Workholding systems

Chip-to-chip tool changes for most CNC mills can exceed 6 sec. By placing more parts under the spindle, tool change time can be amortized over more parts, reducing the per-part tool change time.

Dual-station workholding puts two parts in the same space as a standard vise, effectively cutting tool change time in half. For example, a part that requires 15 tool changes with an 8-sec. tool change yields 120 sec. or 2 min. of tool change per part (15 tool changes times 8 sec./tool change). By replacing a standard vise with a dual-station system, the tool change time per part is divided in half for a savings of 1 min. At a $40/hr. machine rate, the savings per part is $0.67.

If the part requires machining on multiple sides, applying indexable workholding reduces tool change time even further. Indexable workholding can present three sides of a part to the spindle in one clamping. This allows tools used on more than one surface to access all exposed surfaces in one operation without the need for additional tool changes.

Part-change Time

A standard vise presents one side of one part to the spindle for cutting. When a part change occurs on a standard vise, one machined part side is removed with each unclamp/clamp operation. Using a dual-station system, two machined part sides are removed during the part-change process.

The dual-station system amortizes the part-change time over two part sides, reducing it 50 percent. For example, a standard vise holding a part machined on six sides with a 1-min./side part change, yields a 6-min. part-change time – [6 sides times 1 min./side] divided by [1 side/operation]. Switching to a two-station system yields a 3-min. part-change time – [6 sides times 1 min./side] divided by [two sides/operation], saving 3 min. At a $40/hr, the savings per part is $2.01.

For machining parts on all six sides, an indexing workholding system is the recommended solution. In one clamping, this workholding system holds two parts but presents three sides of each part to the spindle. With each part-change operation, a total of six machined sides are removed.

For example, an indexing solution making the same six-sided part would yield a one minute part-change time – [6 sides times 1 min./side] divided by [6 sides/operation]. Using an indexing workholding system instead of a standard vise provides a savings of 5 min./part. At $40/hr, the savings per part is $3.35.

To reduce chip-cleaning time, it is important to eliminate nooks and crannies that trap chips, including all bolt heads, blind holes and protruding objects. Unlike standard vises, modern workholding provides a chip-trap-free environment, allowing the coolant to wash away chips. This can reduce part-change time by an additional 10 percent.

An indexable system is recommended when a part requires machining on adjacent sides (as compared with opposing sides). There are two possible solutions for reducing the time spent awaiting the rotation of an indexer – either obtain a faster indexer or place a second indexer on the machine table. The second solution virtually eliminates index time.

Indexing, multistation workholding presents three part sides to the spindle with only one clamping. Using multiple units eliminates index time because while one unit cuts, the other indexes.

Introducing more parts into the machine creates another time element – traverse time, or the time waiting for the tool to move from one part to another. When cutting a single part in a vise, all motions to and from that part are considered tool change time. Adding more parts requires the tool to move from part to part prior to a tool change.

The preferred practice is to mount parts close as possible to each other to reduce the travel time from one part to another. Multistation workholding systems hold parts close together to minimize traverse time between parts.

Workholding must consistently and accurately place the workpiece under the spindle. To minimize cut time, the workholding must clamp the workpiece rigidly and transfer cutter vibration away from the tool. By using modern workholding with machinable jaws, shops can create a part interface that perfectly matches the part being machined. This provides a rigid hold using minimum clamping force.

In addition, when using a single-piece jaw design, any vibration transmitted by the part into the jaws is not reflected back into the workpiece. The vibration continues away from the part, through the workholding and into the base of the machine. This reduces the occurrence of harmonics and part chatter, allowing for increased feeds and speeds.

No-operator time occurs when the machine is waiting for an operator to take an action. This can occur between shifts or during a shift.

Reducing the number of times a machine stops during a shift minimizes no-operator time. Adding more parts to the machine envelope decreases the per-part cycle time and increases machine in-cycle time. The fewer times a machine stops, the less the chance an operator will not be around to put it back into cycle.

In addition, if a machine can run between shifts and through breaks, additional hours of manufacturing time are gained, thus increasing the machine's overall efficiency.

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