Monitoring force ensures perfect joint

Thanks to an innovative swing joint, flat-panel displays not only swivel left and right and tilt up and down, they also now pivot 90° for portrait and landscape viewing. The mechanism lets viewers dynamically adjust the LCDs to better meet ergonomic settings for small and tall viewers, while securely holding the display in place once positioned. Orbitally formed rivets and the WatchDawg Monitoring/Control System from Orbitform, Jackson, Mich., help ensure the swing joints firmly attach the flat-panel monitors to their pedestals and perform precisely as intended.

Orbital forming was key to the design of the joint, says Orbitform's Gary D. Anderson. For the joint to function properly components inside the joint must be precisely compressed. Too much compression on the wave and nylon washers immobilizes the joint while not enough leaves the mechanism unable to hold the screen steady once positioned. The trick to just the right amount of compressive force on the components, says Anderson, is forming the rivet to a force rather than to a positive or hard stop.

Forming the rivets to a force compensates better for tolerance differences in individual washer thickness than do rivets formed to a hard stop. That's because in traditional rivet-forming operations that form to a hard stop, components must have extremely tight tolerances because the rivet shaft will always be the same length once formed. Thickness variations will result in too much or too little compression of the components. And if stack-up tolerance of the components is too large, the rivet may not be able to be properly formed.

In contrast, forming to a force, says Anderson, eliminates most concerns with joint component stack-up tolerances. The rivet shaft can be longer or shorter depending on part thickness variations, but the compressive forces pushing the components together is always the same.

The flat-panel swing joints use 1.97-in.-long, 0.39-in.-diameter flathead shoulder rivets. The WatchDawg system precisely monitors and controls forces applied to the joint components. The system's load cell guides 1,300 lb of forming pressure through an eyelet peen to the 0.325-in.-diameter semitubular rivet. WatchDawg gives a visual status of the forming tool and workpiece, workpiece height and size, minimum and maximum forming forces, and variable rates of the forming force.

At the heart of the system is a load cell mounted inside the spindle assembly of the powerhead. The load cell measures forming force applied to a part. Depending on machine size, a linear-displacement transducer (LDT) or linear encoder integrated into the powerhead measures the unit's stroke. These rapidly responding sensors measure form depth or position at peak force. They are said to be easily calibrated, which helps simplify the types of fixtures needed for interchangeability and smooth production-line integration.

The WatchDawg System monitors rivet/form collapse and rivet length. It also compares rivet hardness to set limits, monitors how much of the rivet has been formed, and checks that the loadcell trigger point is met. The system uses complex feedback-gathering procedures and process/control monitoring to precisely measure rivet dimensions before and after forming. Parts that fall outside of preset limits are rejected. Other data points can also be saved and displayed during the machine cycle. These can then be sent to other ports or directly to a computer or network. Programmable controls in the system also compensate for part variation.

Orbitform's B-310 orbital riveting machine is used to assemble the swing joint used in the Weight Adjustable Height Adjustable Stands (WAHAS) from Constant Force Technology LLC, Blaine, Minn. A forming peen attaches to the unit's rotating spindle, inclined at a slight angle and focused toward the spindle centerline. As the peen gradually contacts the rivet, steel gently rolls out and over the plate forming to a height of approximately 0.150 in. For the swing joint, spindle speed is 1,140 rpm with a cycle time of 2.3 sec.

The WAHAS are designed so that viewers can lift and adjust monitor orientation to reduce or eliminate musculoskeletal stress disorders. The stands adjust easily with only one hand needing only 5.1 lb of lifting force — the lower limit of the fifth percentile female, according to ergonomic standards.

CFT's stands have a maximum travel lift of 5.1 in. which meets the eye height variance for the 5% female to the 95% male viewer. The monitor tilt mechanism allows for easy positioning at the proper focal distance. The design also frees up desk space by lifting the monitor off the desk. Repositioning is possible either standing or sitting. An energy pivot balances the weight of the monitor and assists the user during the tilt.

Orbital-forming advantages

Engineers tend to rely on traditional fastening methods to join parts, but they can often glean better results and lower costs with orbital forming. Sometimes referred to as spin or radial riveting, orbital forming most commonly serves to fasten an assembly by heading, crowning, swaging, flaring, or forming a column or projection of material. The process can replace fasteners such as screws, snap rings, threaded caps, and rivets.

The technique is similar to impact and compression forming where the tool applies a compressive axial load to plastically deform the material. The difference is that orbital-forming machines rotate the tool or spindle at a fixed angle — typically 3 to 6 — and apply both axial and radial forces to progressively move malleable material into a desired, predetermined shape. In contrast to impact and compression forming, where the process completes in a single pass, orbital forming requires several tool revolutions and typically takes 1.5 to 3 sec. The spindle stroke precision is <0.001 in. This gives designers the option for solid joints with selectable torque resistance or moving swing joints such as those in scissors and handcuffs.

Compared to other rivet-forming processes, the small contact area that the orbital tool makes with the material being formed reduces axial compressive loads by as much as 80%, which has several advantages:

• Reducing compressive loads lowers internal stresses in both the fastener and mated parts, helping to boost the assembly's fatigue life.
• Orbital forming produces a smooth surface finish and, in some applications, eliminates cracks caused by impact riveting.
• Cold-head forming without damage to the fastener shank is possible because the axial loads required by orbital forming do not exceed the fastener's column strength.
• Because less axial load is needed during forming, smaller machines can be used. This reduces equipment tonnage, floor space, and cost typically associated with fastening large parts.
• Fixtures for holding the assemblies don't have to be as robust as those for impact or compression forming thanks to the lower forming forces needed by orbital forming. Tools also last longer.
• Orbital forming is quieter than other cold-forming processes including peening and impact forming.