Here are some useful CNC machining tips to keep manufacturability and cost in mind when designing a part.
CNC parts are widely used in different industries because of their versatility, and that is why the market for these services is continuously growing more and more. We see these everywhere, from small scale parts for DIY projects, components for a prototype, custom parts for aerospace, to toolings used for manufacturing.
Now, If you are looking to fabricate your part through CNC, you'd have to learn the basic rules and concepts in designing for CNC parts. Designing is not as simple as plotting out the shape and profile you want on CAD software because CNC Machines also have limitations and restrictions. You have to be mindful of the different factors such as toolings, material clamping, and other constraints regarding material subtraction.
How does a part design affect the cost of CNC Parts?
A reasonably designed part will save you a fair amount of time and money. What do we mean by this? There are two main drivers of cost for producing a part; these involve the part complexity and machinability. Every feature and profile you put into your design affects what kind of tools and approaches will be used throughout the machining process. These will impact machining time and the capital used for the toolings. The more complex your part is, the harder it will be machined. The harder a part is machined, the more expensive it gets.
Where are the common Design Restrictions Coming from?
Not all curves, profiles, holes, slots, and other features can be created through CNC machining. They also have limitations that should be carefully considered during the design phase of a product, hence the term "design constraints." Being aware of these constraints will guide you, or anyone designing a part, to carefully incorporate reasonable and justifiable features into your design. Below are the major factors you should be considering when designing for CNC parts.
● Part Set-up: This is defined as how a part is clamped or held in place during the machining process.
● Cutting Tool Geometry: This includes the diameter, nose radius, length, and profile of different cutting tools.
● Tooling Access: Of course, surfaces that are not accessible by the tool cannot be machined.
● Part and Tooling Stiffness: Stiffness is a great factor that drives vibration into both the tooling and work part. Vibration causes undesirable surface finish and inaccurate dimensions.
Tips to optimize your design for manufacturing
Although there are no declared laws or standards when it comes to designing CNC parts, there are these common rules of thumb that fabricators in the machining industry have long used. We will be sharing some of these below:
1. Know the application of your part
When designing your part, you should first establish the limits and scopes of its application. This will help you determine what kind of part you will be creating, whether a fixture, a tool, a decoration, etc. In other words, it will define whether you need a complex design or not. It will also help you decide on the material, necessary features, controls, finishes, and tolerance for your design. Finally, it enables you to eliminate unnecessary and unreasonable fluffs in your design, which, in turn, will save you a huge amount of cost.
2. Consider your tooling capabilities.
Design your parts so that the profiles can be cut by standard or common toolings available. You have to think things through whether creating a special tool is justifiable for that specific part. If possible, minimize complex profiles because these will drastically increase your quoted price of up to 3 to 5 times than regular parts.
3. Be knowledgeable of the length and diameter of your toolings.
When designing, you should be knowledgeable on how your parts will be processed in the CNC and what tools will be used. Having this in mind will guide you through the limitations regarding tooling access and tooling rigidity. In addition to this, tooling length and diameter also affects the machining time. To optimize the CNC's capability, it is worth noting that short end mills with a large diameter will give the fastest machining.
4. Establish clear dimension and location references on your designs
Doing this will help you communicate your design intent and critical part features to the fabricator. This involves defining reference planes or points that will stay constant all throughout so that whenever there's a deviation within the tolerance, the design intent remains preserved.
5. Incorporate Radius on internal corners
This tip branches out from the tooling geometry constraints; when using a milling process, it is impossible to have a squared sharp edge on a part's internal corners. Given the shape of toolings, internal corners will always have a radius no matter how small the tool is. However, if an internal part has a mating component that requires sharp edges, then simply design a dog bone corner on the edges.
6. Minimize feature heights and pocket depths as much as possible
Whenever possible, avoid designing parts with a high aspect ratio because they are prone to chatter and dimensional inaccuracies due to the vibration and deflection of the toolings.
7. Dimension your holes in sync with the standard sizes of drills
When designing for holes, make sure that the diameters you set have a corresponding standard sized drill. Special dimensions call for special tools or other processes that may significantly add to the cost of fabricating your product.
8. Control your hole and tapping depths
There are many rules of thumb established for creating these types of features, and these are:
● Do not tap holes to more than three times its major diameter because anything beyond that value is unnecessary.
● Thru holes are always preferred for tap holes because of the chip evacuation during the tapping procedure.
● Whenever blind holes are needed, leave a clearance from the end of the tapped thread to the end of the drilled hole of at least half the major diameter.
9. Consider designing your part to be modular or assemblable.
Some parts can be too difficult to machine as one piece, resulting in higher material waste. It can be helpful to subdivide a whole part into components to lessen complexity in the machining process.