Originally developed as a process for deburring, mass finishing has greatly impacted the manufacturing world to produce uniform, high-quality parts faster and at a lower production cost. As mass finishing has evolved with equipment, media, and technology advancements, industries have benefited noting minimal disadvantages. With many parts being processed at the same time during mass finishing, precautions can be taken to prevent the risk of part impingement, or denting, during polishing. Part impingement can be described as when two parts are hitting each and leaving a mark during the finishing process. Part impingement occurs when the part is struck with enough force by another part or media to leave an impression, or valley, on the surface. 

There are two main causes of part impingement during the finishing process. The first is impingements caused by the parts themselves, or part-on-part impingement. The geometry of a part, in addition to its size and weight, can affect the finishing process. For example, part impingement can occur if the parts are heavy or have sharp edges. 

Tumbling media can also cause impingements during the finishing process. When the parts come into contact with a really hard media the centrifugal force that is exerted can create impingements. This is similar to the way a hatchet works. The heavy end of the hatchet will do the most damage because it has more energy.  

Imperfections created by impingements can impact the overall appearance, performance, and longevity of the finished parts. Preventing part impingement during finishing can be a very important step depending on the requirements of the final component finishing. In addition, the application and performance of coatings like Titanium Nitride, diamond-like-carbon (DLC) coatings can be affected. Thin film applications require the surfaces to be extremely smooth for successful adhesion and ultimate bonding of high-performance coatings. 

How to Control Part Impingement

While it’s unlikely every finished part will be impingement-free, the effects caused by part impingement can be limited. Evaluation of the parts and developing end result expectations with the customer is an aspect of the overall finishing process. Some parts may contain difficult or hard to reach areas, while other parts may have sharp or protruding edges where impingement may be ok for the success of the product. The size, weight, material, and geometries are all important factors to consider when controlling part impingement. 

The Finishing Process

Determining the most suitable finishing process is the first step to controlling part impingement. Typically, the main methods used during mass finishing are centrifugal barrel finishing and vibratory finishing. While both achieve great results, the method used will depend largely on the composition of the part, tumbling media, and experience with the material. The process may also be dependent on the actual requirements of the part for the particular industry application.  In some aerospace applications, an Ra of 125 is totally acceptable while a medical implant component may require a surface Ra below 2. 

Vibratory finishing is often used for finishing large or delicate parts because it causes less wear on the surface. In contrast, centrifugal barrel finishing works well for parts that require heavy deburring because it has a higher energy output. To prevent part impingement while using centrifugal barrel finishing, parts can be segregated using dividers inside the barrel. The volume of parts to media ratio is another key consideration. In some cases, the mass of the parts is used as a driver with the media to increase the tumbling efficiency.

Another way to control part impingement during finishing is to determine the proper speed of the machine. Both vibratory finishing and centrifugal barrel finishing machines have variable speed control. This allows for the rotation speed to be adjusted according to application. In centrifugal barrel finishing, a higher speed creates more centrifugal force, which eliminates part-on-part impingement. When utilizing vibratory finishing, the amplitude should also be considered. A higher rotation speed with a lower amplitude produces the best surface finish. 

If the media level is too low there is not enough resistance for the part within the barrel and it can collide with media against the wall.  The same is true if the media is at an acceptable level but the speed is too high. The increased speed will drive the part against the media, hitting the wall of the barrel and causing the impingement.

Selecting the Right Tumbling Media

Selecting the right tumbling media during the finishing process is a crucial step to preventing impingement. Media impingement can happen when processing materials like aluminum and copper combined with a hard-ceramic media. In addition to the specifications of the parts, it is also important to consider the characteristics of the media being used. The key is to select media appropriate to the size, weight, shape, and material of the parts but will not cause damage or get caught in small openings and passageways.  

Iso Finishing offers the highest quality surface finishes for cast, molded, machined, fabricated, and 3d printing companies. Our team members have the expertise to tackle any polishing project to produce exceptional results. To learn more about the isotropic finishing process or request free sample processing for your part, please contact our team.