Computer Numerical Control (CNC) machines offer engineers one of the most cost-effective ways to create complex parts, with exceptional levels of accuracy and repeatability. However, generating CAD designs for CNC machining can be challenging to prevent manufacturing inaccurate or defective components.

In this guide from Redline CNC, we explore the key issues that design engineers need to consider when formulating CAD designs for CNC machining, including:

5-axis CNC machining is well-known for its ability to produce geometrically complex components, but that doesn’t mean the impossible is suddenly possible! When creating a CAD design for a part for CNC machining, being aware of the most common design pitfalls will help you avoid complications that could result in an inferior product.

When designing a component, remember that the workpiece will be subjected to significant stress during machining. 5-axis CNC machining uses shorter tools, which minimises vibration, but thinner walls on the workpiece reduce stiffness in the material, which can increase vibration and cause distortion. Maintaining low wall height to length and wall height to thickness ratios reduces this risk but, if very thin walls are a necessity, an alternative machining method may be more suitable.

Overcomplicating the CAD design is one of the most common mistakes engineers make. 5-axis CNC machines can cut highly complex parts but are most cost-effective and efficient when machining simple shapes. Unnecessarily complex components result in longer lead times and higher costs. A solution is to machine several smaller parts and assemble these later, which will reduce dependency on time-consuming precision milling.

While 5-axis CNC machining offers excellent versatility to create complex designs, there are some features on CAD software that cannot be efficiently translated into machined parts. One of these is curved holes, which are not easily formed, even with the additional angles that CNC equipment provides. An alternative method for the machining of curved holes is Electrical Discharge Machining (EDM), which is more adept at creating these features.

Tolerances are an important consideration when creating CAD designs for CNC machining, but they vary between different machines. A common mistake, therefore, is to determine tolerances without an understanding of the CNC machine that will be used, increasing the risk of inaccuracies, and raising costs.

When machining a part, remove needless tolerances from the CAD design in-line with the machine specification and do not assign numeric callouts (such as diameter and radii) to surfaces that do not require tolerances. A standard tolerance of ±0.125mm is acceptable for most features where the tolerance is not specified on the design.

5-axis CNC machining is most cost-effective when the material removed in the process is limited to what’s necessary, rather than carrying out extensive alterations to the workpiece purely for aesthetic reasons. Designers should carefully consider whether complex visual features are required, as this will lengthen the machining process and increase costs. Post-machining methods, such as electro-polishing, may be a more cost-effective way to add aesthetic features later, if necessary.

A benefit of CNC machining is the use of short cutting tools to reduce vibration and improve the surface finish of the part. Generally, cutting
tools can only create cavities three times their length, so a 10mm tool can reliably cut cavities only 30mm deep. Milling deep cavities can cause tool deflection, tool fracture, and difficult chip removal.

While it is possible to add lettering to a metal or plastic machined part using CNC technology, there are rules to follow to ensure a high-quality finish. Embossed text necessitates a considerable removal of material, whereas engraved text – while sometimes more difficult to read – is quicker and more cost-effective to add. CNC tools have a limit to how fine they can cut, so careful thought must be given to the font style and size. Sans Serif at a minimum size of 20pt. and a depth of 0.3mm is the best choice but, if smaller text is required, an alternative method may be appropriate. Alternative methods, such as laser etching, can add text to a part post-production for a lower price than machining.

CNC machining provides exceptional reliability and quality but, if costs are not controlled from the outset, it can prove to be an expensive option. Designers need to consider how to balance the need for superior standards and quick lead times with cost control to keep the project within budget.

Optimising your CAD design and choosing the best materials for the project will deliver greater cost-efficiency with no loss of quality or functionality. What best practices should you follow when planning CNC CAD designs?

CNC tools are cylindrical in shape and form a radius when cutting edges, but multiple passes at a low speed to remove material will increase machining time and costs. An effective solution that delivers greater cost-efficiency is to add a radius at least a third of the depth of the cavity for all internal edges, so that tool changes aren’t required. The corner radius should, ideally, be slightly larger than the tool radius to limit load on the tool.

Thicker walls offer greater stability and are less costly to machine, so CAD designs should avoid excessively thin walls unless the weight of the part is an important consideration. Thin walls can be machined if multiple passes at low cutting depths are made but, due to the increased risk of vibration, machining is more challenging and time-consuming. For metal parts, design walls that are at least 0.8mm thick, or 1.5mm for plastic.

CNC machining is more expensive if threads are longer than needed, as special tooling may be necessary. Threads longer than one-and-a-half times the diameter of the hole do not offer any additional strength; instead, design holes that extended 1.5 diameters to the tap’s deepest point. In blind holes, add a minimum of half of the diameter of the unthreaded length at the bottom.

Standard drill bits can create accurate holes in CNC machining, whereas non-standard sizes require additional machine set up, thereby inflating the cost. For blind holes, the depth should be limited to four times the diameter. Up to 10mm diameter, design holes in increments of 0.1mm, or 0.5mm for holes with diameters greater than 10mm.

Vibrations are more common when machining small features with a high aspect ratio, making it more challenging to achieve a high level of accuracy. Design features with a width-to-height aspect ratio below four are cheaper to machine, while using a bracing support or fixing the part to a wall will increase stiffness and simplify machining.

Reliability, versatility, and repeatability are widely recognised benefits of CNC machining, but effective Design for Manufacture (DfM) can be instrumental in enhancing these advantages to deliver better production efficiency and lower costs.

The principle of Design for Manufacture is to create products in an easier and cost-effective way. Rather than focusing purely on the final part in the CNC machining process, DfM covers every element of the manufacturing process to produce flawless parts at a competitive price, including:

Although Design for Manufacture takes time to get right, the improvements in production efficiency are well-worth the investment and effort:

DfM ensures your product is viable
Some products, even if well-designed, simply cannot be manufactured using CNC technology. Consequently, many CNC prototyping machines incorporate DfM as part of the production plan to ensure the part’s viability before embarking on a long manufacturing run. DfM will help to ensure that essential features required for a part’s functionality will work at the final design stage.

DfM reduces mistakes prior to production
If a part enters production before mistakes are identified, the damage can be costly in terms of wasted time and money. By incorporating all aspects of the part’s design, Design for Manufacture makes it easier to spot problems that could result in poor product quality and allow amendments to be made.

DfM shortens lead times
When your clients expect parts to be manufactured and delivered quickly, it’s vital to anticipate and correct potential mistakes prior to production. Without DfM, however, product quality can only be assessed once production has started, so a series of changes are likely to be required before the final design can be approved for manufacture. With DfM, you can eradicate mistakes early, thereby shortening lead times, saving money, and reinforcing your business’s reputation.

At Redline CNC, we offer a personalised one-stop solution for all your 5- axis CNC machining needs that blends expert knowledge, cutting-edge technology, and an unwavering commitment to quality to guarantee outstanding results every time.

Extensive industry experience: Trusted by businesses in multiple industries, including medical manufacturing, power production, automotive, aerospace, and defence because of delivering consistently deliver high standards, Redline CNC is the first-choice for your project, whatever the demands are. Our CNC machining is backed by our BSI ISO 9001-2015 certification, demonstrating our commitment to delivering exceptional standards.

The latest cutting-edge technology: We use the Edgecam and Alphacam systems to program our CNC equipment and the Factory Master MRP System to control the entire production process, so you can be confident that your project is in safe hands.

Outstanding training and expertise: We provide bespoke training for every one of our team to ensure the highest standards of CNC machining. Many of our staff have been part of our team for many years, so we offer a consistent approach that always puts quality first.

Our friendly and knowledgeable engineers are ready to work in partnership with you to create the complex components you need for your next project using the latest cutting-edge CNC technology.

If you would like to find out more about our services, why not send us your designs, so we can provide you with a free, no-obligation quotation? Please get in touch today by sending us a message and one of our team will be delighted to assist.