Introduction
Computer Aided Design (CAD) software allows engineers and designers to create detailed 3D models and technical drawings of machine parts and other components. The use of CAD has become an indispensable tool in manufacturing and engineering workflows.
CAD models designed for fabrication using computer numerical control (CNC) machining must follow specific principles and best practices. Well-designed CAD models make the CNC programming and machining process quicker, more efficient, and higher quality.
This guide covers the 8 key principles that engineers should follow when using CAD software to design parts intended for CNC machining. By understanding these fundamentals, you can optimize your CAD models to avoid common issues, minimize machining time, reduce cost, and achieve dimensional accuracy.
Whether you are designing precision turned parts, complex castings, plastic injection molds, or machined components, adhering to these CAD best practices will lead to efficient, optimized designs ready for flawless CNC fabrication.
Understand CNC Machining
CNC stands for Computer Numerical Control and refers to machining processes that are controlled using computers and CAD/CAM software. Whereas conventional machining relies on manual operation of machine tools, CNC machining uses programmed computer controls to automate fabrication.
The basic CNC machining process works as follows:
1. The part design is created using CAD (Computer-Aided Design) software. Common CAD programs used for CNC machining include SolidWorks, AutoCAD, and Inventor.
2. The CAD model is converted into CNC code through CAM (Computer-Aided Manufacturing) software. The CAM program analyzes the CAD model and generates code telling the CNC machine how to move.
3. The CNC code is then loaded onto the CNC machine controller. CNC machines read g-code and m-code instructions to guide the machine tool motion.
4. The machinist loads the raw material into the CNC machine and initiates the program. Common CNC tools include mills, lathes, routers, grinders, drills, lasers, plasma cutters, and waterjets.
5. The CNC machine uses rotating cutting tools or other processes to cut away material based on the toolpaths defined in the CNC program.
6. The finished part is unloaded once the automated machining process is complete. Some secondary finishing may be required.
In summary, CNC machining automates fabrication by using computer numerical control programs to direct machine tools. This improves precision, repeatability and efficiency compared to manual machining. Understanding the CAD to CNC process is key for designing optimized parts.
Improve Engineering Drawings
Creating clear, effective engineering drawings is critical for designing machined parts in CAD. Here are some key principles for improving your drawings:
Include dimensions only for critical and measurable features - Eliminate any superfluous dimensions that are not needed to fully specify the part. Focus on adding just enough dimensions to describe the necessary size, position and geometric relationships between features.
Add hole tapping needs including thread size and depth - Specify any tapped holes by calling out the thread size and required engagement depth. This ensures the machinist will create holes ready for tapping at the required sizes.
Consolidate call-outs when there are multiples of the same feature - When a view shows multiple copies of the same feature, group them in a single note instead of multiple repetitive notes. For example, note "All 4 holes tapped 1/4-20 x 1/2 deep" instead of 4 separate call-outs.
Orient views to minimize view changes - Orient plan views consistently to avoid flipping the part. This makes it easier to visualize the true orientations.
Add centerlines to locate holes and features - For holes or features that are not tied to other dimensions, use centerlines to locate their positions accurately.
Following these drawing best practices will ensure your CAD engineering drawings contain just the right information needed for manufacturing, minimizing complexity, errors and confusion. Well-executed drawings efficiently communicate design intent to streamline production.
Communicate Assembly Intent
Clear communication of assembly intent in the CAD model is crucial for efficient machining. The drawing should provide all the information needed for fabrication without ambiguity.
Include Critical Part Numbers
Part numbers are essential for identifying each component correctly during manufacturing. Number all parts sequentially or according to internal part numbering conventions.
Add the part number prominently next to each part in the drawing. Avoid cluttering the drawing with part labels, but make sure each part can be clearly identified.
Exclude Non-Critical Operations
Secondary operations like tapping holes or adding threads often do not need to be specified in the initial drawing.
Providing unnecessary call-outs adds clutter and potentially confusing information. Only include critical specifications needed for the initial fabrication.
Streamline Explanation of Intent
The drawing should be unambiguous about assembly order and relationships between components.
Arrows, exploded views, numbered sequences or keys can clarify component relationships and assembly order.
Remove any redundant or excessive dimensioning that does not aid assembly intent. Simplify and streamline the information for clarity.
By focusing on critical part numbers, minimizing unneeded secondary operation notes, and clearly communicating assembly intent, the CAD drawing will enable efficient fabrication of the designed parts. Remove ambiguity by providing only essential specifications required for machining each component accurately.
Avoid Over-Dimensioning
Over-dimensioning your CAD designs can lead to several issues during machining. Every dimension added to the drawing requires extra time for both the CAD work and CNC programming. Excess dimensions also clutter the drawing, making it more difficult to discern the truly critical features.
Parts should only be dimensioned to the accuracy level needed for correct function. For example, a locating hole for a screw may only require +/- 0.5 mm accuracy, while a bearing shaft may need +/- 0.05 mm. Consult machining handbooks and standards guides for recommended tolerances based on the material, feature type, position, and function.
Adding too many tolerances is a form of over-dimensioning. Tight tolerances increase costs and reduce material removal rates in CNC machining. Only critical features that impact form, fit and function require specific callouts. Non-critical aesthetic features can use default tolerances. Consider both the end use requirements and capabilities of your machining process when assigning tolerances.
Following industry standards helps avoid over-dimensioning. Standards provide guidance on which features typically need dimensioning versus applying default values. They also give you recommendations for standard tolerance grades to call out. Using standard practices improves manufacturing clarity and efficiency.
Overall, focus dimensions and tolerances only on critical features and specifications. Remove any redundant dimensions that over-constrain the design. Lean drafting focuses on conveying the essential information clearly and concisely. This allows smoother transitions from CAD to CNC machining.
6. Design for Machinability
Machined parts must be designed in a way that allows CNC tools to physically cut and create the required shapes. Following some key rules of machinability can optimize designs for efficient and successful CNC fabrication:
Avoid Excessively Thin Walls - Thin walls that are less than 1/4 inch thick can easily warp and bend during machining. They also provide inadequate surface area for tools to cut properly. For most materials, wall thicknesses should be designed to at least 1/4 inch.
Eliminate Undercuts - Undercuts create overhangs and areas that cannot be reached by straight CNC cutting tools without special fixtures. Avoid undercuts in the design.
Avoid Tight Internal Radii - All internal edges should have radii added to them. Tight sharp internal radii are difficult to cut and can fracture. Use radii of at least 1/8 inch.
Avoid Intersecting Holes - Wherever possible, design holes to have some minimum distance between them. Intersecting holes are challenging to machine.
Design with Simple Geometric Shapes - Complex free-form shapes, knots, and spirals are unmachinable with basic CNC tools. Design using combinations of simple machinable shapes like rectangles, circles, cylinders, etc.
Following these rules will ensure designs are optimized for efficient, low-cost, and problem-free CNC machining. Eliminating thin walls, undercuts, tight radii, intersecting holes, and complex shapes goes a long way in machinability.
Control Tolerances
Overly tight tolerances can significantly increase manufacturing time and cost. This occurs because achieving high precision requires slower feed rates, decreased material removal, and additional finishing work.
When designing parts in CAD, do not specify tolerances tighter than the standard guidelines for your material and cutting method. Common tolerance grades are:
Aluminum parts cut on a CNC mill: ±0.005 in to ±0.02 in
Steel parts cut on a CNC mill: ±0.002 in to ±0.01 in
Aluminum parts cut on a CNC lathe: ±0.001 in to ±0.005 in
Steel parts cut on a CNC lathe: ±0.0005 in to ±0.002 in
In general, aim for the largest workable tolerance. Only use tighter tolerances for critical features like locating pins, bearing surfaces, and mating components. Relax tolerances on non-critical aesthetic surfaces whenever possible.
Review your industry's accepted tolerance standards and always design parts with standard tolerance grades. Over-specifying tolerances causes unnecessary machining costs and reduces shop productivity.
Minimize Unnecessary Features
When designing parts in CAD for CNC machining, it's important to minimize any features that are not functionally required in the final design. While aesthetic features like rounds, chamfers, or complex curves may look nice, they require additional programming and machining time that drives up costs.
The key is to strike the right balance between meeting functional needs and minimizing machining time. Only add aesthetic features if they serve a functional purpose, like improving part strength, facilitating assembly, or meeting end-use requirements. But avoid adding extra curves, holes, patterns, or textures just for appearance when they provide no functional benefit.
For example, if a part will be hidden inside an assembly, a plain blocky design will machine faster than one with rounds and scallops on every surface. Or if a part requires holes to mount it, aim for the simplest hole pattern that enables assembly rather than an intricate array of unnecessary holes.
In CAD software, it can be tempting to explore complex designs. But each curve, contour, hole, and surface requires additional toolpaths and machining time on a CNC machine. So developing a basic functional design first, then only adding secondary features as needed for function or appearance, helps to minimize costs. A clean, elegant design aligned to manufacturing and functional needs takes skill but will machine efficiently.
Optimize Cavity Depth-to-Width Ratios
When designing machined parts in CAD, pay close attention to the depth-to-width ratio of any cavities, pockets, or holes. An optimal ratio allows for efficient machining by ensuring the cutting tools can fully access the entire feature.
As a general rule, aim for depth-to-width ratios between 1:1 and 3:1. A 1:1 ratio means the depth equals the width. A 3:1 ratio means the depth is three times the width. These ratios provide enough clearance for cutting tools to machine the entire cavity.
For example, if a pocket is 2 inches wide, its depth should be between 2-6 inches. If the depth exceeds 6 inches, it becomes difficult for cutting tools to reach the bottom of the pocket. The tools can deflect, reducing accuracy and leaving behind unwanted material at the bottom.
For holes and slots, a depth around 2-3 times the diameter is ideal. This prevents the tools from rubbing or binding along the sides as they drill or mill into the feature.
In some cases, a ratio greater than 3:1 may be necessary based on part functionality. But this increases machining time, cost, and risk of tool failure. When possible, redesign the part to allow an optimal ratio.
For deep cavities, consider creating access holes, scallops or slots. This provides an entry point for tools to start machining the bottom surface. Then secondary operations can finish the full cavity design.
By optimizing depth-to-width ratios in the CAD design phase, you can dramatically improve the machining process. Well-designed ratios minimize tool strains, prevent tool breakage, reduce cycle times, improve accuracy and lower costs.
Conclusion
Designing parts for CNC machining using CAD software requires attention to detail in the modeling and drawing process. By following the key principles outlined in this guide, you can create optimized designs ready for efficient and accurate machining.
To summarize, focus first on the fundamentals of engineering drawings like dimensions, tolerances, and annotations. Communicate the complete assembly intent through part numbers, callouts, and assembly drawings. Avoid common oversights like over-dimensioning, under-dimensioning, or unrealistic tolerances.
Understand the basics of CNC machining and how your CAD model will be translated into machined parts. Design your parts specifically for manufacturability, considering limitations of CNC tools and materials. Control tolerances to balance costs while still meeting design intent. And optimize features like wall thicknesses and cavity ratios for accessibility and machining time.
With practice and application of these guidelines, you'll be able to design CAD models that machine like a dream. Your parts will go from concept to physical reality quickly, accurately, and efficiently. By mastering CAD for CNC, you gain creative freedom in design along with confidence that your parts will machined right the first time.