Workholding for nonround parts

Author Cutting Tool Engineering
Published
June 01,2012 - 11:15am

Related Glossary Terms

  • centers

    centers

    Cone-shaped pins that support a workpiece by one or two ends during machining. The centers fit into holes drilled in the workpiece ends. Centers that turn with the workpiece are called “live” centers; those that do not are called “dead” centers.

  • chuck

    chuck

    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.

  • 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.

  • facemill

    facemill

    Milling cutter for cutting flat surfaces.

  • fixture

    fixture

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

  • flat ( screw flat)

    flat ( screw flat)

    Flat surface machined into the shank of a cutting tool for enhanced holding of the tool.

  • gang cutting ( milling)

    gang cutting ( milling)

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

  • grinding

    grinding

    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • metalworking

    metalworking

    Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.

  • 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.

  • tolerance

    tolerance

    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

  • 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.

When it comes to workholding, one size definitely does not fit all. For example, workholding setups for round parts have similarities to setups used for all other prismatic (nonround) parts, but the latter has unique challenges. 

The following basic rules of workholding apply to all parts. 

  • Use a locating scheme that places all the work inside the locators.
  • Use clamping that allows tool access to areas to be machined.
  • Stock edges, such as bar stock flats or formed flats, can be used for locating and clamping.
  • Workholding is for accommodating part cutting requirements, not just for holding a part.
  • For workholding, one must consider machine characteristics, including size and capacities, cutting capabilities and options for maximizing throughput. 

s4.tif

A nonround part fixtured for horizontal machining of the front and back in one setup. Image courtesy J. Mason.

For the nonround parts discussed in this column, I am assuming workholding for either horizontal or vertical milling.

Clamps should be placed directly above part locators because that minimizes the effect of cutting forces on a part. It is not good practice to clamp over voids and open bores that require finishing because that can limit tool access and, more importantly, damage the part by bending or otherwise distorting it. Metal has a tendency to move when you least expect it.

When roughing, use no more than three clamps on the fixture unless it is a very large part. A three-point locator and clamp setup is the first choice. Larger parts may require work supports in other unsupported areas to dampen machining forces. Also, roughing and finishing toolpaths must not interfere with clamp locations.

A fixture must be robust enough to handle the nonround part’s cutting requirements. For example, if a facemill requires a 15-hp cut with 200 lbs. of side load force, the fixture must be able to absorb those cutting forces.

Workholding is all about minimizing vibration. Many horizontal machines with pallet changers use tombstone-type workholding, with parts set on rest pads and side locators and hydraulically or manually clamped. This provides stability and helps minimize vibration.

Workholding can be hydraulic, pneumatic or manual. Hydraulic automatic clamping generates higher clamping forces than pneumatic or manual workholding. Hydraulics should be used for high-volume nonround parts; large, heavy parts made of steel or iron; parts requiring large amounts of material removal; and parts requiring very high clamping forces. 

For hydraulic and pneumatic fixtures requiring lines for hydraulic fluid or air, internal lines are the best choice. Using manifold plates can aid placement of internal lines in the fixture. If necessary, leave space between the part and the back of the workholding for chip shedding.

Pneumatic workholding is an alternative to manual workholding. Clamping forces are about the same for both, but pneumatics can speed up clamp and unclamp times. Pneumatics are a good choice for vertical machines where load/unload is part of cycle time.

Manual workholding is the least costly option and ideal for low-volume or one-off nonround parts. Unlike powered workholding, manual setups require the operator to apply force to secure the part. Torque wrenches should be used that can consistently apply the same pressure.

The type of workholding for nonround parts is directly related to a shop’s tooling budget. If a more-expensive hydraulic fixture can be cost justified, that’s the route to go. If not, then less costly pneumatic or manual alternatives must be used.

Vertical machining centers lend themselves to single-part workholding systems. In these machines, vises and fixture plates with predefined locators are often used for workholding. Both can be cost-effective, but dedicated workholding systems are a better option when part volume reaches 500 and above. 

Also, chip control is a workholding concern because chips collect and interfere with locators and clamps. Fixtures must be designed to be effectively cleared with air or coolant.

Compared to VMCs, horizontals are better suited to medium- and high-volume production and unattended machining using automatic pallet changers. However, fixture costs tend to be higher as multiples fixtures are required.

Whether it is used for a VMC or an HMC, all workholding must accommodate roughing and finishing. A key question is whether or not the part can be machined complete in one setup. Parts with large, flat areas requiring fine finishes and tight flatness tolerances may need additional work supports to neutralize cutter vibration during finishing. Generally, it is better to finish a part in one clamping because tolerance error between operations can be limited to machine positioning and not be affected by the workholding. 

In addition to basic workholding setups for nonround parts, milling workholder options include:

  • Fixture plates for 4th- and 5th-axis positioning and contouring are used for parts that can be prequalified with a precision locator and mounted to the machine’s rotary axis.
  • Turn/mill machine workholding that allows the turning chuck to be used as a static fixture for milling with a spindle or live tool.
  • Magnetic and vacuum workholding systems, typically used for grinding, have advantages in milling operations for parts with light finish cuts and large surface areas. Additional clamps and work supports are not required. 

Milling fixtures are unique to each part and its manufacturing process. An effective part process and workholding design can help make the part making process faster, more accurate and, ultimately, more profitable. CTE

About the Author: Joe Mason is cost estimator and process engineer for United Machining Inc., Sterling Heights, Mich., a supplier of high-heat manifold, turbine and exhaust components, and a division of Wescast Industries Inc. He has worked in the metalworking industry since 1978, starting as a machinist. Contact him at cncgcode@earthlink.net.