CNC MACHINING DESIGN GUIDE

CNC (Computer Numerical Control) is a subtractive process where material is removed from a solid block (blank or workpiece) or a pre-formed part using a variety of cutting tools.

1. The final part geometry is defined by a CAD model normally supplied by the customer
2. The Machinist uses CAM software to prepare the cutting paths, and tool selections to needed to achieve the part
3. The paths are then output as Gcode which give the absolute machine instructions such as location and orientation of machine head, part and speed of any movement.
4. The machine is set up and the blank jigged in position.
5. The machine executes the Gcode to remove material creating the part, usually with minimal supervision.

There are many types of CNC machines including some that use electron beams, electro- chemical, water, ultrasound, and lasers.

For this manufacturing guide, we will focus on the more common machines which remove material using cutting tools. These can be separated by the number of Axis they have.

3 axis machines move the cutting tool relative to the part along the x,y,z axis.

Multi Axis machines add rotation to one or more axis allowing the part to be cut from more angles.

This allows more complex parts, and reduces setup as the part can be repositioned dynamically.

CNC Milling

CNC Mills are very common and can be used for many geometries.

The workpiece is held rigidly in a jig or vice, and the mill head moves in 3 axis, x, y,z, relative to it to remove material using high speed rotary tools or drills.

Due to the limited range of motion they are relatively easy to operate and program, so set-up costs are low compared to other CNC processes.

The limited range of movement means that some features are not possible to create.

This can be partially overcome by manually reorienting the part, however each movement adds time and risk of error, so machining costs can increase quickly.

CNC Milling Process

1. Machining G Code is created, either from a supplied CAD model, or translated from drawings by the operator.
2. The blank is cut to size and attached firmly and precisely to the platform.
3. Specialised high speed rotational cutting tools are used to remove material. Often this is in multiple stages, first passes are often high speed lower accuracy, using special roughing cutters, with later passes to remove smaller amounts of material at higher accuracy for the final part.
4. After machining the parts are de-burred (often manually)
5. Critical dimensions are inspected.
6. Post processes (e.g. surface finishes) are applied.

CNC Cutting tools

Different cutting tools are used for different geometries and stages of the machining process.

Here are some of the most common.
1-3 Flat, Bull, and Ball heads are the common cutting tools for making slots grooves cavities and vertical walls. They are selected by the required form or bottom radii of a cavity.

4 Drills are generally selected for standard size holes.

5 Slot cutters have a larger diameter head than shaft so can create undercuts, removing material from the sides of vertical walls.

6 Taps are used for threaded holes. These are often applied manually.

7 Face milling cutters remove material from large flat surfaces.

3 Axis - Multiple Set-ups

Some features cannot be created in a first pass with 3 axis machines.

Examples would be holes perpendicular to the bed, or features on the reverse side of the part

In these cases the parts is manually lifted off the bed, and reoriented and the previous steps are repeated.

CNC Turning

CNC lathes are widely used. They can produce parts at a higher rate than CNC milling, and in turn lower cost, which makes them very useful for large numbers of parts.

The work-piece is held on a spindle and rotated at high speed. The cutter is normally a blade unlike the rotary cutters in a mill, and moves gradually towards the part to describe its profile.

Lathes can only produce “revolved” or “rotationally symmetrical” parts along a central axis e.g. cylindrical parts and threads.

Often features are created on a CNC lathe, and the part is then transferred to a mill for areas which cannot be achieved.

CNC Turning Process

1. Machining G Code is created, either from a supplied CAD model, or translated from drawings by the operator.
2. The blank (usually cylindrical) is cut to length, held in the spindle and rotated at high speed.
3. The Cutting tool generally moves along x,y axis relative to the part to gradually removing material to create the required profile.
4. After machining the parts are de-burred (often manually)
5. If needed the part is moved to a mill to add features not possible on a lathe
6. Critical dimensions are inspected.
7. Post processes (e.g. surface finishes) are applied.

Indexed 5- Axis Milling

Also known as 3+2 CNC milling machines these systems were developed to reduce the setup time of multiple orientations during machine time they work as a conventional mill in 3 axis.

Between operations the bed and/or tool-head can rotate giving access to the work-piece from a different angle.

The ability to reorient the work-piece automatically allows the creation of more complex parts with better accuracy, reducing machine and operator time.

Continuous 5 Axis CNC Milling

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

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

This process delivers the highest quality and most complex parts, but comes at the highest cost, requiring specialist operators.

Mill-Turning CNC Centers

Mill Turning Centers are a hybrid of a lathe and a mill.
They offer the benefits of high volumes and speed from CNC turning and the flexibility to create some complex geometry offered by 5 axis milling.

These systems are ideally suited for parts with fundamental rotational symmetry and additional features, such as impellers and can create these at much lower cost than other 5 axis CNC machines.

The work-piece is attached to a spindle and either rotate at high speed like a lathe or can be precisely positioned like a 5 axis CNC.

Lathe and milling cutting tools are use to remove material from the work-piece.

Vertical Radii

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

Tip

Increasing the corner radii (e.g. +1mm) allows the tool to follow a circular path rather than 90 degree angle. This reduces load on the tools, allows a higher quality finish and slightly lowers cycle times.
Smaller radii can be achieved by using smaller cutters, either for the full program or as a second tool pass after roughing. This will increase time and cost.

Cost will increase as small tools cannot remove materials as fast.

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 gets thinner vibrations are increased due to the reduced stiffness. This reduces accuracy. Thicker walls are recommended for plastics because they are:

• Less stiff
• Prone to warping from residual stress
• Soften as temperature increases.

Every material is different based on its properties.

Refer to this table for more details

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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None 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.

CNC TAP CHART

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 change at this scale.

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

CNC TAP CHART

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.

CNC TAP CHART

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.

CNC TAP CHART

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.

Tip

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.