This month I have a question for YOU. My last column responded to a reader who wanted to specify the size of a through-feed vibratory finisher for a production requirement of 2,000 parts an hour. The parts are cast iron, 6×4×1/2 inch, face ground on each side, with several tapped 1/4 -inch and 1/2 -inch through holes. The machining coolant is a water-soluble oil. My question is: What process would you recommend?
One reader wrote to suggest blast finishing with plastic media; another reader wrote to suggest thermal deburring. Each of these persons felt that the alternate process might be a better choice. Their points were valid. My short answer is that this column is about mass- finishing methods, and my response assumed that the vibratory method had already been chosen. Mass finishing is a method in which parts, along with any additional ingredients of the process, are not fixtured or stationary during the process. This is accepted to mean vibratory, tumble, and centrifugal disc processing. We also include blast finishing because the vast majority of parts in that process are loosely tumbled. Other processes may use the same, or similar, ingredients as with spin finishing, but the parts are not loose, in a mass, during processing.
Nonetheless, it is good to have knowledge of, and consider, other deburring methods for every application. The world famous and prolific writer on the subject of deburring processes, LeRoux Gillespie, identified more than 100 deburring methods. The 80/20 rule says that 80% of the applications will be satisfied by 20% of the processes. It is even more out of balance regarding deburring—probably 90%+ of deburring is done by mass finishing.
Those who have been aware of my advice over many years know that I advocate blasting as the most cost-effective method. I suggest that you investigate that first. So, let's consider it here. Wheel blasting with metallic media is the least expensive surface conditioning and deburring method, but it won’t work here because the finish is unacceptable on the face ground surfaces. Wheels can throw non-metallic media, but not with sufficient velocity to debur cast iron. This moves us to consider non-metallic media propelled by compressed air. This may meet the customer’s deburring specification.
The machining coolant must be removed before blasting, adding a washer and drier to the cost. The airblast system may require 12 or more nozzles to process at the desired production rate. Each suction type nozzle uses about four hp of compressed air, so at least a 50-hp compressor is required. Pressure nozzles consume about twice as much compressed air. The vibratory system will be conveyor loaded and unloaded using inexpensive handling equipment. The blaster will require more sophisticated, perhaps robotic, handling and may even require re-positioning the parts to debur all sides and edges.
Following a blast operation, it may be necessary to clean and rust-inhibit the parts. These steps can be integral to the vibratory system, perhaps using a 10-hp warm air blower over the discharge conveyor. The requirements vary, but airblasting cast iron often requires dry air—a costly option. The thermal deburring process has been well established. I plead ignorance, however, as to its current state-of-the-art. Some of the problems I recall from years ago included high initial cost, breaking of thin cross sections, the need for a subsequent cleaning process, and some safety issues. I don’t know if the soluble oil residue would be a problem. If you have actual manufacturing experience with this method, I would appreciate your input.
As a matter of comparison, the through-feed vibratory finisher selected was a 40-cu-ft model. It required a loading conveyor and hot air blow-off over a discharge screen conveyor. Total capital requirement was $90,000. The estimated hourly operating costs are $7.50 for media; $9.00 for compound in flow-through mode and $3.40 for power. Additional capital would have provided a closed-loop compound system, but it was not initially justified.
Many other processes can debur these parts. Some examples are spin finishing, drag finishing, electro-chemical, abrasive flow machining and wire brushing. Again, I ask your input in these areas. You are a manufacturer and have the most valid information. If you know how to accomplish the presented case at lower initial and ongoing cost, please share your ideas with me. Be sure to include ancillary equipment, pre-treatment, post treatment and inspection costs. Your input will be shared in a future column, although we will not reveal your specific company, or any confidential information.