Facing Hard Facing

Published Date
January 15,2008 - 11:00:am

Greenleaf Corp. introduced its WG-300 ceramic composite  tool material about 25 years ago. The ceramic matrix  is reinforced with silicon-carbide "whiskers” that boost  toughness. Initially applied in the relatively constant cutting  conditions of turning operations, the composite tools  provided a significant increase in productivity when roughing  aerospace alloys. Shops are now using WG-300 tools in hard  milling operations. 

Tom Mahusky, shop supervisor at Cleveland Hard Facing  Inc., Cleveland, is familiar with machining hard materials."We've dealt with hard materials for the past 20 years, so we've learned a lot of tricks," he said. A recent job put that knowledge to use. It involved hard  facing and machining hammer components used to forge  titanium parts. To enable the four roughly 20" x 16" x 8" L-6  tool steel hammers to survive the high temperatures and  pressures of the forging process, hard facing specifications  called for application of a 1/8"-thick layer of low-carbon steel  followed by a 7/8"-thick layer of nickel-base Hastelloy and  finally a 1/4"-thick layer of Waspaloy, another nickel-base  material. Cleveland Hard Facing applied the layers using the  metal-inert-gas arc welding method. After the welding was completed, Mahusky rough milled  the parts. The hammers had pyramid-shaped tapered faces, so  the cutting tool had to machine through all of the weld layers  as it descended the sloped sides of the hammers. "Hastelloy  and Waspaloy don't like to be cut," Mahusky said. The alloys  are not unreasonably hard; "only going in the low 30s [HRC]  as welded," he said, but added that when machined, the  materials "will workharden just like a 300-series stainless.”  This increases the hardness by 10 HRC. Without a large-capacity  CNC mill in his shop, Mahusky used a 7 1/2-hp manual  vertical mill to rough the welded overlays. Running at 1,400 sfm and a feed rate of 18 to 19 ipm, he took DOCs of up to  0.100" with a 4"-dia. Excelerator milling cutter tooled with  four round, negative-geometry WG300 ceramic inserts. "A  positive insert is not strong enough to withstand roughing  in this case," he said.  Mahusky worked with Denny Carpenter, a Greenleaf sales  and service engineer, to set up the operation. Carpenter made  sure the machining parameters were "within the capabilities of the machine, the part and the rigidity of the setup," Mahusky said. "It depends on the configuration  of the part. If it's big and overhanging, you  need as rigid a setup as possible. You have to take  all that into account when milling with ceramics."  Machine stiffness is crucial. "Even if you are  taking small finishing cuts, you still need a rigid  machine," he said. "If you are going to take bigger cuts, it's not just a high-speed spindle you need—you must have rigidity and torque at the spindle."  Mahusky left 0.020" finishing stock on the hammers  and subcontracted finish machining to job  shop Quality Industries Inc., also in Cleveland.  Quality Industries finished the hammers on an  ACRA FVMC-610 CNC vertical machining center using  a 1.5"-dia. positive-geometry Excelerator endmill  tooled with three RPGN-43 inserts. The tool ran at  3,800 rpm, 80 ipm and a 0.020" DOC. Vice President  Jim Kaplan said the light DOC produced a minimal  flow of chips. Greenleaf's Carpenter added that  "compensating for the chip thinning effect is 100  percent crucial” in maximizing tool life in the operation.  Mahusky said he would not consider hard milling without  ceramics. "Not today, when you know what you know, and  you remember how you did it back when. You'd get carpal  tunnel syndrome just changing inserts when it was carbide.”  He added that according to a sales representative for the  weld materials applied to hammers, some shops don't attempt  to mill the hard welds and grind them instead. "That's  gets pretty expensive because you are getting into wheels,  too," Mahusky said.

Related Glossary Terms

  • alloys

    alloys

    Substances having metallic properties and being composed of two or more chemical elements of which at least one is a metal.

  • ceramics

    ceramics

    Cutting tool materials based on aluminum oxide and silicon nitride. Ceramic tools can withstand higher cutting speeds than cemented carbide tools when machining hardened steels, cast irons and high-temperature alloys.

  • computer numerical control ( CNC)

    computer numerical control ( CNC)

    Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

  • 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

    feed

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

  • gang cutting ( milling)

    gang cutting ( milling)

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

  • hardness

    hardness

    Hardness is a measure of the resistance of a material to surface indentation or abrasion. There is no absolute scale for hardness. In order to express hardness quantitatively, each type of test has its own scale, which defines hardness. Indentation hardness obtained through static methods is measured by Brinell, Rockwell, Vickers and Knoop tests. Hardness without indentation is measured by a dynamic method, known as the Scleroscope test.

  • 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.

  • machining center

    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.

  • milling cutter

    milling cutter

    Loosely, any milling tool. Horizontal cutters take the form of plain milling cutters, plain spiral-tooth cutters, helical cutters, side-milling cutters, staggered-tooth side-milling cutters, facemilling cutters, angular cutters, double-angle cutters, convex and concave form-milling cutters, straddle-sprocket cutters, spur-gear cutters, corner-rounding cutters and slitting saws. Vertical cutters use shank-mounted cutting tools, including endmills, T-slot cutters, Woodruff keyseat cutters and dovetail cutters; these may also be used on horizontal mills. See milling.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • stiffness

    stiffness

    1. Ability of a material or part to resist elastic deflection. 2. The rate of stress with respect to strain; the greater the stress required to produce a given strain, the stiffer the material is said to be. See dynamic stiffness; static stiffness.

  • 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.