Q. I am a manufacturing engineer for an automotive parts manufacturer. We purchase cast and stamped products, machining where necessary, and assembling the final products for automotive manufacturers. We are spending too much on such things as deburring and cleaning of parts after machining or grinding. My assignment is to take a fresh look at our mass finishing operations and come up with a cost saving plan. What are the most significant things I should look for? R.J.
A. I’ll boil this down to four main areas. The famous 80/20 rule applies here: 80% of the problems will be found in 20% of the operations. If you encounter specific problems not covered, please query me again. In my initial assessment of mass finishing operations, I usually identify potential cost savings of 30%, or greater. Here are the areas to study first:
Clearly define the degree of deburring and/or surface finishing you actually need to satisfy your customer. Sometimes, I will remove a part from a vibratory finisher, or other mass finishing process, when it has only been processed for half of the specified time. I will take this part to quality control and ask if passes inspection. More times than not, it will be acceptable. Right there is a 50% time reduction, often saving thousands of dollars that might be spent to purchase additional capacity. You may find that the department was running two or three shifts to meet a requirement that can be met in half the time.
Measure the flow rate of the compound. In all mass finishing machines there is an optimum volume of compound for efficient finishing. In vibratory flow-through systems it is generally 1- 3 GPH for each cubic foot of working mass. Many times you will find these machines running two or three times that rate, and it is easy to calculate the savings that will result from turning down the flow rate. You may hear the argument that the compound is being recirculated, and therefore a high flow rate does not waste any compound. High flow rates, however, cause the parts and media to float, reducing cutting efficiency, glazing media, and finally increasing the time cycle. Also, high flow rates contribute to a higher retention of particulate contamination in the solution, resulting in dirty parts. Sometimes, dirty parts require a subsequent cleaning operation that could have been avoided in the first place.
The cost of finishing parts is increased dramatically when machines are not filled to their optimum capacity. In batch operations, look for ways to accumulate enough parts that each machine will be run with a full load of parts. I teach that machines should be full to capacity with as high a ratio of parts to media as can be possible without causing damage to the parts. We often see two and three shift operations when all the work could have been run in one shift had the machines been running at capacity. With some in-line manufacturing operations the part supply is far below the capacity of the mass finisher. You may not be able to change this right now, but when specifying new equipment, be sure to properly size the machine to the job.
Use the correct media (size, geometry, weight, abrasive content) for your parts. The wrong media can easily double the time cycles. Almost without fail, if you find cylinder shaped media you can reduce the time cycle dramatically just by changing to a more efficient shape. This was covered in some detail in this column in the July 2010 issue. Media is offered in a variety of compositions, mostly having to do with the cutting rate or the final finish required. Often, you can go to longer lasting, lower cutting media and still meet the requirement. This is particularly true if the media being used is a cylinder; you can change the shape and the composition for a substantial saving. The subject of heavy media has also been mentioned many times in this column, and ceramic media is available 70% heavier than it was just a few years ago. This is important in mass finishing because cutting action is similar to grinding or filing where the pressure applied on the workpiece dramatically increases the cut rate. And, in burnishing or coldworking operations, the result is also dependent on the pressure applied. While on the subject of media, you want to become aware of part-on-part finishing. If that can replace media finishing in any operation, the savings are significant.
This short list will cover 80% of the cost saving opportunities. As you gain experience with the processes, you may want to question whether the process selected is the best for the application. In one case, four automated centrifugal disc machines were replaced with two vibratory finishing machines set up for part-on-part processing. In another case, two spindle finish machines were replaced with a single bowl vibrator having rotating dividers.
When choosing vibratory media, understand the size, shape, starting roughness condition and metallurgical structure of the part.
It has been shown that the inexpensive chemically accelerated vibratory surface finishing (CAVSF) process can reduce the average surface roughness.
Metal fabricators that laser-cut with oxygen take steps to prepare parts better for powder coating.