Those of us who have played the particle counting game know that there are some tough calls when it comes to harvesting particles as part of the cleanliness testing procedure. In some cases, adhering strictly to the specification would result in the rejection of every part tested. Although I do not believe that rules are “made to be broken,” there are cases where interpretation not only makes sense but is absolutely necessary. Since once the harvesting phase is complete, a particle is a particle, it is sometimes appropriate to take note of the nature and source of a “particle” at the harvesting stage. I know many of you are saying “Huh?” so let’s take a minute to talk about these special case particles
“Attached” Particles -
“Attached” particles are especially prevalent in machined parts but examples can be found resulting from nearly any manufacturing process. These particles, many of which are technically called “burrs,” may actually be a contiguous part of the substrate. A few of many examples are chips resulting from tapping, broaching and turning operations as well as flash that results on molded parts at the parting line of the mold. The obvious and simple solution is that these parts should be “de-burred” prior to the cleaning process preceding particle harvesting for cleanliness evaluation. Despite tight manufacturing processes, some of these “attached” particles will make it through intact. The worry, of course, is that although they are attached now, they may not be well enough attached to remain that way. The important thing to realize at the point of cleanliness testing is that these particles are NOT due to any shortcoming or anomaly of the cleaning process. They are a manufacturing issue. No amount of “cleaning” will remove or prevent them yet they pose a real risk.
“Captured” Particles -
Captured particles are particles that may be from any source that are firmly wedged or captured by geometric features of the part being tested for cleanliness. Chips that become wedged or “sprung” into tapped holes and other part features may not be dislodged by the brushing, flushing or ultrasonic techniques described in the preceding blog and yet may be freed by conditions present in the application of the part with potentially disastrous results. Again, the real answer is “chasing” threads, more aggressive cleaning techniques directed specifically at the potentially troublesome features of a part and/or better inspection techniques applied prior to cleanliness testing. These particles are NOT really the result of inadequate cleaning and likely won’t be prevented by modifications to the cleaning process.
Particles on Non-Critical Surfaces -
Many parts have both critical and non-critical surfaces. “Valve bodies” are good examples of this kind of part. These parts often start out as castings with relatively rough exteriors. Machining processes further develop the part into its final form. “As cast” surfaces are notorious for particle shedding yet particle harvesting techniques previously described do not discriminate between critical and non-critical surfaces. A particle from any source is still a particle. Because the surface area of the non-crital part exterior (especially in the case of a casting with internal machining for example) may be many times that of the critical interior machined surfaces, harvesting particles indiscriminately may result in a huge number of particles most of which came from surfaces that don’t really need to be that clean. In some cases, therefore, it is not inappropriate to specify concentrating particle harvesting on only the critical surfaces of a part by masking or otherwise preventing or avoiding access to the non-critical areas.
The Forever-Shedding Part -
Some substrates never stop shedding particles. Cast iron is one example of a material that will confound any particle harvesting protocol due to its structure which includes encapsulated free carbon. Highly polished surfaces produced by buffing, surprisingly, also shed particles. Buffing tends to smear the metal resulting in layer upon layer of thin laminations which flake off. Although we will talk about particle analysis in more detail in upcoming blogs, “flakes” are often traced to aggressively buffed surfaces. In these cases, the only solution to passing a cleanliness specification that includes an aggressive particle harvesting procedure may be selection of a different substrate material or a change in finishing procedures.
The above is not an exhaustive list of particles that may not “count,” but hopefully will help the reader understand that such cases do exist and may not be cleanliness issues at all. The question is, do you break off that burr, for example, and collect it as a particle or do you leave it there with confidence that since it is actually attached to the substrate it won’t be a problem?
- FJF -