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THE ORBITAL FORMING PROCESS

Orbital Headforming Machines
have been used to assemble a variety of stamped and blanked components for 30 years.

With the introduction of flexible manufacturing and advanced productivity concepts, including just-in-time (JIT) manufacturing and measurable process control, the use of orbital headforming in production is increasing.

Orbital headforming, which is sometimes called orbital forming, is a clean, silent, non-impact, and vibration-free cold forming process. Often, it can be used as an alternative to conventional staking, peening, crimping, pressing, swaging, spinning, rolling, riveting, welding, upsetting, and other fastening operations.

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Figure 1

An orbital headforming machine flares studs, pins, posts, hubs, spacers, rivets, and other fasteners quickly.

If one or more shoulder pins must be secured to a blanked plate, a single-spindle machine can be used to secure one pin at a time, or a multi spindle system can be used to flare any combination of pins, hubs, or posts on the same work piece.

(See Figure 1)

The process also can be used to assemble components without fasteners.
For example, Figure 2 illustrates semi-pierced studs on a fine blanked part being flared over a mating plate.

On an appliance part, two to four bent-up tabs on a stamped mounting plate may be designed as spacers that fit through through matching square or rectangular holes of a mating cover plate.
An orbital headforming machine then can be used to flare these tabs for a secure and correctly aligned assembly without fasteners.

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Figure 2

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Figure 3

High-torque joints in solid or tubular form in stamping assemblies may be produced in many ways with orbital headforming machines.

Blanked holes in a plate may be designed in D or double-D configurations, often with provisions for specific part orientation.

Shafts or tubes with the matching D or double-D shapes (see Figure 3) can be made to protrude through the stamping for assembly on the headforming machine.

Similarly, round shoulder pins may be flared in punched square holes of a mating plate, allowing the headforming machine to cause available material to flow into the corners of square holes. Further, one or more round posts or square tabs protruding through punched holes in stampings can be headformed. Notches or serrations may be blanked into round holes to increase the contact area and improve torque resistance after the part assembly.

Compared to any press method, the orbital process requires 12 to 15 percent of the forming forces. the reason is the same as that used to explain how a 100-pound woman can gouge dents into hard flooring when she wears spiked heels, while a much heavier person wearing flat-bottomed shoes does not. Orbital headforming uses less force but concentrates it on a continuos radial line emanating from the center of the shaft, rather than over the entire area to be formed. This prevents damage to the opposed, threaded shaft end of mating part, facilitates the part's support, and allows for the use of simplified fixture tooling.

Likewise, orbital headforming often eliminates the need for separate hardware; blanked ridges, bosses, and integral projections of malleable material can be formed out to anchor components in position. For example, a leaf spring may be captured on a stamping by flaring two blanked rib sections with a single form tool.

The Orbital Process

In orbital headforming, a form tool, mounted off-center in a revolving spindle much like a tool in a jig-borer, is inclined at a slight angle toward the center of the spindle. the tool axis of the form at the working end of the tool intersects with the true centerline of the spindle. the machine spindle rotates, but the tool in the orbital head or chuck is free to rotate in its bearings. the drive spindle advances, bringing the tool into contact with the work piece.

At this point, pressure is applied, and the line contact between the non-spinning tool and the work piece never varies. At each revolution of the spindle, the same line of contact is maintained to flare the material really. Because the same point on the tool is always in contact with the same point on the work piece, almost no friction and no tearing of the work material result, regardless of the component shape.

The orbital movement can be combined with controlled pressure, stroke, or cycle time (work speed). Depending on the application, one, two, or all three factors-each of them infinitely adjustable-may be used in conjunction with the orbital movement. A combination of a preset pressure and a cycle time is also sometimes used, usually when two or more parts to be joined by studs, pins, or rivets are subjected to broad tolerance variations, or when a brittle base component such as ceramic, glass, or phenolic is used.

Orbital headforming machines and modular units generally are stationary systems. Usually, parts and components must be brought to the machine. The use of portable headforming units in a gantry-type setup is rare and limited to the assembly of aircraft structures (fuselage and wings), railroad cars, truck bodies, and large turbine installations.

Although orbital headforming is used extensively, conventional rivet-setting machines with automatic rivets feeds are sometimes more effective for some applications.

Types of Orbital Headforming

Multiple-Point Forming: Multiple-point forming on a single work piece or gang heading multiple parts is possible, with many variations. An orbital headforming machine can be configured to carry as many tools as needed and with centers as close as 3/16 inch to meet multi-task applications. Larger multi spindle systems may have tooling plates up to 20 inches across, often with tools working at different heights or heading levels and on more than one part simultaneously.

Changeable tooling sets allow simple conversion from one heading pattern (job) to the next. Standard multiple attachments, in-line and random pattern tooling and two, three, and four variable-center-distance spindles--fit most machines.

Double-Ended, Opposed Heading: For double-ended, opposed heading, two modular heading units generally are mounted horizontally on a machine bed, facing one another. This configuration allows cut rods, shafts, or tubes to be used, as well as cutoff and feed devices to load the machine.

Swing-Joint Forming: The precise stroke control on orbital headforming machines makes possible the production of pliers, scissors, pocket knife joints, surgical sutures, gear trains, bobbins, handcuffs, or any swing-joint assembly as "tight swing", "loose swing", or "floating", as desired.

Non-round Headforming: Single- or double-D solid shafts (square, rectangle, and oval) can also be orbitally formed. Three rectangular studs can be simultaneously headed with a single large, round tool in a single-spindle orbital head, for example.

Aspects of the Process

Cycle Time: As a rule, shorter cycle times in manufacturing operations result in better production rates. In general, cycle times for orbital headforming run from 0.5 to about 2.0 seconds on solid steel studs of higher tensile strength. This includes tool approach, heading dwell, and spindle return but not part loading.

The softer and more malleable a material is, and the smaller its diameter, the shorter the work cycle is. However, even on steel pins 1.0 inch in diameter, cycle times are about 2 seconds. When automatic slider plates or index-type fixturing are used, the time required to load parts manually or automatically has a larger impact on production rates than does cycle time.

Heading Capacity: Heading capacity for any size of machine is governed not so much by the diameter of the figuring as by the total surface area to be formed and the material's tensile strength. For example, a machine that can flare a 5/16-inch-diameter solid shoulder pin in mild steel can be used to swage over the shell of a Type D flashlight battery to crimp or seal its end. It can also flange a tube or a 3-inch-diameter hollow aluminum body that has a wall thickness of 0.030 inch.

Fixturing: Parts placed or fed into the fixture of an orbital headforming machine usually can be left freestanding and require no clamps or hold-downs. No spinning force is transferred from the work head to the parts, so they remain where they are placed through the cycle. For example, a two-plate assembly to be joined by one rivet usually requires a simple locating nest with a pocket to position the pre-formed rivet head.

Stud Placement: Hard-to-reach pins can be dealt with by orbital headforming. A shoulder stud may be secured close to a vertical wall or in a recess of a part. The reach of the tool is limited only by the clearance around it while it is orbiting. Changing the angle of the tool by as little as 2 degrees can reduce the clearance requirement.

Selectable Pressure/Time Settings: On orbital headforming machines, cycle time and heading pressure can be set for specific tasks. Both are infinitely adjustable. Depth control can be set in increments of 0.001 inch. This makes it possible to form either firmly fixed joints (with selectable, increasing torque strength) or smoothly moving swing joints (which can also have the chosen amount of required built-in resistance).

Many manufacturers assemble thin or fragile materials, especially those that produce electrical or electronic parts. Common problems they encounter are breakage and loose assemblies.

A typical example is a multi contact thermostat assembly. The cumulative thickness tolerances on each stack of parts may be (+/-) 0.030 to 0.040 inch. The selectable pressure and time settings on an orbital headforming machine can help eliminate breakage and loose assemblies. For pressure to build up, there must be resistance to the force being applied. When the orbital tool is allowed to move as far as it wishes in its stroke, and its movement is stopped only after a certain pressure is built up in its chamber, the machine controls can compensate for any variations in the thickness of the stacked assembly.

Pressure can be adjusted until it is sufficient to form a rivet, locking all the elements of the assembly in place without pressure building and cracking the ceramic insulators. In addition, pressure is maintained at a preset limit (set by a relief valve) long enough to prevent the material being formed from springing back after the pressure is relieved.

Materials: In general, any malleable material up to Rockwell 35C can be formed orbitally. This includes most ferrous and nonferrous metals, stainless steel, zinc and aluminum die-cast material, some sintered metal, and many types of engineering thermoplastics.

In addition, thin case-hardened steels and plated, painted, or plastic-coated materials can usually be orbitally headformed, because material displacement is microscopic during each tool revolution. After orbital headforming, the coating surface is usually left in its original condition. The luster of some plated surfaces actually improves.

Microphotographs show that orbital headforming does not disrupt the molecular structure of metals. However, compressing the grain structure work hardens the material somewhat to make a stronger connection in the case of rivets or a harder contact surface in the case of flared, flanged, or swaged parts.

Tool Life: Because the orbital headforming process is nonimpact, tools sustain little wear. Instead, the process action causes the formed ends to become polished, work hardened, and nearly maintenance-free.
A flat-faced tool to form mild steel studs may last for years without requiring any maintenance. For more complex tool shapes or those that are used on certain aluminum or brass grades, periodic polishing may be required.

Operator Skill: An orbital headforming machine usually requires a few minutes of work by a setup person, along with some trial and error to establish operational parameters. After it is setup and its controls are set to automatic mode, the machine can be operated by skilled or unskilled workers.

Process Control with Orbital Headforming

Process control with orbital headforming is available for many stand-alone machines and modular units for system integration. Because automatic parts assembly must provide continuous production of quality parts, monitoring components at the heading points is necessary. Faulty or missing pins, studs, shafts, bushings, posts, rivets, eyelets, and other fasteners must be identified reliably and with minimal effort.

One of the objectives of process control may be to measure the height of a fastener component, or a target height. The headforming unit, with its non rotating tool, approaches and sense the presence of a rivet or other fastener and compares that data with the machine's stored program values. Depending on the readout, one of two actions occurs:

1. The form tool starts the orbital process and completes its heading task.

2. The process is aborted, and the drive spindle retracts to its idle position.

The target height for any given rivet is determined and set with an adjustable limit switch (proximity switch/linear potentiometer). The switch is activated for a designated period by the approaching spindle quill of the drive unit. The acceptable (within tolerance) rivet configuration allows the switches to make contact for the required time sequence, relaying the signal to the control panel to start the orbital forming cycle.

If the rivet is too long, a sensing signal to proceed with the forming task is not sent to the controller.If the contact sequence is disproportionate, the rivet is either too short or missing; in either case, the result is reported to the control monitor, which stops the work cycle and initiates the rejected parts mode.

A holder bracket, usually mounted on the side of the drive unit, may position the adjustable limit switch. This monitoring function by a limit switch is similar to ram up switches on presses or in-station signal switches used on automatic indexing systems.

If rivets are very thin, long, flimsy, structurally weak, or held in an unstable manner in the mating parts, less contact force can be used for monitoring purposes. The dual-pressure mode requires a second pressure circuit and additional control elements; its sequencing increases the cycle time minutely.

Electronic time control and monitoring of the forming cycle are other available functions of volume production in the automation process. A predetermined target time for heading a specific rivet type in any given station environment may be measured to identify and eliminate rivets that indicate a material hardness that is either too soft or too hard to meet the application criteria. The target time is usually established through trial-and-error evaluations or pilot runs.

If the cycle time is excessive or below the selected timing standards, the process controls signal the existance of a defective assembly, or the rivet will be discarded and replaced with a new one.

Quality assurance in the automation process also includes monitoring the heading or forming pressure with electronic pressure transducers to ensure that the headforming force falls between set minimum and maximum levels. A no-go decision is relayed to the system's controls to initiate the rejected parts mode if the pressure falls outside of the set range. A successful assembly count is recorded on a display screen and/or printout.


Conclusion

Orbital headforming is an efficient and precise process for assembling stamped parts. It provides strength, an attractive finished appearance, and batch-to-batch uniformity.

Werner R. Stutz

Taumel Assembly Systems designs and builds Orbital Headforming equipment.

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