CNC Design Rules
How has subtractive manufacturing shaped how products are designed?
2-5 min read
Subtractive manufacturing is the process of creating a part by removing, instead of adding material.Common methods revolve around CNC(computer numerical control), and the advent of this technology in the 60s coupled with early CAD(computer aided design)+CAM(computer aided manufacturing) transformed traditional manufacturing into an incredibly fast, reliable, and accurate process. Because of the benefits, the aerospace and automotive industry rely on this technology for rapid prototyping and reliably achieving micron level tolerances to consistently deliver on important projects.
Developed at MIT initially to manufacture helicopter blades
Of course, there is a catch. CNC machining adds extra design consideration for your part to work as intended and work with the machining process. Complex geometry of the desired part can add excessive machining operations, increase total runtime, increase failure rates, and can increase part cost. Fillets and compound chamfers are examples of features that must work with the machining process and with your initial design, and the operations tied to these features can often be expensive and risky.Depending on the required tooling and size of these features, deflection and wear might be imparted on the tool which will affect tool longevity and dimensional accuracy. If an inside fillets radius matches the diameter of the selected bit it will make too much contact with the fileted corner. So now you run the risk of causing excessive flute engagement which severely impacts the quality of the bit performance for future operations.
Pivoting to the effect of different CNC machining systems on how our part is manufactured, we need to understand the types of CNC machines. Common CNC machines have 3 axis of freedom, the X,Y gantries and the Z for the toolhead. And then we have 4 axis machines which include 1 rotational axis that turn the X and Y called the A axis. Finally, if you talk to the right people you might witness a 5 axis machine in person. This kind of machine includes the same X,Y,Z,A and the new B axis, which rotates the A axis around the X axis. I don’t know how they pull it off either. With the types of machines outlined it’s important to understand that the specific machine chosen for a job will influence the work holding of a part which will also determine which operation can or cant be performed easily.
Work holding is the process of setting up your part, in most cases stock material. The name of the game is to do as little work as possible, the more you need to reorient your part so your machine can reach details on different faces of your part, the more inaccurate that part becomes positionally, just by moving it around. Depending on the machine used, minimal work holding can be achieved. More axis of freedom means more face accessibility which equals higher dimensional accuracy and often faster operation. However no matter what machine you use, milling a true 90 degree corner is close to impossible, there will always be a fillet. Again we face another detail that must be accounted for in our design. The corner dog-bone is a common way to achieve precise surface contact between 2 parts, and is an artifact seen in parts designed for CNC machining methods. Still, the process of machining dog-bones needs to be considered, milling all the dog-bones in the same operation(via contour, etc.) could catch the bit in the middle in the round corners. You can easily solve this problem by simply drilling each dog bone out, drilling is the easiest operation to perform.However drilling can take longer than a simple contour for example depending on the conditions.
Because of all the dependencies on the CNC machine and tooling , it’s critical that the engineer or designer or whoever, is well informed on the process, and in tight communication with their machinists.Even on the machine side, effects like spindle vibration or the force exerted by the toolhead can deform the work piece, features that are to thin can tear and can get wound up around the bit, it’s so important that we design parts to account for things like this.What tooling to use in accordance to what operation is another important element of CNC machinable design. Undercuts are typically avoided because of the tooling required and complex work holding orientations. Engraving vs embossing text are also both examples of the same thing. Embossing text works by milling away the surrounding material, which as you can imagine takes up a ridiculous amount of time and wears on the bit. Because of this, engraving operations are usually performed with a v-bit or ball nose which allows the machine to simply trace the text instead of removing excessive material. So with different operations requiring different tooling, consider the specific bit material and if it’s coated or treated, this will affect machining different materials like titanium or plastic. It’s also worth noting that for every length that the bit is doubled, the deflection is increased 8 fold, and adjusting speeds can again affect runtime. This makes it important to design parts that don’t require unnecessarily long tooling.
In closing, I think these are some ways that subtractive manufacturing has shaped the way we design products. This article highlights that everything is designed with a purpose.
So the next time you pick up a CNC machined part you’ll know why it’s shaped the way it is.
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