Casting Surface Finish Tips for the OEM & Foundry
Clear and precise communications between a casting end-user and its foundry are vital if a casting is to be launched successfully. As a casting buyer, have you remembered to include discussions about the cast surface finish requirements? Very often this subject is overlooked, so it is important that both the buyer and seller know and understand the surface finish expectations before any castings are produced.
Misunderstanding the surface finish requirements leads to incorrect assumptions during the quoting stage, which can then translate into higher casting costs after production begins. Requiring a casting to have high levels of visually defect-free surfaces will increase the foundry’s scrap rate and can easily add 15% or more to the cost of a casting.
Cosmetic surface finish requirements, such as the amount and type of visual defects allowable on the casting, should be well understood. The casting designer should make every effort to define the surfaces that are touch points or need to be defect-free. When the foundry engineering team is made aware of tighter defect requirements, they can engineer its process to improve the surface finish. Some steps that the foundry can take are placing ingates and risers in the correct location in relation to the critical casting surfaces, or the use of alternate metal filtering methods.
In some cases, drawings or supporting specifications will contain wording such as: “part to be finished in a workmanlike manner,” or “casting to be clean and free of adhering sand, parting fins, and gate/riser contacts.” This doesn’t really address the surface finish requirements completely and are sometimes vague at best. The foundry and OEM need to develop an inspection criterion so they both understand and agree on what is and is not acceptable with these systems.
To facilitate this common understanding, several surface finish comparator plates are commercially available that can be used to specify the desired surface finish of the casting. Surface finishes related to texture are directly related to the molding and finishing processes used and should be discussed during collaboration with the foundry at the quoting stage. For gauging surface roughness or texture, the C-9 Microfinish Comparator plate can be used. It shows nine different as-cast surfaces covering molding methods from diecasting to green sand.
For allowable surface defects, the use of SCRATA plates is widely used in the industry. These consist of plastic surface replica plates covering various types of defects in several levels of intensity. Another standard that is used is MSS-SP55, which is a photographic representation of defects in varying intensities. All of the comparator systems require operator training to make sure these are applied correctly.
Closely related to the topic of surface finish are the finishing requirements for the casting. In this context, we are covering two specific areas—grinding and cleaning. Both can have a significant impact on the surface finish and physical appearance of the casting. Make sure the foundry won’t be performing more finishing work on the casting than is necessary to fulfill the design intent.
In the area of grinding, does the parting line need to be ground flush with the surrounding surface, or can it be left as-cast? Similarly, can riser and ingate connections be ground smooth but left as a raised pad instead of being completely removed? It is often difficult to have an even transition from the ground surface to the as-cast surface.
With regard to cleaning, normal foundry processing would generally involve shot blasting the casting after shakeout, then perform grinding operations. An additional shot blast operation after grinding will have the effect of smoothing the ground areas but will definitely be adding cost to the casting.
Reviewing all these topics before the foundry starts processing their quote could seem like a lot of work at first glance. However, the more work that is done up front in terms of understanding the requirements and expectations of a casting will result in far less rework and frustration once the part is in production. CS
Click here to see this story as it appears in the May/June 2020 issue of Casting Source.