CNC machining is the process of machining a block, forging, casting using a 3-axis, or a 5-axis machining center. It is a subtractive machining process and CNC
milling aluminum is one of the most machining methods. Besides aluminum, the machined raw materials plastics, steel, and alloys. It is principally used in the die-mold, automotive, and aerospace industry. The answer to the quest for quicker prototyping with lesser material waste and higher accuracy lies in CNC milling.
Types of CNC milling processes
The process is machining flat surfaces or a contour by fixing the work-piece on a table of a CNC machine. The milling processes can be broadly categorized as:
⦠Plain Milling- where the axis of rotation of the cutting tool is parallel to the work-piece surface.
⦠Face Milling- where the axis of rotation of the cutting tool is perpendicular to the work-piece surface.
⦠Angular Milling- where the axis of rotation of the cutting tool is inclined to the work-piece surface.
⦠Contour Milling- where complex contours are achieved by tilting the cutting toll or/and table.
⦠Straddle milling- where the cutter machines on 2 opposite sides. An example is dovetail milling. This balances the load on the cutter leading to better accuracy and surface finish.
What materials can be used for CNC milling?
The metals which can be milled are
⦠Mild Steelrn⦠Aluminumrn⦠Brassrn⦠Stainless steelrn⦠Alloy Steel
The Plastics which can be milled are
⦠ABSrn⦠Nylonrn⦠Polycarbonatern⦠PTFE (Poly Tetra Fluro Ethylene)
⦠HDPE (High-density Polyethylene)
What kinds of tools can be used by CNC machining?
The tooling of the CNC machining center is varied having different forms and sizes. They are held in a magazine called ATC (Automatic Tool Changer) where normally 50 or more tools are housed and the CNC calls the tool as per the required operation and the change time from one tool to another is lesser than 0.2 sec. The most common tools are :
a) Cutters- which is essentially coated Tungsten-carbide inserts mounted on a fluted holderrnb) End mills- fluted Carbide with coated Tungsten-carbide tips.rnc) Ball-mills- slender tools for finishing purpose to contour the profile or to machine sharp corners.rnd) Drill bits- to create holes.rne) Taps- to create an internal thread in a hole.
It must also be noted that due to the geometries of the component and tool-path -always an off-the-shelf tool from the cutting tool providers like Sandvik, Seco, Widia, etc might not be the right answer due to interferences and chatter. Sometimes a component-specific tool holder might be required.
What parameters you should know about CNC milling?
CNC milling is a complex process that depends on the mechanics of cutting. And deformations due to temperature, the rigidity of clamping, the rigidity of machine-tool, and cutting-forces are to be considered. Expertise is required to optimize the machining process. The advantage of CNC machining is being able to machine a prototype from the given design and if necessary be able to modify it with offsets in the CNC milling process.
To achieve the best surface finishes, the trend now is to avoid or completely do away with grinding. This is achieved by adopting the latest techniques like Adaptive Milling cycles using certain software where constant chip load and cutting forces are maintained by designing the tool path accordingly. Studies show up-to twice better surface finish is achieved with the corresponding reduction in tool-wear.
Tool-life depends primarily on feed, depth-of-cut, and rpm. To maximize tool-life it is recommended to increase the depth-of-cut rather than increase the feed. Taylor’s law of tool life is used for optimizing and calculating the expected tool-life which is
VTn=Krnwhere V= cutting speedrn T= tool lifern n and K are constants.
CNC Machining process of a specified prototyping component requires expertise in selecting the CNC machine tool, work-holding, machining sequence, cutting-tools, cutting-parameters, CNC program, and an expert technician to realize the prototype. It is crucial to optimize the variables of the cutting process which are:
a) Feed (mm/rev)rnb) Spindle-speed (rpm)rnc) Depth of Cut (mm)rnd) Work-holdingrne) Tool radiusrnf) Cutting fluidrng) Temperature control
This will avoid tool-wear and frequent changing of inserts, eliminate vibrations, improve surface-finish, maintain tolerances and flatness, eliminate deformations and also reduce the cycle-times.
Factors to be considered for cost-effective when CNC machining
⦠Machining time: The machine hour rate of a CNC machining center is high and so- higher the cycle time- more will be the component cost.
⦠Design cost: The number of man-hours required to design the component and edit it for revalidate.
⦠Material Cost: If the prototype material is expensive- care should be taken to have the least machining allowance in the raw input blank.
⦠Form of the prototype: If it requires high surface-finish or tolerance and if it has thin walls - special tools and reduced speeds might be required which increases the cycle time and the cost of the prototype.
Conclusion
CNC machining process is the best available solution for high accuracy and medium to low batch productions. Rapid prototyping with Additive manufacturing Processes is the new challenge to CNC prototyping but is limited as of now with the accuracy of the finished components. The next generation of prototyping is a hybrid of Additive manufacturing and CNC Machining. This results in quicker prototyping with lesser material waste and higher accuracy.