Additive manufacturing has a lower cost when compared to CNCs, while CNC machining is more precise.

Overview of CNC :

             Computer numerical control (CNC) is a common type of subtractive manufacturing, meaning it carves out needed components from large blocks of material. MIT introduced this technology in the 1950s, and it has since become a staple of the manufacturing industry. For this reason, you will also hear people call it traditional manufacturing.

CNC machining can produce parts that are strong and have precise dimensions. It tends to be best for engines, airplane machinery, and other fields where robust parts are needed. It is common in industries such as woodworking, lettering, and engraving

Working of CNC:

          First, an engineer makes a 2D or 3D model using CAD, or computer-aided software. The CAD file then gets translated into instructions using a computer-aided manufacturing (CAM) program. Once the instructions are created, a post-processing program converts them to specific commands and transfers them to the CNC machine for execution.

This process typically begins with a block of material, which an array of rotating tools and sharp blades cut away. Some of the more basic machines move along three axes, while more advanced machines have four or five-axis. Stepper or servo motors ensure the movements will be precise.

Some common types of CNC machines are:

  • Mills , which use rotary cutting tools to sculpt components
  • Lathes – which rotate the material on a spindle
  • Grinders – which cut using an abrasive wheel
  • Drills – which use a spinning drill to carve stationary material
  • Routers – which cut large-scale components from wood, plastic and sheet metal
  • Plasma cutters -which use a plasma torch to cut two-dimensional shapes into sheet metal.
  • Laser cutters – which are powerful cutting tools good for cutting metal, plastic and wood

CNC machines often involve the use of multiple tools, such as drills and saws, to make the needed cuts. CNC machines may combine various tools into single “cells” to maximize efficiency. Most new CNC machines are completely electronic.

Overview of Additive Manufacturing:

            3D printing (sometimes referred to as Additive Manufacturing (AM)) is the computer-controlled sequential layering of materials to create three-dimensional shapes. It is particularly useful for prototyping and for the manufacture of geometrically complex components.

            3D printing is the popular name for additive manufacturing, which refers to building products from the bottom up, adding one layer at a time. Invented at MIT in the late 1980s, it found its first uses in prototyping, but its applications have since expanded into a variety of fields.

3D printing in the medical field

            3D printing is of great use in the medical field, where it can create organs, prosthetics, and implants. The food industry has also made use of this technology, producing foods by squeezing them out layer by layer. Artists regularly use it to produce artwork, and fashion designers use it to create their apparel and jewelry.

            3D printing has some exotic applications, as well. People can use it to make 3D selfies, and create coral-shaped structures to help repair damaged reefs. NASA has even developed a zero-gravity 3D printer to manufacture parts on their spacecraft.

Working of Additive Manufacturing:

            First, an engineer makes a 3D model using CAD. Another option is to use a 3D scanner, which is a device that analyzes the shape and appearance of a real 3D object and creates a digital model from it. A third choice is using photogrammetry software, which helps construct 3D objects by just analyzing photographs.

            The completed model then gets analyzed for errors, which commonly include holes and intersecting faces.

Then, a program called a slicer prepares the model for the 3D printer by turning it into a series of thin, 2D layers, producing a G-code file. This file includes a set of instructions for the printer to carry out.

Next, a 3D printer reads the G-code file and lays down the material, one layer at a time, to produce a three-dimensional object. The way the printer deposits these layers varies, but here are some common methods:

  • Fused deposition modeling – which uses a plastic filament to produce the product
  • Selective laser sintering – which fuses metallic powders by heating them
  • Selective laser melting – which fuses metallic powders by fully melting the materials
  • Stereolithography – which uses light to cause chains of molecules to link and form polymers

            Depending on the size and complexity of the model and the method used, the printing process can last from a few hours to a few days. Sometimes post-processing is necessary as a final step. The component may need to get washed, polished or sealed before anyone can use it.

Main Aspects of 3D printing and CNC:

Accuracy:

          CNC is well known for its dimensional accuracy.

While metal 3D printing, in particular, can be associated with thermal distortion, cracking, and other defects. Thankfully, there are now software technologies to help 3D printing engineers simulate, predict and reverse potential defects in advance. These software improvements have the capability to entirely negate past inaccuracies.

Complexity:

          Pre-programmed CNC machining can turn around complex projects speedily.

But, one of the main advantages of 3D printing is its ability to handle almost any complex geometry. This reduces the number of parts that are usually needed in an assembly when using CNC. So, for complex parts that are difficult to manufacture, 3D printing must be considered first.

            An added bonus is that there are now software technologies that take advantage of this feature to help designers choose the minimum weight and optimum stiffness through topology analysis before printing.

            3D printing technologies differ in terms of the tolerances they can handle. And CNC is generally considered superior. Although, when considering the minimum layer thickness achieved by some 3D printing technologies, this advantage becomes less noticeable

Environmental Impact:

          Just how sustainable is 3D printing in comparison to CNC machining?

            3D printing generates less waste as the material is usually fed and any leftover material can be recycled. This makes it cleaner than CNC which usually involves cutting from material and creating. However, 3D printing is limited to the build size. So, when it comes to the maximum size of parts 3D printing cannot compete well with CNC.

            There is almost no limit to the type of material that can be used in 3D printing such as plastic and metal. There are 3D printing examples of food, chocolate, vitamins, and even human tissues. Although CNC can be used for a wide range of materials there are still materials that CNC cannot do, such as certain superalloys.

User-Friendliness:

          Generally, 3D printing is easier to use as it is automated.

Once the engineer orientates the part, defines the slicing & scan path, and generates support, if needed. This can all be automated using software technology so that the printer works unsupervised until it is finished.

            CNC usually requires a skilled operator to set up the speed, cutting path, take manual measurements as well as re-positioning. And the operator must supervise throughout the entire process.

            It’s worth noting that both manufacturing processes may require post-processing and therefore this cannot be used for comparison.

Cost:

The right process for the right production volume.

            This is largely dependent on the volume of printing and related to the speed of delivery. Therefore, for low quantities (preferably in 100’s), 3D printing offers a lower cost and is faster. Other factors require further consideration such as CNC parts repair and replacements. But for large quantities, of 500 and more, other manufacturing should also be considered, such as casting.

Benefits of CNC Machining Over 3D Printing:

            3D printing is a younger technology, and although advancements will undoubtedly occur in the future, it is still relatively limited in the materials and benefits it offers. Here are some of the advantages of CNC machining.

  • Cost: CNC machining is more cost-effective at most quantities. While the per-unit cost stays the same with 3D printers, it becomes cheaper with CNC machining.
  • Speed: If you need your product faster and a high level of quality isn’t necessary, CNC machines can work at faster speeds.
  • Variety: CNC can make a product with almost any material you desire — and in practically any size you’d like.
  • Strength: CNC machines can make parts strong enough for engines and airplanes.
  • Quality: They can make products of high quality and precision today’s 3D printers cannot match.
  • Consistency: Due to their high degree of precision, they will make your part the same way every time.
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