The following feature focuses on the 10 most important topics for machine shops performing deburring and is based on a conversation with LaRoux Gillespie, Dr. Eng, FSME, CMfgE, PE. Gillespie, a past president of SME, is a researcher, engineer, manager, consultant and author of more than 265 publications and 45 books, 13 of which are on burrs and deburring. He was interviewed by Alan Rooks, senior editor of Manufacturing Engineering.
Gillespie began his research into deburring in 1971. “I got started when my shop superintendent came to me and said, ‘LaRoux, we’re rejecting 50,000 parts per month for burrs.’ These were mostly stainless steel parts that could fit on your fingernail, and some had a requirement that no burrs could be seen on any edge at 30× magnification; the 8 µin. surface finish couldn’t be affected; edges could not be broken more than 0.001″ [0.025 mm], and some had tolerances of 0.0001″ [0.0025 mm]. Nobody had looked at how to deburr to those specifications.”
1. The Two Most Critical Issues in Deburring are Quality and Economics
Many machine shops don’t have a good understanding of deburring’s role in part quality. For one, that role varies by the size of the part. When removing a burr from a very large part, the operator might be able to remove burrs and break the edges with a file without any problem, but if the part is 0.020″ (0.508 mm) in diameter, and is part of a critical micro assembly, he may not be able to break the edge (impart a chamfer or radius) more than 0.001″ without affecting part function. There are 20 major categories where burrs cause problems and 124 different deburring processes (hand deburring is one); of that total, only two of the processes don’t change part dimensions, which may affect part quality. Most deburring processes affect surface finish in ways that may be advantageous or disadvantageous.
When choosing a deburring process, a shop must address several issues, including the work material, edge radius, stock loss and surface finish. Many questions need to be asked, including how tall and how thick the burr is. Most burrs on precision parts are between 0.001 and 0.003″ (0.025 and 0.0762-mm) thick and can be removed via conventional deburring processes. If a burr is 0.005–0.010″ (0.127–0.254-mm) thick, it will usually need to be machined off. All of these issues factor into choosing a process that meets economic and quality requirements.
Most shops could use a better understanding of deburring economics. As outlined in my book,The Economics of Burrs and Deburring, there is a difference between the cost of deburring and the salary of the deburring operator. If you ask a deburring operator what he does, particularly in a small shop, you will find that he is taking burrs off, sanding the surface because it is rough, removing a broken drill from the part when needed, and cleaning the parts. The last three have nothing to do with deburring, and shops need to factor that in when calculating deburring costs.
2. How Deburring Affects the Part
Almost every deburring process produces a radius or chamfer while removing the burr. Vibratory finishing and other mass finishing operations change external dimensions on all exposed surfaces, improve residual stress issues, and generally change the surface finish and texture. Some deburring processes etch part surfaces. Thermal processes change some of the metal structure—think of laser or EDM heat-affected zone (HAZ) when cutting and deburring, for example. Some deburring processes deposit, or can deposit, foreign material or oxides on the part surface. Deburring processes that use abrasives can impregnate microscopic media into surfaces.
Shops must also use the most appropriate deburring tool to minimize the effect of deburring on the part. There are many to choose from. For example, while hand deburring is just one of many deburring processes, there are more than 10,000 hand deburring tools in different sizes, materials and configurations. Then there are sanding papers and sanding tools in hundreds of different geometries, as well as hundreds of abrasive brushes. That’s not to mention the vast number of tools and types of equipment used in the other 123 deburring processes. All of these tools affect the part, and shops need to understand those effects, at least in general terms.
3. Minimize or Prevent Burrs
Burrs should be minimized or prevented during machining whenever possible—particularly on the most expensive or difficult parts. Another book I wrote, SME’s Deburring and Edge Finishing Handbook, includes almost 100 pages on preventing and minimizing burrs when grinding, turning, milling and drilling. The part design and manufacturing processes both influence how that can be accomplished. Shops should use available resources like the Handbook and the many published articles on deburring in trade journals.
For example, downmilling can minimize the creation of burrs on key surfaces, though it doesn’t prevent all of them. If, for example, you are endmilling a slot, there are 8–10 different edges in the slot and there is nothing you can do to prevent burrs on all of them. But when the top surface is being milled, applying downmilling will minimize burrs. The late David Dornfeld, mechanical engineering professor at the University of California, Berkley, conducted research demonstrating that, with the right toolpath programming, operators can greatly minimize the size and number of burrs by downmilling. Also, when end milling, most shops use an end mill with a helix that pulls the chips and burrs up, but if burrs are a major concern on the on top surface, use a helix that forces the chips down and flush the chips out with high-pressure coolant.
4. Design Parts to Minimize Burrs and Put Them Where They Can Be Easily Removed
When a tool exits the workpiece over a 45° chamfer, very few or no burrs are produced. Feeds and speed and cutter sharpness greatly impact the thickness of burr produced, and deburring difficulty (or time required) is proportional to burr thickness. Reducing the cutting forces just as the cutter exits the part by slowing feed rates will greatly reduce burr size. Putting the burrs on a side or area where they are easily removed saves hours of time in removal each month. Also, have edge breaks specified that match what your deburring process can easily produce after it removes the burr. Do not machine through threads where burrs are difficult to remove—redesign the part at that area instead.
5. Leave Burrs When They Don’t Need to be Removed
Another way to minimize the need for burr removal is to identify opportunities where burrs do not need to be removed at all. For example, do your parts really all have to be burr free when they will be sent to another shop for additional machining? In another example, say you are pressing a stainless steel sheet and you’ve trimmed off the edge, but you have a press burr on the edge of the part. In the next operation, the edge will be rolled over to make a tube-like round gripping surface, and that burr will be inside of the tube where no one can be hurt by it. Another example is an electron beam welding operation where two parts will be fitted together. A note can be added to the job to not remove a burr if it is standing vertically since the parts will fit closely together. As long as the part can slide down into the next part, where it will be welded, that burr not only doesn’t have to be removed, it provides additional material to fill the weld.
6. Abrasive Brushing Offers Economical Deburring
A brushing operation on a milling machine is usually the least expensive approach to removing the most accessible burrs, and this approach works particularly well in the automotive industry. For example, when face milling across a transmission housing, which has many surfaces, burrs are typically generated on every edge. The milling machine can be tooled with abrasive-filled nylon disk brushes containing hundreds of fibers and programmed to rotate with the bottom of the fibers 1/8″ (3.175 mm) below the workpiece surface. The brush rotates, going down into the pocket and coming out, and takes about one to two minutes to remove 80–100% of loose burrs, depending on their thickness. This process doesn’t work for intersecting holes and deep holes, but takes care of the majority of deburring needed for exposed surfaces. The brushes typically last a long time, prevent the need for additional deburring handling, and eliminate waiting for the next deburring operation.
There are probably 100 different brush combinations for deburring, and disk brushes come with different fiber lengths, fiber diameters and grit sizes. An operator can choose different speeds and feeds and different depths. There are hundreds of variables, so if one approach doesn’t work, try another one. Go back to the brush company and get their input—don’t give up if it doesn’t work the first time.
7. Deburring Microparts
There are relatively few solutions for deburring microparts (1 mm or less in size), which can have walls 0.001 or 0.002″ (0.025 or 0.051-mm) thick and holes 0.003″ (0.076 mm) in diameter. For example, there can be major burr problems in intersecting microholes, such as a 0.010″ hole breaking into a 0.015″ (0.381-mm) hole in stainless steel. How will you deburr that? If your shop doesn’t have an electrochemical deburring machine, consider using an inverted burr ball to go down through the hole and use the top side of the ball to cut off the burr. Another option is to EDM the burr off. With today’s microEDM machines, operators can make a tool that will only cut with the very end of the tool (electrode), then lower it down to where the burr is in the intersecting hole. While EDMing is a relatively slow process, keep in mind that the burrs on microparts are small and can be removed relatively quickly.
8. Managing Breakthrough When Drilling to Minimize Burrs
When drilling holes, breaking through the bottom of the workpiece typically produces heavy burrs—particularly with stainless and even aluminum. The process may also create a small burr or raised metal at the hole entrance. The force at which the drill breaks through the hole exit makes a difference in how big the burr is, as does the tool geometry. One solution is to reduce the feed rate when the drill is within 0.003 to 0.005″ of the bottom of the part to reduce the breakthrough force; this will minimize the thickness of the burr and make it easier to take off. If the drill is going through 1″ (25.4 mm) of material, and feed rate is reduced for the last 0.003 to 0.005″, very little cycle time is added to the process.
Also, most drills have 125 to 130° points. Dubbing the corners of the drill point (grinding a different angle at the corner) changes the geometry and reduces the force as the drill breaks through the workpiece. Orbital milling, which produces smaller burrs than drilling, can also be used to create holes. Keep in mind that orbital milling is a slower operation than drilling and produces a lower quality hole. However, if burrs are a big problem and you can live with the quality a milling tool provides, orbital milling offers a good solution. It can make a big difference in the size of the burr produced on an intersecting hole or even at the bottom of the hole.
9. Consider EDM Deburring
As mentioned above, EDM deburring can be an effective way for a shop to efficiently remove burrs on a limited number of parts. All it takes is designing a tool (electrode), adjusting the EDM to approach the burrs, and turning on the EDM. An HAZ may be created, but if it doesn’t affect how the part works or looks, and the HAZ is concealed, EDMing may be a good option. A conventional EDM only affects 0.002–0.004″ (0.051–0.102 mm) of the material, and if that spot is at the corner of a part in a hard-to-reach place, EDMing will save time and money.
10. Electropolish Deburring Also Imparts Surface Finish
Electropolish deburring is another option. This electrified process submerges the part in a bath of phosphoric acid, which is comparatively mild and inexpensive. The process works well in stainless, copper, and other materials. It produces a beautiful, shiny part, and will remove burrs up to 0.003″ thick. Electropolish deburring removes material from all surfaces, but predictably. Tabletop machines are available and no tooling other than alligator clips to hold the parts is required. In the electrified process, all the burrs act as antennas and the energy is directed first to those antennas. The process can be completed in three to five minutes and multiple parts can be processed at the same time.