The importance of Machining for precision sand castings

Published - 27th Feb 2024

How we improve tolerances and refine critical features 

Haworth Castings’ specialist machining centre is key to the production of high-quality sand castings, and is a crucial part of our UK foundry facilities. In this article we look at why some parts require machining after casting, and how this is carried out. 

Machining centre a key part of the metal casting foundry 

Haworth Castings is an aluminium sand castings and gravity die castings foundry and machining centre. We specialise in manufacturing fully tested, machined and finished precision castings to the defence, aerospace, marine, medical, energy, automotive and safety-critical industries. 

Sand casting is a highly versatile and cost-effective method of producing a wide range of metal parts. However, machining of the cast part is often essential to achieve the precise dimensional tolerances and fine surface finishes required for many applications.  

What does machining achieve? 

Machining after casting allows for the addition of various features to the castings, enhancing their functionality and performance. External or internal machining will provide: 

• Precision and accuracy: machining enables us to the achieve precise dimensional tolerances and intricate geometries demanded by our customers. 
• Surface finish: machining allows for the creation of smooth surface finishes. These are essential for applications requiring high aesthetic standards, or improved functionality such as corrosion or fatigue resistance. 
• Holes and threads: machining is used to create holes, threads and other intricate geometries specified in the design requirements, which may not be achievable through the casting process. 
• Optimised performance: machining can reduce friction and enhance the mechanical properties of the castings, leading to improved performance and durability. 

How does the need for machining affect the casting design? 

During the casting design process, care will be taken to ensure that allowances are made for any post-production processes that may alter the size or nature of the part.  

Where machining is planned, a ‘machining allowance’ is incorporated at the design stage, so that the casting can eventually be machined back to the exact specified dimensions. 

When is machining done? 

Machining takes place once any fettling or heat treatment has been completed but before any finishing processes such as anodising or painting.  

What does the machining of castings involve? 

Haworth Castings offers the full range of machining services to turn a casting into a finished component.  Our machining services include turning, milling, grinding and drilling, and are carried out either manually or using automated tools under computer numerical control (CNC). 

1. Machining castings: Turning

Turning is used to describe a variety of operations on both the internal and external surface of the casting. These include: 

• Facing: removing a thin layer of the material to create a smooth, flat surface. 
• Drilling: creating a hole inside the work piece. The turned hole is always cylindrical in shape and circular in diameter. 
• Boring: enlarging a drilled hole to meet precise specifications in terms of size and surface finish. 
• Reaming: sizing and finishing a hole that has already been drilled or bored. 
• Thread cutting: cutting threads into the inner and outer surface (e.g. for nuts and bolts). 
• Grooving: creating deep grooves on the external and internal surfaces to specific dimensions. 
• Knurling: cutting a serrated edge onto the surface (e.g. for a hand grip). 

How is turning done? 

A cast component is positioned in a chuck or turning fixture in a lathe. The component is then rotated at speed and a specialised cutting tool manufactured from ceramic or carbide material is traversed to the work piece along various axes of movement. The tool removes any unwanted material from the inside or outside of the casting to create the desired shape, dimensions and features. 

The choice of tool is determined by a number of factors, including the component material and the required finish. 

CNC or Manual turning  

Manual turning requires expert skills on the part of the operator, who manually operates the lathe. It is very useful for producing one-off components. 

The machining of large quantities is more typically carried out using CNC machines, which are programmed by our experts to carry out specified turning operations. This offers a more cost-effective and consistently reliable technique for medium- and high-volume requirements. 

2. Machining castings: Milling

Milling is one of the most popular machining techniques used at Haworth Castings. In turning, the component is positioned in a chuck and rotated at speed while the cutting tool is traversed to the workpiece. Milling works in reverse – the workpiece remains stationary, while the milling table or fixture moves and the cutters rotate. 

Turning is used primarily to create circular diameters and bored holes, and is a quicker process because the tools are not required to move. Milling is far more versatile. It is used to produce asymmetric and non-circular features and shapes, such as off-centred holes, slots, radial corners and square pockets.  

How is Milling done? 

In milling, the component is securely clamped to the machine table. The cutting tools, which use changeable carbide or ceramic inserts, move and rotate at optimum speed across the work piece to generate various features on the face of the casting.  

Milling can be carried out on either horizontal or vertical machines. Horizontal machines are often preferred because features can be machined onto multiple surfaces without resetting the component. Vertical machines can only work on one surface (the upper side) at a time. 

During the milling process, the engineer must ensure that the workpiece is clamped to the fixture securely without causing distortion. This is vital otherwise inconsistencies with the shape and dimensions may result. 

Traditionally, turning and milling have been carried out separately, but we also have machines that can now perform both operations together – reducing set-up and tooling times and costs. 

3. Machining castings: Surface grinding

Surface grinding is a machining process used to enhance the dimensional accuracy and improve the surface finish of sand castings. This can be vital for applications requiring high aesthetic standards, corrosion resistance, and enhanced mechanical properties. 

The cast component is secured on a magnetic plate or, with non-ferrous castings, a holding fixture. The outside edges of the castings are ground using a carborundum grinding wheel, which revolves at a rapid speed to produce the required precision flat finish. 

4. Machining castings: Manual drilling

Manual drilling is used to drill or tap holes into the component. The positioning of the holes is determined using cast dimples which are introduced during casting manufacture, or by bespoke drilling fixtures. 

Machining expertise at Haworth Castings

Haworth Castings’ machine shop includes a large selection of turning and milling machines, live tooling machines (which can carry out both operations simultaneously), and a wide range of CNC machines that can carry out precise machining techniques efficiently and accurately. Click to see our current plant list. 

Our machining team has the expertise to operate and supervise all our machines, checking that components are properly secured, and that the tools are working effectively and are not worn. We also have expert CNC programmers, who ensure that automated processes are carried out according to specification 

Machining castings: Inspection 

Inspection of the machined castings is important to ensure that the machining work meets the specified criteria. Casting inspection is carried out using automated equipment and probes during the machining process and post manufacture. 

Coordinate measuring machines (CMMs) provide an extremely accurate way to check the measurements of the machining – down to a few microns. Specialist CMM software ensures that the part is checked in the same way each time and also verified against the pre-entered drawing specifications. This eliminates any chance of human error. 

Mechanical gauges such as thread and plug are also used to check certain features on the casting, such as tapped holes and bore diameters. These features are quicker and easier to check manually rather than by CMM. 

The inspection process is used to identify a number of different potential defects. This could include issues with positioning of features, sizing, or distortions within the casting. In some cases the part can be modified to address these issues, but in other cases, it may need to be re-cast.  

Certification 

All of our machined parts are certified before leaving the machine shop to verify that they conform to the highest standards. 

Machine-finished castings as an alternative to machining from solid 

As experts in sand and gravity die castings, the team at Haworth Castings is capable of producing high quality sand castings that can replace the need to machine parts from solid.  

Where parts are produced in a subtractive manufacturing process the consumption of raw material is significantly higher, and production cycle times tend to be much longer. It is therefore usually more cost-effective to cast parts to near net shape, and finish them with machining, for high volume production requirements. Haworth has a track record of helping customers achieve significant cost downs by converting machined-from-solid parts to castings. 

Precision casting and machining at Haworth Castings 

At Haworth Castings, we understand the critical role of machining in the post-casting process. We provide comprehensive machining services that complement our exceptional sand casting capabilities, ensuring that we deliver high quality metal castings that meet even the most exacting specifications. 

For more information about our machining services and to explore our complete range of capabilities, please contact Haworth Castings on 01794 512685 or email sales@haworthcastings.co.uk.