CNC Machining Design Guide

CNC machining is a subtractive manufacturing process that removes material from a solid block (called a blank or workpiece) or a pre-formed part using various cutting tools.

1. The desired part geometry is defined by a CAD model, typically provided by the customer. A machinist then uses CAM software to prepare the cutting paths and select the appropriate tools needed to create the part.
2. These cutting paths are converted into G-code, a set of precise machine instructions. The G-code defines the machine's actions, including the location and orientation of the machine head, the part's position, and the speed of any movements.
3. Once the machine is set up and the blank is securely positioned using jigs, it executes the G-code to remove material and shape the part. This process requires minimal supervision, as the machine operates autonomously to produce the final component.

Various types of CNC machines exist, including some that use electron beams, water, ultrasound, and lasers. However, for this manufacturing guide, we will focus on the more common machines which remove material using cutting tools. These types of CNC machines can be categorised according to their number of axes.

3-axis machines move the cutting tool relative to the part along the X, Y, and Z axis.

Multi-axis machines add rotation to one or more axes, allowing the part to be cut from more angles. The most common of these carry out 5 axis CNC machining. This technique allows for more complex parts and reduces setup as the part can be repositioned dynamically.

CNC Milling

CNC mills are among the most common CNC machines and are highly versatile and capable of producing a wide range of geometries, including most CNC machine designs.

The workpiece is securely held in place using a jig or vice while the mill head moves along three axes—X, Y, and Z—to remove material using high-speed rotary tools or drills.

Thanks to their limited range of motion, CNC mills are relatively simple to program and operate, resulting in lower setup costs compared to other CNC processes.

However, this limited range of movement restricts the ability to create certain features. Manually reorienting the part can address this limitation, but each reorientation takes time and increases the risk of errors, leading to higher machining costs.

CNC Milling Process

CNC milling is a subtractive manufacturing process that involves the precise removal of material to create a part with specific dimensions and features. The CNC milling process follows a series of well-defined steps, from preparing the blank to finalising the part with post-processing. Below is a breakdown of the key stages:

1. G-Code Creation: Machining G-code is generated either from a CAD model provided by the customer or manually translated from technical drawings by the operator. This code contains precise instructions for the milling process.
2. Material Preparation: The blank (raw material) is cut to the required size and securely fixed to the milling machine platform, ensuring precise alignment and stability.
3. Roughing Passes: In the initial roughing stage, high-speed rotational cutting tools remove large amounts of material. This process prioritises speed over precision, using specialised roughing cutters to shape the workpiece.
4. Finishing Passes: This stage focuses on achieving high accuracy and a smooth surface finish, removing smaller amounts of material at lower speeds to achieve the final shape and precise dimensions.
5. Deburring: After machining, the part is deburred—typically done manually—to remove sharp edges, burrs, and other imperfections caused during cutting.
6. Dimensional Inspection: Critical dimensions are measured and inspected to ensure the part meets the specified tolerances and quality standards.
7. Post-Processing: Any required post-processes, such as applying surface finishes, heat treatments, or coatings, are performed to enhance the part’s functionality or aesthetics.

CNC Cutting Tools

Different cutting tools are used at various stages of the machining process to achieve specific geometries and features.

Here are some of the most common examples:

Flat, bull, and ball end mills (1-3) are commonly used to create slots, grooves, cavities, and vertical walls. The choice of tool depends on the cavity's required shape or bottom radius.

Drills (4) are typically selected for creating standard-sized holes with precision.

Slot cutters (5), which have a larger diameter head than the shaft, are used to create undercuts and to remove material from the sides of vertical walls.

Taps (6) create threaded holes. While they can be applied with CNC machines, threading is often performed manually.

Face milling cutters (7) are designed for removing material from large flat surfaces, making them ideal for smoothing and levelling workpieces.

3 Axis - Multiple Set-ups

Some features cannot be created in a single pass using 3-axis machines.

For example, holes perpendicular to the machine bed or features on the reverse side of a part require additional steps. In such cases, the part is manually removed from the machine bed, reoriented, and then securely repositioned before repeating the machining steps to achieve the desired features.

CNC Turning

In CNC turning, the workpiece is mounted on a spindle and rotated at high speeds. The cutting tool, typically a blade rather than the rotary cutters used in milling, gradually moves towards the part to shape its profile.

CNC lathes are commonly used for producing parts at a higher rate than CNC milling, making them more cost-effective for manufacturing large quantities of parts.

CNC turning is ideal for producing "revolved" or "rotationally symmetrical" parts along a central axis, such as cylindrical components and threaded parts.

In many cases, features are first created on the CNC lathe, and the part is then transferred to a CNC mill to complete features that cannot be achieved through turning alone.

CNC Turning Process

CNC turning is a process used to create rotationally symmetrical parts with precise control over shape and dimensions. The process follows a series of steps to convert raw material into the finished component. Here’s an overview of the key stages:

1. G-Code Creation: Machining G-code is generated from a CAD model supplied by the customer or manually translated from technical drawings by the operator. This code provides the machine with precise instructions for machining the part.
2. Material Preparation: The blank (typically cylindrical) is cut to the required length and securely fixed in the spindle, which will rotate at high speed during machining.
3. Cutting and Shaping: The cutting tool moves along the X and Y axes relative to the rotating part, gradually removing material to form the desired profile.
4. Deburring: After machining, the part is deburred to remove sharp edges, leftover material, and imperfections. This process is often performed manually.
5. Transfer to Mill (if needed): If additional features are required that cannot be created on the lathe, the part is moved to a CNC mill for further processing.
6. Dimensional Inspection: Critical dimensions are measured and inspected to ensure the part meets the required specifications and tolerances.
7. Post-Processing: Any necessary post-processes, such as surface finishes or coatings, are applied to enhance the part’s functionality or appearance.

Indexed 5- Axis Milling

Also known as 3+2 CNC milling machines, these systems minimise setup time for multiple orientations during machining. They operate as a conventional 3-axis mill.

However, between operations, the bed and/or tool head can rotate, providing access to the workpiece from different angles.

This ability to automatically reorient the workpiece allows for creating more complex CNC designs with improved accuracy, reducing both machine and operator time.

Continuous 5 Axis CNC Milling

Similar to indexed 5-axis CNC, continuous machines allow for the movement of all five axes simultaneously during machining.

This allows very accurate and complex free-form CNC designs to be created.

This process delivers the highest-quality, most complex parts but is also the most expensive and requires specialist operators.

Mill-Turning CNC Centers

Mill Turning Centres combine the capabilities of a lathe and a milling machine. They offer the high-speed production and efficiency of CNC turning alongside the flexibility of 5-axis milling to create more complex geometries.

These systems are particularly well-suited for parts with rotational symmetry, such as impellers, and allow for additional features. Compared to other 5-axis CNC machines, they provide a cost-effective solution, as they reduce the need for multiple machines, decrease setup time, and streamline production processes.

The workpiece is mounted on a spindle and can either rotate at high speeds, like a lathe or be precisely positioned, similar to a 5 axis CNC machine.

Both lathe and milling cutting tools are used to remove material from the workpiece, enabling versatile machining capabilities.

Creating an efficient and cost-effective design for CNC machining requires careful consideration of various factors that impact both the manufacturing process and the final product quality. This section of the CNC machining design guide explores the best practices for CNC design, focusing on strategies that ensure optimal performance, reduce production time, and minimise costs. By adhering to these design guidelines, you can achieve high-quality, precise parts while maintaining efficiency throughout the machining process.

Corner Radii

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A cavity typically requires an end mill tool, which is cylindrical and leaves a radius.

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Increasing the corner radii (e.g. +1mm) enables the tool to follow a circular path instead of a sharp 90-degree angle. This not only reduces the load on the tool but also improves the surface finish and slightly lowers cycle times.

Using smaller cutters can achieve finer details for smaller radii. These can be used either throughout the program or as a second tool pass after roughing. However, this approach may increase both time and cost.

Cavities

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A cavity will typically require an end mill tool. End mill tools have a limited cutting length (typically 3-4 times their diameter)

Longer tools will flex under full cutting load, reducing accuracy or damaging the part.

Deep cavities dramatically increase cost as a lot of material needs to be removed, and its harder to extract the chips.

Minimum Wall Thickness

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As the wall thickness decreases, vibrations tend to increase due to reduced stiffness, which can lead to decreased accuracy in machining. Thick walls are especially vita in plastics, as they are:

• Less stiff
• More prone to warping due to residual stresses
• Llikely to soften as temperature rises

Each material behaves differently, so it’s essential to consider their specific properties when designing for CNC machining.

Refer to the table below for more details on material behaviour and recommendations.

Hole Diameters

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Holes are machined using an end mill where possible.
Standard drill bits are often used and will achieve the best accuracy under 20mm.

Common sizes can be found here in metric and imperial.

CNC TAP CHART

Hole Depths

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Non-standard diameter holes must be machined with an end mill. In this case, cavity guidelines apply.

Where deeper holes are required, specialised drill bits are required. These usually are limited to minimum 3mm diameter.

Minimum Hole Diameters

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Most CNC services can achieve 2.5mm. Below this is considered Micro-machining. Micro-machining requires speciality tools, and cutting physics changes at this scale.

With specialism comes cost, so consider if your project really requires this.

Thread Depths

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For pull out force, most load is taken by the first few teeth (1.5 Nominal Diameter) Longer than 3x Nominal diameter is not usually necessary.

Where a hole is blind, 1.5x Nominal diameter needs to be added at the bottom CNC threading tools can be threaded throughout the full length.

Thread Diameters

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Machinists prefer to use CNC threading tools as they are less prone to breakage M6 is typically the smallest.

• Internal threads are cut with Taps.
• External threads are cut with Dies.

These are generally limited to minimum 2mm.

Tolerances

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The tolerance defines the acceptable limits of a measurable or important dimension. CNC as a process has amongst the tightest tolerance capability. 

Bear in mind the tolerance of any parts which will mate to ensure fit. 

Check the functionality for extremes cases of both parts by calculating the effect of the deviation.

If no tolerance is specified, most machine shops use ±0.025 mm

Text and Lettering

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Applying accurate text adds cost and time due to the requirement for small tools. Achieving tight internal radii will require small cutters. There will be radii on the internal vertical edges.
Text, especially Embossed, reduces the use of profile cutters and roughing tools which increases the CNC time.

Achieving high quality surface finish at the foot of text is often a challenge.

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As a rule text should be avoided as it adds cost.

Embossed (raised) text is often preferred as less material has to be removed to create the feature, and gives better results if the part is for injection mould tool-ing.

BOLD Sans-Serif e.g. Arial, Verdana, or Helvetica are recommended as they have fewer sharp features and often are pre-programmed CNC routines.

The text should be carefully checked all the CNC rules will apply, including thin wall, cavity depth cavity width etc.