In my previous article, I introduced you to the inefficiencies caused by the result of media attrition when vibratory processing gears. If you recall, I noted that media attrition is the result of frictional wear and a function of machine time. Every day, every hour of machine run time decreases ever so slightly the volume of processing media in the vibratory bowl. If the level of media in the bowl is not periodically topped-off, low media volume will result in a plethora of mechanical inefficiencies. These include part-on-part contact damage, increased processing time, decreased applied refinement force on gears being processed in the vibratory bowl, media lodging, loss of gear tooth metrological form, and bowl fluid retention.
What is deceptive about media attrition is that it is continuous and disconcertingly too subtle to detect. A regular regime of media addition, however, will keep processing balanced. But how frequently does that need to occur? How can you predict what attrition loss will be? How much media do you need to add when you do notice that the level in the machine is too low? Let us take a quick look at a few tricks that will help you stay ahead of the attrition efficiency bandit.
What is the Attrition Rate of the Media You are Using?
The key tidbit of information relative to predicting how much media to add to the machine and when — as well as when media needs to be ordered from your supplier — is understanding the media’s attrition rate. However, this bit of information, as I can attest, has seldom appeared in media catalogs. Media catalogs, printed and distributed by media manufacturers, instead describe their categories of media offerings in euphemisms. Statements such as “… provides a fast refinement speed,” or, “… generates an acceptable pre-plate finish,” are common descriptions. However, these statements offer zero data with which you can predict a suitable replenishment time and purchasing schedule.
There are two characteristic components that go into making vibratory media. They are the ceramic base, i.e. the binder, and the loose abrasive grit, typically, aluminum oxide. Like sandpaper purchased at the local hardware store, media manufacturers offer a variety of compositions designed to meet processing parameters for deburring applications. The different formulations are also referred to as “bonds”. Understanding the approximate formulation of the bond will help you to understand the media’s attrition rate.
For traditional abrasive ceramic formulations, it is the aluminum oxide (or other) abrasive added to the ceramic binder that mechanically abrades the parts being processed to remove adherent burrs and appropriately radius sharp edges. By varying the ratio of ceramic binder to aluminum oxide grit, the media manufacturer can produce assorted compositions (different bonds) that offer an assortment of refinement speeds and finishes. By increasing the percentage of grit and lowering the percentage of ceramic binder which, as its name implies, is the glue that holds the media together, the manufacturer creates a media composition that aggressively removes burrs in a short period. However, since the ceramic binder content is reduced, the media has a higher attrition rate. Reversing the composition characteristics such that there is a higher binder content and lower abrasive content, results in a slower refinement time (for traditional, abrasive-only processes), a lower attrition rate, and potentially a higher-quality final finish. Table 1 is a table that captures this information for the traditional media compositions.
How to Calculate Media Consumption Using Media Density and Media Attrition Rate
Here is a hypothetical example of how to calculate your attrition rate. Let us assume that a 20ft3 vibe bowl with 16ft3 of usable capacity is in use. Additionally, let us assume that 80 percent of the usable volume is media and 20 percent of the usable volume is the gears being refined. Let us start by calculating the volumetric displacement of the media and parts:
• Media = (16 ft3)(0.8 usable volume) = 12.8 ft3 media
• Parts = (16 ft3)(0.2 usable volume) = 3.2 ft3 parts
• Let us now assume for this example:
• The media being used is a fairly aggressive 40-bond deburring media
• The processing time is 2.0 hours
Using the data in the above table, it is possible to determine the actual weight of media consumed; therefore, the weight of media sludge generated for the two-hour cycle is as follows:
• From the table, 40-bond media has a weight density of 80 lbs/ft3
• From the table, 40-bond media has an attrition rate of 1.2%/hr
A two-hour processing cycle of 40-bond media will consume the following weight of media:
Volumn of Media x Media Density x Run Time x Attrition = Rate Media Consumed
12.8 ft3 usable volumn x 80 lbs/ft3 x 2.0 hrs x 0.012/hr = 24.6 lbs
Media is commonly sold in 50-pound boxes or sacks. If a two-hour processing consumes approximately 25 pounds of media, you can project that for every four hours the bowl is operating, it will require one each 50-pound box of raw media for replenishment. The above formula can be used to determine media consumption for any media bond that is represented in the table.
As maintaining the proper media level in a vibratory bowl or tub is critical to efficient and effective processing, understanding your media attrition rate is a prerequisite to proper vibratory process maintenance. Using an appropriate media attrition rate calculation will allow you to plan your media replenishment frequency and quantity. High media attrition rates, where possible, should be avoided. The use of chemically accelerated mass finishing technologies such as isotropic superfinishing has the distinct advantage of being able to achieve comparable or superior material removal rates to highly abrasive (high attrition rate) medias while using low or non-abrasive medias—significantly reducing media attrition rates and simplifying your media maintenance requirements.