Metal CNC Machining 101: Basic Machining Guide
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Metal CNC Machining 101: Basic Machining Guide
Machining is the process of using a cutting tool to remove material to achieve a desired geometry and surface finish, which are in-line with the part’s intended application. CNC on the other hand, stands for Computerized Numerical Control, and refers to the instructions entered in the machine to effectively control its movement and tool speed to achieve a desired machining result.
There are a number of machining processes with are numerically controlled. Some of the most used machining, used to create parts in automotive and consumer electronics applications, are milling and turning.
What is CNC Milling?
CNC Milling is a machining process in which the workpiece is fixed, and a rotating tool is advanced onto the workpiece to remove the desired material. The image below shows a diagram where both the workpiece and the rotating tool can be observed:
What is CNC Turning?
CNC Turning, as opposed to milling, rotates the workpiece which is held by a holding device known as a chuck. As the workpiece revolves the tool is advanced onto the workpiece to remove material radially along the longitudinal axis of the workpiece. The image below shows the common configuration of a lathe, a turning machine, and the workpiece position with respect to the tool:
Manufacturing Guide of Metal CNC Machining
1. Designing and creating a drawing
Like it was mentioned in the introduction, machining is a process by which we remove material to get a desired geometry and surface finish. The desired geometry comes from the work or function we intend our part to perform. In order to define that geometry designers, create mechanical drawings that specify dimensions, geometrical features, and tolerances expected from the machining process.
Looking at this mechanical drawing you will notice a few elements. There is a colored view, which is called an isometric view, this is meant to provide the machinist who will be producing the workpiece an overall view of the intended finished.
On the left of the drawing there are three views which convey the overall and detailed dimensions of the part. The features in this part are:
- The thickness of the part
- Two indents
- Two through holes
If we were to go step by step of how this part would be created the machinist will take the following approach. The stock material, in this case 6061-T6 Aluminum, is cut to the overall dimension of the part to create the rectangle-shaped block. This part would then be fixed and the two indents would be cut, followed by the drilling of the two through holes.
2. Metal Material Selection
Every part in a specific assembly serves a purpose. In order to achieve that purpose, its material which will define its structural integrity, malleability and wear resistance needs to be considered carefully. For example, consider the piston of an internal combustion engine.
A piston in an internal combustion engine is expected to withstand temperatures of approximately 1,150°C at the peak of its work. For this specific application the selected material would need to withstand the heat of the combustion chamber in order to transfer the chemical energy into mechanical energy that makes an engine work.
In our drawing example we had aluminum for the support feet of our fixture. If we consider aluminum for this application, we will quickly realize that a material as soft as aluminum, with a melting point of 660°C would melt at high operating conditions and thus not perform an adequate job. Usually carbon steel is chosen for such an application because of its improved material performance.
When it comes to machining then, we found ourselves at a contradictory situation. This is because a tougher material in a certain application means a tougher material to machine. Higher pressure, and cutting forces are required to machine carbon steel as opposed to aluminum. This is where the ingenuity of the engineer is needed to find a common ground between machinability of a material and proper functionality in a specific application.
3. Tools Selection
Tool selection in the machining world is a whole topic of its own. There is a high number of machining tools for every application whether you are removing a large area of material, cutting an engraving into your part, or creating an aesthetically appealing surface finish. All these scenarios will require different types of tools.
Let’s talk about some commonly used types of tools for specific applications. The first tool we will cover is known as an end mill. An end mill is the most commonly used tool during milling. This tool has a rectangular profile with helicoidal cutting features. When spun at high angular velocity it makes for the perfect cutting tool. It can remove material from above a workpiece and for side cutting as well.
Another commonly used tool is the face mill. This tool has a large surface area and is able to remove large chunks of material at a time. It is commonly used for scenarios where large amounts of material need to be removed in a quick machining operation. Looking at the drawing in figure 3, this type of mill would have been used to create the general rectangular shape of the part.
There’s also the consideration of material selection for a cutting tool. Depending on the material machined, the hardness and tensile strength of that material, a different tool material might be needed. Most machining tools are made with cobalt steel alloys for their material properties. If machining, however, a tougher material like stainless steel a tungsten-carbide insert is usually needed to properly machine the workpiece.
4. CNC machining
Now that we have covered the basics of machining, let’s dive into what computerized numerical control means in machining and how it plays a role in the fabrication of our part. Before starting the machining process, a machinist will need a drawing detailing the dimensions of the part. Furthermore, since machines are numerically operated through a machining program, designers will also generate a 3D model that the machinist can use to create a numerical control program.
In the image above we see a 3D model, like the one we generated for the drawing in figure 3. A 3D model is processed in a manufacturing software like Masercam where the features of the part are recognized and a corresponding numerical control routine is generated. This routing consists of a set of commands and coordinates that allow the tool to advance in the workpiece to create the desired geometry.
The figure above shows a sample numerical control script. This script is transferred into the lather or mill, the stock material is setup and the program started to get the desired part.
5. Surface Finishes
Surface finish in machining refers to the appearance of the surface of a part after the machining process. The surface finish is dependent of multiple factors. The cutting speed of the tool and the temperature of the workpiece as it is being cut are the main variables that will impact the surface finish. The cutting speed is directly controlled by the machinist when the numerical control program is generated. A slower-cutting process will take more time and thus be more costly, but it will render a better surface finish.
On the temperature side, the forces that allow the material to be removed are shear and friction forces on the material’s surface. The energy exerted on the material is dissipate through heat on the workpiece’s surface. Referencing our material comparison earlier between aluminum and carbon steel, you can imagine that a softer material like aluminum will have a higher malleability due to machining temperatures than carbon steel. One way to control this is to use coolant as the part is being machined.
The image above shows a jet of coolant stream dispensing oil on the cutting area. This effectively transfers the surface heat to the liquid and cooling the part surface. This directly improves the surface finish of the part, leaving a smother and more appealing finish.
6. Tolerance & Quality Control
Tolerances are the allowable deviation of a dimension on a part. For example, if I need my part to have an overall size of 100mm, it will be difficult if not impossible for the machinist to get the part at exactly 100mm. What designers do in order to have a realistic expectation of this dimension’s size, is attach a tolerance to it. Instead of specifying 100mm, a tolerance of say 1mm is added, so the part’s size could be in a range between 99mm and 101mm, where the target is 100mm. This dimension with an attached tolerance is expressed as 100mm ± 1mm.
When a machinist produces hundreds or thousands of parts using this dimension and attached tolerance, it will see a predictable behaviour. Most of the parts will have a dimension close to the target, which is 100mm. A lesser amount of the parts will have a deviation close to the 1mm. Controlling the amount of parts that fall too far away from the target, or nominal dimension, is called quality control.
The image above shows a bell curve, which is the expected trend of the measurements of parts. This trend is expected to become more pronounced the more parts are produced.
Design Guide of Metal CNC Machining Parts
1. Wall thickness & Geometric considerations
As a designer there are several considerations that must be made before commissioning a part for machining. When a tool cuts material from a workpiece, it is effectively pushing and stressing the material all around the workpiece. The geometry of the workpiece, as it is being machined, must be able to withstand the forces exerted by the tool and not fail under these stresses.
The image above shows an image from a research paper of how a thing wall would deform under machining stresses. We can see that the higher the wall is from a support structure in the workpiece, the more deformation it will see. This effectively happens to every feature in our workpiece and it needs to be thought through before finalizing a design.
Considering the other topics covered in this article, material selection, machining settings and workpiece geometry will all play a role in the quality control of the final part. Machinist will provide designers with a minimum wall thickness (i.e., 1mm for stainless steel, 3mm for aluminum) depending on their specific machine settings and capabilities.
2. Chamfer & Bevel
A designer needs to be conscious of the fact that there are limitations to the geometry that can be created through a machining process. The main limitation being the inside corners of a pocket feature. A pocket is any cavity in a part that is created through the consistent lowering of an end mill onto the workpiece effectively creating a cavity. There is no physical way that and end mill, which has a round cutting profile, can cut a 90 degree corner.
As a compromise, designers need to round or fillet the inside corners of these pocket features. One thing that must have occurred to you is, that the larger the tool is the larger that fillet needs to be. This is also directly proportional to the time it will take to cut that cavity and thus how much it will cost to produce. As a result designers will strive to have large fillets in their pockets to allow for large tools to be able to cut these features.
3. Holes & Threads
Lastly, one of the most common features found in parts are holes and threaded holes. Considering that screws are one, if not the most, popular method of fastening parts in an assembly holes are everywhere in parts. One might think that drilling a hole into a part using a mill is as simple as plunging the tool vertically to create said hole. However, there are multiple considerations that a designer must make before dimensioning a whole in their part.
Similar to how the size of a part will dictate the size of a fillet in a pocket feature, the diameter of that tool will dictate how small that hole can be. The smaller the hole the harder it will be to machine. This is because a small hole will require a tool with a small diameter. The reduced size of that tool makes it more fragile and prone to breaking while drilling that hole. Which is why for narrow holes a machinist will tend to use thin but short drill bits, this in turn limits the depth of the hole.
The designer needs to find a balance between hole diameter and hole depth that will provide the machinist a feasible cutting scenario. This will reduce the number of tools that could potentially break during machining, and the number of parts that could be scraped from holes out of specification. This also applies to threaded holes since the process to manufacture them is similar in nature.
Conclusion
In conclusion machining is a balance between considerations of the application intended for the desired part and the variables that influence the machining process of that part. Careful material selection, tool selection and geometry design must be performed to allow a machinist to create a proper routine for producing your part. The more consideration a designer pays to these elements, the better outcome they will have with regards to cost and part quality.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.