Workholding for nonround parts: Turning Performance

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.

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.