CNC machining sand castings

How your casting’s final finish can benefit from CNC machining.

Introducing the final sand casting features with CNC machining

CNC machining is used in the sand casting process to add features that can't be introduced by sand casting alone, due to the extremely small dimensional tolerances. A machining allowance is incorporated at the design stage, and specified for production.


In order to get a casting into its final state it will almost certainly need to be machined. Machining offers a much finer degree of control over part dimensions and surface finish. This can include areas of the casting which join with other components, or have specific performance requirements. For instance, features such as mating surfaces and tapped holes will need to be machined.

GW_Sand Casting Process Map v2


how much does cnc machining cost?

Machining can add significant cost to the casting project, and this will be specific for each casting project.

For tight tolerance dimensions on a finished part, any casting process requires additional
CNC machining after fettling or any heat treatments have been completed.

  • It’s important to consider machining requirements early on in the sand casting process, including the need for pick up points and datums.
  • Specialist tooling may also be required depending on the part topology. This may have a significant impact on lead time and cost

Read more: The GW Machining Buyer's Guide.

Machining Photo LinkedIn

Machining a smooth surface: does it matter?

While surface smoothness can certainly be an aesthetic feature, it can also be important for final optimal part functionality (for example, for a sealing face). Surface finish can impact part function, including corrosion resistance, friction, fatigue resistance and strength.

Considering these properties, and the CNC machining specification at the design stage of the sand casting process is important.

Read more: What are the post-processing options used in sand casting?

How can you reduce lead time and cost of CNC machining?

Machining processes can add significantly to lead time and cost. There are ways to reduce machining impact on lead times for your sand cast part, if these are taken into consideration at the start of the casting process.

These can include

  • set up times
  • tolerance specification
  • temperature control
  • casting design (considering pick up points and datums).

Can CNC machining produce parts that sand casting can't?

Machining is used to create features on a casting with tight tolerances (often measured in microns), surface finishes or to achieve design dimensions that would be impossible to produce using casting alone.

Read more: What is a micron and how much does it cost?

CNC machining allowances in sand casting design

A machining allowance is needed at the design stage to ensure the casting can be machined back to the specified tolerance.

This depends on factors including casting size, material used and production volume. Balancing is also important. This is where critical features on the cast part are prioritised for measurement to balance the casting.


Want to know more about the sand casting process? 

Download our free ebook, 'The Basics of the Sand Casting Process'.

Read the sand casting ebook