CNC Machining Plastics A Guide to Materials, Tolerances and Design

CNC machining is one of the most precise ways to make a plastic part. A computer controlled cutting tool removes material from solid stock until the finished shape remains. Whether you are prototyping a single component or specifying a production run, three decisions shape the result. These are the material you choose, the tolerances you set and how you design the part. This guide covers all three so you can send a job to your plastic manufacturer with confidence.

What is CNC machining of plastics?

CNC machining covers several related processes and the right one depends on the shape of your part.

• CNC milling uses rotating cutters to produce pockets, slots, holes and complex 3D shapes. It is the go to process for housings, manifolds, jigs and precision blocks.
• CNC turning spins the material against a cutting tool to create round parts such as bushes, spacers, rollers and seals.
• CNC routing is ideal for cutting profiles and features from plastic sheet, such as panels, guards and machine covers.

For parts with angled features or complex geometry, multi axis CNC machining can reach several faces in a single setup, which improves accuracy and reduces cost.
Machining has no tooling cost, so it is usually the most economical route for anything from a single prototype to a few hundred parts. At higher volumes injection moulding may become more cost effective. Our guide on how to choose the right plastic manufacturing process explains the trade offs in more detail.

Which plastics are best for CNC machining?

The material you choose affects strength, stability, surface finish, cost and how tightly the part can be held to size. Here is a quick comparison of the plastics we machine most often.

Start with what your part has to do. Think about the loads, temperatures, chemicals and conditions it will face, then weigh the mechanical properties against cost. Sectors such as food and beverage and medical and healthcare may also need certified or compliant grades.

You can explore the full range on our materials page, including antistatic, high temperature and food grade options.

What tolerances can you achieve when machining plastics?

Tolerances define how much a finished part can vary from its design dimensions. For machined plastics, general tolerances typically sit around ±0.1mm and ±0.05mm is achievable on stable materials such as acetal. Tighter tolerances are possible on specific features but they add machining time and cost.

Plastics behave differently from metals and this affects how tightly they can be held.

• Thermal expansion: Plastics expand and contract far more than metals as temperature changes. Heat from cutting and changes in room temperature both affect measured sizes.
• Moisture absorption: Materials like nylon absorb water from the air and can grow slightly over time.
• Internal stress: Extruded and cast stock contains stresses that can cause the part to move as material is removed. Using stress relieved (annealed) material and roughing the part before finishing both help keep it stable.
• Flexibility: Softer plastics can bend under cutting forces or clamping pressure.

Practical advice. Apply a general tolerance class to your drawing, such as ISO 2768 medium (m). Then call out tighter tolerances only on the dimensions that matter, such as bearing fits, mating faces and sealing surfaces. This lets the machinist focus precision where it counts and keeps your costs down.

Design Considerations for Machined Parts

Wall Thickness

Thin walls are more prone to warping or cracking during machining, particularly in softer plastics. Keeping wall thickness consistent across the part, reduces the risk of distortion and makes for a more reliable finished component.

Undercuts and Features

This process involves heating a plastic sheet and forming it over a mould using vacuum pressure. It is highly cost-effective for medium to large components and is ideal for enclosures, trays and packaging. Vacuum forming is particularly useful when tooling costs need to be kept low.

Surface Finish

CNC routing uses computer-controlled cutting tools to shape plastic sheets with high precision. It is ideal for detailed parts, signage, and components requiring tight tolerances. This method works well for both prototyping and small to medium production runs.

Fixturing

Laser cutting offers excellent precision and clean edges, making it perfect for intricate designs and thinner materials. It is commonly used for display products, panels, and decorative elements where detail is essential.

Getting It Right From the Start

For a fast and accurate quote, please send us the following.

• A 3D CAD model (STEP files are ideal).
• A 2D drawing showing critical dimensions and tolerances.
• The material and grade, or a description of the application so we can recommend one.
• Quantities, including any expected future volumes.
• Finish requirements and any extra work such as assembly and packing.

Advanced Plastic Technology

UK plastic manufacturer based in Milton Keynes. CNC machining, laser cutting, injection moduling and fabrication services, ISO 9001 certified. Get A Quote.

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