CNC (Computer Numerical Control) machining is a process of removing block materials via high-speed rotating cutting tools to shape components based on 3D drawings. As the cutting tools directly contact the part and remove excess material to obtain the desired part, they play a vital role in CNC machining.
The most common CNC cutting tools, such as end mills and drill bits, have cylindrical tool shanks, specific tooltip shapes, and limited cutting lengths. These two characteristics limit the cutting ability and prevent the machining of deep narrow holes and undercuts. So based on this, we'd like to provide some commonly used cost-saving design tips for CNC machining parts.
Tips on Minimum Radius
The CNC milling cutter has a cylindrical shape. When cutting internal walls, a radius will appear at the vertical angle. Small tools can be used at a lower speed to achieve a small radius. But this will result in more time and higher costs.
Therefore, when designing CNC machining parts, it's best to increase the depth radius and use a similar radius for the inner edges. The minimum R (Raduis) for all internal right angles of the workpiece will be processed under a certain rule if you do not specify it in the 2D drawing. Different vendors of CNC machined parts in China apply different rules. Normally, responsible vendors will figure it out while offering their quotation.
If possible, make the R angle as large as possible. The workpiece must retain right angles if the spark erosion deburring is necessary (deburring requires CNC machining of copper first and then spark erosion machining, which is expensive), or voids can be created in the workpiece for the R angle, which can be machined directly by CNC machines at a lower cost if the workpiece is designed differently. If there are through holes on both sides of the workpiece, deburring can be performed by cutting and machining on a wire cutting machine, which is more expensive.
Tips on Threads
1. To reduce communication costs and prevent processing errors, remember to design threads according to standard drilling hole diameters and rolling outer diameters and specify the thread parameters in detail. When placing orders, try to process matching internal and external threaded parts in the same order.
2. Each engineer draws a 3D thread bottom hole with a different diameter. For instance, the standard diameter of the bottom hole for M3x0.5 is ∮2.5. During programming, the bottom hole is directly programmed to be drilled with ∮2.5, followed by automatic tapping. If the bottom hole is drawn as ∮3, an oversized bottom hole may not be suitable for threading (although it is possible to fix some by adding a threaded sleeve). If you work with a vendor of CNC machined parts in China, it's better to use international standards to avoid misunderstanding.
3. For special threads, physical samples need to be provided for fitting.
4. Strong thread connections should be designed in the first few threads. In some cases, very long thread lengths are unnecessary. Long threaded holes may require special tools and additional processing time and cost. It is preferred that the thread length does not exceed three times the hole diameter. In the case of blind holes, it is advisable to leave at least half the hole diameter as unthreaded length at the bottom of the hole.
Tips on Cavity Depth
Machining deep cavities increases the cost of CNC parts due to the extensive material removal and time-consuming process. CNC cutting tools have limited cutting lengths, and the optimal machining results are achieved when the cutting depth reaches 2-3 times its diameter. For instance, a ø12 end mill can safely cut cavities as deep as 25mm.
Cutting deeper cavities (5 times or more than the tool diameter) can lead to issues like tool deflection, vibration, chip evacuation challenges, and tool breakage. It necessitates the use of specialized tools or multi-axis CNC systems. In addition, when cutting cavities, the tool must be inclined to the correct cutting depth, and a smooth entry requires adequate space.
Limiting the depth of all cavities to five times their length (i.e., the maximum dimension on the XY plane) can help minimize machining costs.
Tips on Wall Thickness
Machining thin walls requires multiple shallow cutting passes, which can lead to vibration and result in deformation or breakage. As a result, precise machining of thin walls is challenging and may increase processing time.
Recommended wall thickness for CNC machining:
Metal parts: the best is to be at least 0.8mm (with a minimum of 0.5mm)
Plastic parts: the best is to be at least 1.5mm (with a minimum of 1mm).
Tips on Tolerances
The tighter the tolerance, the higher the machining cost, as it increases both processing and quality inspection time.
If specific tolerances are not indicated on the part drawing, standard tolerances (±0.1mm or higher) will be applied for machining. For any special tolerance requirements, it is crucial to provide a 2D drawing with corresponding annotations.
Tips on 2D Files
A 2D drawing is the optimal way to communicate specific design aspects. It includes precise annotations for key features such as tolerances, surface roughness, assembly methods, critical inspections, and quality control. It provides a valuable reference for selecting the best machining method and process route, ultimately leading to lower costs.
Also, make sure to include synchronized annotations for thread hole dimensions and depth.
While reviewing engineering drawings, we will compare 3D and 2D drawings. Any conflicts that arise can be promptly addressed through effective communication and feedback.
However, every project is unique. Besides the above tips you can consider while designing CNC machined parts, you can involve your vendors early to take advantage of their expertise. As a CNC machined part in China, with over a decade's experience, our CNC machining engineers at X Rapid Technologies are always ready to kick your project off. Get in touch today!
