Cost-Effective Internal Deburring

Q. We are a secondary machining center providing mostly boring, honing, and cross-drilling on iron and aluminum castings. Most of the castings are housings and bodies for small engines, valves or compressors. We also provide vibratory finishing operations to debur after machining.

We have improved the mass finishing over the years, but we continue to be challenged by internal burrs. Some of the internal deburring processes will work under one set of conditions but not another, and most processes either involve a substantial capital investment or very high costs by outside contractors. We often use brushing, either by hand or set up on a CNC lathe, but this basically uni-directional finish leaves some areas untouched or requires relocating the part for a second pass. Can you offer any suggestions, or are you aware of any new developments that address this issue? 

A. I’m reminded of a survey I conducted at a seminar in the late ’70s. I asked attendees what was the most important issue that we, the experts in deburring and surface finishing, could work on. The answer was overwhelming: A good, cost-effective method for internal deburring!

At the time, there were several, mostly expensive, options: thermal energy deburring, orbo-resonant deburring, electrochemical deburring, abrasive flow machining, wire brushing, and of course, hand deburring. Most of those methods are still available, and many have been considerably refined over the years, yet they continue to be expensive. The problem continues to beg for a better, cost effective, answer—until now.

Mass finishing generally refers to methods in which the parts and media, if there is any, are loosely contained in the process. The four common types of mass finishing equipment are tumbling barrels, centrifugal tumbling barrels, vibratory finishing and centrifugal disc machines. A more liberal list also includes blast finishing, but in this column we have almost always stayed with non-fixtured, loose processing because that is the stricter interpretation of mass finishing. The basic four types, as well as blast finishing, often involve fixturing of the part. The process I am about to describe is a fixtured version of blast finishing. I think you will agree it deserves special recognition because it answers the need for a cost effective internal deburring process.

Another departure from the tradition of this column is that I must name names so that you can learn something new about internal deburring, if this is your interest. Hammond-RotoFinish is the result of a merger of a line surface finishing company, and a line tumbling/vibratory finishing company. Both have been well known for many years. Probably every single reader of this column is familiar with one or the other of these companies.

Perhaps someone at RotoFinish was playing billiards and noticed that a single ball can strike a collection of other balls and send them all in different directions, at velocities sometimes almost as great as the striking ball’s. The balls that took the first hit often transfer their energy with almost equal force to other balls. He probably noted that balls were going in many directions and frequently impacted the side cushions, only to bounce a little further. The original striking ball then bounces off in a direction of its own, perhaps re-striking another ball or two. Maybe the observer mused at how dramatic this scene would be if more than one ball were driven into the racked balls. Or, what if the balls at each end of the table were driven toward one another with equal force? Balls would be bouncing right out of the arena! Further suppose the size of the container, the tabletop, were made smaller and formed into a closed unit. What chaos would ensue?

Now, this person, being an intelligent deburring expert, realized that if the balls were smaller, and were shot at each other in the confines of a closed container, the little balls would bounce all around the inside of the container, repeatedly striking each other and all surfaces and edges inside the contained space. Wow!

So, an idea was conceived at RotoFinish: Replace the cue sticks with blast nozzles and propel the balls directly at each other inside a chamber. Let’s say the balls are made of steel, and the chamber is the inside of a valve body with intricate passageways and intersecting holes. Where are all those little steel balls going to go? I’m sure that you are imagining all those internal burrs getting blasted away! And so, a new process is born.

The idea is to position two blast nozzles so that they each blast directly into the other—inside the chamber. Only an inch or two separates the blast nozzles. The spacing depends on the diameter of the nozzle outlets, the configuration of the confined area, and a few other factors. In this case, the nozzles enter the blast chamber from opposite ends of the part. The result is an explosion of steel balls impacting every surface and removing every burr.
These fixtured nozzles are moved through the chamber while maintaining their position relative to each other. They can travel the length of a bore in a single pass, go back and forth, pause at various intersections of internal passageways. It all depends on the intensity of action needed.

It’s also conceivable that the nozzles can be positioned at various angles to each other, while still blasting directly into each other’s abrasive stream. This design adds a hellish vortex of particles within the chamber, giving a somewhat different yet still highly effective blast pattern. This option is useful when the part being deburred does not have available openings at 180° angles.

The blast media is not limited to steel balls, although they are extremely efficient. Up to the point of fracture, the less resilient the material, the greater will be the transfer of energy. Sometimes the surface finish resulting from steel media is too rough, so other materials, such as polycarbonate pellets, may be utilized. Media that fractures easily, however, is not ideal.

We are talking here about internal deburring and deflashing. Consider also that the parts may be sand castings with a need to remove internal, burned in sand. Or, they may be heat treated parts with internal scale to be removed. The action we have described is extremely aggressive. It can do all these things and more.

In my opinion, this is one of the greatest deburring developments in many years. RotoFinish simply calls it “Reciprocating Blast System.”