Precision CNC Machining for Sand Castings

Precision CNC Machining for Sand Castings

Machining Makes Sand Castings Great?

Great CNC machining of castings is critical to the entire casting process. Poor machining can lead to higher costs, poor finishing, incompatible components and delayed deliveries . The difference between a great casting and an average one can is often laid at the foot of the machining facility.

Casting is a highly versatile and cost-effective method of producing a wide range of metal components. However, the machining of sand castings, in particular, it is often left to the CNC machining to achieve the precise dimensional tolerances and finer surface finishes.

Why Machining Matters

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

• Design for Machining: Sarginsons place an enormous focus on the concept that better inputs create better outputs. And Sarginsons unique DfM forms a vital part of getting castings right for machining at the design phase, simplifying datum strategies and omitting technical CNC problems.

• 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.

It’s Not Just The Machining

Programming and operating the CNC machine is a small part of the skillset required for great sand cast machining. The work to set up and prepare the CNC is often more important than what comes later. The goal is minimising cycle times as that is where the cost is.

• Fixturing – Great fixturing dictates everything that comes afterwards. One op not two is always critical. As is the choice between hydraulic, manual or pneumatic holding. Simple, repeatable and reliable fixturing makes everything faster and cheaper. Sarginsons has its own fixture and pattern manufacturer, Pattern Solutions.

• Tooling Strategy – the amount and variety of specialist and driven tools has totally changed what’s possible in CNC machining of sand castings. And it improves virtually every day. A comprehensive knowledge of tooling is a prerequisite of any CNC strategy, as correct tooling can save fortunes over the course of a year; although specialised tooling is expensive. In many ways the challenges increase with the limitations of short run or prototype machining.

• Right CNC Machine - To properly accommodate wide variety of casting shapes and sizes, it is essential that the machine shop includes a wide variety of different machines. There can be nothing more costly than using expensive machinery for simple machining jobs.

Design for CNC is essential?

There are a huge range of factors that influence CNC that are best incorporated into the design of the casting.

• Casting Allowances - During the casting design process, care must 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.

• Datum Locations – Datums strategies must be an integral part of any CNC machining operation, with Datum points allocated at the design phase with a view to CNC machining demands so as to minimise complications and avoid difficult and inconsistent location.

• Standardized Measurements – using standard dimensions and geometries in any casting design is a simple way to keep costs down through simpler machining and cheaper tooling. Minimum wall thicknesses, cavity and hole sizes, Chamfers and fillets should all be designed with standard tolerances in mind, whilst undercuts are best avoided.,

• Overengineering – Simplify design and avoid tolerance variation or implementing more design or process than required for the part to fulfil its need. This can rear its head in many aspects of design and production. One such area is dictating tighter tolerances than required, examine the tolerances to which your part can operate acceptably, and then require that tolerance. Another area is unnecessarily complex design, which can lead to increased machining time, use of five-axis machining when three-axis would have sufficed, and, as a result, higher setup costs.

CNC Machining Mistakes Cost More

Machining is the last stage of the casting process, before coating or anodising so it is the most expensive phase at which to make mistakes or get things wrong. Which is why it is so important to focus on simplifying every stage of this process.

CNC machining of castings can involve manual turning and machining and CNC turning, but most typically it involves 3 to 5 Axis milling, the most expensive and complex machining phase.

Sarginsons, through its subsidiary, Numachine, offers a full range of machining options but it is the 14 large, up to a bed size of 1.5 Metres, five and four axis milling machines that do most of the casting machining.

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).

Turning involves placing a cast component 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.

Machining castings: Milling

Milling is one of the most popular machining techniques for castings. Milling involves a stationary workpiece kept stationary by, normally, specialist fixturing, while the milling table or fixture moves, and the cutters rotate.

Milling is a versatile process. It is used to produce asymmetric and non-circular features and shapes, such as off-centred holes, slots, radial corners and square pockets.

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.

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.

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.

Sarginsons is an Aluminium Foundry but…

Sarginsons is a specialist low porosity and sand foundry that offers four different sand casting techniques, gravity, and low pressure casting. It has spent millions developing and researching new techniques, across every aspect of casting technology, from digital twin simulations, perfect sand castings to smarter directional solidification. And it has developed some of the most sophisticated CNC machining capabilities in the sector.

Numachine is Sarginsons CNC machining subsidiary, but it has kept its own identity and continues to offer machining services to a wide range of clients – from laboratory equipment to roulette wheels. Sarginsons believe that encouraging it to compete, hones its skills and knowledge so that it is better able to provide Sarginsons with a state of the art technology. Too often foundry machinists become the poor relations, soon after which standards, creativity and innovation disappear.

Numachine is one of the most vibrant, cutting edge and dynamic machinists in their industry. And whilst it is a leading force in machining components from a range of materials, it really when minimising machining times and solving seemingly intractable machining, fixturing and datum issues.

Patterns Solutions are Sarginsons sand pattern and fixturing experts. As with Numachine it was felt that allowing them to keep their own identity would enable them to stay abreast of the latest technology and techniques. This also avoids the invariable sense of being an also ran in the shadow of their big brother. Having an inhouse pattern manufactures gives Sarginsons an increased ability to react quickly, tackle complex components and to indulge in rapid iterative design of sand castings.