Deburring and Polishing Technical Articles

Dry organic mass finishing systems have been popular with jewelry manufactures for some time. This is because the light weight materials have a tendency to gently work parts in a wiping action similar to a buffing wheel and produces an almost hand buffed appearance on finished parts. Similar results can be accomplished with a wet systems using heavier steel or porcelain media, but they are not exactly the same. Normally smoothness and color are best accomplished by hand buffing or using treated dry organic media in a mass finishing system.

Wet deburring and burnishing systems do accomplish fast good looking results and provided that the parts are properly worked/deburred and have a smooth surface finish of at least a 16 to 18 RMS, a burnished surface finish is normally acceptable for the an end product. However, burnishing really does not remove edge or surface irregularities. This type of wet processing will only modify features and move material, but not remove it. Therefore, what you put into a burnishing operation is basically what you are going to get out of that machine system. If you start out with a rough porous part, after burnishing, you end up with a shinny, slightly work hardened, rough porous part. Some minor surface improvement can be accomplished, but not as much as is possible with just dry organic media materials.

Because most jewelry begins with a molded part or casting, the surface finish of these items normally exceeds a 35 RMS (Root Mean Square- a measurement of roughness or profile-peaks and valleys-of the materials surface). To get a good looking part, that surface roughness should be improved to at least an 18 RMS, before using a dry organic or burnishing mass finishing system. This is also true for hand polishing or using a buffing wheel system. The fastest way to remove the most amount of material in the shortest period of time is to use a wet mass finishing system with a coarse ceramic media. However, the final surface finish will only be as good or as fine as the roughness or size of the abrasive within the media. Meaning that a fast cutting media made with 60 to 80 grit may still not be fine enough to accomplish an 18 RMS and may better be accomplished with 100 to 120 or finer grit size media.[1]

For speed and efficiency, it is common practice in jewelry to use at least two or more processing steps to accomplish a mirror surface finish. That again is because of the abrasive grit within the media. Wet mass finishing systems remove more material in a shorter period of time because of its weight and abrasion characteristics than dry systems. That means that most processing of jewelry starts out with a wet system and because of convenience it is relatively easy to change from one wet media operation to another. Whereas dry processing requires at least 2 machine systems or a greater amount of down time to change over from a wet to dry operation. The reason for this increased down time is because proper maintenance care must be taken to close off the liquid systems inlet and outlet as well as to dry or replace the work chamber. Generally speaking to run a wet system requires less cleaning care and maintenance and normally produces an acceptable part.

Another reason for using an all wet systems is because of the nature or characteristics of the dry organic materials. That is, up to now, the particle size of dry organic materials has been a lot smaller and lighter than wet preformed ceramic or plastic shaped media. The dry organic small size particles lend themselves to small lightweight or thin flat parts and detail, but not speed. In fact, if the part or jewelry has some deep recesses, these small particles can get jammed or stuck in the detail and not allow for a uniform surface finishes; therefore, larger organic shapes are sometimes preferred to work small detail from the outside. That is because normally media is smaller than the part being worked, but larger than a recess within the part. That means that the media will be pushed into the detail without getting stuck, yet be small enough to be easily separated from the parts through a screen system when processing is done.

To achieve a good surface finish, a wet processing system using a preformed ceramic shaped media takes 2 to 12 hours or more in barrel systems, maybe 1 to 2 hours in vibratory equipment and a matter of minutes in high energy systems. Processing time cycles are influenced by equipment, which effects how energy is transferred, the size of the media, its coarseness, and the roughness of the part being worked. In comparison with wet systems, dry organic abrasive systems normally take anywhere from days in barrel systems, 4 to 24 hours of processing time in vibratory equipment, and about an hour or 2 in high energy systems. Even these time cycles can vary greatly depending on the roughness of the part. In this time period a dry abrasive system will only improve a surface finish by 10 or 12 RMS and I would not normally[2] recommend using a dry abrasive system for parts rougher than 25 RMS

Up to now, we have been talking about wet and dry abrasive material removal systems. Because of the difference in processing times there is little incentive to use a dry process system.

However, there are a couple of ways to speed up dry processing systems besides using different equipment. The most common way to improve the speed of a dry process is to add organic wood shapes. In fact, the time cycles quoted above reflect this type of processing. When used by itself, the wood shapes do not have much capability to deburr; therefore, it is used in conjunction to both organic and inorganic compounds to increase its ability to be abrasive or polish. The wood shape is significantly larger than the small loose random shaped sawdust like particles and therefore has more mass. The specific gravity of the bulk shape versus the particles also improves the performance of the mix and that effects the time cycle of the processing in relationship to just the smaller particles alone.

Wood shaped media is cut from a wood slab instead of molded; therefore, shapes are normally limited too either a peg, wedge, cube or diamond shape, but length and thickness can change. These shapes perform and behave identical to preformed ceramic and plastic shaped media except they are not really considered abrasive. The wood shape can be used by itself with additive compounds and without the loose particle mix. In fact, if there are a lot of deep details, holes, or tubing, the wood shape with compound is sometimes the preferred method of processing. However, most dry organic materials are used like an epoxy mix in a 5:1 ratio of shapes to particles. That is because these two media forms compliment each other. By itself, the loose random particles behave like water or a river, going around objects something like a lubricant. When organic wood shapes are added to the small particles, the analogy of water, becomes a river choked with ice that bangs into everything causing greater pressure than water alone can provide by itself.

Cut wood shapes weight about 24 pounds per cubic foot of material. Surprisingly, in bulk, all organic particles become heavier the finer they become, but their specific gravity density decreases. By itself, the heaviest and hardest particles of the dry organic media is loose walnut shell which weights about 35 to 45 lbs./cu.ft, next is corncob at about 23-33 lbs./cu.ft., and then wood saw dust comes in around 21-27 lbs./cu.ft. Even though these materials are similar in weight and size, shell and corncob products are granular; whereas, wood saw dust is more fibrous. Normally that means that the granular products flow easier but can possibly leave dimple marks, but the wood behaves more like a buffing wheel because of its wiping action and because it is loose, it sticks to the product more. When pretreated with either an abrasive or polishing compound, the weight of these dry organic materials changes a little, but they are still within the upper range shown

The wood shapes versus the loose random particle mix can achieve nearly identical surface finish end result, but not in the same time frame. Cycle times are reduced when the wood shapes are added by about 10%, because the shapes have greater surface area and specific gravity. In either case, we are still talking long time cycles here. To improve time cycles of the loose random shaped particles even more, some people have added preformed ceramic and plastics shapes to the organic materials. That is because ceramic media weights approx . 85 to 100 lbs./cu.ft. and plastic media from 55 to 90 lbs./cu.ft. Weight plays a very important roll in any mass finishing system because the relationship or function of Z, pressure, to the X and Y movement is a greater factor in part processing. The greater the pressure, the more material is remove and the faster the process. However, there is a limit to this rule. It is not recommended to use steel media with dry organic materials because weight difference is too great and that causes complete separation of both types of media within a machine system and parts will not move uniformly between the two distinct layers of media.

Because we are talking about wet and dry systems and weight as an important factor in processing times, there is always the possibility of using either abrasive ceramic or plastic media by itself or with pumice. It is possible to run inorganic materials by themselves, but it is not a good, efficient practice. By themselves, the media will hog off a lot of material in a short period of time, but within an hour or two, the media begins to load up with contaminates and deteriorates at a very high speed. A better solution is to add a fine inorganic pumice to the mix, which reduces time cycles and media life is extended; however, besides dust, the biggest problem is the surface finish of the part. The parts maybe deburred by this procedure, but when they come out of the process, they will look like castings or sand blasted parts and will probably be rougher than when they were put into the equipment.

For organic dry deburring, adding ceramic or plastic preformed shapes to predominately dry organic materials may be a good idea in some instances, but there are a lot of drawbacks. Preformed abrasive media by its very nature is or has a rough surface. The light weight particles of organic media gets stuck on the abrasive media and eventually clogs the whole outer surface of the abrasive making it become ineffective after short while. Therefore, this application maybe good for short term results, but not for continuous use. Another reason for the adhesion to the abrasive problem was stated earlier, most sawdust like particles are pretreated with either an abrasive additive binder or polish to improve their function. That additive compound then normally penetrates the plastic or ceramic media and soaks into or closes the porous surface of the abrasive thereby decreasing its ability to abrade. Ceramic porcelain media does not contain abrasive particles; therefore, it can be used with organic materials very effectively. Another exception to all of this is a little used special media recommended and used with barrel finishing systems. There is a special serrated nylon peg shaped media that is specifically designed to take or carry an abrasive paste. It is similar to wood pegs, but it has grooves or serration’s along its sides to help keep the abrasive in place and it behaves like that of a file or rasp.

Now, with all of that done and said, there have been some recent new developments in dry organic finishing that a lot of people do not know about and that may be of some interest. About five years ago, a company developed a patented method for making dry organic preformed media shaped materials with abrasives that look and behave just like normal ceramic and plastic deburring media, but it is used dry. The resin bond media is classified as a dry organic shape, but it can actually have more inorganic abrasive than organic materials in its preformed shape and composition. The composition formulations weight from 65 to 85 pounds per cubic foot and they behave the same as wet inorganic media used in processing parts. Time cycles are comparable to that of wet processing media, but it is only used dry. In addition to improving the cutting or speed of the material removal, the amount of material removed is or can be greater, thereby leaving the parts smoother. However, the biggest advantage of this new dry organic media is that it lasts 5 to 20 times that of most wet preformed inorganic media.

Like any new product on the market, there are pluses and minuses. Because this is a relatively new product, the costs are high at around $12.00 per pound. However, if you take into consideration the cost of media attrition, water, waster treatment, chemicals, maintenance, and labor relating to wet processing, the costs of this new media is compatible. Also, because attrition rate is lower, there is a substantial cost savings for freight over a long period of time. Some other indirect tangible savings are: there are no water marks on parts, no rusting, no sticking together of flat or light weight parts, and basically parts are smoother, brighter, dry, clean, and ready to use if there is a next operation.

Another negative feature is the inherent features of dry processing. That is, there is the dust problem of air borne particles and drag out of particles because of static electricity. The air borne particle problem is significantly reduced with any closed systems. That means that barrel system should not have a problem. However, it is recommended that any open top machine systems should have a dust cover or an adequate air collection system nearby. Even though I mention this as a problem, there is also a benefit created by this problem. The dust and drag out of small organic particles creates its own self purging system. That is, as the media wears down it becomes smaller, finer, and ineffective: therefore, there are no or few problems relating to the disposing of media due to porosity contamination. New media is constantly added to the mix unless there is a size problem with holes or recesses within the parts.

Up to now, we have been talking about both wet and dry abrasive systems. Until this new resin bonded dry media was developed, a wet abrasive system was the only alternative for efficient material removal using mass finishing systems. Now there is a choice. As the title indicates, it is now possible to efficiently run all equipment “ All Dry, All The Time”. The word convenience now does not carry the same weight or mean the same thing as it did earlier. There is no longer a major problem or difference between abrasive and secondary processing operations, as long as one does not have to switch from wet or dry systems to finish a part. Both methods are efficient, but now you have a choice of wet or dry. With more and more government regulations governing waste treatment and handling, I think the handwriting is on the wall.

As for a secondary burnishing or polishing processes after deburring, the choice of either a wet or dry system is also not as simple as it once was. Again speed and weight is in favor of steel and to a lesser extent porcelain media processes for achieving aesthetic results. Provided a good cut down of the part is done prior to this operation then a wet system is again a lot faster than dry processing. However, as mentioned before, brightness can be accomplished on even the roughest part going into the process. In other words, a burnishing process is not necessarily a smoothing of the surface finish. It does not remove material, but modifies or moves it and in the process it surface hardens the materials being worked. Burnishing cycles normally don’t exceed 1 to 2 hours in vibratory machines, more in barrels and minutes in centrifugal systems.

If time is not a factor and surface finish is, then I would recommend the fine organic materials alone as the better way to go. Unlike wet systems, dry systems will remove and improve surface finishes; however, the finish depends a lot on the additives. That is, most dry organic materials are treated or pretreated prior to processing with some form of polishing additive. The same polishing rouges used in hand buffing or wheel systems, can be used in mass finishing systems and are normally sold as an additive paste, liquid, or blended into the organic mix as a pretreatment. The most common additives are red iron oxide, green chromium oxide, and white or blue aluminum oxide rouge.

One of the most important rules about surface finishing is this. The RMS of the surface finish can only be as fine as the media that is being used; therefore, dry organic materials are very effective in producing smooth surface finishes, especially in high energy centrifugal force type equipment. Normal time cycles using dry organic materials in high energy systems rarely exceeds 2 to 4 hours, hence its increased popularity among jewelers as the equipment of choice for surface finishing. Then again, there are some high horse powered vibratory machines systems that are comparable to these high energy system in aggressiveness.

The biggest difference between wet and dry surface finishing is now no longer a question of time. Wet systems are work well and may still have the advantage of shorter time cycles; however, when it comes to a question of smoothness, then the dry systems are normally superior. With the growing concern for the environment, I am sure you will see more and more companies considering the dry finishing systems alternative.

For more information or inquires, please contact A.F.Kenton at Nova Finishing Systems Inc. in Huntingdon Valley, PA. 19006, or call 1-800-942-4474,e-mail novafinish@aol.com.

Mike Cantwell
Finishing Associates, Inc.
1610 Republic Rd.
Huntingdon Valley, PA. 19006

[1] It is interesting that there are no studies or graphic charts stating the expected surface finish or time cycle using a specific grit media. This is all derived from hands on information.

[2] See exception on new dry organic shapes later in article.

Nova Finishing
PO Box 185, Hatboro, PA 19040 * 1610 Republic Rd. Huntingdon Valley, PA. 19006
215-942-4474 * 800-444-4159 * Fax 215-953-1342
novasales@novafinishing.com