According to deburring expert Dr. LaRoux Gillespie, “Everything on burrs has to do with the ductility of the material that you are machining.” He pointed out that machining glass, which has zero ductility, would produce no burrs, while unalloyed copper is highly ductile and therefore prone to forming thick burrs that are difficult to take off.
Gillespie, a past president of SME, first dealt with machining and burr removal when manufacturing parts and drilling printed circuit boards (PCBs) at Bendix Corp. (now Honeywell) in 1966. He began intensive study of burr formation and removal and authored a few technical papers, increasingly supported by SME. To date, Gillespie has written 146 technical papers and 13 books on deburring. He has compiled a list of 124 different deburring processes and notes that every deburring process has one or more physical or economic limitations.
He pointed out that the best way to eliminate the problems caused by burrs is to not make them in the first place. A general rule of burr minimization and elimination is to use sharp cutting tools. “As long as you have a sharp tool and can keep it sharp, then you are in pretty good shape,” he said, “As a tool gets dull it creates larger burrs.”
Concerning advanced materials, Gillespie said minimizing burr production on composite materials is complex. “In an aircraft, you’ve got an aluminum skin on the outside and inside you may have a layer of carbon fiber, then another metal layer, maybe aluminum or titanium. You may have five, six or seven layers then a reinforced plastic of some kind on the inside bulkhead. Instead of one problem with burrs you’ve got multiple issues.”
A shop may know how to minimize burrs when drilling aluminum, but when the next layer is carbon fiber, the drill that was good for aluminum is not necessarily good for carbon fiber. The subsequent layers can be other materials that may be hard or soft. “Drill makers are experimenting with different geometries but it all depends on what each layer is and also on machining parameters that may change with each layer,” Gillespie said.
Fifty years ago Gillespie faced a similar situation when drilling PCBs. The boards comprised of a copper layer, on top of a fiberglass layer, atop another layer of copper. Drilling the boards produced burrs at the top of the hole and on the breakout side. In between, the holemaking “fluffed up” the hole’s inside edges. Most of those holes were to be plated, Gillespie said, and fluffy edges negatively affected hole size and the plating process.
One way to minimize burrs was to stack three boards at a time and use a small pressure pad around the drill to squeeze the boards tightly together, making it more difficult for the drill to tear layers apart. In another technique, a stack of three boards was backed by a phenolic plastic throwaway layer about 0.031″ (0.79-mm) thick. The copper could not push the hard phenolic out of the way, minimizing burr formation. Gillespie said the same sacrificial idea can apply to drilling carbon fiber and other materials. The aircraft industry has experimented with the tooling to keep the layers around the drilling area tightly compressed.
With composites, Gillespie said, every company must decide the quality required for each level and what can be done to minimize burrs. The aircraft industry is the biggest user of composites. According to Gillespie, the National Institute for Aviation Research at Wichita State University is among the groups working with aerospace manufacturers on a wide range of issues, including finding ways to better produce the composite parts. Every aerostructure producer has worked or is working on the burr issue.