Burrs have always been a "problem" for part manufacturers
, and they're not the only thing that can be a problem. Sometimes parts are required to have sharp, non-continuous edges, and other times they are required to have rounded edges with a specified dimensional accuracy. In addition, the deburring process must not damage nearby surfaces, change the color of the part or deposit oxides or other materials on the part's surface.
Many part manufacturers are looking for more cost-effective deburring methods, more precise edge trimming processes, faster processing speeds and less negative impact. However, the complexity of the part combined with the difficult material properties often make deburring a difficult challenge. Some manufacturers must remove burrs quickly and completely. For example, to match the part's production rhythm and avoid slowing down the production line, many automotive part manufacturers must completely deburr a part in less than four seconds. Other part manufacturers aim to ensure that part edges have precise and repeatable geometry.
Here are 10 of the most challenging deburring processes and the methods used to achieve success, although all of these deburring methods may have shortcomings of one kind or another.
(1) Composite Materials
Composite parts are among the most difficult to deburr. Some composites consist of two metals, while others (such as circuit boards) consist of different layers of plastics, ceramics and metals. Some aerospace composites (such as carbon fiber/aluminum composites) have extremely high strength/weight ratios, but the edges of aircraft parts must meet stringent requirements for flight safety.
The open edges of multilayer composites (i.e., the exposed surfaces at the top and bottom) are particularly susceptible to cracking, excessive stress, delamination peeling, moisture absorption, continuous wear, and chemical corrosion. All of these problems can adversely affect the life or performance of the product. Deburring, on the other hand, has the potential to both cause and avoid or ameliorate these problems.
Figure 1 Deburring a composite part consisting of seven layers of material is extremely challenging
In order to achieve acceptable part edges and surfaces, each material in the composite needs to be treated differently. Therefore, the best process to improve the quality characteristics of one material may be detrimental to the others.
There is another, more subjective question: What determines the edges when processing different layers of material? It is generally accepted that the edge is the contour seen on the outer surface. However, for composite materials, each transition from one layer of material to another may form an edge. Thus, composite products may have many internal edges, and each edge may be critical to product performance.
When deburring composite materials, it is important to understand the properties of the various materials in the composite and what the user's quality requirements are for each interface in order to minimize the burr. While there may be several machining/finishing processes that can all meet the minimum quality requirements, the best process should be carefully selected from among them.
The most appropriate tool should be selected for the deburring process, which requires close cooperation with the tool manufacturer to take full advantage of their knowledge base. In addition, optimal cutting conditions should be used to ensure that the layers of material do not delaminate and peel off.
Cross-holes create a three-dimensional edge structure, and the varying thickness and height of the burr at each point on the intersection line means that deburring processes based on the same size burr may not be effective.
In addition to rotary handheld tools and some specialized deburring tools, thermal deburring is an efficient deburring method for many metal parts with cross holes, including small diameter holes. If surface finishing of cross holes is required, the electrochemical deburring method and the extrusion honing abrasive flow process developed by Kennametal can also be used to obtain high quality edges on almost any part configuration. For large diameter cross holes, deburring can be accomplished quickly and efficiently using abrasive-filled rubber or paper slug-type tools.
Figure 2 Comparison of cross-hole surfaces before (left) and after (right) treatment with a deburring tool
Deburring becomes more challenging when the hole diameter is less than 3.175 mm and the depth is greater than 10 times the diameter, because most deburring tools or processes have difficulty getting into these deep and small cross holes. For example, deburring a deep cross-hole in an automotive engine crankshaft is very difficult, and the speed required for deburring at production speeds makes it even more difficult, as all deburring must be done in four seconds or less.
(3) Transmission housing
An automotive transmission case is a complex "maze" of narrow passages, and burrs are usually present on each side of these machined passages. The intricate passages make it a difficult challenge to completely remove these burrs. Abrasive-filled nylon brushes and other types of brushes can reach under the edge and remove the burr by moving back and forth over the edge. The same deburring technique is used for milled edges on automotive engine blocks that measure hundreds of inches.
Figure 3 Abrasive-filled nylon brushes efficiently remove burrs from narrow passages in automatic transmission housings
(4) Integral impeller of a jet engine
The impeller disc is an important component of a jet turbine engine with a number of shorter turbine blades. The overall impeller is usually machined directly from a single forging, instead of machining the individual blades separately and welding them in place one by one, and then finishing the impeller. The material used to make the impellers is a heat-resistant super alloy capable of withstanding temperatures of 1650°C, which is typically a difficult material to machine.
These valuable discs must have a precise profile to form the airflow, and each edge must be flawless, meaning free of burrs and capable of forming a specific edge curvature (which can vary from row to row of blades). Robotic handling of work pieces coupled with CNC machining programs that minimize burr generation provide the best solution to meet these stringent requirements.
(5) Polyetheretherketone (PEEK) resin
Because of its excellent physical properties, chemical inertness, and biocompatibility, PEEK is used in a wide range of industries such as aerospace and medical devices. However, processed PEEK parts often leave feather-thin burrs, and many manufacturers find it difficult to remove these burrs without compromising the part's surface finish, dimensions, color and other requirements.
Patrick Byrne, sales and marketing manager for Comco, a manufacturer of micro abrasive blasting equipment, explains that while it is difficult to safely remove burrs from PEEK parts with small tools, a micron-sized soft abrasive can be used to remove PEEK burrs at high speeds without disrupting the geometry and surface finish of complex parts. A low-temperature tumbling process can also be effective in removing external burrs from some PEEK parts.
Figure 4 Removal of burrs on PEEK parts
(6) Teflon (polytetrafluoroethylene)
Teflon parts tend to have burrs that are difficult to remove, which shift when they are removed, and burrs that bend when they are sandblasted. The key to removing such burrs is to use an extremely sharp tool or to pre-freeze the part before cutting or sandblasting. Frozen Teflon parts are less tough and can prevent or reduce burr formation during machining. The same approach applies to many other plastic parts, but when freezing, the proper temperature must be selected to avoid damaging the part.
When machining threads
, burrs may be created at the entrance and exit of the threaded hole. These burrs are usually located on the top sides of the threaded tooth and at the edge of the groove. Burrs at the beginning and end of the thread are thicker than those at the top of the thread and are located on different planes. Therefore, a single deburring process is usually not effective. Rolling for batch finishing can remove some of the burr on the thread, but not on the top of the thread.
Because of its ability to remove some of the burr, the rolling process is still widely used, but care must be taken to prevent burr debris from being left in the thread groove, which could cause interference during assembly. Manual deburring, electrochemical deburring and SurfTran thermal deburring processes are very effective for all thread burrs.
Fig. 5 Threads in the orifice are more prone to burrs
When designing parts to be more tolerant of burrs can also solve some problems. The thread rolling process can produce perfect burr-free threads. In addition, thread grinding is also effective in preventing larger burrs.
(8) Aircraft wing beam parts
In order to reduce the weight of the aircraft which will increase the fuel cost, many structural parts of the airframe have a number of milled recesses. The machined parts are usually thin-walled, and any bends, scratches, or other defects can cause the parts to fail, as these stressed parts will be used on the aircraft for decades, and their safety requirements are extremely stringent. The fact that many of the recesses are deep and may also have high requirements for sidewall surface smoothness makes deburring a difficult process.
Vibratory polishing is a frequently used deburring process, but requires the use of larger polishing machines. Brush deburring is the preferred process, which brushes away the burrs while the workpiece is clamped to the machine. The key to the process is to choose the appropriate brush wire shape, embedded abrasives, brush wire length and processing parameters.
(9) Micro mechanical parts
Manufacturing micro parts (such as micro gears, extremely thin rods and escapement clamping plates) has always been a difficult challenge, these parts require burr-free edges under a 30x microscope and a smooth edge radius of no more than 0.025 mm. today, micro parts are required to be smaller in size, thinner in thickness, machined in quantities ranging from a few to millions, and in a variety of workpiece materials previously unimaginable. Tiny part sizes and tolerances, extremely demanding edge quality, and the challenge of machining advanced titanium alloys, shape memory alloys, and other new materials in metals and plastics further complicate edge finishing of micro parts.
Chemical deburring, electrolytic polishing, centrifugal roller tumbling and manual deburring are applicable to many micro parts, but sometimes, the only solution is: to prevent the formation of burrs in the areas where deburring is most difficult. The micro-stamping industry is probably the best example of the use of this burr-free processing method. Solutions have also been developed for laser processing of micro parts, such as burr-free processing on many materials with picosecond and femtosecond lasers. Similarly, leaders in the microforming industry have spent considerable time conducting experiments to study the flow characteristics of fluids in miniature molds and to minimize the amount of flutter generated on miniature parts. Although flutter can be quickly removed with a laser, dealing with part flutter takes time and money that could be spent elsewhere.
(10) Miniature medical devices
Medical device manufacturing is another area with high deburring requirements. For example, after laser processing of vascular stents, "laser burrs" (molten metal debris, to be precise) can be successfully removed from the edges of the stent by electrolytic polishing. Other medical devices that need to be implanted in the body (such as bone screws, fixation splints, pacemakers and analgesic administration devices) must also be free of burrs to prevent metal particles from falling into the blood vessels and entering the heart, as well as irritating or cutting the surrounding tissue. After the deburring process, the edges of the device must be surgically and rigorously cleaned. Since these devices or edges may be only slightly larger than the diameter of a human hair, a magnified inspection is required to determine if burrs are present.
Figure 6 To prevent metal particles from entering the blood circulation system
It is important to ensure that the human implant (e.g., the bone screw in the figure) is free of any burrs
In addition, microsurgical instruments have strict requirements for edge quality. The edges of the surgical instruments must be clear, flat and burr-free, which requires extremely fine edge finishing on the tiny edges. Likewise, bacteria and other pathogens must be removed from the instrument surface.
Efficient deburring of parts faces a variety of difficulties, but even the most challenging processing requirements
can be met with appropriate deburring processes and tools.