CNC Machining Design Guidelines

Understanding CNC Machining: The Foundation of Modern Manufacturing

Computer Numerical Control (CNC) machining has revolutionised the manufacturing landscape across Atlantic Canada, enabling businesses to produce complex components with unprecedented precision and repeatability. For engineers and designers working in Nova Scotia's growing manufacturing sector, understanding the fundamental principles of CNC machining design is essential for creating parts that are both functional and economically viable to produce.

CNC machining encompasses various subtractive manufacturing processes, including milling, turning, drilling, and grinding. Each process removes material from a solid workpiece to achieve the desired geometry. The key to successful CNC machining lies not just in the programming or operation of the machines, but in the initial design phase where engineers make critical decisions that affect manufacturability, cost, and quality.

Throughout the Maritime provinces, manufacturers are increasingly adopting advanced CNC technologies to compete in global markets. Whether you're designing components for the shipbuilding industry in Halifax, agricultural equipment in the Annapolis Valley, or precision instruments for the aerospace sector, following established design guidelines will help ensure your projects succeed.

Material Selection and Machinability Considerations

Selecting the appropriate material is one of the most critical decisions in CNC machining design. The machinability of a material directly impacts production time, tool wear, surface finish quality, and overall manufacturing costs. Understanding these relationships helps engineers make informed choices that balance performance requirements with economic constraints.

Common Machining Materials and Their Properties

Aluminium alloys, particularly 6061-T6 and 7075-T6, remain among the most popular choices for CNC machining due to their excellent machinability, favourable strength-to-weight ratio, and corrosion resistance. These materials machine approximately three to four times faster than steel, making them cost-effective for many applications. In Atlantic Canada's marine environment, the corrosion resistance of aluminium is particularly valuable.

Steel and stainless steel grades offer superior strength and durability but require more careful consideration during design. Materials like 1018 mild steel and 4140 alloy steel machine relatively well, while stainless grades such as 303 and 316 present greater challenges due to work hardening. When specifying stainless steel for Maritime applications, designers should consider:

• 303 stainless steel for non-critical applications requiring good machinability

• 316 stainless steel for superior corrosion resistance in marine environments

• 17-4 PH stainless steel for high-strength applications requiring heat treatment

• Duplex stainless steels for extreme corrosion resistance in offshore applications

Engineering plastics including Delrin (acetal), PEEK, and nylon offer unique advantages for specific applications. These materials typically machine faster than metals but require attention to thermal management, as plastic can melt or deform if cutting parameters generate excessive heat. Feed rates and spindle speeds must be carefully optimised to prevent material degradation.

Machinability Ratings and Cost Implications

Machinability ratings, often expressed relative to 160 Brinell B1112 steel (rated at 100%), provide a useful benchmark for comparing materials. Aluminium 6061-T6 carries a rating of approximately 300%, meaning it machines three times more efficiently than the baseline. Conversely, titanium alloys rate around 22%, indicating significantly longer machining times and higher costs.

When material selection involves trade-offs between performance and cost, consider that machining time typically represents 60-80% of total part cost for complex components. Choosing a material with superior machinability can substantially reduce project costs without compromising functionality.

Dimensional Tolerances and Geometric Specifications

Specifying appropriate tolerances is perhaps the single most important factor affecting CNC machining costs. Overly tight tolerances increase machining time, require more expensive equipment, and necessitate additional quality control measures. Conversely, tolerances that are too loose may result in parts that fail to function as intended.

Standard Tolerance Guidelines

For most CNC machined parts, the following tolerance guidelines represent a balance between precision and economy:

• General dimensions: ±0.125 mm (±0.005 inches) achievable on standard equipment

• Precision dimensions: ±0.050 mm (±0.002 inches) for critical features

• High precision: ±0.025 mm (±0.001 inches) requires careful process control

• Ultra-precision: ±0.010 mm (±0.0004 inches) demands specialised equipment and significant cost premium

As a general rule, tolerances tighter than ±0.025 mm should be applied only to features where they are absolutely necessary. Each incremental improvement in precision can increase costs by 25-50% due to slower feed rates, additional finishing passes, and increased inspection requirements.

Surface Finish Specifications

Surface finish, typically expressed in Ra (roughness average) values, significantly impacts both functionality and cost. Standard CNC machining typically achieves surface finishes of 3.2 µm Ra without special processing. Finer finishes require additional operations:

• Standard machined finish: 3.2 µm Ra (125 µin) — no additional cost

• Fine machined finish: 1.6 µm Ra (63 µin) — moderate cost increase

• Ground finish: 0.8 µm Ra (32 µin) — significant cost increase

• Polished finish: 0.4 µm Ra (16 µin) or better — substantial cost premium

Specify surface finish requirements only on surfaces where they matter functionally, such as sealing surfaces, bearing interfaces, or aesthetic faces. Leaving non-critical surfaces at standard machined finish can reduce costs by 10-20%.

Feature Design Guidelines for Optimal Manufacturability

Designing individual features with manufacturability in mind ensures that parts can be produced efficiently and to specification. The following guidelines address the most common features encountered in CNC machined components.

Holes and Threaded Features

Holes are among the most common features in machined parts. Standard drill sizes are readily available and cost less to produce than non-standard diameters. When possible, specify hole diameters that correspond to standard drill sizes in metric or imperial measurements.

For threaded holes, depth limitations depend on the threading method. Cut threads using taps typically allow depth-to-diameter ratios of 3:1 or less for reliable results. Thread depths beyond this ratio require thread milling or other specialised techniques. Consider these guidelines:

• Specify standard thread sizes (M6, M8, M10 or 1/4-20, 5/16-18, 3/8-16)

• Design thread engagement length of 1.5 times the nominal diameter for steel, 2.0 times for aluminium

• Include thread relief grooves for threads that run to a shoulder

• Avoid specifying threads finer than necessary, as coarse threads are stronger and faster to produce

Internal Corners and Radii

CNC milling operations inherently produce radiused internal corners due to the cylindrical geometry of cutting tools. Designing parts with this limitation in mind prevents manufacturing problems and reduces costs.

Internal corner radii should be at least 1/3 of the pocket depth, with a minimum of 3 mm for most applications. Smaller radii require smaller tools, which are less rigid, wear faster, and must operate at slower feed rates. For deep pockets, consider specifying larger radii or incorporating relief cuts at corners to accommodate assembly requirements.

Wall Thickness and Thin Features

Minimum wall thickness depends on the material and overall part geometry. Thin walls can deflect under cutting forces, causing dimensional errors and poor surface finish. General guidelines include:

• Aluminium: Minimum 1.0 mm for short walls, 1.5 mm for walls over 50 mm tall

• Steel: Minimum 0.8 mm for short walls, 1.2 mm for taller features

• Plastics: Minimum 1.5 mm due to lower material stiffness

When thin walls are unavoidable, discuss the design with your machining partner early in the process. Specialised workholding, optimised toolpaths, and careful process planning can often achieve results that might initially seem impossible.

Design for Assembly and Functional Requirements

CNC machined components rarely exist in isolation—they must interface with other parts, support specific loads, and function within larger systems. Designing with these considerations in mind from the outset prevents costly redesigns and ensures successful project outcomes.

Datum Features and Inspection Points

Establishing clear datum features in your design provides reference points for manufacturing and inspection. Primary datum surfaces should be large, flat, and easily accessible for measurement. Consider how the part will be inspected and ensure that critical dimensions can be verified using standard measurement equipment.

For complex parts requiring coordinate measuring machine (CMM) inspection, include tooling features or reference points that allow consistent orientation during measurement. This is particularly important for parts manufactured in quantities where statistical process control methods will be employed.

Assembly Considerations

Design features that facilitate assembly and accommodate manufacturing variations. Chamfers and lead-ins on mating features ease assembly significantly—a 0.5 mm x 45° chamfer on hole entries and shaft ends costs little to add but provides substantial benefits during assembly.

When designing press-fit or interference-fit features, account for the tolerance stack-up between mating parts. Specifying an interference fit of 0.025-0.050 mm requires that both the hole and shaft be manufactured to tight tolerances, significantly increasing costs. Where possible, design assemblies that use slip fits with mechanical fastening or adhesive bonding.

Cost Optimisation Strategies for CNC Machined Parts

Understanding the cost drivers in CNC machining enables engineers to make informed trade-offs during the design phase. The following strategies can substantially reduce manufacturing costs without compromising part functionality.

Minimising Setup Operations

Each time a part must be repositioned in the machine, setup time is incurred. Designing parts that can be machined from fewer orientations reduces these non-productive costs. Where possible, concentrate critical features on accessible faces and design geometry that permits machining in two or three setups rather than five or six.

Five-axis CNC machines can access complex geometries in a single setup, but this capability comes at a premium hourly rate. For parts requiring multiple orientations, calculate whether the higher machine rate is offset by reduced setup time and improved accuracy from single-setup machining.

Standard Tooling and Stock Sizes

Designing parts to utilise standard tooling and stock material sizes reduces costs in several ways. Standard end mills, drills, and taps are readily available and less expensive than custom tooling. Similarly, designing parts to nest efficiently within standard bar, plate, or rod stock minimises material waste.

In Atlantic Canada, material availability can affect lead times and costs more significantly than in larger manufacturing centres. Consulting with local suppliers about commonly stocked materials during the design phase can prevent delays and reduce procurement costs.

Quantity Considerations

Manufacturing costs per part decrease as quantities increase, but the relationship is not linear. Setup costs are amortised across larger quantities, and programming and fixturing investments become more economical. For prototype quantities of 1-5 pieces, consider simplifying designs to reduce programming complexity. For production quantities of 100 or more, invest in optimised tooling and fixtures that improve cycle time.

Quality Assurance and Documentation Requirements

Comprehensive documentation supports successful manufacturing outcomes and provides traceability throughout the product lifecycle. Working with experienced engineering partners ensures that quality requirements are clearly communicated and consistently achieved.

Drawing Standards and Specifications

Engineering drawings remain the definitive communication tool between designers and manufacturers. Ensure drawings include complete dimensional information, geometric dimensioning and tolerancing (GD&T) callouts where appropriate, material specifications, and surface finish requirements. Reference applicable standards such as CSA, ASME, or ISO as appropriate for your application.

For parts requiring certification or traceability, specify these requirements clearly on the drawing. Material certifications, dimensional inspection reports, and first article inspection (FAI) documentation may be required for regulated industries such as aerospace, defence, or medical devices.

Partnering with Experienced Engineering Firms

Successful CNC machining projects result from collaboration between designers and manufacturers. Engaging with experienced engineering partners early in the design process allows for design for manufacturability (DFM) reviews that identify potential issues before they become costly problems.

For businesses throughout Nova Scotia and the Maritime provinces, working with local engineering expertise offers advantages in communication, lead time, and the ability to collaborate closely throughout the project lifecycle.

Partner with Sangster Engineering Ltd. for Your CNC Machining Projects

Designing for CNC machining requires balancing technical requirements with manufacturing realities. At Sangster Engineering Ltd., our team of experienced engineers in Amherst, Nova Scotia, provides comprehensive support for your manufacturing projects—from initial design consultation through to production and quality assurance.

Whether you're developing new products, optimising existing designs for manufacturability, or seeking expert guidance on material selection and tolerance specification, we bring decades of experience serving clients across Atlantic Canada and beyond. Our understanding of local manufacturing capabilities and supply chains ensures your projects are completed efficiently and to specification.

Contact Sangster Engineering Ltd. today to discuss your CNC machining design requirements. Our professional engineers are ready to help transform your concepts into manufacturable, cost-effective components that meet your exact specifications. Let us put our expertise to work for your next project.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.