Making the Grade With Mechanical Media
An amateur woodworker who purchases sandpaper at a hardware store will find it available in a variety of grades. Likewise, the vibratory operator is faced with a variety of choices when choosing a suitable media for the finishing task to be accomplished.
The most commonly employed mass finishing media is abrasive ceramic media, composed of a ceramic binder and abrasive grit. By altering the ratio of grit to binder or by choosing different grit grades, the media manufacturer can alter the aggressiveness of the media.
The ceramic binder is itself a blend of sand, clay and feldspar. Also, by changing the ratio of these binder sub-components to one another, the media manufacturer can alter the kiln-firing, vitrification temperature, and consequently the media’s attrition rate.
In general, the higher the vitrification temperature during kiln firing, the denser the media and the lower the media’s attrition rate. High-temperature, vitrified media maintains its size and shape longer because of this durability.
Polishing belts are purchased by choosing the abrasive grade content of the aluminum oxide resin impregnated on the belt, first and foremost. Likewise, media abrasiveness is usually specified by different grades, more commonly referred to as bonds.
Table 1 shows that, as the ratio of abrasive content increases in the media bond, consequently the ceramic binder content must decrease. Since the abrasive is less dense than the ceramic portion of the media, the higher the abrasive concentration, the lower the media’s density.
Since the ceramic binder is the glue that holds the media together, the media’s deterioration rate will become higher as its content decreases with an increase in the abrasive content. Simply stated, there is less glue present to hold the piece of media together.
The rate at which media deteriorates is known as its attrition rate. Unfortunately, when vibratory department personnel are considering the selection of a new media, the manufacturer’s literature never details the attrition rate of the media in anything other than euphemistic phrases such as “fast cutting,” “moderately abrasive” or “long lasting.” The sales brochure will seldom state the media’s actual attrition rate. Therefore, it becomes a guessing game of trying to interpret the verbal descriptions vs. the task at hand.
Table 2 has been compiled as a result of this sales literature citation deficiency. Based upon the author’s experience, attrition rates have been assigned to the previously cited media bonds in Table 1.
A Simple Technique
for Calculating Media Sludge Generation
Let us assume for illustrative purposes that the vibratory department has a 20-ft3 vibe bowl with 16 ft3 of usable capacity. Let us also assume that the vibe operator loads the machine at an 80-percent-media-to-20-percent-parts ratio. It is now possible to calculate the volumetric displacement of the media and parts:
Media = (16 ft3)(0.8 usage) = 12.8 ft3 media
Parts = (16 ft3)(0.2 usage) = 3.2 ft3 parts
Now let us make the following additional assumptions for this example:
1) Media is a 40 bond.
2) Processing time is 2 hours.
Using the data found in Tables 1 and 2, it is possible to determine the actual weight of media consumed and, therefore, the weight of media sludge generated for the two-hour processing cycle as follows:
1) From Table 1, 40-bond media has a weight density of 80 lbs/ft3.
2) From Table 2, 40-bond media has an attrition rate of 1.2 percent/hr.
A two-hour processing cycle will generate the following weight of media sludge:
(12.8 ft3)(80 lbs/ft3)(2.0 hrs)(0.012/hr) = 24.6 lbs of media sludge
Media vERSUS Part
To maximize the rate of part refinement in the vibratory bowl, one must additionally maximize the media-to-part-contact efficiency. When considering the efficiency of contact between these opposing surfaces, there are three types of contact patterns possible: planar, linear or point contact.
Planar contact occurs when the flat portion of the part being vibratory finished is contacted by the flat face of a piece of media. This type of contact pattern offers maximum surface-to-surface abrasive contact and, consequentially, usually the shortest processing time.
Linear contact is the result of curved-surface-to-flat-surface contact. The resulting contact pattern is also known as a tangent line. There are three combinations of media and parts that result in linear contact:
1) A flat part being contacted by a curved piece of media.
2) A curved work piece being contacted a flat media face.
3) A curve part being contacted by curved media.
Linear contact is not as efficient as planar contact, because a smaller percentage of the part’s total surface area is being contacted per unit time.
Point contact occurs when either spherical media contacts a flat part or when flat media contacts a spherical part. If the part to be vibratory finished is flat, then planar contact can be attained by using flat media. Spherical media is typically used for peen polishing operations and, as such, is not typically a media choice that is used during normal abrasive ceramic vibe operations. n
William Nebiolo is with REM Chemicals Inc. (Southington, Conn.) Contact him at 860-621-6755. This information was presented at the NASF Sur/Fin Conference in 2010.
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When choosing vibratory media, understand the size, shape, starting roughness condition and metallurgical structure of the part.