Post-machining (sometimes known as secondary machining, or simply ‘finishing’) is a crucial process which ensures that every casting meets the quality standards that OEMs demand.
Cast metal components are a staple of advanced technologies - such as those seen in aerospace and medicine - where specifications must be exact. Not only is there no room for error, but parts need to be completely free of debris from manufacture, too.
Once a casting has been initially cast or moulded, it may need some refinement to meet these standards – and that’s where post-machining comes in.
Trimming. This process involves removing any excess material from the casting. Initial removal of the running system and overflows from the casting process is typically achieved using press trimming, manual break off or sawing. These processes generally still leave a visual witness of the gate area, so for more exacting applications, subsequent finishing or milling of these features may be require to blend them in.
Milling and Turning. Whilst diecasting produces a near net shape of the end product, the dimensional tolerances of the process are still not as accurate as machined parts. Therefore if you want the economic advantages of diecasting, but tighter dimensional accuracy than the process can deliver, secondary milling or turning of the part can remove cast material and hold tighter tolerances. The casting process also requires taper on some faces, and if this isn’t wanted on the finished part, then secondary milling or turning can remove this.
Drilling and tapping. In some cases, multiple castings may need to be assembled together or combined with other components. Post-machining may involve operations such as drilling, tapping, or welding to facilitate the assembly process.
Deflashing and Deburring. During the casting process, small burrs or rough edges around the joint line of the tool, known as flash, may form on the casting. Deburring and Deflashing is the process of removing these imperfections to achieve a smooth and clean surface.
Surface Finishing. Casting surfaces may undergo additional treatments to improve their appearance or functionality. This can include processes like polishing, sanding, grinding, or applying coatings to achieve the desired texture, smoothness, or protective properties.
It’s important to remember that initial dimensions of castings can vary slightly, due to factors such as shrinkage and thermal effects during the solidification process. Post-machining, therefore, allows for precise removal of excess material and adjustment of critical dimensions to meet the required spec.
Castings can also have complex geometries and intricate features that may be challenging to achieve with the die alone. Post-machining enables the creation of precise details - such as threads, holes, or fine surface textures - which may be critical for the functioning or aesthetics of the final part.
Post-machining operations provide opportunities for thorough inspection and quality control, too. Machining can uncover any defects, porosity, or material inconsistencies that may have been hidden within the casting. This non-destructive testing can be critical to ensure that the final part meets the required quality standards.
When manufacturing a casting, several design considerations should be made in regard to post-machining, to ensure a successful and efficient process.
When your casting is in the design phase, it’s important to provide sufficient allowances for machining. This includes leaving extra material at critical areas where post-machining operations will be performed.
These allowances ensure that the final dimensions and surface finish can be achieved without cutting into the desired features, or weakening the casting.
In addition, the casting design should also consider the accessibility of the areas that need to be machined. Adequate space and clearance should be provided to allow machining tools to reach and operate on the required surfaces. This includes considering the orientation and positioning of the casting during machining operations.
Furthermore, the choice of casting material can impact the ease and effectiveness of post-machining. Some materials may have better machinability properties, while others can be more challenging. Material selection should always consider the desired final properties of the part, as well as the ease and availability of machining tools – not to mention the techniques for that specific material.
Post-machining operations can add to the overall manufacturing cost and lead time. This is why it’s important to balance the desired level of post-machining with your functional requirements and cost constraints.
It’s vital to work with a casting partner who will not only optimise designs for the process at hand, but carefully assess machining operations to minimise excess material removal – as well as reduce machining time and maximise the utilisation of resources. This will not only help to reduce costs and lead times, but ensure that your casting meets the needs of its final application.
Are you considering a die cast part for your next project?
MRT Castings have specialised in the manufacture of high-quality aluminium die castings for over 75 years. From design for manufacture to post machining, mechanical assembly and beyond, we have the experience needed to meet every expectation for your component.
If you need help to choose a casting process, or you’d like to discuss the next steps, contact our team today.
Download our free guide: Notes for Designers of Non-Ferrous Castings
