This final segment of our series covers work holding, part handling, and industry trends By Robert B. Aronson, Senior Editor
Mounting parts on fixtures and worktables, then getting them to and from machine tools, has gained importance in manufacturing. This trend is the result of higher-volume production emphasizing speed even as smaller-volume production and precision parts demand minimal setup. Both require greater attention to part handling.
KGM 181 grid encoder from Heidenhain (Schaumburg, IL) can do more than check circles. It performs circular interpolation tests with small radii and curved paths to inspect the dynamic performance of a control.
Work Holding
Two major subcategories of workholding are jigs and fixtures. Although definitions vary, generally jigs guide a cutting tool to the work piece, often for a drilling or boring operation. A fixture has much broader application in positioning a part relative to the cutting tool. Tooling can refer both to the cutting element or the jigs and fixtures.
The earliest work holding methods were chiefly pins, bolted clamps, and straps that held the part to the work table. These straps were commonly bars held in place and manually tightened with screws. Clamp elements of various shapes were built up to support the part. T-slots were an early addition to the machine table to allow easier placement of clamping elements. For complex parts, specialized fixtures were made. Fixtures. They are used for complex parts and often need adjustable supports in addition to clamps to prevent distortion during the cutting operation. These supports are often manual or semi-automatic jack screws. Fixtures can be dedicated, semi-dedicated or temporary. Fixture design is a key issue in determining clamping force needed. Ideally the holding clamps should take only 10 to 25% of the cutting force, with the remainder of the force being absorbed by the fixture, stops, or locators. Dedicated fixtures are typically custom made where the economics of a long run make the investment practical. The middle ground is the modular fixture. These fixtures are pre-machined plates, clamps, supports, risers, and locators that can be repositioned to accommodate moderate variation in part configuration. Temporary fixtures can be built up from standard pieces, usually for a single part or low-volume run. One of the early innovations in clamping was the hydraulically actuated swing clamp. This has a rotateable arm that would be automatically swung out of the way to simplify the part placement and removal. As in the use of other simple push or pull hydraulic cylinders used to transfer force on a fixture, they require a connection to a power supply by utilizing flexible hoses that could move with the table.Pallets.
With the addition of cells and the multifunction machine tool has come the requirement for more consistent, higher-volume part supply. Pallet systems were developed to help fill this need.
Pallets were introduced to allow preparation of the part off line and therefore facilitate increased machine spindle run time. This has also helped lead to today's multipallet or pallet-spooling systems.
The earliest pallet versions were two-sided systems. The machine worked on parts on one pallet, or half of a large pallet, while the other was being prepared. Later, multipallet, and pallet spooling systems were developed. Many simple measuring tools now have digital readouts that connect to data recorders. The displays on this L. S. Starrett (Athol, MA) unit help ensure accuracy, while the recorders store production data.
Another innovation that came with pallets was the decoupler. Once the part was secured to the pallet by hydraulically actuated clamps, the hose could be removed while the clamps retained their holding power. Pallets could therefore be stored for a time off line.
Today's systems are more complex, requiring such things as multiple system clamping pressures on the same fixture. Some manufacturers are now offering live power options on their machines. One of the ways this is done is by the addition of rotary unions or valves built into the machine. This again allows a constant connection to the hydraulic power supply, and eliminates the need for a pallet decoupler. One of the benefits of this approach is that it permits the clamping pressure to be monitored and re-supplied as needed while the machining takes place as well as permitting multiple clamping pressures on the same fixture. It can also eliminate the time and labor required to couple and uncouple the pallet each time a part is changed on the pallet. Because of the potential labor costs involved in installing a complex pallet fixture with external hydraulic lines, several manufacturers now offer manifold mountable devices. The external plumbing can not only be costly to install, but it can also act as a buildup area for chips to collect on the fixture. By manifold mounting, external plumbing, chip build-up, and installation labor can be greatly reduced or eliminated.
Manifold mounting means that the fluid passages and/or ports are bored within the pallet or on a separate plate. The hydraulic fluid is directed to the devices by drilling a supply feed hole that intersects these passages, and sealing the connection with an o-ring on a face of the clamping device. Pneumatic clamping is not as common for machining applications because it typically can not generate the holding force of hydraulic systems. Pneumatic clamps usually need more space to provide the same clamping force, due to lower operating pressure. However, for smaller parts and lighter materials such as plastics and composites, and the use of higher speed machine spindles, pneumatically powered holding is becoming more practical.
Space is at a premium on a fixture. The goal is high-density workholding or presenting as many parts to the cutting element as possible to maximize the machine run or cut time. It is also important to expose as much of the part as possible to the cutter or drill. Holding power. Clamping design is a compromise between the need to keep the part secure and to allow the part to be machined or drilled. Clamping force is another compromise between the amount of force needed to secure the part and the need to prevent part distortion. This has traditionally been a matter of operator experience but more recently, computer programs are being developed that take into account part size, material, and production speed, with the type of machining operation a dominant factor. Face milling would put much higher loads on the part than a drilling operation, for example. One source gives a rule of thumb that total holding force should be twice the maximum cutting force.
Vises. Provided in either manual or automatically actuated, vises are major workholding elements. Designers can increase jaw friction with serrations or by diamond coating the clamping surface. Jaws can also be carved or machined to better grip irregular parts. Work holder design. It involves a number of factors, including:
The number of parts per work cycle required
The number of operations involved on each surface
The sequence of operations
Cutting forces on the part
Optimum holding forces
Number and type of clamps per part
Clamp position
Time required to clamp and release a part
Penalty of error