In the realm of manufacturing, choosing the right fabrication process is key to ensuring the best outcome for your parts. In this article, we explore the 2 common methods in traditional manufacturing - CNC Machining and Sheet Metal Fabrication (SMF).
Each offers distinct advantages and approaches to bringing your designs to life. CNC Machining boasts precision and versatility, while SMF excels in time- and cost-effectiveness. We will explore how each process is performed and offer a few guidelines, so you can make a more informed decision on which is better suited for your product.
Machining is a form of subtractive manufacturing, where excess material is removed from a starting work part to create the desired geometry. CNC stands for Computer Numerical Control, where the cutting tool is controlled by pre-programmed computer software. This allows for great precision and repeatability to be achieved.
The ability to work with various materials and achieve unparalleled accuracy allows CNC Machining to cater to a virtually limitless range of products. The Medical, Optics and Semiconductor industries rely heavily on this precision manufacturing to produce high-quality components.
The two most common machining processes are turning and milling. In turning, a cutting tool removes material from a rotating workpiece to generate a cylindrical shape. In milling, a rotating tool is fed across the work material to create a straight surface.
Machining is usually done in a two-step process. Roughing cuts are used to remove large amounts of material in a short amount of time to produce the general required shape. Finishing cuts are then used to achieve the final dimensions and tolerances.
SMF processes are performed on thin sheets of metal, with a typical thickness ranging from 0.5 to 10mm. SMF is usually performed at room temperature since the force required to deform these thin metal sheets is relatively low. SMF is often a more time- and cost-effective option when compared to CNC Machining.
Common SMF products include base plates and thin covers, as well as hinges and brackets. These parts are relatively simple and can be fabricated from a single sheet of metal. SMF parts are crucial to an array of industries, from Automotive and Aerospace to Robotics and HVAC systems.
Cutting is achieved by the shearing action of the sheet metal between two sharp cutting edges. This is similar to cutting a piece of paper with a pair of scissors. Today, many of these cutting operations are carried out with the use of lasers, which offer greater precision compared to mechanical stamping machines. This also allows for more complex shapes to be produced.
When cutting out shapes along a closed outline, either blanking or punching are to be done. Blanking involves cutting the work material to form the desired product, and the remaining stock is discarded. This is similar to using a cookie cutter on a sheet of dough; the cookie is the desired part while the remaining dough is discarded.
Punching is done similarly to blanking, except that it produces holes and the remaining work material is the desired product. This is similar to making doughnuts: the hole in the center is discarded while the remaining disc is kept.
Forming is defined as the process of applying force to permanently change the shape of the sheet metal. This is similar to folding a flat piece of paper to create 3D shapes. This is further broken down into bending and drawing.
Bending is done by straining the sheet metal along a straight axis. Most produce little to no change in the thickness of this metal sheet.
In drawing, a piece of sheet metal is pushed into the opening of a die cavity to create a cup- or box-shaped part. Usually, up to 25% thinning of the side wall may occur. The drawing process requires a die and mold to be made to accommodate the desired shape of the product. This makes drawing relatively expensive and better suited for mass production, given the fixed costs of the die and mold.
As we compare CNC Machining and Sheet Metal Fabrication, it is crucial to recognize their distinct approaches to modern manufacturing, as well as their individual strengths and limitations. When deciding between these 2 fabrication processes, you should take note of the following general guidelines:
CNC Machining is the better option for parts that have complex geometries and intricate designs. These complex shapes are not easily achievable through SMF, where parts generally have to maintain a uniform thickness throughout. Sharp edges are difficult to achieve through SMF processes, as a minimum bending radius must be accounted for to minimize the possibility of defects.
SMF processes are also limited to a certain range of thickness, usually from 0.5 to 10mm. Thicker sheets (10mm and above) require more robust equipment to perform cutting and forming operations, while thinner sheets (0.5mm and below) pose challenges such as being prone to deformation.
For more detailed information on design guidelines for SMF processes, please refer to our Sheet Metal Design Guidelines.
CNC Machining can be applied to a wide variety of materials. Virtually all solid metals can be machined, as well as plastic and plastic composites. On the other hand, SMF processes are limited to mild metals that are more easily bent into shape.
Mild steel and stainless-steel sheets are most commonly used. Aluminum 5052 has a high alloy content and can be bent without cracking occurring in the material, which makes it ideal for SMF processes. This is preferred over Aluminum 6061, which is prone to cracking during the bending process.
Do carefully consider the properties of the materials you wish to fabricate your parts out of, and choose the corresponding fabrication process accordingly.
CNC Machining is able to achieve very close tolerances, with the most advanced machines being able to meet tolerances of a few microns (0.003 - 0.005 mm). SMF, however, is unable to achieve as close tolerances, with the general tolerances being a few hundred microns (0.1 - 0.2 mm). The tightest tolerance achievable will increase for metal sheets of greater thickness.
As such, we would advise you to pick CNC Machining should your part require very tight tolerances.
