What Are the Top 12 Design Considerations for CNC Machining?

Welcome, savvy industrialists and precision enthusiasts! If you’ve been grappling with the question, “How do I optimize my design for CNC machining?” you’ve come to the right place. This post aims to cut through your queries like a well-calibrated CNC machine.


Good design and meticulous planning are the bedrock of successful CNC machining. In this blog, I’ll guide you through the top 13 design considerations that will facilitate the machining process and save you time and money.


So, if your idea of a fun time involves turning a CAD model into a tangible masterpiece—or a highly lucrative product line—then buckle up. This ride is about to get technical.

1.How Should the Part Geometry Be Designed to Facilitate Machining?

Opt for Standard Geometries


Standard geometric shapes like rectangles, circles, and simple curves are your friends in CNC machining. Custom shapes often necessitate custom tools or more complex tool paths, which can drive up your costs and extend lead times.


Consider this: a rectangle can be milled with a standard end mill, whereas a custom shape might require a form tool that needs to be specially manufactured. The latter increases your tooling costs and adds time for the tooling to be made. Custom shapes may require multiple tool changes during the process, leading to increased cycle times. 


Depth-To-Diameter Ratio


The depth-to-diameter ratio of a feature is a crucial consideration in preventing tool deflection. As a general rule, keep the depth no more fantastic than four times the diameter of the cutting tool you’ll be using. Going beyond this ratio may result in tool deflection, adversely affecting the machined part’s dimensional accuracy and surface finish.


The key takeaway? Stick to shallower depths for more accurate and faster machining. This is particularly vital when working with more rigid materials that cause more excellent tool wear.


Avoid Undercuts


Undercuts are hidden geometries not visible from above the part and can’t be machined with standard tooling. They usually require specialized tools and multiple setups, adding complexity and time to the machining process. If the design permits, steer clear of undercuts or try to simplify them as much as possible.


If you need to include an undercut, keep the size standardized to use off-the-shelf tooling. Remember, every design element should serve a functional purpose. If it doesn’t, consider eliminating it to reduce cost and complexity.


Z-Level Machining


When you design with Z-level machining in mind, try to segment your part geometry into layers that can be easily machined. This will result in less tool wear, lower chances of tool breakage, and a more efficient overall process. 

2.Are There Any Tool Access Considerations in the Design?

Tool Length and Reac

More extended tools can access deeper pockets and cavities, but there’s a catch. The longer the tool, the more susceptible it is to vibrations and deflections, which can affect the quality of your finished part. Balancing tool length and reach is crucial to ensure efficient and precise machining.

Pro Tip: Consider staggered depth cavities to accommodate shorter tool lengths and reduce the chances of tool deflection. 

Tool Angles and Clearances

Sharp internal corners may be aesthetically pleasing but can be a headache to machines because standard milling tools are circular. Aim for rounded inner corners with a radius that can accommodate the tool for better tool access.

Remember: The smallest internal radius should ideally be 1.5 times the diameter of the tool you plan to use.

Clearance Zones

Provide enough clearance for maneuvering the tool, especially in confined spaces or deep pockets. Tight clearances can lead to inefficient tool changes and even collisions, slowing the machining process and potentially damaging the tool or part.

Quick Tip: For internal features, provide a clearance of at least the diameter of the tool for smooth operations. 

3.Should the Design Include Draft Angles?

Draft angles are slight tapers applied to a part’s walls that need to be released from a mold. You might ask, “Why would I care about draft angles in CNC machining, which isn’t a molding process?” Draft angles can also facilitate tool access, minimize tool wear, and enhance surface finish in CNC machining.

For the Uninitiated: A draft angle is measured in degrees and represents the angle between the vertical line and the tapered wall.

Improving Tool Life

Draft angles can extend the life of your cutting tool. When walls are perfectly vertical, the tool engages with the material at a 90-degree angle, which can be harder on the tool, especially with more rigid materials. A slight tilt allows for more efficient cutting action, reducing tool wear.

Tool Savvy: Implementing even a 1- or 2-degree draft angle can significantly affect tool longevity. 

Easing Material Removal

Draft angles ease removing material, which can be particularly useful in pocketing applications. With a draft angle, the amount of material to be released decreases progressively, allowing for a smoother, more efficient cutting process.

Smooth Operator: Think of it as slicing a piece of cake. It’s easier when your knife is angled rather than perpendicular to the surface. 

Cosmetic Advantages

Let’s remember aesthetics. A well-designed draft angle can also enhance the surface finish of the machined part, providing a smooth texture that’s pleasing to both the eye and the touch.

Aesthetically Yours: A smoother surface can reduce or eliminate the need for secondary finishing processes, saving time and money. 

When to Skip the Draft Angle

While draft angles offer multiple benefits, they are only sometimes appropriate. A draft angle would be counterproductive for parts that require precise vertical walls for mating or assembly.

Precision Over Ease: Know when to prioritize dimensional accuracy over machining 

4.What Are the Considerations for Internal Corners?

Beware of Sharp Angles

Let’s get this straight—sharp internal corners are a no-go. Why? Because most milling tools are round. Trying to mill a sharp corner with a rounded tool is like fitting a square peg in a round hole; it just doesn’t work.

Keep It Real: If your design has 90-degree internal corners, consider adding a radius at least as large as your cutting tool. This way, your design remains machinable without needing a magic wand. 

Tool Diameter and Corner Radii

Your tool’s diameter plays a significant role in determining the smallest internal radius you can machine. As a rule of thumb, the internal corner radius should be at least 1.3 times the diameter of the milling tool.

Radius Smarts: This ratio ensures the tool can navigate the corner smoothly, making your machinist happy and your parts more accurate. 

Depth Considerations

When it comes to internal corners, depth can be a tricky business. Deep corners limit tool access and may require specialized tools. These, in turn, can increase your production costs and time.

Go Deep Wisely: If your design demands deep corners, ensure the vertical walls allow sufficient tool clearance. Your budget will thank you. 

Wall Slope

Have you ever considered adding a slight angle to your internal corner walls? Known as a draft angle, this can facilitate tool movement and enhance surface finish. While it’s only sometimes feasible due to part requirements, it’s a real game-changer when it is.

Sloping to Success: A modest 1- to 3-degree draft angle can make a difference, especially in deep pockets. 

5.How Should Holes Be Designed for Efficient Machining?

Diameter and Depth Ratios

Size matters when it comes to hole design, particularly the diameter-to-depth ratio. The deeper the hole, the more difficult it becomes to remove chips, leading to a higher likelihood of tool breakage.

Size Wisely: Generally, the hole depth should be four times the diameter for twist drills unless you fancy spending extra on specialized deep-hole drilling techniques.

Hole Tolerance

Tolerance is the allowable variation in dimensions, and tighter isn’t always better when it comes to holes. Super-precise tolerances mean slower speeds, more passes, and higher costs.

Loosen Up a Bit: Unless you need that high-precision hole, consider specifying a more generous tolerance. Your timeline and budget will thank you. 

Through-Holes vs. Blind Holes

You’ll encounter two main types of holes: through holes and blind holes. Through-holes go entirely through the material, while blind holes stop at a certain depth. Through-holes are generally more accessible and cheaper to machine.

See the Light: Opt for through-holes to keep costs down and simplify chip evacuation. 


Threaded Holes

If you add threads to your holes, ensure you understand the tapping process. The tap needs enough room at the bottom of the hole for the threading operation.

Tap Wisely: For blind holes, leave a space equivalent to at least 1.5 times the thread pitch at the bottom. 

6.What Are the Key Factors for Designing Slots and Pockets?

Slot Width to Tool Diameter

The width of the slot should be a tad larger than the diameter of the tool you plan to use. This isn’t just for the tool’s well-being but also for more efficient chip evacuation.


The Rule of Thumb: Make your slot at least 1.25 times the diameter of the milling cutter. This gives you room to breathe and ensures a cleaner finish. 


Slot Depth

Much like holes, deeper is only sometimes better for slots. Deep slots require multiple passes, which ramps up machining time and can lead to tool deflection.

Go Easy on the Depth: A depth-to-width ratio 4:1 is a sensible rule. Anything more profound, and you’re inviting trouble. 


Pocket Corners

Sharp corners in pockets are akin to kryptonite for your standard milling tools. Much like internal corners, adding a radius can save the day.

Rounding it Off A small radius—equal to or greater than the radius of your cutting tool—can make the machining process smoother and faster. 


Pocket Drainage

If you’re working with materials that require coolant, think about how it will escape from the pocket. No one likes a soggy pocket!

Exit Strategy: Incorporate a drainage channel or slightly tilt the pocket to facilitate coolant and chip evacuation. 

7.How Can Wall Thickness Be Optimized?

Uniformity is Key

Uniform wall thickness across your part is generally beneficial. This aids even cooling and reduces the risk of deformities and internal stresses.

Recommendation: Aim for uniform wall thickness whenever possible.

Minimize Variability

Minimize the difference between the thickest and thinnest walls when different wall thicknesses are unavoidable. This will help to reduce internal stresses and improve the overall quality of the part.

Recommendation: Keep the variability within 40% of the general wall thickness.

8.How Should Threads Be Designed for Machinability?

Type of Thread

There are various types of threads, like V-threads, square threads, and Acme threads, each with its own applications and machining considerations.

Recommendation: Choose the thread type that aligns with the functional requirements of your part and is also machinable. For general purposes, V-threads are often a good choice. 

Tread Depth

The thread depth is crucial for both the strength of the threaded feature and the machinability. Too shallow or too deep threads can lead to issues.

Recommendation: As a general rule, aim for a thread depth that is 55% to 65% of the nominal diameter for most materials. 

Thread Pitch

The pitch of the thread affects both its load-bearing capability and the ease of machining. A finer pitch may provide more grip and be more difficult to machine.

Recommendation: Use coarser threads for easier machining and fine threads for parts that require higher precision or load-bearing capability. 

Tolerance and Fit

The tolerance and fit of the threaded features are critical for the assembly and functionality of the final part.

Recommendation: Always specify the fit class and tolerances on the design drawing. This will ensure that the threads meet your assembly requirements. 

Thread Relief

Thread relief areas provide a space for the cutting tool to retract and can prevent tool breakage.

Recommendation: Include a thread relief at the end of the threads to enhance machinability. 

9.What Are the Guidelines for Designing Undercuts?

Understanding the Limitations

First and foremost, understand that not all CNC machines can handle undercuts. This depends on the machine’s axis capabilities.

Recommendation: Always consult with your machining provider to determine what undercuts can be executed. 


Tooling Requirements

The geometry of the undercut dictates the tooling required. Specialized tools may be needed, which could increase costs.

Recommendation: Try to standardize the undercut geometry by using commonly available tools. 


Depth of Undercut

The depth of the undercut directly impacts its machinability. Deeper undercuts require more extended tools, which are more prone to vibrations and deflection.

Recommendation: Limit the depth of the undercut to enhance machinability and tool stability. 

10.How Should Features Like Ribs and Bosses Be Designed?

Rib Thickness

Just like wall thickness, the thickness of ribs needs to be optimized to avoid issues like warping and longer machining times.

Recommendation: A good rule of thumb is to keep the rib thickness to 60% of the wall thickness to which it is attached. 

Boss Diameter

When designing bosses, especially those intended for screw threads, the diameter is a critical aspect.

Recommendation: Ensure that the boss diameter is adequate for the type and size of the thread that will be used. 

Rib-to-Rib and Boss-to-Boss Spacing

Spacing between multiple ribs or bosses must be adequate for tool access and material removal.

Recommendation: A spacing of at least two times the diameter of the cutting tool is generally advisable. 

Aspect Ratio

The aspect or height-to-width ratio for ribs and bosses can influence their machinability and structural effectiveness.

Recommendation: Low the aspect ratio to ensure easier machining and better structural support.

11.What Are the Guidelines for Implementing Text and Engravings?

Font Selection

Yes, aesthetics matter, but not all fonts are equal in machining.

Recommendation: Stick to simple, sans-serif fonts for better readability and easier machining. Avoid thin or script-like fonts that can compromise legibility. 

Text Size and Depth

The size and depth of your text or engravings can dramatically affect visibility and machinability.

Recommendation: Aim for a minimum text size of 16 points and a depth of at least 0.5mm for clear, readable text. 


The location of your text matters, especially about other features like holes, edges, or corners.

Recommendation: Place text at least a few millimeters away from any other feature to prevent overlapping or structural issues. 


The orientation in which the part will be machined can affect the quality and visibility of the text or engravings.

Recommendation: Consider the machining orientation when placing text, as this can affect the tool’s ability to render the text accurately. 

12.Should the Design Include Chamfers and Fillets?

Purpose of Chamfers and Fillets

These features often remove sharp edges, making the part safer to handle and reducing stress concentrations.

Recommendation: Use chamfers and fillets judiciously in areas that will be frequently handled or are prone to high stress. 


Size Specifications

The size of the chamfer or fillet must be carefully selected to not interfere with the part’s function or fit.

Recommendation: Keep the size proportional to the overall dimensions of the part for a balanced design. 


Designing for CNC machining is not just an act; it’s an art form that balances aesthetics, functionality, and machinability. From part geometry to minute details like text and engravings, each element plays a critical role in the final masterpiece that is your machined part.


Ready to turn your impeccably designed parts into a reality? At Worthy Hardware, we’ve got the machinery and the expertise to make it happen. Don’t leave your project to chance; partner with the pros. Contact Us Now for a quote, and let’s start machining excellence, one part at a time.