The Workholding is the Hardest Part

XL Machine was one of sister brand Modern Machine Shop’s Top Shops winning companies in 2017. The manufacturing company was also part of a panel discussion with representatives from the other winning shops that year at the inaugural Top Shops Conference in Indianapolis, Indiana.

In checking back with the company, I learned about a neat robotic cell the Three Rivers, Michigan, shop had added to machine three- and four-blade cast propellers (a family of 20 part numbers) for outboard boat engines. I mentioned that holding such contoured parts for five-axis machining had to be tricky. In fact, that was the most challenging part of the project, according to the company.

That, my friends, is a story hook, so I visited to learn more.

Cell Structure

XL Machine was recently rebranded as Burke Porter, a part of Ascential Technologies. The manufacturer began as a prototype machining company. It still does prototype work today, but pairs that with production machining capability. The shop serves various customers in markets such as automotive, heavy industry, entertainment and marine.

At first, the customer with the propeller job turned each part’s hub in-house. The shop would then mill either side of the contoured blades on two stand-alone three-axis VMCs and a stand-alone HMC that blended the verticals’ cuts near the hub. When its customer asked it to turn the hubs as well, creating a multi-machine, automated process seemed like a logical next step.

The castings as they are received by the shop are made of “nibral,” an alloy consisting of nickel, bronze and aluminum. It is useful for marine applications because it is durable and resists corrosion in seawater. Machining-wise, it’s said to be like stainless steel in that it’s a bit gummy.

The U-shaped cell designed to finish these parts features four Okuma machines: two side-by-side 2SP-V760EX vertical lathes for op10 and op20 hub-turning work, and two MU 6300V five-axis machines for op30 and op40 blade-milling work. These machines are served by a large FANUC M-900ib robot.

The cell includes two multi-level racks with drawers into which the cell operator places raw castings and removes completed parts. To start the process, the robot opens a drawer to grip a raw casting by its hub. A camera mounted above the rack views the part number on tabs on the end of each blade to verify the robot is picking the correct part. (These tabs are also used for locating and clamping the parts in the five-axis machines’ fixtures.) Another camera on the robot’s arm then detects the part’s orientation so the robot properly positions the gripper without contacting the blades.

Next, the part is delivered to the op10 vertical lathe for internal and external machining of the hub half facing upward. The other half of the hub is secured by a custom power chuck from Co-Op Tool, a division of Hammill Mfg. The machine also uses a custom plunge tool from Cline Tool, which combines four operations into one cutter body. This is helpful because many tools for the turning and boring operations are so large that adjacent automatic tool changer (ATC) pockets must remain empty to accommodate them. The custom cutter saves three ATC pockets. The machine also makes use of flood coolant and through-tool air delivery to help remove chips from the insides of the freshly machined hubs.

Once op10 work is completed, the robot grips the part, the power chuck disengages and the robot takes the part to a flip station. The station then lowers, and the robot, now with the gripper, facing down, grips the other side of the hub. It then takes the part to the second lathe for op20 hub work, which is similar to the op10 work.

Five-axis blade milling is next. The op30 machine mills one side of each blade, and the op40 machine mills the other side. This is where the trick fixturing created specifically for this application shines.

Workholding Props

In addition to the custom lathe workholding devices, Co-Op Tool supplied the two unique hydraulic workholding fixtures for the op30 and op40 blade milling work. Dave Bermudez, the company’s senior design engineer, created the fixtures. When pallets of fixtured parts shuttle in and out of machine workzones, the pallet with locating elements that seat into the machine is typically separate from the fixture. However, given the size of the propellers, and being that Z-axis space is limited in such five-axis machines, the pallets had to be integrated into the fixtures themselves. Bermudez says this reduced the height of each fixture by approximately 4 inches. But he notes it also meant that hydraulic circuits that normally would be routed internally in the fixtures had to be plumbed externally.

These fixtures feature a center collet chuck designed by Co-Op Tool onto which a propeller locates and is secured after the robot places it in the fixture. (Prior to clamping, the robot rotates the part slightly so the tabs contact stops to ensure proper positioning in the fixture.) Hydraulically actuated swing clamps from Vektek then move into position to pinch the tabs at the end of each blade. Next, spring-loaded workholding supports under each blade lock into position once they contact the underside of the part to prevent vibration during machining.

Because each casting varies as to how much flash or extra material it might have, the single finishing pass is preceded by a roughing pass that removes extra material. This pass might do a good bit of air cutting, but it reduces the chance of tool breakage on the finishing pass.

Touch probing is performed for both the op30 and op40 blade-milling operations. Once a fixture with part is loaded into a machine, a spindle-mounted touch probe locates the center and top of the hub to ensure it is properly positioned and seated in the fixture. It then touches off on the tabs to ensure the propellers are oriented correctly in the fixture. These probing routines add 10 minutes to the overall cycle time (bringing the total to 40 minutes), but the shop justifies this because they ensure proper positioning in the fixture. In addition, all cutting tools are probed to check for insert breakage before being delivered back to the ATC carousel. If it is determined that a tool did break, a redundant tool is called up and that operation is repeated.

The final operation is milling away the tabs on each blade. This is done with the hydraulic swing clamps still pinching the tabs. The central collet chuck on its own provides sufficient holding force for these relatively light machining operations as each tab is cut off.

A Fitting Finish

The propeller customer performs some subsequent polishing to the machined blades prior to assembly on its outboard engines. That said, the company says the customer appreciates the quality finish of the machined blades because it minimizes polishing, especially in the corners where blade profiles transition to the hub. This required a bit of five-axis toolpath ingenuity. Specifically, a ball end mill is moved up and down the corners vertically rather than back and forth across the transition, which would leave behind multiple steps that would have to be removed. This little bit of toolpath ingenuity here takes care of the polishing.