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Posted on Jul 13, 2020
The importance of high-quality, high-precision CNC parts and CNC machining and in today’s economy cannot be overstated. Just about any industry that relies on CNC machined parts uses high-precision CNC machines itself or it relies on professional machining services that create custom machined parts from raw materials into individual components.
The manufacture of CNC machining parts involves computer numerical control machining and is an automated machining process that enables high accuracy and precision. CNC machined parts, especially when they are complex, usually contribute to cost-effectiveness. On top of that, you can achieve a degree of complexity and intricacy that traditional machines could never accomplish.
An important process before actually machining your part with your CNC machine is the design of the part or component. During this process of designing a CNC machining part, there are a lot of things that you need to consider. For instance,
you should design your parts so that the tools of the largest diameters can machine them. You should also avoid designing cavities that are more than four times deeper than their width and make sure that your designs meet the requirements of one of the six principal directions. If this is not feasible, you should consider using 5 axis CNC machining.
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There is no set of standards accepted and used in all industries in the CNC machining world. The instruments and thus its capabilities used in CNC machining of parts also always evolving. In designing CNC workpieces, this uncertainty is a challenge.
However, following some general values and common practices can help you ensure that you come up with a high-quality and feasible design.
Both drill bits and end milling tools are used by machine operators to machine CNC parts. Use standard drill bits of dimensions measured in metric or imperial units as a guide when deciding on the diameter of the holes in your design. It is technically possible to have any diameter above one millimeter. When holes need to be very tolerant, the engineers use finishing reamers and boring tools. It is recommended for a hole less than about 20 millimeters in standard diameter that require high precision.
The recommended thread size is M6 or higher for the design of CNC machined parts, but you can only use M2 as a minimum. Machine operators use CNC threading tools for cutting threads down to M6, thus reducing the risk of tap disrupting. Could cut to M2 taps and dies.
Three times the nominal diameter the recommended thread length and a minimum nominal diameter are 1,5 times. You need to add an unthreaded length equal to 1,5 times the nominal diameter at the bottom of the hole for all threads below M6. The hole can be threaded over its whole length for threads larger than M6.
The recommended vertical corner radius for internal edges is one third or more of the cavity depth. When using the recommended corner radii, a diameter tool is used that complies with the recommended cavity depth guidelines. The slightly higher corner radii than the recommended quantity make it possible to cut along a circular path rather than a 90-degree angle that provides a superior finish for CNC machining parts.
The recommended depth of a cavity is four times its width because the cutting length of end mills is limited. Usually, this limit is 3-4 times the tool diameter. Increased vibration, deflection, and chip evacuation are the result of low depth-to-width ratios.
You can use a variable cavity depth to solve this problem if you need greater depths for your particular design. With special tools, you can have a ratio of 30:1 for tool diameter and cavity depth.
The walls of your CNC machined component should ideally be of a thickness of at least 0,8 mm for metals or 1,5 mm for plastics, although a thickness of 0,5 mm is technically doable, too.
Thinner walls increase the vibrations during the processing process and reduce the degree of precision. Thinner walls of plastic may also warp due to residual stress or adjust due to increasing temperatures.
You can add a few small features to your design, but you should keep some general rules in mind. Usually, any hole in a design should be at least 2.5 millimeters in diameter. This way you can be sure that machine shops with standard tools will be able to machine the holes in your part or component with high accuracy. With specialized tools, other diameters are also possible.
:: Read more: How to Machine Precision Parts?
When designing parts you should also keep in mind machining tolerances, in engineering and machining the word tolerance refers to the amount by which a dimension is allowed to vary. Basically, it determines the minimum and maximum limits of a specification.
If you don’t provide an information sheet or drawing of your part, most companies will use general tolerances for their CNC machined parts. These tolerances differ between shops, and some might not even provide default tolerances. In this case, you need to provide them yourself.
Companies that provide general tolerances may have them for wall thickness, tapped holes, and or surface treatments, so mostly general dimensions. A typical tolerance standard for metal parts would be +/- .005 inches.
You can ask a company to use tolerances other than your component tolerances. The tolerance you want to use will be determined according to your CNC machining part design, shape, and function.
If you want tighter tolerances then you should also expect a higher cost. The higher your requirements the longer the CNC machining of your parts might take and the more advanced CNC machine tools and inspection equipment must be used.
Limitations due to the Part Size
There are some limitations during CNC machining of metal parts due to the size of the component. These limitations depend on what type of CNC machining process is used to turn your design into an actual part.
If you are using a milling machine to produce tour CNC machined part, its height is determined by the size of the part features. The dimensions of a build space are not necessarily identical to the size of the parts that can be machined. Even though a machine may have the theoretical ability to travel a certain length on the Z-axis, you might not be able to machine a part with the same length, you will be limited to a little less than what that number is.
With a lathe, you need to pay attention to the diameter and the length of the workspace. These determine the largest part size you can machine. CNC machining parts service providers that have lathes with driven tools can machine parts with more intricate features while keeping the lead time low.
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