Finessing fins

Author Cutting Tool Engineering
Published
May 01,2010 - 11:00am

Related Glossary Terms

  • 3-D

    3-D

    Way of displaying real-world objects in a natural way by showing depth, height and width. This system uses the X, Y and Z axes.

  • bandsaw blade ( band)

    bandsaw blade ( band)

    Endless band, normally with serrated teeth, that serves as the cutting tool for cutoff or contour band machines.

  • chatter

    chatter

    Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

  • chuck

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    Workholding device that affixes to a mill, lathe or drill-press spindle. It holds a tool or workpiece by one end, allowing it to be rotated. May also be fitted to the machine table to hold a workpiece. Two or more adjustable jaws actually hold the tool or part. May be actuated manually, pneumatically, hydraulically or electrically. See collet.

  • computer-aided manufacturing ( CAM)

    computer-aided manufacturing ( CAM)

    Use of computers to control machining and manufacturing processes.

  • coolant

    coolant

    Fluid that reduces temperature buildup at the tool/workpiece interface during machining. Normally takes the form of a liquid such as soluble or chemical mixtures (semisynthetic, synthetic) but can be pressurized air or other gas. Because of water’s ability to absorb great quantities of heat, it is widely used as a coolant and vehicle for various cutting compounds, with the water-to-compound ratio varying with the machining task. See cutting fluid; semisynthetic cutting fluid; soluble-oil cutting fluid; synthetic cutting fluid.

  • endmill

    endmill

    Milling cutter held by its shank that cuts on its periphery and, if so configured, on its free end. Takes a variety of shapes (single- and double-end, roughing, ballnose and cup-end) and sizes (stub, medium, long and extra-long). Also comes with differing numbers of flutes.

  • feed

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    Rate of change of position of the tool as a whole, relative to the workpiece while cutting.

  • fixture

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    Device, often made in-house, that holds a specific workpiece. See jig; modular fixturing.

  • gang cutting ( milling)

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    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

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    Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

  • inches per minute ( ipm)

    inches per minute ( ipm)

    Value that refers to how far the workpiece or cutter advances linearly in 1 minute, defined as: ipm = ipt 5 number of effective teeth 5 rpm. Also known as the table feed or machine feed.

  • lathe

    lathe

    Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

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    machining center

    CNC machine tool capable of drilling, reaming, tapping, milling and boring. Normally comes with an automatic toolchanger. See automatic toolchanger.

  • milling

    milling

    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • outer diameter ( OD)

    outer diameter ( OD)

    Dimension that defines the exterior diameter of a cylindrical or round part. See ID, inner diameter.

  • turning

    turning

    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.

OMW Corp., Novato, Calif., is a job shop that focuses on prototypes and small to medium production runs. Founder Joe Osborn noted that many customers ask OMW to make parts for automated manufacturing equipment. “We don’t design the machines, but we might typically make a hundred different parts for a particular project,” he said. “That type of work is not really prototype machining, because these machines aren’t prototypes, but it is a lot of different parts at low volumes.”

One challenging machine part was a cylindrical component used to separate small plastic parts after ejection from an automated molding machine. The 9 "-long, 303 stainless steel part has a dozen 0.6 "-tall × 0.060 "-thick radial fins that extend about a quarter of the way around the circumference of its 5⁄8 "-dia. shaft. In use, a computerized control rotates the shaft back and forth to selectively stop and release parts as they move beneath it.

OMWPrtT5-10.tif

Courtesy of OMW

When OMW machined this 9 "-long component for an automated manufacturing system from 303 stainless steel, the shop overcame challenges that included finding a way to quiet a dozen 0.060 "-thick fins that “rang like a bell” during a milling process.

OMW worked with a customer-provided SolidWorks model and programmed the part in SurfCAM. The first operations, performed in an Okuma turning center, involved turning a 2 "-dia. bar to the final OD of the fins and then stepping down to 5⁄8 " for 3 " at one end of the shaft. After flipping the bar in the lathe chuck, OMW turned the other end to 5⁄8 " in diameter for 1.5 " and then to a 3⁄8 "-dia. bearing surface for the shaft’s final 0.5 ". 

Next, the bar was fixtured vertically in a Hwacheon Sirius-U vertical machining center with the 5⁄8 " end up. That end was drilled with a center hole and milled for a length of 1 " into a round-sided triangular shape. 

OMW then returned the bar to the lathe to cut the 12 0.3 "-wide × 0.6 "-deep grooves to produce the fins. For support, the lathe’s tailstock was put in the center hole drilled earlier because “The grooving tool puts a lot of force on the part,” Osborn said. A Kennametal 0.120 "-wide grooving tool with an 0.008 " radius cut the grooves at 838 rpm and a 0.0025-ipr feed rate, making three roughing and two finishing passes with flood coolant. The tool also created a 0.312 " radius between each fin and the bottom of the groove and put 0.10 "-long, 45° chamfers on the top edges of the fins. 

Then it was necessary to cut excess flute material from about three-quarters of the part diameter. To hold the bar horizontally on the VMC, OMW fabricated a square fixture plate with a central hole that matched the triangular shape at the part’s one end. The other end of the shaft went into a fixture that allowed it to rotate. The fixtures were clamped in vises on the VMC’s table, and OMW was able to flip the part in the vises and index it at 90° intervals. The shop probably could have accomplished similar results with an indexing table, Osborn said, but preferred to combine the accuracy of indexing via the square plate with the control of CAM software. 

Cutting the excess flute material from the shaft, however, was “a tricky thing,” Osborn said. The fins were “too thin, only 0.060 " wide. When you cut them away, they rang like a bell. That created chatter and terrible surface finish.” In addition, when the fins began to vibrate, the endmill often would catch on a fin and bend it. Osborn called the initial fin-cutting efforts “a disaster.” 

The shop’s creative but simple cure for the ringing fins was aluminum spacers, about 0.3 " wide × 0.75 " tall × 0.5 " long, pushed between the fins. The spacers were “kind of jammed in there and they wanted to fall out,” Osborn said, so they were attached one to another with a thin band of aluminum. “They stuck up about 1⁄8 ", but that didn’t matter because we weren’t machining exactly where they were. They were there to keep the fins from vibrating, to make the machining possible.” 

The fix worked; a 4-flute, 3⁄8 "-dia., coated, solid-carbide bull nose endmill with a 0.0156 " corner radius removed the excess fin material at 4,000 rpm and 20 ipm. 

In addition to removing about three-quarters of each fin from around the shaft, Osborn said: “We had to go back and machine the last part of each fin away, perfectly matching the contour of the bar. It almost had to look like the fins were welded onto the bar. All we were allowed to leave was the bar’s round shape.” 

Osborn said the indexing fixture developed in-house enabled the part to be indexed accurately, and, with the CAM software, “We did 3-D machining, planar passes, just to clean up the last bit of the fins. One of the nice things about 3-D machining” is that it enables a user to “save setups and basically simulate other types of machining.” 

Except for the 3⁄8 "-dia. bearing surface and the triangular section on the ends, OMW bead-blasted the part after machining to finish it. CTE

For more information about OMW Corp., call (415)382-1669 or visit www.omw corp.com. 

About the Author: Bill Kennedy, based in Latrobe, Pa., is contributing editor for Cutting Tool Engineering. He has an extensive background as a technical writer. Contact him at (724) 537-6182 or by e-mail at billk@jwr.com.