Q. In a ceramic media deburring operation, what effect does compound lubricity have on deburring rate? Our machine is a 25-ft bowl used in single-pass mode with a 15- to 20-min process time. K.J.
A. Compound lubricity affects the media cutting rate as much as 30 to 50 percent or more. You can approach burnish finishes even with abrasive media if the compound is extremely lubricious. You will find, however, that some edge breaking occurs even in a lubricious mass, as pointed out in this column in April 1993, written by my predecessor, John Kittredge: “Deburring occurs with non-abrasive media for the first 10 to 15 min or so of a vibratory cycle at a rate almost equal to that of the abrasive products.”
Kittredge used the term “deburring,” but I believe it would have been more accurate to use “edge breaking” because fixed or pliable burrs are not removed. There is some deburring, however, because loose burrs are knocked off and sharp edges are blunted. A predominant factor is the hardness and/or pliability of the material being worked. In these first few minutes, compound lubricity and media abrasiveness are not overriding factors since loose burrs and rough edges quickly yield to the impact of the media. The heavier the media, the more the edge breaking. For this reason, steel media can be used for light deburring or edge breaking, even though there is no abrasive action at work. Compound lubricity plays a major role in cutting after the initial effect. Edge radiusing and cutting are improved by less lubricious compounds when all other factors—compound flow rate, amplitude, frequency, media depth, parts ratio, etc.—remain constant.
My experience has been that deburring time cycles are reduced 33 to 50 percent when changing from lubricious to non-lubricious compounds, particularly when time cycles are in excess of 30 min. In shorter time cycle operations such as with many single-pass machines, it is a close call and should be settled with actual testing.
Your question caused me to reassess recommendations for aggressive media and the propensity to look at published media cutting rate charts as the “bible” for selecting media. I researched old and new media selection and media cutting rate charts, and all I could find were based on long-time cycle tests. In many cases, the cutting rate was based on tests conducted over a 24-hr period. This data leads us to make recommendations that do not account for the advantage of heavier, longer-life media in those light deburring/edge-breaking applications. The 80/20 rule comes to mind: 80 percent of the work is done in the first 20 percent of the time.
Let me suggest to you readers who have light deburring or merely edge-breaking applications, and who are using any of the medium or fast-cutting media that you re-evaluate the media requirement and test the result you would get by using one of the heavier, longer-life products now on the market. The media cost savings will be significant, media dust problems will be reduced, and finish may be improved. All this, and you may still meet the actual edge and surface requirements. (When you’re re-evaluating media, include factors such as media shape and specific gravity to get the most effective contact with all surfaces.)
When choosing vibratory media, understand the size, shape, starting roughness condition and metallurgical structure of the part.
Choice of equipment, media and compounds has a major impact on your finishing applications.
Precision shot peening brings an entirely new concept to the field of microabrasive blasting, and it is complementary to its larger cousin. Using glass bead media, several companies have been shot peening for years with microabrasive blasting technology.