at a distance equal to the radius of the curl plus the material thickens to avoid tearing of materials.
A foreman on the shop floor is obviously not qualified enough to understand all that. CAD nesting reports depict all these characteristics for sheet metal fabrication business. The DWG and DXF files generated in CAD have detailed information about bending radius, pressure and much more. The nesting reports will be directly accessible to CNC bending machines and press brakes. This will reduce the machine setting time and overall manufacturing time can be reduced to almost half the original time.
Clearly, CAD drafting processes give a good picture of fabrication process needs. A CAD designer having a fundamental knowledge of fabrication processes can inculcate them while preparing drafts. Since sheet metal fabrication processes aren’t like any other and to not get fooled by it, better explore your theoretical designing processes with CAD drafts and nesting reports to stay profitable.