What is CNC Milling?

What is CNC Milling?

CNC milling machines interpret numerical code to move a rotary tool with the right angles, speeds, and directions according to their axes to cut a piece of material into the desired shape.

CNC milling machines function in a similar way to all their CNC machine tool siblings. They interpret numerical code to move a rotary tool with the right angles, speeds, and directions according to their axes to cut a piece of material into the desired shape.

Milling is a common method of CNC manufacturing thanks to its versatility, accuracy, and relative simplicity to program when working with three axes. Here we look at CNC milling in closer detail and discuss why it’s so popular in mass production.

What is CNC milling used for?

CNC milling is used to create highly precise parts with a variety of features. Milling machines cut create slots and holes, threads, and a wide variety of features as well as flat surfaces. Due to the wide range of things a CNC milling machine can do, they find their way into a lot of different manufacturing focuses like creating machine components, toys, car parts, and much more.

Milling machines can be used with materials from wood and plastics to ceramic and metal. Tools of differing design and durability exist to specify a milling machine for their jobs, as one suited for woodwork will not necessarily be able to handle something like steel.

CNC milling machines may be used to cut and shape most or all of the features on complex parts, as changes of tools and utilising different axes mean that a single machine can handle many tasks within the same job.

Milling machines can carry out important refinement work like deburring, which removes rough edges and other imperfections from a finished piece. For more cosmetic edge work, chamfering and bevelling can be done at accurate angles, and countersinking helps the later process of adding screws to a piece.

Why is CNC milling used?

CNC milling machines vastly simplify parts that would traditionally take a wide variety of tools and a ton of skill to make reliably and to specification, and it can create them in higher volumes and at faster speeds.

Milling machines are used for tasks such as profiling and cutting cavities, where specific depths and radii need to be met with often tight tolerances. In the case of profiling, this can require sloped sides to concave or convex shape that require exact angles and smoothness to suit their purposes.

What are the advantages of CNC milling?

CNC milling machines hold some clear advantages over using traditional milling machines and processes that require more human input.

Immediately of note is the fact that more human input equates to more human error. Even though CNC machines can still fall prey to this, with incorrect code or poorly touched-off tools resulting in the crashes that all CNC operators dread, the nature of an automated machine doing the cutting work means that no mistakes are made with following the instructions themselves. If a movement is coded in, the machine will follow it without slipping or misjudging distances.

CNC milling is highly accurate even when working with complex geometry, and it works at higher speeds than handheld tools could ever achieve. Working with CAD files enables checking of the design before committing to production, allowing seasoned experts to appraise files and perhaps even run simulations to ensure no mistakes will happen during production.

CNC machines can produce parts hundreds of times faster than manual processes, meaning faster turnaround for the business producing parts that passes on to their customers. Faster order fulfilment means more orders fulfilled, and with higher accuracy guiding the work, this means more jobs being done ‘right first time’.

Higher accuracy of work additionally leads to lower levels of waste, saving costs within the business and leading to a better return on raw material investments.

Ultimately, CNC milling machines benefit from the speed and accuracy of the microcomputers that aid them and achieve much higher throughputs than traditional alternatives. They also feature enhanced safety that protects operators from their much higher power.

Are there any disadvantages to CNC milling?

It could be argued that CNC milling often demands more cost than manual machines, both in terms of upfront cost and the ongoing upkeep in demands of power and maintenance. This also means that CNC machines are costing money every second that they’re switched on, so there must be as few mistakes as possible whenever the machine is running.

In the case of places where manual machines are replaced with CNC-operated alternatives, fewer staff will be needed to run the new machines and as such the workforce will likely be downsized and some jobs would be lost.

This also means that manual mill skills are not as widely taught or necessary, as the requirements of running CNC milling machines have more and more relevance. Whilst the impact on manufacturing as a whole is debatable, this may be of concern to those who want to see manual skills preserved.

CNC machines do lack the autonomy of a manual worker, who can stop and reassess their work. CNC machines, on the other hand, will follow their coding until a crash occurs or the operator stops the machine.

Many of these concerns won’t affect machining centres that never employed manual machines nor the people operating them, but for workshops considering an upgrade, these setbacks will need to be weighed against the business needs and the potential gains of using CNC milling.

What is the difference between a CNC mill and manual mill?

Manual milling machines require total operator input to function. The quality of their output and the accuracy of the work they do relies on the skills of the user. Manual machines also lack some of the inbuilt convenience and function of their CNC counterparts, such as extraction.

Many CNC machines feature material and swarf extraction built into their design, removing the waste products of cutting as the machines work automatically. Manual machines typically require an added solution for this, which can give the operator more considerations to bear in mind as they work.

The biggest difference between CNC milling machines and their manual counterparts is that CNC machines work automatically and with computer-controlled accuracy and speed. Manual machines rely on their operator more than CNC machines.

CNC machines have changed the need for certain designs of milling machine, but some that exist with manual machines include:

  • Ram type

Ram type milling machines move their machine tools along the XY axis. The spindle (the machine’s rotating shaft with the fixture for the tool or, alternatively, the workpiece) is fixed to a moving arm, allowing the machine tool to manoeuvre along its axis.

The ram type machine is named for the specific inclusion of the ‘ram’, which allows the machine’s column to move. Universal horizontal milling machines are a common type of ram type milling machines.

  • Knee type

Knee type milling machines are in some ways a direct inverse of the ram type. The spindle of a knee type milling machine is static. Instead, the worktable can be adjusted vertically on its ‘knee’, which moves the workpiece up and down as required to meet the tool.
Horizontal plain milling machines are often knee type machines, their spindles positioned so that the tool can be used parallel to the workpiece.

  • Bed-type

Bed-type milling machines hold their workpieces stationary by having the worktables directly affixed to the machine bed. This stops the workpiece from moving along the Y and Z axes, whilst the cutting tool is often capable of full XYZ movement.

Bed-type milling machines might come as simplex cutters, working with a single spindle; duplex, using two spindle heads with one on each side of the workpiece; or triplex, with two spindles on either side and a third positioned on a cross rail or turret.

  • Planer-type

Planer-type machines are typically more heavy-duty milling machines and have a similar setup to bed-type machines in that their worktables don’t move along the Y axis or Z axis, but their tools are able to move along the three or more axes.

Planer-type machines use multiple tools simultaneously with a high degree of movement and positioning, making them well suited to handling complex parts.

CNC machines also possess better safety features such as doors that prevent the machine from operating if they’re opened, protecting the operator and other bodies nearby from flying material and running tools.

What are the different types of CNC milling machines?

CNC milling machines vary in their approach to cutting and the breadth of control that they afford. Some of these include:

  • Horizontal milling

Horizontal milling machines are named so because of the orientation of their spindles. Horizontal mills can make deep, heavy cuts with speed thanks to their shorter, thicker cutting tools.
Horizontal CNC milling machines excel at plain milling, making them well suited to the grooving, slotting, chamfering, and similar operations. Horizontal mills are better suited for projects that take longer, and their orientation lends some help to keep swarf clear of the workpiece through gravity.

  • Vertical milling

Vertical milling machines have their spindles aligned vertically, placing their tools perpendicular to the workpiece and thus making them great at face milling.

Vertical machines excel at drilling and high-accuracy features such as cavities, recesses, and blind holes as well as contouring. Vertical mills tend to have higher accuracy than horizontal mills.

If a turret mill is used, the tool’s spindle (the machine’s rotating shaft with the fixture that holds the tool) can be stationary whilst the workbench moves. If a bed mill is used, the workpiece only travels along the X-axis and the tool will travel back and forth on the Y-axis.

  • Face milling

Face milling positions the cutting tool rotation axis perpendicular to the surface of the workpiece (think of a drill aiming into the flat face of a piece of plastic, for instance). Face milling is used to create contours on a finished piece as well as cut flat surfaces.

Face milling is used both for cutting and finishing, and the applicable cutting tools often feature teeth both on the face of the tool itself and its sides (known as the peripheral teeth).

  • Plain milling

Plain milling refers to cutting that takes place with the milling tool oriented parallel to the face of the workpiece. Plain milling tools often resemble machine gears with sharper teeth along their edges for cutting, though wider tools are also used for correspondingly wider cutting areas.

Narrow cutters are better for making deeper cuts to create more depressed ridges and slots, whereas wide tools are useful for cutting large surface areas and removing a lot of material.

For cutting wide surface areas, slower speeds tend to be used with a wide cutting tool, whereas the narrower cutting tools require higher RPMs.

  • Form milling

Form milling deals with irregular workpiece surfaces such as contours, curves, and parts that feature a mix of flat and curved areas in one piece.

Form milling tools can achieve some of the more complex cutting jobs like semi-circular cavities, half-round recesses, and asymmetrical contours, as well as cutting small spherical parts such as beads.

One common part produced with the help of form milling are helical gears, which have slanted teeth rather than parallel.

  • Angular milling

Angular milling employs cutting tools positioned at specific angles relative to the workpiece, such as at 45-degrees rather than wholly perpendicular or parallel.
Angular milling allows for bevels and chamfers which would not be possible by employing plain or face milling alone. They also enable the cutting of shapes such as dovetail joints with accuracy and precision, ensuring that both jointed pieces in question fit together perfectly.

Using each of these different processes appropriately involves more than simply changing the tools and adjusting the machine’s cutting angles. The feed per tooth – the amount of material each of the tool’s cutting teeth should remove as it rotates and makes its way through – needs to be calculated accurately.

Is CNC milling hard?

Not with an experienced machining centre behind the process.

Here at GA Profiplast, we have the tools and know-how to mill a wide variety of parts, from simple batch jobs to complex and demanding parts that need to meet tight tolerances. We are the experts at machining plastic parts fast and cost-effectively.

To learn more about CNC milling for plastic components, contact us today.

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