Character Matters in Mechanical Finishing
How physical characteristics of vibratory bowls affect processing efficiency.
The operating channel of a traditional vibratory bowl is toroidal in shape. Parts to be refined are placed into the machine’s channel, along with the processing media of choice. During machine operation, parts roll within the channel and make laps around the machine’s center hub. A combination of the vertical rolling motion and horizontal sliding motion results in a helical motion path.
Machine manufacturers have adapted a number of engineering elements into machine design to increase the machine’s functional performance and operator ease, especially during unloading while increasing the efficiency of part processing. Physical characteristics that can be customized at the time of a machine build include the durometer value of the lining, the presence of OD wall ribbing, the shape of the machine’s walls, the presence or absence of a part unloading ramp and the use of a sound cover.
How do design characteristics affect machine performance? What should be considered when ordering a machine?
Vibratory bowls are outfitted with hot-poured polyurethane liners. The durometer hardness of the liner is an oft-cited machine characteristic. For certain specialty applications, the durometry hardness can be specified by the purchaser at the time an order is placed for the machine.
Durometer hardness is the measure of an object’s ability to resist indentation by a probe that is pressed into its surface using a prescribed load. The technique was developed by Albert Shore in the 1920s2.
Durometry is the principle technique used to quantify the hardness of assorted gels, elastomers and rubbers. While a Rockwell hardness tester is used to determine the hardness of a metal specimen, a durometer is used for measuring the hardness of gels, plastics and elastomers.
Durometer hardness is measured using a durometer tester, which uses a spring to depress an indentor pin into the surface being measured. The spring has been calibrated to apply a specific load of force with every test. The softer the surface being measured, the further the indenter pin can deflect the surface1.
Since the spring applies the same load force with each measurement, the differences in surface deflection between surfaces of varying hardness can be compared with a known measurement value1,2.
Indentor pins of varying widths and shapes are used for the different hardness range scales depending upon the hardness of the substrates to be measured2. For example, a sharp-pointed pin is unsuitable for a gel-like substrate because it would penetrate rather than deflect the surface when the measurement is made2.
Likewise, the indentor spring used for a harder substrate measurement varies across the hardness scales. A more robust spring is used to measure Shore A versus Shore 00 surfaces and a more powerful spring is employed for Shore D measurements2.
The unit of measure for the hardness reading is known as Shore Hardness, honoring Albert Shore. Different Shore Hardness scales are used for substrates in different hardness ranges. The most commonly used scales are the Shore 00 Scale for gels, the Shore A Scale for rubbers and Shore D Scale for plastics.
Liner durometry is also a consideration when sending a legacy piece of equipment out for relining. Relining vibratory equipment is an expensive proposition and the durometry hardness specified should correlate to the intended processing application.