What is CNC Machining and How Does it Work

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A new high precision computer-controlled manufacturing technique called CNC machining has spawned. CNC machines or computer numerically controlled machines are electromechanical devices that can manipulate tools around a varying number of axes, usually three or five, with high precision per instruction from a computer program.

The first CNC machines were developed in the 1940s and 50s and relied primarily on a data storage technique known as punched tape. The code to control the machines would be manually punched into a data card and fed into a system that would then interpret the data.

Needless to say, these early machines were rudimentary and their functionality was limited. CNC machining technologies rapidly grew as technological capability only accelerated in the late 20th century.

CNC machining is one of two ways that engineers, machinists, or makers can generate a physical part from a computer design file, with the other being 3D printing known as additive manufacturing.

The contrast between these two techniques is stark. CNC machining, like other machining processes, is a subtractive process where the material is removed from stock, 3D printing, on the other hand, is an additive process essentially functioning in reverse.

CNC machining is a way to transform a stock piece of material such as aluminum steel or titanium into a finished product or part. It relies on digital instructions, specifically referred to as G code. 

Before modern computer-aided manufacturing and computer-aided design programs, such as Autodesk fusion 360 were around, machinists would manually write the G code to control these machines. CAM programs today will allow you to make a 3D model and automatically generate G code that will drive the machine with little input.

When you compare the capabilities of automated CNC machining to the manual alternative, you can start to see the benefits.

  • CNC machines simply run faster at higher precision and accuracy, while simultaneously allowing the transformation of a digital design into a physical part.
  • CNC machines are precise and are measured in thousandths of an inch, referred to as thou. 
  • Standard machining can provide tolerances on parts around ∓ .005 inches. Fine machining can produce tolerances of about ∓ .001 inches. In specialized processes like polishing, it can offer up repeatable tolerances as tight as ∓ .00005 inches.

Many designs or specific machining processes require the use of multiple tools to make cuts. One tool doesn’t function universally. For this, the machinist will often build digital tool libraries that interface with the physical CNC machine. These machines, often costing thousands of dollars, can automatically switch tooling when directed by their digital instructions, allowing for them to become manufacturing workhorses.

Basic CNC machining will move one or 2-Axis referred to as the X-Axis and the Y-Axis. You’ll often hear the terms 2.5-Axis, 3-Axis, and 5-Axis CNC machines which simply refer to the degrees of freedom a machine can make cuts in. 

A three-axis machine will move in the X, Y, and Z-axis whereas a five-axis machine can move in these three axes along with two additional rotational axes. The possibilities of production with five-axis machines are practically endless. 

Five-axis machines used to be relegated to high precision work but as they have become more affordable, they are quickly becoming standard in many shops. 

There are three conventional machining technologies that you need to understand to grasp the basics of how the material is removed from stock in CNC machines.

  • The drills that function by spinning a drill bit and moving a bit into contact with a stationary stock. 
  • The lathes function in reverse to drilling. Lathes spin a block of material against a stationary drill bit or cutter to remove material in a circular or fluid path. The shape capabilities on lathes are more limited than other techniques, but modern technology does allow these machines to create things such as square holes or non-circular shapes. 
  • The CNC milling machine involves the use of rotary cutting tools to remove material from a stock piece. These machines function similarly to drills with their tooling capabilities encompassing much more variety. 

Almost any material can be used in CNC machines from plastic to titanium. Different materials have different properties. Machinists and engineers will overcome their unique challenges by altering machining variables like tool selection, rpm, feed rate, and coolant flow among an extensive variety of other factors. 

CNC machined parts are all around you, they hold together your car and perform vital functions aboard SpaceX rockets. CNC machining is arguably the only way to produce high-precision metal parts for engineering designs across the world.

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