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MECHANICAL ENGINEERING 122 Processing of Materials In Manufacturing

Waterjet Project

Josh Kim | Reilly Marin | Ian Shain | Jega Vigneshwaran April 3, 2014 Group 11


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CNC Waterjet Process Process There are two major types of waterjet cutting processes: pure waterjet cutting and abrasive waterjet cutting, as seen in Figure 1. Pure waterjet cutting is used primarily for softer materials that do not require the abrasives, such as disposable diapers, toilet tissue, and automotive interiors.1 Abrasive waterjets are used to cut harder materials; they act like liquid band saws that can begin cutting anywhere on the sheet of metal. Ultra-high pressure water is directed into the waterjet cutting head. Once inside the head, the water goes through a small orifice in order to increase the velocity of the water to supersonic levels (an application of the continuity equation). The supersonic water jet is then mixed with the abrasive in a mixing chamber before being directed through a focusing tube to straighten the flow of the water. The abrasive-mixed water jet is then expelled onto the sheet of material being cut, usually at pressures between 50,000 – 60,000 psi (newer systems can cut at pressures up to 90,000 psi).2 In order to obtain a good surface finish, the mixing chamber is usually 0.01 to 0.2 inches above the work piece.

Figure 1: A pure waterjet versus an abrasive waterjet.3

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"Pure Waterjet Cutting." Pure Waterjet. N.p., n.d. Web. 24 Mar. 2014. <http://www.flowwaterjet.com/en/waterjettechnology/pure-waterjet.aspx>. 2

"How Does Waterjet Cutting Work?" ESAB Cutting Systems. N.p., n.d. Web. 24 Mar. 2014. <http://www.esabcutting.com/the-cnc-cutting-blog/waterjet-cutting/how-does-waterjet-cutting-work.html>. 3

http://en.bhdt.at/


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The waterjet, unlike a band saw, can begin cutting in the middle of a sheet of metal (or any other desired material except for diamond, certain advanced ceramics, and tempered glass). This is because of the CNC, or Computer Numeric Control, which directs the waterjet head’s motion. All CNC machines, whether it is a waterjet machine, a turning center, or a machining center, have three major parts: the physical machine, the actuators/sensors/feedback systems (which act as the body of the machine), and the controller (which acts as the brain of the machine). Most CNC machines contain closed loop feedback systems instead of open loop. Open loop systems do not have a method to correct the position of the tool if there is an error—the controller sends the necessary information to the actuator and the actuator sends back a signal to the controller only when the job has been completed. The actuator in an open loop system does not communicate information about how the job was performed. A closed loop system allows the controller to correct any errors in the tool path; the system constantly sends back signals to the controller about the head’s location and speed, allowing the controller to make changes, as it deems necessary. There are two types of tool paths that can be programmed into the controller, which then instructs the machine how to manufacture the part. A point-to-point control path instructs the tool to move from point to point and the tool performs an action at each point, such as drilling or boring. A continuous control path instructs the tool to remain in constant contact with the work part and is how the machine cuts contour shapes in the sheet. The flow of computer-aided CNC begins with the development of the 3D model of the part using CAD software, like SolidWorks or CATIA. The designer then needs to decide the specific machining operations, cuter-path directions, and tooling that will be required to manufacture the part. In order to produce the CNC part program, which will be used to guide the CNC machine, the CAM software is executed. It is important to review and verify the accuracy of the part program before it is downloaded to the machine to ensure there are not any errors. Once the program has been downloaded to the CNC machine, the part program should be checked again for accuracy to ensure there were not any conversion errors. If the part program on the machine is correct, the machine can begin producing the part.4

Advantages One of the greatest advantages of waterjet cutting is that it does not superheat the material around the cut. This is especially important for certain metals, where excessive heat can alter a metal’s properties or compromise its material integrity. As a result, waterjetting can be used to cut a wide variety of materials, such as metals, stone, plastics, and glass accurately, with a tolerance of 0.005 inches obtainable. Furthermore, the kerf of 4

“Manufacturing, CNC, EDM”, Slide 18. ME 122 Spring 2014; Professor K. Youssefi


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the cut can be adjusted by changing nozzles. With kerfs as thin as 0.5 mm, very little material is wasted and parts can be nested very close to one another on the sheet metal stock. Another advantage is that the process does not create harmful dust during the machining process due to the liquid media cutting, which suppresses airborne particles. It also uses very little water because of its closed-loop recycling system that recirculates the water. Waterjet cutting can also be easily automated for production purposes, and is also desirable because it leaves good edge finishing that does not require secondary operations. Not only this, but complicated shapes can be cut using a single tool, which reduces production time. Sheets of material can also be stacked to speed up the production time even more.

Disadvantages However, as with any process that has so many benefits, there are drawbacks. The main disadvantage is the high cutting rate variability between materials: there are a limited number of materials that can be cut economically. For particularly hard materials, the cutting speed must be reduced drastically, which in-turn slows down production time and a bottleneck occurs. Also, waterjet cutting has a high entry-cost compared to laser cutting methods, and the abrasive granules used to cut harder materials can be quite expensive; the orifice and mixing chamber also need to be replaced periodically. The minimum kerf is also still larger than laser cutting: 0.02 inches for abrasive water jetting vs. 0.006 inches for laser cutting. Furthermore, retaining dimensional accuracy in thicker parts can be a challenge: the water jet dissipates slightly (taper) as it cuts through the part due to a loss of energy, resulting diagonal cuts or incorrectly sized dimensions. Another flaw, â&#x20AC;&#x153;stream lagâ&#x20AC;?, occurs when the entering point cuts faster than the exit point. Parts that are too thick can even result in wavy cuts, which would render them useless. Conversely, very thin sections do not yield very good cuts; the most economical range for waterjet parts is around 0.25 to 2 inch thick parts. Lastly, waterjets can only be used to cut parts with uniform cross sections.

Discussion: Yellowfin Tuna The geometry, precision, and overall look of the finished Yellowfin Tuna part were nearly identical to that of the CAD drawing. However there were a few areas of the finished part that did not turn out as desired.

Surface Finish & Burrs The first area of concern can be seen in Figure 2. When looking closely, a small lump is visible in the relatively straight right edge of the upper part of the largest fishâ&#x20AC;&#x2122;s tail. This imperfection is very noticeable when touching this area of the part. Upon further inspection, it seems as if the waterjet head moved erroneously while cutting the material.


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This left a bump of material that was not removed by the machine. However, areas of much higher detail turned out flawlessly.

Figure 2: This is the bottom surface of the part. The imperfection is circled.

Another area of potential concern is the surface finish of the cut surfaces on the sides of the part. The surface turned out much rougher than expected, likely due to the grit of the abrasive granules used in the cutting. The sparkly shine of the rough side surfaces is visible in Figures 3 and 4. For a part like the Yellowfin Tuna with an emphasis on aesthetics, the surface roughness of the sides is negligible. A finer abrasive material and slower cut rate may be the best solution to solve this roughness issue. Nevertheless, the surface roughness is something to consider for projects requiring very high precision from waterjetted parts.

Figure 3: View of tapered edge on top narrow fin.


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Figure 4: View of tapered edge on bottom narrow fin.

Cross Sectional Taper The final and most significant issue with the part and waterjetting in general is the cross sectional taper. As the stream cuts deeper into the material, it slows down, expands, and bends, causing more material to be removed from the bottom of the plate than the top. This taper is most visible at the tips of the narrowest fins, shown in Figures 3 and 4. Additionally, the smallest hole (the small fishâ&#x20AC;&#x2122;s eye) had the desired amount of material separating it from the edge of the fishâ&#x20AC;&#x2122;s head on the top surface. On the bottom surface, however, the material separating the hole and the edge is much thinner. This is due to the expansion of the hole and the shrinking of the small fishâ&#x20AC;&#x2122;s outline due to taper. This difference in material is visible in Figures 5 and 6. These figures also show the size of the largest hole (representing the eye of the largest fish) on each side of the part.

Figure 5: Top surface view of fish eyeholes.


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Figure 6: Bottom surface view of fish eyeholes.

The top surface of the part (Figure 5 exhibits a smaller hole and the bottom Figure 6) has a larger hole, which is more evidence of taper. Overall, the top surface of the part has much sharper edges than the bottom surface. It can therefore be argued that the bottom surface is a â&#x20AC;&#x153;badâ&#x20AC;? surface in terms of its dimensional accuracy. The taper of the waterjet removes more material on the bottom surface than is desired by the initial design. To help alleviate this issue, a thinner material could be used for the part to reduce the dimensional inaccuracy of the bottom surface. Despite these three issues, the final part turned out very well. By using the recommended fillet radius and minimum hole diameter (as specified in the attached part drawing), there were no major cutting errors into the material. The details in the ridges of the top dorsal fin of the outermost fish and the precision in distance between the eyehole and outer edge of the innermost fish are quite impressive. In hindsight, the only major change to be considered would be to use thinner material to avoid the taper that occurred on the bottom surface of the part.

Discussion: Nautilus Shell For our second part, we chose to go with a nautilus shell design. This iconic design is aesthetic, intricate, and capable of pushing the limits of water jet cutting. While the design turned out great as a whole, there are many imperfections that could be improved upon in later iterations.

Surface Finish To begin with surface finish, the top and bottom of the part both have the same finish from the 0.25-inch aluminum stock plate. The sides, however, exhibit a different finish due to


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the abrasion of the water and abrasive mixture. This abrasion is almost entirely uniform around the shell, except for a few places where the piece looks like a secondary machine operation as seen in Figure 7 below.

Figure 7: A view of the secondary machining operation on the perimeter of the part.

The part was more than likely belt sanded to remove the excess connecting material in these two regions on the exterior of the shell. The machinists in the shop most likely did this because there were burrs from where the piece was separated from the stock plate.

Taper Many imperfections arise from the nature of the procedure as well. The waterjet cutter propels a stream of water out of a nozzle, which inherently will taper outwards as the water loses velocity. Due to this tapering of the jet, the shapeâ&#x20AC;&#x2122;s geometry on the bottom of the piece will be slightly different than the geometry at the top. More specifically, the features of the bottom will be slightly thinner than those of the top because the kerf increases with the depth of the part. With a part as thin as ours, the taper is not significantly noticeable, but on certain bands of the shell, more imperfections can be seen on the underside of the shell.

Burrs Along this same line is the amount of burrs produced from the jet of water. Because metal is being forced out the bottom of the stock plate, burring will occur. These burrs are produced from the metal that is blasted by the water jet but is not actually removed from the part. The water jet therefore produces a jagged edge along the entire outer and inner perimeters of the part. Some metal, which would normally burr, will shear off surrounding metal, causing the unintentional removal of metal as seen in Figure 8.


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Figure 8: A view of the shearing imperfections on the underside of the part.

Fillets & Edges The waterjet machine is capable of producing inside fillets with a radius of .04” and through holes with radius of .06”. In designing our part, we had to verify that all corners and sharp edges accounted for this radius so the machine would not error. In retrospect, we should have added a larger fillet radius to the corners of the piece so the machine was less strained and the cuts came out cleaner. Outer corners, however, should theoretically be quite sharp but this was not apparent with this particular part. The one sharp outer corner, the tip of the shell, has a slight round to it that looks to be approximately .01-.02” in radius. Perhaps this corner, shown in Figure 9 could have been sharper depending on the tool path but it is a small enough radius that it serves its purpose and shows a clear point on the shell part.


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Figure 9: The outer corner and inner corner, both rounded.

When modeling the shellâ&#x20AC;&#x2122;s bands on CAD, the smallest fillet radius possible became of particular interest. For each of the shellâ&#x20AC;&#x2122;s internal sections, we had to add the proper fillet, which looks better on the acute angles than the obtuse angles. We added the same radius for all corners in these internal sections, but we should have increased the fillet radius on the obtuse angles, so the part looks more natural and has less jagged cuts. During the actual CAD modeling, the addition of the fillet radii to the design altered the actual outline of the part. Some of the sections in the shell became too indiscernible with the minimum fillet radius and so we had to freehand the model a bit. This is an example of how the machine capabilities affect the design of the part and showcases the principle of design for manufacturability.

For the Future Overall, the part came out with few noticeable errors. In a future iteration, we would most likely increase the overall thickness of the bands of the shell, as those are the regions that contain the most imperfections. By increasing the thickness of these bands, we could limit the deflection and shearing of the metal, which would produce a cleaner cut.


Waterjet Project | ME 122

Yellowfin Tuna Fish

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Waterjet Project | ME 122

Nautilus Shell

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Waterjet Project