Splitting Methods for Metal and Plastic Parts in CNC Machining

CNC machining has become one of the crucial manufacturing processes for products in the 21st century. The widespread use of 5-axis CNC machines, in particular, has enabled this process to produce more complex structures.

However, CNC machining still faces some limitations. Factors such as the CNC machine’s travel range, material thickness, cutting tool length, and part structure can prevent certain parts from being machined in one piece. To overcome these limitations, methods of splitting parts are often used in CNC machining.

Splitting Methods by Material Type

In the machining process, methods of splitting parts vary according to the material’s properties and machining requirements. Typically, materials are categorized into metals and plastics, each with its specific splitting methods to suit their machining characteristics and usage needs. In this section, WayKen will share the different splitting methods for metal and plastic workpieces and their specific applications.

Metal Workpiece Splitting Methods

For metal materials, common splitting methods include welding and designing holes for screw fixation. At the early stages of structural verification and testing, some metal prototypes are split by designing screw holes on the back and connecting them with screws fixation. Compared to the method of welding split parts, it is easier to assemble and less prone to deform.

splitting car reflector with screw studs

For example, this complex aluminum alloy car reflector. Based on its structural features, it is necessary to split it during CNC machining. So we can design screw studs in suitable gaps to connect parts without affecting the assembly with other components. In addition, the design of the screw studs should not affect the appearance of the reflector. Therefore, they are usually designed on the back of the part when splitting metal workpieces.

Plastic Workpiece Splitting Methods

Plastic parts splitting can be divided into two categories: Not using glue and Chemical bonding or Glued.

Splitting Not Use Glue

Similar to metal part splitting, this method is split by designing screw holes and using screws for fixation.

Chemical Bonding or Glued

Splitting plastic parts with chemical bonding can be further divided into non-transparent and transparent plastics.

Non-Transparent Plastics

For non-transparent plastic parts, the main concern in splitting is the firmness of the bond. Common splitting methods include the Zigzag joint (left side of the picture below) and the Step joint (right side of the picture below).

zigzag joint and step joint

Both the two joints ensure that the split parts are structurally constrained, maintaining structural integrity. At the same time, they also increase the contact area between the split parts, ensuring a stronger bond. However, excessively large bonding areas can make it difficult to apply glue evenly, resulting in inconsistent bonding strength.

Typically, the step joint will be based on the part’s material thickness of 1/2 * 2mm for splitting, such as the material thickness of 3mm is usually split into 1.5 * 2mm step for jointing. The zigzag joint typically has a height of 3-5mm and the length is adjusted according to part structure, usually 30-50mm.

Transparent Plastics

In common, the transparent plastic parts are split along the contour at the corners of the parts with straight cut method. However, you should pay attention that it is necessary to avoid light transmission and guiding areas when splitting car lamp transparent components.

Take the following headlights as an example, the large curved transparent surface on the top of the headlights should be split as a whole. However, splitting this curved surface and bonding it would cast a shadow of the split line when the light is on, affecting the light transmission.

transparent parts for headlights

Also, check out the transparent parts below. Although they have been split into several different components and then machined and polished before being bonded as a whole, you can see that the bonding lines are barely visible in the final part, making the product appear as a single piece.

splitting and bonding of transparent parts

Why Splitting Machining is Mostly Used in Prototyping Stages?

You may wonder why splitting is often employed in CNC prototype machining, especially for plastic prototyping. It is rarely used in mass production due to the following reasons:

Different processes are chosen for different production stages. CNC machining is used for prototyping, while vacuum casting is used for trial production, and injection molding for mass production.

Even if CNC machining is used from prototyping to mass production, the areas requiring splitting in the prototype stage will be adjusted in the trial production stage. This ensures the parts can be machined as a whole in mass production to improve production efficiency.


Splitting parts in CNC machining helps overcome limitations related to machine travel, material thickness, tool length, and part structure. It helps to broaden the usage of CNC machining and reduce costs. Therefore, these splitting method holds significant application value in CNC machining.

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