CNC machining parts generally start from designs or drawings that can be created by CAD software and saved as a DXF file, which is converted to G-code (CNC programming language), running and controlling the machines and cutters. Here we bring CNC machining design guide with steps, methods, rules, tips, and restrictions.
CNC is the acronym of Computer Numerical Control, the computerized automated machinery can be used to remove material from a blank as per specific design or perform certain machining operations according to the programming, the computer will convert the CAD design into numbers (graphic coordinates) to control the movement of the cutting tool of the CNC machine (mill, lathe, router, etc.)
The process of CNC machining:
– Design a drawing of CNC part
– Convert the design to programming language
– Run on the CNC machine
1. Search, collect and classify the sketches, images, photos, ideas or any useful resources for reference when creating an ideal CAD model.
2. Determine the position for locating part zero, and put the part in the CAD design, make the 0, 0, 0 origin is at part zero.
3. Create an initial model of the CNC machining part.
4. Add essential information or parameters like dimension, tolerance required.
Even though there is no common standard for the features and parameters of the CNC part, considering the processing availability and difficulty, some rules and recommendations are usually applied in actual situations.
Tolerance limits the permissible dimensional error or boundary, it’s related to the materials and geometry of the part, the standard tolerances are ±0.125 mm (0.005”), but some tight tolerance can achieve ±0.0125 mm (0.0005”) and even 0.0001”.
2. Hole size
The standard diameter (larger than 1mm) is recommended, check out the drill bit sizes and use a drill bit to process a hole first and then use a reaming or boring tool to finish the hole. If you used end mill to drill non-standard diameter, maximum depth (4 x nominal diameter) should be applied.
3. Thread size
The recommended thread size is M6 or larger, which is cut with CNC threading tools, and the thread length is better between 1.5 x nominal diameter and 3 x nominal diameter.
4. Thin wall parts
It is recommended that minimum wall thickness for metal thin wall parts is 0.8mm and 1.5mm for plastic parts, to ensure the material stiffness and reduce vibrations during machining, as well as improve accuracy.
Before designing a part for CNC, there are some tips you can master to speed up the process and avoid lots of problems.
1. Consider the geometry of the cutting tool used to remove material in the machining and features you designed should fit the processing method of the CNC machine.
2. When designing a part with Scan2CAD, remember to define the scale of your vector image, the system units can be set as millimeters. Reduce the vector lines to the lowest number of nodes and only export drawing parts into the final DXF file. Convert splines and arcs, remove spaces and draw one coherent cut path. Delete duplicate copies of objects.
3. Cavity depth should be under four times the tool diameter in order to reduce the machining difficulties and possibility of tool breakages.
4. Engraved text is better than embossed text because it reduces the waste of material and small features should be avoided.
5. Try not to use specific tolerances, because when requiring higher accuracy, more time and labor will be needed and causes increased cost.
6. In CNC programming, all the dimensions and positions of points, lines and surfaces are based on the programming origin. Therefore, it is better to mark the coordinate dimension directly on the part drawing, or use the same datum to introduce dimension as much as possible.
7. The conditions of geometric elements should be complete and accurate in programming.
8. In CNC machining, the working procedure is often concentrated, so it is very important to locate the same datum. It is often necessary to set some auxiliary benchmarks.