Mr. Gary Ward
USF Surface Preparation Group - Walther Trowal
Mass finishing is usually the most neglected process in any plant. It's the operation even the maintenance employees don't want to work on. If you are ISO certified, chances are the "mass finishing" part of the process is not included in your control plan.
USF - Walther Trowal seeks to be a global leader in the mass finishing process. We have an extensive "Mass Finishing Technical Center" in West Chester, OH where you can see finishing brought to a science.
There are four (actually five) main factors which dictate the success of a mass finishing operation. These are:
- The Equipment
- The Media
- The Compounds
- The Work piece load factor
- The Operator
Equipment
The four main type of mass finishing equipment are:
1. Barrel Tumbling
2. Vibratory Bowl or Tub
3. Centrifugal Disc
4. Drag (or plough) finishing
Both barrel tumbling and vibratory are considered "low energy" techniques. These take considerable time to use, but they offer an extreme variety of finishes possible.
Centrifugal Disc finishing increases the effectiveness of the media, and usually accomplishes the desired finish from 5 - 12 times faster than barrel or vibratory.
Drag finishing maximizes the action of the media, and is best suited to larger parts. It is usually 20 - 40 times faster than vibratory finishing, and because of fixturing can render selective finishes to various surfaces of the part. With vibratory finishing about 70% of the total energy is wasted in contacting media with media (rather than media with parts). In drag finishing over 90% of the energy is directly used to finish the part.
Media
There are more than 5000 different media avaiable for mass finishing operations. Gnereally, media is of one of the following classes: Ceramic, Plastic, Synthetic, Abrasive, or Biological. The media serves many functions including: supporting the part, separating the parts, carrying the compound, abrading, cleaning, polishing, and transmitting energy from the equipment.
Media will (in less time than most realize) "load up" and become ineffective. This is probably the most overlooked problem with mass finishing. While the media can be changed (to solve this problem) it can also be rejuvinated (much like you dress your grinding wheels). For many hard media, a washing in "SPIC & SPAN" will remove the metal loading and restore the media to its original condition and effectiveness.
Compound
Compounds are essential to the successful control of mass finishing. Amoung its critical functions are: providing lubricity to the media, cleaning parts, suspending oils and other particles, cushioning impacts between parts and media, and brightening.
To be controllably effective, compounds should be carefully measured or metered into the mass finishing system.
Load Factor
For any type of mass finishing (functional, cosmetic, or both) there is an ideal "load factor" which describes the ratio of media to parts in the finishing system. Deviating from this ideal will always result in less than satisfactory (or at very least - less than efficient) finishing.
A notebook full of handouts was given to each attendee which was prepared by the "Mass Finishing Technical Center" which relates detailed data in each of these four major areas. Similar information may be requested from the website www.surfacepreparation.com.