Understanding Vibratory Finishing: Part 4 of 4: Media

The vibratory finishing process has four key elements: media, compound-water solution, equipment and parts. Previous installments of this series have dealt with the first three of these.

The "Tetrahedron of Interdependence" (Fig. 1), was shown as a structure simulating the vibratory finishing process. When a problem occurs with any one of the four elements, as this illustration shows, the problem might severely affect the entire process. The elements are dependent upon one another. The process, like the structure, will fall apart if any one element fails.

Without question, the most important of the four elements is the parts. Without the parts to be finished there would be no process. And, depending on the requirements for the parts, the process is designed, equipment is chosen, media and compound solution are selected. Only when a new process is set up to utilize existing equipment is the procedure changed. Normally, the parts dictate all else.

In a previous article, we looked at the likelihood that variations in the composition of media would be sufficient to cause noticeable differences in the process. The conclusion was that this is rare today, and not very likely a cause of problems. The same can be said about the compound solution and the equipment. Concentrations could change quite easily or settings could change, but the materials themselves are infrequently a really significant variable.

But are the parts a significant variable? Yes, they are. Why? Do dies ever have exactly the same sharpness? Can the burrs these dies form then ever be quite the same size? Don't they very slowly grow in size as the sharpness of the tooling decreases? And how about the quality of die-cast surfaces? Doesn't it gradually deteriorate and develop heat-check? Doesn't this cause a slow, progressive deterioration of the as-cast surface? "Yes!" to all the above!

Now before all of you manufacturers of high quality parts nip off my head at the neck, let me explain loudly and clearly that we do not imply that these gradual changes are always bad. If they were, we'd have to do everything by hand. And we don't, so relax! But at least agree that slow, gradual, often unnoticeable changes take place in the surfaces of parts. If this concept is accepted, we will have no misunderstandings.

In vibratory finishing, deburring of die-cast parts is commonplace. When new parts are made, the manufacturer sends some of them off to his favorite supplier(s), requesting quotes on finishing. Cycle times are established, machine capacity calculated, media and compound solution selected and costs totaled. The manufacturer can then determine his manufacturing costs, including the deburring operation. He, the deburring shop operator, orders equipment, media and compound, and prepares space for the operation.

Three to six months later, equipment is installed. Media and compound are put in and parts are brought to the machine for final check-out. Then, all hell breaks loose! Parts are rejected. Every last one of them. What happened?

If the equipment was set up and run as it should have been during the demonstrations months before, and if the same compound and the same media were used and used properly, the problem must be with the parts. Are they, in fact, the same? Are the burrs the same and is the tooling the same?

Fig. 2 shows the concept of burr size versus time as a function of die wear. Note that time, or the number of parts produced, may be quite long, while burr enlargement might be quite small. Or they may be very changeable: short time or few parts and a big change in burr size. Fig. 3 shows what this must do to time cycles in the vibratory process! Again, the differences between T₁, and T₂ may be very small. But the point is, they must vary directly with the difference in burr size, B₁, and B₂.

The identical effect takes place with surfaces. As dies get worse, the surfaces cast against them are rougher and the parts after plating look duller. Quality diminishes. It must! Vibratory finishing is a very stupid process. It can't think. Can't vary the job even when it should. It has to be consistent. Sounds like a pretty good quality control tool, doesn't it? It is.

When everything else checks out as okay, check the parts. They may have changed. So be sure to measure them now so you'll recognize when they do change and by how much.

Table I — Media-to-Part Volumetric Ratios
Media-to-Part Ratio by Volumn	Normal Commercial Application
0:1	No media. Part-on-part. Used for beating off burrs. No media for cutting.
1:1	Equal volumes of media and parts. Forgings, sand castings; to produce crude, very rough surfaces.
2:1	More gentle, more separation. But still allows relatively severe part-on-part damage
3:1	About minimum for non-ferrous parts. Considerable part-on-part contact. Fair-to-good for ferrous metals.
4:1	Probably average for non-ferrous parts. Good for ferrous metals.
5:1	Good for non-ferrous metals. Minimal part-to-part contact.
6:1	Very good for non-ferrous parts. Common for preplate on zinc with plastic media.
8:1	For higher quality preplate finishes.
10:1 TO 20:1	Even better. Used for very irregularly shaped, fragile parts.
Infinate	Absolutely no part-to-part contact. One part per machine or compartment or the part is fixtured.

As a result, manufacturers are taking advantage of the uniformity of the vibratory process to tell them when to make adjustments in tooling. For example, a process is set up to deburr parts in 30 min. In time it is found that it takes almost 40 min to deburr the same parts. And, it is also determined that 40 min is too costly-that a cycle of 35 min could be tolerated, but no longer. Therefore if this time is exceeded, it's time to sharpen tooling! Fig. 4 presents this more graphically.

The company in our illustration has had something very important happen to it. Its personnel have begun to rely on the process, its uniformity and precision. Vibratory finishing is no longer a burden fraught with constant problems. It has become an important machine tool in a profitable plant. Now that's what it's all about!

No discussion of parts would be complete without reviewing what it is that the vibratory process can do for them. We'll look at the seven significant purposes of vibratory finishing:

1. Clean.
2. Deburr or radius.
3. Improve surfaces.
4. Brighten.
5. Inhibit.
6. Dry.
7. Transfer.

1. Clean. This subject has been well covered in previous parts of this series. But cleaning is an important and rapidly growing capability of vibratory finishing procedures. Cleaning can be done with little or no increase in cost, quickly and without heat. Typical parts cleaned commercially in vibratory units today include all kinds of machined engine components, stampings, copper plumbing fittings, brass forgings, radioactively contaminated parts, metals, plastics, ceramics, rubber and wood.

2. Deburr or radius. This is the best known use of vibratory finishing. All metals, ceramics, and some plastics can be deburred. Cast, threaded, stamped, forged or wrought materials are routinely processed. Anything sharp that needs to be dulled can be accommodated. Deburring prevents people from cutting themselves, and makes parts feed through automatic feeders more easily. It helps to prevent accidents, and assures that baby brother won't cut his lip on the new toy. And deburring parts keeps OSHA off your back, too.

3. Improve surfaces. Rough surfaces need to be smoothed before they take a high-quality electroplate, paint or anodize. Zinc die casting for jewelry, musical instrument keys, sports equipment, automotive and appliance parts are smoothed. Surface roughness values below 10 microinches (0.25 micrometers) AA are commonplace and, with good preplate plastic media, values below five microinches (0.13 micrometers) are commercially achieved.

In any discussion of surface improvement, the concept expressed in Fig. 5 must be emphasized. This graph simply shows that the better you, the manufacturer, make your parts' surfaces, the cheaper it will be to finish them. Surface roughness and vibratory process time go hand in hand. They MUST! The stupid process can't do it any other way.

4. Brighten. Brightening or burnishing of metal surfaces is a good, viable application of mass finishing. It is done most easily to the more malleable metals by using nonabrasive media such as steel or the nonabrasive ceramics. Aluminum, copper, brass and mild steels can be brightened in 10 to 30 min depending on finish quality required and starting surface quality. Harder metals such as stainless, some brasses and harder steels take 30 min to several hours. These metals may require modified techniques, too. Brightening may be done for product appearance or quality requirements as well as for product identification.

5. Inhibit. Any time water, air and extremely active metal surfaces are brought together, corrosion must take place unless some form of inhibition is used. Inhibition is usually the result of an additive in the compound solution, but it may be achieved partly as a post treatment.

Very sensitive metals including powdered iron, cast iron, copper, brass, zinc and steels of all types will corrode to some degree unless protection is provided. Metal surfaces will look cleaner and brighter for longer periods of time if they are inhibited properly.

6. Dry. To prevent corrosion or discoloration or to improve parts handling in subsequent plant operations, parts should be dried. Many types of drying equipment can be used, including spin baskets, hot-air conveyors and dryers filled with ground corn cobs. The latter may be the round vibratory or the rotary type with screw auger. The cob meal types efficiently wipe surfaces and eliminate water spotting, but because they use media, lodging can sometimes occur.

7. Transfer. Modern mass finishing is capable of any degree of automation desired. Fully continuous systems put through vast quantities of parts at extremely high rates and at low costs. Simple batch systems reduce initial investments, but require more labor. With sufficient forethought, you may be able to add transfer systems later. Using technology to reduce cost while achieving acceptable product quality pays great dividends.

The easiest transfer to use, and one of the most effective, is the simple conveyor belt. It is often used to load parts into the vibratory machine. The belt can be loaded at any time during the cycle. Proper height and depth of loading on the conveyor insures that the machine is being properly utilized. When the vibrator is empty, the conveyor slowly feeds parts into it, distributing them throughout the mass. No part-on-part impingement occurs. The operator does not have to be there at the beginning of the cycle.

In-house handling of parts is also important. A manufacturer of extremely high quality products could not use his vibratory finishing process for preplate on zinc because the parts were treated like forgings. The defects caused by poor handling could not be removed fast enough to make the vibratory process economical! But didn't he have to do more work on those parts--by polishing or buffing? You bet! It didn't make a bit of sense!

Parts. Bless 'em, they keep us all in business. So give them a break and think about what they go through. And save yourself some money as you do this.

Parts and Media

The effect media has on parts was discussed in Part 1. Now, let's look at what parts do to media! Sound confusing? It isn't.

Big, heavy parts can crush media, leading to premature media breakdown and higher costs. Crushed or whole media can stick or lodge in parts and be carried out of the vibrator by the parts, causing an apparent high media "wear. "

But one of the worst effects parts can have on media is to soil the media. Oils are especially bad for media if the compound solution can't remove them rapidly enough. With oil on media surfaces, cut stops. Parts emerge dirty or dark in color. Gradual changes occur in the process and the efficiency of the process and satisfaction with it declines. It shouldn't.

An important relationship between media and parts is known as the media-to-parts volumetric ratio. In a "mass" finishing process, the amount of contact between parts is controlled by the ratio of parts to media. Machine settings control the force of these contacts. This is a statistical phenomenon based on the probabilities involved. More parts and less media allow more contact. Typical ratios are as shown in Table 1.

As noted, changing from a ratio of 4:1 to 3:1 makes a dramatic difference in results. This, in a machine with random distribution of parts, is a fact of life. Users who continually put in a "few extra parts" decrease surface quality. There is no choice.

So if you want smooth parts, keep media levels up. Don't overload the machine with parts. Put only enough parts in the machine to allow producing good results. Calculate the number of parts you can safely put in per load and stick with it. Otherwise, don't complain about quality! Parts and Compound Solution

Putting dirty parts into the vibrator when the process can't handle them is poor. Putting new soils into the machine, if the compound solution is not ready for these new soils, is just as poor; but if the proper compound solution is used, the process is stronger; it can do more. The criticalness of the compound solution and its effect should not be overlooked.

Parts and Equipment

The relationship was discussed in Part 3 of this series of articles. Keep in mind the fact that most good commercial equipment can do most jobs. But some of the equipment can do some jobs better, faster, at less cost or more efficiently. What degree of automation do you need now as well as next year? Does the equipment have the capability to provide for it when you need it? Did you buy a cheap machine or a low-cost one? How much do you have to "fuss" with the process after it's installed? How is your equipment warranty? Does it cover the process as well?

These questions involve parts and equipment. They should be answered early in the decision making process.

Parts and People

People make parts. Or at least they control the machines that make the parts. When the quality of the parts is maintained, the vibratory finishing process is consistent. If the part quality tends to stray, expect the process to stray, too. It's really that simple.

People make decisions. They make changes that affect the parts. The process can't. Take another look at Fig. 6, the Tetrahedron of Interdependence, with people in the middle where they belong. People will muddle the process or keep it simple. The choice remains in the hands of the people!

Smart people take every possible advantage of their stupid process. They insist that it do the same, uniform job day in and day out, right on through coffee breaks and lunch. People rely on the vibratory finishing process and they learn how to use it as an excellent quality control tool (Fig. 4). The choice is up to people! Take it! PF