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Machinists Can Reduce Or Prevent Burrs in the Metalcutting Process

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Machinists Can Reduce Or Prevent Burrs in the Metalcutting Process DECEMBER 2006 / VOLUME 58 / NUMBER 12 ➤ BY LAROUX K. GILLESPIE able burrs with some metals, though it chining variables affect burr may produce microburrs. properties. n Selective hardening prevents Machinists can control burr plastic deformation from occurring size by reducing cutting forces macroscopically at part edges. Any as the cutter exits the part. Tac- environment that prevents plastic de- tics for reducing cutting forces formation will prevent or minimize include using sharp tools and burrs. Hardening techniques include tools with appropriate geom- laser, mechanical-localized, chemical etries (positive rake angles, for and thermal. example), preventing built-up n Prevent large exit burrs when edge and lowering feed rates drilling by using sacrificial backup ma- at tool exit points. terial that is harder than the workpiece. To minimize burr size, feed The backup material, positioned at the rates must be reduced only as bottom of the hole, pushes back on the the tool exits the workpiece. bottom of the workpiece, which helps Higher feeds are used when prevent burr formation. In this proce- Figure 1: Since a form tool is always in the cut, drilling a hole until the drill dure, it is essential to keep the mate- there is no place for a burr to be formed except is about to break through the at the tool’s leading and trailing edges. rials held tightly against each other. workpiece. The corner of the drill’s cutting edge Figure 4 shows the impact must cut into the backup to effectively on burr formation of slowing reduce burrs. For drills smaller than the feed per tooth. In this in- 1 ⁄8", machinists should drill 0.010" into stance, the feed is held con- the backup. A rule of thumb is that the stant, but when the spindle Machinists can reduce larger the drill diameter, the deeper into speed is increased, the feed or prevent burrs in the the backup material the tool should cut. per tooth is reduced, which Figure 2 shows how drill exit-burr size reduces the cutting force. This metalcutting process. varies when a backup material is used. in turn reduces burr thickness. Burr In this case, the material being drilled Producing too small a chip in is 303 Se stainless. strain hardening materials will cause the tool to rub rather Making Small Burrs than cut, producing large and If burr formation cannot be pre- hard-to-remove burrs. vented, the goal should be to leave Down burrs so small that simple and rapid Solving Part Geometry s long as there has been not acceptable and require edge a burr to be formed except at the finishing processes can remove them Impact metalcutting, there have treatment. tool’s leading and trailing edges. and round the cut edge slightly. Any Endmills and facemills A been burrs—an un- Burrs on leading and trailing edges deburring process will readily remove Figure 2: Impact of backup material’s hardness on have complicated geometries wanted side effect of the process. Burr Prevention can sometimes be prevented if di- burrs that measure 0.000010" in thick- burr size of drilled holes. that make burr minimization Barring a revolutionary change, There are several common burr ameters are tapered. Another way ness or height. challenging. Consider the task burr formation will always be a prevention techniques, including to prevent lathe burrs is to generate Burr size varies according to cutting of facemilling the top of an en- problem. However, machinists can the following. part edges via a corner-rounding forces, workpiece material and the gine block. To minimize burr use several strategies to prevent n Form or extrude parts instead path rather than simply facing and geometry of both the part and the cut- size, the programmer must burrs or reduce their size. Some of machining them. Extruded sec- turning. This removes any burrs ter. While the height and thickness of create toolpaths that minimize burr prevention and minimization tions prevent burr formation at part that form during the process and most burrs are from 0.001" to 0.002", tool exits over edges. Basi- strategies can be complex, but oth- edges because high-pressure form- creates chamfered or radiused others can be quite large. A good burr cally, end users must down- ers are relatively simple and easy ing does not produce chips. edges rather than sharp ones. In- minimization program should be able mill into an edge and program to implement. n Cast part edges. Casting saves stead of feeding the tool along the to prevent 90 percent of burrs that are the tool to exit the edge over With a few exceptions, conven- finishing costs, such as deburring. lathe axis and then at a right angle over 0.020" long. an area that has already been tional machining techniques al- However, flash will form at die to generate shoulders, feed straight In most cases, a burr forms when a machined. This directs the cut- ways produce burrs. Burr preven- openings and generally must be re- out and then trace a radial path on cutting edge passes over a part edge. ting force into the workpiece. tion requires both conventional moved. Casting techniques include the outside of each shoulder to When a drill produces a through-hole, When the cutting edge exits and nonconventional approaches. P/M forming and other operations round the edge. It takes only 1 or it makes a burr at the hole’s entrance the workpiece over a previ- Some nonconventional approaches that use dies. 2 seconds, but prevents the need to and exit. Similarly, when an endmill is ously machined area, there is are outlined in Table 1. However, n Use form tools to produce deburr. applied to make a through-slot (Figure no force to push a burr from even when burrs are not produced, part features on a lathe (Figure 1). n Ultra-high-speed machin- 3), it produces eight edges—each with Figure 3: An endmilled through-slot has eight the edge because the cutting sharp edges left as a result of the Because a form tool is always in ing (cutting velocities higher than a different burr. It is important to know edges, each with a different burr. edge is not cutting at that lo- machining process generally are the cut, there is simply no place for 10,000 sfm) produces no observ- exactly which mechanism forms which burr and recognize how different ma- cation. In addition, always cut into the sur- Typically makes burr or Process Typical edge radii produced (in./mm) face. Instead of applying a cutter that leaves molten material covers the full width of the workpiece and making a single pass across the Abrasive jet machining No 0.003/0.08 entire workpiece, select a cutter with Chemical machining No Unknown a smaller diameter, lay out a sample Photochemical machining No 0.001/0.03 toolpath and look for areas where the tool is cutting over an edge. Modify Electron beam machining Yes Unknown the path so that when the cutter passes Electrochemical discharge machining Unknown Unknown over the edge, it occurs over a previ- Electrochemical grinding No 0.003/0.08 ously milled surface. Because no chip is being created at that point, it cannot Electrochemical machining No 0.001/0.03 roll over to form a burr. This approach Electrochemical honing No 0.0005/0.013 is demonstrated in the following face- milling example. Electrical discharge machining Yes Unknown When a facemill is fed straight into Electropolishing No 0.001/0.03 a straight edge, some teeth cut as they Electrostream machining No 0.002/0.05 exit the part edge. To prevent that exit cut, feed the tool into the workpiece Hot chlorine gas machining No 0.002/0.05 in a circular arc so the teeth are only Ion beam machining No 0.00005/0.0013 cutting when entering the workpiece. Figure 4: Effect of workpiece hardness and spindle speed on drill burr thickness. Material is AISI 1018 steel machined dry at 6 ipm with a 0.25"-dia., HSS twist drill Laser beam machining Yes Unknown The tool can be fed around the work- having a 118° point and 27° helix. piece perimeter continuously after it Plasma arc machining Yes Unknown has entered the cut in Ultrasonic machining No 0.001/0.03 this manner. Teeth will never be cutting as Waterjet machining Yes Unknown they exit, which solves Table 1. Burr formation in nontraditional machining processes, where burr/molten material is visible under 30× magnification. the problem when fa- cemilling a solid work- hole in a brass ammunition cartridge ers. The helix angle on endmills nor- with each spindle rotation. This does piece. case. mally causes an exit burr on either the not require traversing all the edges; The key is to select The makers of the case used a ball- top or bottom of the part being pro- rather, a simple back-and-forth motion a tool diameter smaller end pushrod to indent the inside of filed because the helix pushes material over the entire surface brushes all the than the workpiece the case where the hole would later out. Compression routers (also called top edges. This process produces a thickness and to offset be drilled. This simple indentation upcut/downcut tools) have a helix that small radius on these edges and also the tool. To calculate prevented burrs in two ways. The results in cutting into the part on both removes small as well as long, thin the WOC for this con- Figure 5: A ball indenter prevents a drill-exit burr from indentation lowered the ductility of the top and bottom of the part.
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