Introduction to Computer Numerical Control
Computer numerical control, or CNC machining, is a computer-aided, high-accuracy manufacturing process. Pre-programmed CAD software is used to automate the controlled machining and eliminate the need for an operator. The main advantage of CNC machines is their ability to run unattended during the machining cycle and manufacturing process, allowing the operator to carry out other tasks elsewhere.
This drastically reduces human error during the controlled machining process and allows for high accuracy manufacture of the different parts. Another benefit of CNC machining is consistent and accurate workpieces.
The CNC machining operations of today benefit from not only high accuracy machine tools and code controls, but also the ability to repeat multiple manufacturing processes on separate occasions. The flexibility of CNC programming easily allows CAD files to be tweaked and changed to produce multiple different parts.
All CNC machines work based on a 3-axis motion control process. The X, Y, and Z axes are positioned with high accuracy along their length of travel. Most axes are linearly positioned, but some are also rotary, meaning that they move around a circular part. CNC machines work with a range of motion actuated by servomotors and guided by computer-aided code controls.
Overview of types of axis machining
CNC machining operations work with 3 main types of computer-aided axis machining.
The most simplistic of the three types is 3-axis machining, where the workpiece is fixed throughout the manufacturing process and able to move in the standard linear X, Y, and Z directions. This type of controlled machining is largely used for manufacturing processes in 2 and 2.5 dimensions.
4-axis machines work with the standard X, Y, and Z axes, and add another axis (often known as the A-axis) of rotation around the part being manufactured. This type of manufacturing process is used as a more cost-effective method of computer-aided controlled machining that would theoretically be possible with a 3-axis machine, but require more time and conde controls.
Finally, 5 axis machines work by use of 2 out of 3 possible rotation axes. There are two main subtypes of 5 axis CNC machining operations: 3+2 machines, or fully continuous 5 axis machines.
In 3+2 machines two rotation axes (either the A and C axes or B and C axes) will operate independently from each other, allowing the workpiece to be rotated by a compound angle relative to the main cutting tool. In a continuous controlled machining manufacturing process, the two rotation axes can be simultaneously altered alongside the machining and cutting tools moving in the standard linear axes.
3 axis machined parts
3 Axis machining is suitable for fairly simple 2D parts that don’t require much depth or detail, such as basic brackets, plates with holes in them, or simple aluminium moulds.
3 axis machining works using the following operations and principles:
- automatic or interactive operation
- milling slots
- drilling holes
- cutting sharp edges
4 axis machined parts
The main advantage of 4 axis machining is that utilising the A axis of rotation eliminates the need for multiple fixtures, and fixture changes, which reduces the overall cost of the manufacturing process. It allows for the production of angled parts, which are not otherwise possible in standard 3 axis CNC machining. It should be noted that all angled features must lie about the same axis for the manufacturing process to be optimised successfully.
The two subtypes of 4 axis machining are index 4-axis machining and continuous 4-axis machining. In the first, the axis rotates when the machine is not cutting the material; in the latter, the material can be cut and rotated simultaneously.
The following components are suited to 4 axis controlled machining:
- Cam lobes
- Plane type part
- Variable bevelled parts
- Curved surface parts
5 axis machined parts
5 axis machining specifies a workpiece that can be manipulated from 5 sides at a time. This type of complex machining is commonly used in the automotive, aerospace, and boating industries. It is best suited to complex solid components that would otherwise need to be cast.
5 axis machining requires more complex CNC programming and code controls. It is most effective for high feature accuracy, increased productivity, higher quality finishes, cutting intricate details, and machining complex shapes.
Some of the machined parts used within the manufacturing process for 5 axis CNC machining are:
- End mills (flat, ball, bull, and chamfer) face mill
- Corner rounding tools
- Slot tools
- Spot-centre drill
- Twist drill
Alongside the different axes the CNC machining manufacturing process operates within, there is also an extensive variety of CNC machine tools allowing a wide variety of cuts and incisions to be performed.
CNC machine tools are at the cornerstone of all CNC machining operations. In the most basic terms, a cutting tool is a tool affixed to a CNC machine that is then used to remove material from the workpiece by shear deformation. The machines work by having the tool rotate at rapid speed, making cuts and chips at the workpiece at either singular or multiple points. The machine tools used within the manufacturing process affect the size of the chip removed from the material. The speed and feed rate will also influence the final result of the controlled machining.
An extensive list of all CNC machine tools is outwith the scope of this article, but some examples are listed below:
- End mills (Flat, Ball, Bull and Chamfer)Face mill
- Corner Rounding tools
- Slot Tools
- Spot-Center Drill
- Twist Drill
Flat nose mills are used for milling 2D contour pockets. Ball nose mills are used for 3D milling, and bull nose end mills have a radius corner. Chamfer mills have an angled nose used to create a chamfer to deburr parts.
A face mill has a cutting insert that is replaced when worn. These are rigid and may have up to 8 or more cutting edges, suitable for quick removal of material. They are often used for the first machining operation to create a flat finished face on the part.
Corner radius tools are used to place a fillet on the outside corner of a part
Slot Mill/Slotting Saw
Slot mills include side milling cutters and Woodruff key cutters used for creating slots.
Hole-making tools: Centre spot drills
Short and rigid drills are used to create a conic on the face of the part.
Countersunk drills are used to create the conical face for a machine screw, and combined countersunk drills create the screw clearance hole and the countersunk in a singular motion. Twist drills are available in many lengths and are made of high-speed steel, carbide, or cobalt coated with titanium nitride for a longer lifetime. The tip angle is 118 degrees.
This type of CNC machine tool forms threads by shearing material away. Form taps work by forming the metal into shape. They produce no chips and are used for soft materials such aluminium, copper, brass, and plastics. Bottoming taps are used to tap blind holes. Spiral point taps push the chip ahead and out the bottom of a hole. Care should be taken to select the correct drill size for drilling the holes to be tapped.
Reamers are used to create holes of precise shapes and excellent surface finish. Reamers provide high accuracy and are best used for ground pins and bushings. They also require a specific hole size to be drilled before use.
A counterbore looks similar to an end mill with a pilot in the centre. Within CNC manufacturing processes, it is used to spot face holes. The function of the pilot is to ensure the spot face is centred on the hole.
Cutting Speeds and feeds
The cutting tool moves through the material at a certain rotational speed defined in revolutions per minute (measured in RPM) and feed rate (measured in mm per minute through the linear feed of the material). It is important to select proper speeds and feeds for the material and part. This selection is more difficult than a manual mill, in which the operator can feel the pressure and alter the feed based on the cutting force. CNC mills require speeds and feed to be programmed via code controls in advance. The tool supplier provides guidelines for the RMP and feed rate of specific tools.
When it comes to the manufacturing process, there are specific formulae for the speeds and feeds used in CNC programming of the CAD software in advance.
CNC milling refers to a type of computer aided controlled machining that uses rotary tools to make cuts at materials during the manufacturing process. The machine reads the geometric code from the CAD file and replicates the design using the machine tools with high accuracy.
CNC Mills are very common and can be used for many geometries. The workpiece is held rigidly in a jig or vice, and the mill head moves in the 3 axis plane to remove the material using high-speed rotary tools or drills.
Due to the limited range of motion, they are relatively easy to operate and program, so set-up costs are low compared to other CNC processes. However, the limited range of movement means that there are some limits to the manufacturing process.
This can be overcome by using the machine tools to reorientate the part, however, each adjustment in the manufacturing process adds extra time and risks possible error, meaning that costs can increase quickly.
CNC turning is a high accuracy computer-aided manufacturing process in which the material making up the workpiece is held and rotated by the machine while the tool chips at it to create the desired shape. This type of controlled machining produces parts at a higher rate than CNC milling, which makes it a highly cost-effective process and particularly useful for manufacturing large numbers of units.
CNC turning machines work by holding the workpiece on a spindle and rotating them at high speed. The cutter used is typically a blade, unlike the rotary cutters used in CNC milling.
Due to the nature of the manufacturing process, this kind of CNC machining can only produce revolved or rotationally symmetrical parts along with a central access (eg. cylindrical parts and threads). If a more complex design is required, the part will often then be transferred to a CNC mill for further controlled machining.
There is a wide range of CNC machining operations suitable for different manufacturing processes. When choosing a partner for producing your CNC machined parts, don’t leave quality to chance. Upload your files today, and get started with a free quote from Geomiq.
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Disclaimer: The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of Geomiq. Examples of analysis performed within this article are only examples. They should not be utilized in real-world analytic products as they are based only on very limited and dated open source information. Assumptions made within the analysis are not reflective of the position of any Geomiq Employee.