CNC Machining Introduction

Jack Lie CNC machining expert

Specialize in CNC Milling, CNC Turning, 3D Printing, Urethane Casting, and Sheet Metal Fabrication Services.

CNC machining is widely applied for subtractive manufacturing process. In CNC machining, raw materials are removed form solid materials stocks by various cutting tools, in order to create final parts with designed geometries in CAD files. These CNC materials include both metals and plastics. CNC machining can produce final parts with high tolerance and excellent properties, especially suitable for one-time usage and low to medium volume production with high repeatability. While comparing to 3D printing, CNC machining still has some natural restrictions in reason of its subtractive method.

CNC Machining Method

There are two main way of CNC machining method, CNC milling and CNC turning, each method has advantages for different manufacturing geometries with unique characteristics. Other machining methods likes 5-axis machining, 3-axis machining can be defined as combinations of these basic methods.

CNC milling process

CNC milling as the most popular CNC machine machine architecture, are normally synonymous with term of CNC machining. In CNC milling process, raw metal blocks are mounted on CNC machines bed, and rotational cutting tools remove unneeded materials for final geometries. Basic CNC milling process as following:

  1. CNC machines operator will transfer technical drawings of CAD models into a series of commands, which can be interpreted by CNC machines(G-code).
  2. Material blocks or work-pieces will be placed on platform and cut into desired size. Precise positioning and alignment are the key of accurate CNC parts manufacturing, so we need special metro-logy tools such as touch proves to achieve this purpose.
  3. Specialized cutting tools with high speed of thousands of RPM will remove materials to desired geometries. Normally we need several passed to create the designed part, firstly, quickly remove block materials form blocks to approximate geometries at lower accuracy, then one or more finishing passes to produce the final parts.
  4. Once models with complex features, which is difficult to reach at just one single setup by cutting tools, such as slots on back side, we need to flip parts and repeat above step until final parts are created as requirement.

After milling process, deburring is required for parts finishes. Deburring is a manual process of small defects removing on sharp edges. If higher tolerance is specified in technical drawing, we also need t inspect the critical dimensions. At last, the parts is get ready for use or post-processing.

Most CNC milling systems have normal 3 linear degrees, X,Y and Z axis. Advanced system of 5 degrees adds more rotation on bed or tool head as A and B axis. 5-Axis CNC machining is able to produce complex geometric parts without multiple machine setups.

CNC turning process

CNC turning is a process, which apply stationary cutting tools to remove materials of blocks on rotating chuck. Finally produce parts with symmetry along center axis. Normally, turned parts can be produced faster than milled parts at a lower cost. The main steps of CNC turning as following:

  1. G-code will be generated form designed CAD model, then select suitable diameter size cylinder stock materials and load in CNC machine.
  2. Material stocks rotate at high speed, and stationary cutting tools will trace its lineament, remove materials progressively until final design geometry. Holes features on center axis can be manufactured by center drills and internal cutting tools.
  3. Once complex parts need to be flipped or moved, repeat cutting process to create final required geometries. Otherwise, processed parts can be cut form stock for usage or post-processing.

CNC turning system also called as lathes, which is applied for cylindrical parts production. In addition, Non-cylindrical parts also can be produced by CNC modern multi-axis CNC turning centers, which can be equipped with CNC milling tools. These systems combine CNC turning high productivity with CNC milling capability to create large range of geometries with looser rotational symmetry, like camshafts and compressor impellers.

Since CNC milling and turning system are blurry, so we will focus on CNC milling because of it is a more common manufacturing process.

CNC machine parameters

CNC machines operators determine machining parameters in G-code generation process. Which normal are the build size and accuracy.

CNC machines have large build size, CNC milling system can produce dimensions up to 2000×800×100 mm, CNC turning system can manufacture diameter up to 500 mm.

CNC machining can provide machined parts with high accuracy and tight tolerance. Our standard tolerance is accuracy of ± 0.125 mm, tight tolerance can achieve to accuracy of ± 0.025 mm.

CNC machine parameters

CNC machines operators determine machining parameters in G-code generation process. Which normal are the build size and accuracy.

CNC machines have large build size, CNC milling system can produce dimensions up to 2000×800×100 mm, CNC turning system can manufacture diameter up to 500 mm.

CNC machining can provide machined parts with high accuracy and tight tolerance. Our standard tolerance is accuracy of ± 0.125 mm, tight tolerance can achieve to accuracy of ± 0.025 mm.

CNC cutting tools

CNC machine utilize various cutting tools to create different geometries, we will introduce the most common machining tool in CNC:

Mill tools including normal 3 types: flat head, bull head and ball head. These different end mill tools are applied for slots, grooves, cavities and other vertical walls manufacturing with different feature details. Ball head tool are also widely used in 5-axis CNC machining to create surfaces with curvature and freedom geometries.

Drills are common used to produce holes, for non-standard diameters, plunging flat head end mill toll can applied on helical path.

Slot cutters have smaller shaft diameter than its cutting edge, this permits slot cutters to create T-slots and other undercuts in process of removing materials form vertical wall sides.

Taps are applied for thread holes production, this need precise control of rotational and linear speed. Manual tapping is still widely applied in some machine centers.

Face milling cutters are high effective tools for materials removing form large flat surfaces. They can machine larger areas with fewer passes and less time, in reason of larger diameter than normal tools. Face milling is often in machining early cycle to prepare block dimensions.

Geometric Complexity& Design Restrictions

Although CNC machining provides more design freedom, unlike 3D printing, there are still some restrictions, parts with high complexity will increase manufacturing steps and final cost.

The main restriction in CNC is connect to geometry of cutting tools. Such as internal edges of slots are rounded, because the cutting tools have cylindrical profile.

Tool access is another major CNC machining restriction, block materials only can be removed in tools reach area. As in 3-axis CNC machine system, all features must be designed to access directly form above direction. 5-Axis CNC machine systems can create more intricate parts, because angles between parts and tools can be adjusted, in order to get into difficult reaching area.

Thin walls or fine features are difficult to create by CNC machines. Thin walls are prone to break in reason of vibration and cutting force in machining process. We recommend minimum wall thickness is 0.8 mm for metal and 1.5 mm for plastic.

CNC Machining Characteristics

CNC machining is able to produce parts with excellent material properties in a wide materials area, including all engineering materials. In contrast with 3D printing, CNC machined parts have fully-isotropic physical properties as bulk materials. CNC machining is widely used with metals in prototyping and high-volume production. Plastics are more difficult to produce in reason of its low stiffness and melting temperature.

CNC machining materials

CNC machining materials cost varies greatly, in metal materials, Aluminum 6061 is the most economical option, which for plastics, ABS has the lowest price. In addition, materials physical properties also affect CNC overall cost, such as hard metal stainless steel is much more difficult to machine than Aluminium.

Aluminum 6061Good strength-to-weight ratio
Good machinability
Low hardness
Stainless Steel 304Excellent mechanical properties
Goog corrosion resistance
Relatively machining difficulty
Brass C360High ductility
Excellent machinability
Good corrosion resistance
ABSGood impact resistance
Good mechanical properties
Susceptible to solvents
NylonGood mechanical properties
High toughness
Poor moisture resistance
POMHigh stiffness
Excellent thermal, electrical properties
Relative brittle

Post processing & surface finishes

CNC machined parts always left visible tool marks, post-processing is an effective method to improve surface finishes, enhance wear, corrosion or chemical resistance and optimize appearance. Our main post processing methods including anodizing, bead blasting and powder coating.

Benefit of CNC machining

  1. CNC machining provides high accuracy and repeatability, which is ideal for high-end applications.
  2. CNC materials have fully-isotropic physical properties, which is suitable for engineering applications.
  3. CNC machining is the most effective manufacturing method for low-to-medium volume parts production.

Limitation of CNC machining

  1. In reason of subtractive method, CNC machining is expensive or impossible for certain geometries manufacturing.
  2. CNC machining has high start-up cost than 3D printing, especially for low-cost prototyping in plastic materials.
  3. CNC machines need expert knowledge to operate, it has longer lead time (10 days) than 3D printing (2-5 days).