What is CNC?
Using a computer to aid in transforming a stock piece of material to a finished product is CNC.
CNC stands for computer numerical control, and started in the 1940’s and 50’s using punched tape or perforated paper tape.
It quickly transitioned to analog and then digital mediums, and now uses CAM or CAD files to give the instructions for the finished parts.
The computer files define the shape and sometimes give more information, like which layers to print and when for 3D printing, to the CNC machines.
Prototyping and other labor intensive mechanical processes can be shortened considerably by using CNC.
Often multiple tools are needed to make the cuts to process a part.
Basic machines use the x and y axis to position their tools, advanced machines use all three x,y,and z axes to interact with their materials.
Multi axis machines can even flip their materials to allow access to the underside of parts.
This eliminates steps that workers would have to perform, where they would flip the parts in between the machining done by the CNC machine.
Typically manual cuts do not have the accuracy of a computer cut, and they have a much higher chance of creating flaws and irregularities.
Finishing work and etching are often better performed by hand, as well as simple cuts that would otherwise take extensive design work to accomplish.
Conventional technologies for CNC include drills, lathes and milling machines.
Although novel technologies like electrical or chemical machining or other cutting mediums, like laser cutting, use CNC as well.
Materials and uses
Materials like brass, aluminium, copper, steel, and titanium as well as wood, fiberglass, foam, and plastics are used, although almost any material can be cut with the right machines.
Rapid prototyping by hand was filled with large error margins before the use of CNC and after the technology took off, options for large and accurate productions of prototypes exploded.
CNC has a competitor in 3D printing, with advantages for each.
3D printing can create internal structures and complex shapes with greater ease than CNC.
CNC, on the other hand, has a wider selection of materials available, as 3D printing is limited by what can be adapted into filaments suitable for 3D printing.
CNC can also be adapted for niche manufacturing purposes with electrochemical machining as an example.
For one-off and medium-volume production CNC is reliable and cost effective.
Process:
- Creating a CAD model
- Converting a CAD file to a CNC program
- Preparing the CNC machine
- Producing the finished part
Process step-by-step.
A 2D vector or 3D solid part file is needed to start the process of CNC, these are produced in CAD or computer aided design programs.
AutoCAD, Sketchup, Solidworks. Autodesk Inventor, CATIA, PTC Creo, FreeCAD, and OpenSCAD are all CAD programs.
Some of them are even freeware, like freeCAD, so finding a CAD solution is easy.
This file contains the measurements and coordinates of the parts, showing the space they occupy.
Designs have to be made with the restrictions of the CNC machine in mind.
For example most CNC tooling is cylindrical, so corners will be curved in the finished part.
Properties of the material, tooling design, and workholding capabilities of the machine further restrict the design possibilities.
Features like minimum part thickness, inclusion and complexity of internal parts and features, and maximum part size are all limited by these factors.
Once a compatible design is finished it is exported to a CNC-compatible file format.
Some common formats are AI, PDF, DXF, DWG for 2D and STL, OBJ, DXF, STEP, IGES, DWG, and 3DM for 3D.
Manufacturing the part:
The file is then run through a program, usually a CAM program, to analyse the geometry and produce the code that will ruin the machine and manipulate the tooling to cut the part.
Several programming languages are used, including M-code and G-code.
G-code, general or geometric code, is the most well known.
It controls where, when, and how the machine tooling moves.
This includes things like how fast to travel and in what direction, when to turn off or on and what overall paths to take across the material.
M-code, or Miscellaneous function code, controls the auxiliary functions of the CNC machine.
Automated processes are commonly M-coded, like the opening and closing of machine covers.
After the program is finished it is loaded into the CNC machine.
The machine has to be prepped before it can begin CNC production, with tasks like fixing the workpiece into the machine though spindles or vises.
Attaching or cleaning the tooling parts is also a common task.
After setup the program can be run.
The machine then executes the program, following the instructions of the file with its integrated computer.
Producing the desired part.
All of this can be produced in-house as long as the company has the CAD programs and CNC equipment.
Outsourcing to CNC machining service providers is also common.
Industries that use CNC in their manufacturing include aerospace, automotive, agriculture, commercial, defense, electronics, firearms, medical, and optical.
Types of CNC operations:
- Drilling
- Milling
- Turning
Discription of Operations
Drilling uses multi-point drill bits to create cylindrical holes in the workpiece.
The machine moves the drill bit perpendicularly to the workpiece surface plane.
Angular drilling can also be performed with more specialized machines and work holding devices.
Drilling can result in operations like tapping, countersinking, counterboring and reaming.
Milling rotates multi-point cutting tools to shave material from the workpiece.
CNC milling is performed opposite to the traditional method of milling, with the workpiece and the cutting tool moving in the same direction.
Operations performed by CNC milling include peripheral milling which cuts cavities, like slots and treads into the workpiece, as well as face milling which cuts shallow flat surfaces and flat-bottomed cavities into the workpiece.
Turning on a CNC machine uses a single-point cutting tool to remove material from a rotating workpiece.
This is usually performed on a lathe, removing material from the circumference of the workpiece until the desired diameter or shape is achieved.
Boring, facing, grooving, and thread cutting are all operations that CNC turning is capable of.
Some other CNC operations are broaching, sawing, grinding, honing, and lapping.
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