Q. We have several vibratory finishing machines. Some are bowl styles with internal separation, one is a racetrack design and some are straight-through tub designs with external separation; four different manufacturers are represented, and are all at least 10 years old. I am creating a standard by which future machine purchases will be evaluated with the long-range goal being to reduce the variety and improve the performance of the vibratory finishers we operate.
Can you recommend selection criteria for buying vibratory finishing machines? J.C.
A. Is the age of your machines due to purchasing older, used equipment or did you purchase new machines 10 or more years ago? I ask that because it is important to know how long ago your company decided that vibratory finishing was the process of choice when reviewing other mass finishing processes such as blast finishing, centrifugal disc finishing, etc. I will assume the decision in favor of vibratory is valid, and you are only interested only in selecting the equipment features or brands.
All of your equipment will separate the parts from the media at the end of the time cycle, some with internal screen decks in bowl or racetrack designs, and some with external screen decks as with your straight-through, continuous machines. Short time cycles allow you to use single- pass machines, while longer time cycles generally require internal separators that can be activated when needed. You already have each of these designs, and I’m assuming you know when and why to prefer a particular design. I will give you some guidelines to use in selecting equipment of these various designs.
First and foremost has to be reliability, and the availability of spare parts when needed. You already have considerable experience with different brands, making that your primary source of information.
Bowl machines have fewer moving parts and history has proven them to be less expensive to maintain than straight-through machines that have vibrating separation decks, return conveyors and multiple eccentric drives. Eventually motors have to be re-wound, and bearings changed; these costs vary greatly with different brands.
Most good vibratory machines will outlive their linings, and that will be the largest repair cost of all, approaching 50% of the cost of a new machine. When getting reline costs, be sure to specify the material you want, such as hot or cold pour polyurethane, neoprene and rubber; and, specify the hardness required. Bowl machines typically have liner life two to four times greater than tub designs; some of that advantage may be lost to higher relining costs. For each of these repair items, get an estimate of the downtime for the entire repair process.
The second priority on my list is how well the machine can run the necessary process. For example, steel media can be used to polish, clean, or lightly debur, but many machines do not perform with steel media. If parts are delicate, subject to deformation or nicking, or if parts are very large or heavy, the machine must able to process those parts without damage. The machinery must meet any special requirement.
It is likely that process requirements, or techniques will change over the life of the machine, which could easily be 20 years. A third priority, then, is that the machine be easily adjustable to accommodate any future process changes.
Many examples come to mind: changing the vibratory frequency and/or amplitude; changing the time cycle; changing from plastic media to heavy ceramic media; changing to steel media from any other; changing from open to closed drain techniques; introducing a more delicate product line, and so on. These changes may require changing the amount of eccentric force, and you want a machine designed to easily add or subtract weights. The process change may require—in bowl or racetrack styles—that you change the amount of weight on either the top or the bottom. You may need to change the lead angle between top and bottom weights. Some bowl machines are very difficult to adjust, even requiring removal of the bowl from the base to access either the top or the bottom weights; other bowl machines make it difficult to determine and accurately change the lead angle. And, some don’t even allow for any change in certain of these features.
Next, consider the basic design features of the equipment you are considering. In tub-style machines, you will find eccentric weights placed under or adjacent to the work chamber, or even above the media level in the work chamber. You may find two or more eccentric shafts, and two or more bearing and weight assemblies along each shaft. The placement of the eccentric shafts will change the center of gravity in the working mass, and this changes the action within the tub. The location of the eccentric shafts and weights not only has process value, it can make access more or less of a problem. In straight-through designs, you may find the center line of the shafts to be non-parallel to that of the tub center line.
In all designs, you will want a drainage system such that compounds are removed frequently, and without accumulation in the process chamber. Generally, it is good to have at least one bottom drain for every ten cubic feet of mass, or one for every four feet of a single pass design. Another desirable feature is a media discharge door that allows for easy removal of the entire mass.
In bowl machines, the elevation of top and bottom weights relative to the elevation of the working mass is very important. The higher the top weights are, the easier it is to drive media up to, and across, the separator screen. And, the higher the top weights are, the more center post pressure that is applied, making it more difficult to treat parts gently.
The lower the bottom weights are, the better the rolling action will be. One very popular bowl design has the top weight at the level of the bottom of the work chamber, and the bottom weights 12 or more inches below that. That design has many process advantages, but results in a machine profile that often requires a work platform for the operator. These are factors in machine design, and you must decide which is most important to you. In any case, access to both top and bottom weights, and easy changes to the weights and the lead angle is high on my priority list.
The power train is the last consideration on my list. You will find vibrating motors, motors with eccentric weights on one or both ends, variable speed motors, variable speed gear boxes, direct drives, u-joint drives and belt drives. There may be one, or more, eccentric shafts connected by u-joints, or belt drives. Each has advantages, and each has its own maintenance considerations. When comparing maintenance costs, find out if standard off-the-shelf motors, bearings, bearing assemblies and other items such as re-winding motors or relining bowls are available from third-party vendors, or do you have to go back to the OEM for these items.
Learn the reasons for these different designs and select equipment that has the design advantages best meeting your needs while giving you the most future flexibility. A checklist for purchasing can be developed. Each item on the list should be given an importance rating, along with the reason why it is important. This will lead to a total score for each machine considered, allowing you to make and document a good selection.