25 TPJ • DECEMBER 2009 A TPA PUBLICATION

T E C H T A L K

OVER the decades tube rotary drawbending machines have made use ofnumerous devices to clamp a tube in posi-tion as it is being bent. For example, theclamp die is designed to clamp the tubefirmly to prevent it from slipping relativeto the bend die as the bend die rotates.Another example is the pressure die; itsprimary job is to hold the tube firmlyagainst the bend die while at the sametime the clamp die helps pull the tubearound the bend die. Yet another device isthe split die actuator, which must remainclosed while a tube is being bent. All threeof these tools must remain rigid underloading.

Machine tool builders predominatelymake use of one of two approaches to

secure the workpiece: a direct-actingcylinder or an over-center toggle mecha-nism (see Figure 1). Understanding thedifferences between the two approachescan influence bend quality.

DIRECT-ACTING CYLINDERAn example of a direct-acting applicationis a hydraulic cylinder that pushes directlyagainst a clamp die (see Figure 2).

The hydraulic pressure developed by thecylinder has a direct influence on the amountof force that the clamp die exerts against thetube. Increasing the hydraulic pressureincreases the clamping force. The amount ofclamping force can be controlled easily by amicroprocessor, which is advantageouswhen bending thin-walled tubes.

By George Winton, P.E.

TOGGLE OR DIRECT-ACTING CLAMPING?

Options for securinga tube for bending

Figure 1This manual tube bender uses an over-center toggle device to clamp the tube.

An alternative uses a ball screw and anelectric servomotor. This type allows theoperator to adjust the clamping force bychanging the amount of power applied tothe servomotor. It’s a green alternative inthat it uses no hydraulic fluid.

OVER-CENTER TOGGLEMECHANISMAn over-center toggle mechanism is simi-lar to a DE-STA-CO clamp. The over-cen-ter approach makes use of a mechanicaladvantage, like that used by any commonlever. The amount of clamping force itdevelops is much larger than the forceneeded to close it. The clamping force, inturn, can prevent the tube from movingduring bending (see Figure 3).

Most over-center devices have somesort of pressure adjustment via a manual-ly adjusted screw.

However, this doesn’t necessarily pro-vide the fine touch needed to bend thin-walled tubing to a tight centerline radius.A direct-acting clamp or pressure die (orboth) allows the small pressure adjust-ments that often are necessary to make acomplex bend successfully.

Despite the lack of fine adjustment,over-center toggle mechanisms have sev-eral advantages. First, this type of systemgives the operator a visual confirmationthat the mechanism is engaged and pro-viding clamping force. Second, it is suit-able for a range of wall thicknesses; itworks well for thick-walled tubing andcan provide the force necessary to pinchthin-walled tubing.

A TPA PUBLICATION DECEMBER 2009 • TPJ 26

T E C H T A L K

PRODUCINGTHE HOLDING FORCEWhether the clamp is direct-acting or over-center, three types of actuator commonlyare used to create the holding force: pneu-matic, hydraulic, or all-electric. They havea variety of pros and cons.

A direct-acting pneumatic actuatorrequires an air source and generates sub-stantial noise. Also, because air is com-pressible, the air may compress in themiddle of a bend (allowing the clampingcylinder to move), which can lead towrinkles in the bend zone.

A hydraulic system can run quieterand maintains pressure, but useshydraulic fluid, so it comes with the costsassociated with purchasing and disposingof the fluid and the time spent cleaningup occasional spills. An electric system isthe quietest of the three types and doesn’tuse hydraulic fluid, but it comes with ahigher initial price tag and requires a dif-ferent set of troubleshooting skills.

A pneumatically powered, direct-act-ing clamp die may work well when work-ing with an easily bent material, such as1⁄8-in. copper tubing (see Figure 4). Thetubing is light and malleable, and 80-pounds-per-square-inch (PSI) shop air issufficient for applications of this sort.However, 80 PSI wouldn’t handle 1-in.-dia. steel tubing on a 2-D bend. The air’scompressibility may be the cause of sev-eral process problems.

An over-center toggle mechanismworks well for most bending applica-tions. However, some cases require thegentle touch of a direct-acting mecha-nism. When considering purchasing abending machine, it’s important to con-sider the requirements of the bendingapplications, the capabilities of themachine, and the versatility of the clamp-ing system.

George Winton, P.E., is the founder andpresident of Winton MachineCo., 3644 Burnette Road, Suwanee,GA 30024, 888-321-1499, gwinton@wintonmachine.com, www.wintonmachine.com.

Direct-ActingClamp Cylinder

Figure 2This direct-acting clamp cylinder uses hydraulic pressure to apply clamping force.

Figure 3On most toggle-type mecha-nisms, the actuator is mountedbelow the clamp die holder andtherefore is not direct-acting.

Figure 4A pneumatically powered,direct-acting clamp and pres-sure die are suitable for bend-ing 1⁄8-in. copper tubing.Increasing the workpiece sizeor material hardness decreas-es the likelihood that pneu-matic power will be sufficient.

Clamp Die Holder

Actuator