CNC (Computer Numerical Control) machining is one of the most commonly used techniques for rapid prototyping to create physical parts for design validation. Understanding the cost of CNC machining helps reduce the initial product development costs. This article dives into the factors that affect CNC machining costs and offers strategies to reduce them.
Factors Affecting CNC Machining Costs
As an advanced manufacturing technology, CNC machining can provide complete automation without human error after setup. It can operate at high speed and process complex parts. Besides the labor costs, many factors affect the cost of CNC machining.
There are many types of CNC machines, such as lathe, box-type, C-shaped type, and gantry. Different types of cutters will affect the overall cost of CNC machining operations.
Also, the number of axes of CNC machines directly determines the complexity of the parts in CNC machining. Although multi-axis machining provides more manufacturing freedom and capability, using the appropriate number of axes can also reduce costs.
Different CNC machines have different prices, resulting in differences in labor costs. For example, there is a cost difference between a $20,000 machine and a $2,000,000 machine. Using multiple machines to make a part and using one machine to apply different functions to complete it will also result in different prices.
For producing parts with different sizes, CNC machines often have different sizes. Making large-sized components requires more complex CNC machines, and the cost of using a suitable machine tool for CNC machining small parts is lower than that of using a high-end one.
Size and geometry of parts
The larger the part, the more material is needed to manufacture it, and the more waste during processing, thus increasing the cost.
At the same time, producing more complex parts will increase costs, possibly requiring more advanced machinery, longer machining time, multiple assembly and equipment setups, more resources, and closer inspection.
Material selection and costs
The cost of materials is one of the most vital factors of part costs, including raw material costs, material quantity, and processing time.
As the number of parts increases, the unit cost of a set of identical parts decreases significantly. This cost reduction is mainly due to removing duplicate programming costs and the results of machine setup and debugging. Programming and machine setup are all completed once and used for all parts. That is to say, the cost of a single part includes setup and programming costs. Assuming 1,000 pieces are to be produced, setup and programming costs go shares on 1,000 pieces.
Tooling and cutting costs
The price of tools is essential to CNC machining costs. Whenever it is necessary to change the size or shape of a CNC-machined part, corresponding tooling changes are required. Even some tools like toolings and fixtures can be reused, but there are also consumables such as cutting tools.
Manufacturers with all available tools and raw materials can provide faster and shorter delivery cycles. However, once the material needs to be purchased or special tools are required, the delivery time will be longer. Because manufacturers still have to pay corresponding labor costs for longer or shorter delivery cycles. And longer delivery cycles will result in resource waste and higher overall costs.
The fundamental purpose of CNC machining is to eliminate labor costs in manual processes such as turning, moving, or operating tools. However, labor costs are still needed in changing tools, setting workpieces, and for workers' salaries.
When a part is complex and requires highly skilled technicians, its machining costs will rise. These additional skills and experience come at a cost because they are mainly obtained from on-the-job experience rather than formal education. The higher the labor intensity of a part's production process, the higher its price.
Before machining the parts, tolerance and surface roughness requirements need to be clearly defined. Stricter tolerances will make it more difficult to machine and increase scrap rates, which will also increase costs; higher surface roughness requirements may make it impossible for CNC milling machines to achieve them, so grinding machines or even other methods must be used, increasing processes and naturally increasing costs.
After CNC machining, some finishing may be required, such as vacuum plating, to improve the functionality, performance, and aesthetics. These post-processing requirements will increase the cost of parts to some extent.
Another cost-driving factor is applying different surface treatments to various areas of the part. It includes partial electroplating, partial anodizing, bicolor oxidation, oxidation in one area, and electroplating in another.
Strategies to Reduce CNC Machining Costs
After understanding the main factors that affect CNC machining costs, we can now learn how to reduce costs throughout the entire process from design to machining.
Design optimization for manufacturability
1) Avoid thin-wall designs
Thin-walled parts require more time to machine due to their fragility. They are prone to vibration, and deformation, and difficult to keep accurate tolerances. Slow machining, special techniques, and high scrap rates make these thin-walled parts more expensive. The suggestion is to use thicker wall designs with a minimum wall thickness of 0.8mm for metal parts and 1.5mm for plastic parts.
2) Avoid features that cannot be CNC machined
Not all features can be CNC machined, such as 90° internal corners. CNC milling tools have cylindrical shapes, resulting in rounded corners instead of sharp angles. If sharp angles are necessary, it usually requires a more expensive manufacturing process like EDM (Electrical Discharge Machining). For assembly purposes, use filleted corners instead.
3) Use larger internal corner radii
While smaller tool diameters can achieve smaller corner radii, they require multiple passes at lower speeds since they cannot remove material as quickly as larger tools. It increases machining time and costs.
It is better to have a corner radius (R) of at least one-third of the cavity depth (D). Additionally, designing internal edges that do not exert excessive pressure on the tool can help reduce tool wear. A good practice is to have a corner radius slightly larger than the tool radius used for machining the cavity (e.g., a 6.5mm radius for a milling tool with a 5mm radius).
4) Limit the depth of cavities
Machining deep cavities significantly impacts the cost of CNC parts due to the extensive material removal required, which is time-consuming. Deep holes can lead to tool deflection, difficulties in chip evacuation, and tool breakage. When the milling depth exceeds twice the diameter of the milling tool, the feed rate must be reduced, increasing machining time and part cost.
Generally, the milling depth should not exceed four times the tool diameter. The maximum depth should be four times the width of the cavity (e.g., a 15mm wide cavity should not exceed 60mm in depth).
If deep holes are unavoidable, a step-by-step approach with gradually decreasing stepovers can be employed, although it is time-consuming. Additionally, when machining deep cavities, the tool must be tilted to the correct cutting depth, requiring sufficient clearance.
5) Avoid complex surface features
When designing CNC machined parts, simplicity is key. Minimize or avoid complex surface features because they require smaller tools to achieve desired surface smoothness. These smaller tools significantly increase machining time and costs. To minimize costs and machining time, minimize or avoid the design of complex surfaces. For example, when chamfering outer edges, use chamfers instead of fillets unless necessary.
6) Limit thread length
Strong threaded connections primarily occur within the first few threads. Therefore, very long threads may not be necessary and can require special tools and additional machining time, increasing costs. The advice is to limit thread length to no more than three times the diameter of the hole. For blind holes, leave at least half of the hole diameter as unthreaded length at the bottom.
7) Design holes with standard sizes
Standard drill bits can quickly and accurately create holes. From an economic standpoint, it is preferable to use standard hole sizes. Non-standard hole sizes may require drilling with milling tools, which can increase costs. Additionally, the depth of drilled holes should generally not exceed ten times the diameter of the drill bit.
Material selection and waste reduction
Different materials have varying prices, which may differ in diverse market regions. When selecting materials for parts, choose suitable materials based on the part's purpose and functions, rather than opting for expensive materials that exceed performance requirements. For example, stainless steel 316 is more costly than stainless steel 304, even though they both belong to the stainless steel category.
When designing part structures and manufacturing processes, it is essential to consider minimizing material quantity. If necessary, parts can be designed in a way that two relatively simple components can be assembled to form a complex one that requires more material quantity and processing time.
The cost of a part is directly proportional to the time required for its machining. The processing time of a material is related to its machinability. Materials with low machinability require more time for machining and also consume more resources such as cutting fluid, electricity, and cutting tools. For example, softer materials like aluminum alloys are easier to cut, resulting in shorter machining times. However, harder materials like stainless steel require more expensive machining tools and are more prone to tool damage and wear, leading to higher part machining costs.
Streamlining the production process
Minimize CNC machine setups. Flipping or repositioning parts increases manufacturing costs as it often requires manual intervention. Additionally, complex geometries may require custom fixtures, further increasing costs. To reduce the number of parts flips and repositioning, design all features that require machining on a single plane. If unavoidable, consider splitting the workpiece into several pieces that can be CNC machined in a single setup and then assembled using bolts or welding.
Avoid adding text on part surfaces. Adding text through CNC machining only increases machining time and costs. However, if text and fonts are inevitable, you can follow the below rules.
- Adding text on part surfaces through silk screening or spraying is more cost-effective than CNC machining.
- On curved surfaces, use laser engraving as it requires less material removal.
Outsourcing vs. in-house machining
If you have in-house machining capabilities and tight deadlines, it is advisable to prioritize in-house machining.
When cost control is a priority or if you don't have CNC machining capabilities, outsourcing is a better option. A good machining company may not be the cheapest, but it will provide you with the best value for money and results. Some machine shops offer lower CNC machining costs per part but compensate for this by reducing quality. While they may initially seem cost-effective, you may regret it when you receive parts with unacceptable quality. To address the price vs. quality dilemma, find a trusted machining company that can provide you with satisfactory quality and pricing.
Now, we know that many factors influence CNC machining costs. When you don't have in-house CNC machining capabilities, the best option is to find a suitable supplier. Requesting quotes from multiple suppliers can provide multiple references for machining costs.