Orbital riveting enhances auto-part quality.
While orbital headforming of rivets is not new, it's getting more attention lately as manufacturers look for ways to improve quality and reduce assembly costs. These are but two of many benefits found in this fastening technique.
Toronto-based Canadian ASE Ltd is one company that has moved heavily into orbital headforming. If you talk to Nick Torok, production engineer at the Fenmar Drive assembly building, he'll tell you that he's sold on orbital headforming because it gives him the quality level demanded by his customers--quality-conscious automotive companies.
Canadian ASE is a major supplier of seat-back latch assemblies used in automobiles where seat backs recline or fold. A typical assembly may contain as many as 30 to 35 components including 8 to 10 rivets, pins, and retainers that require 16 or more heading operations.
The Ford seat-back latch-assembly line provides a good example of the type of operations performed at Canadian ASE, and demonstrates how orbital headforming is used in joining the various components of the latch assembly.
The seat-back latch-assembly line is described by Nick Torok as "semiautomatic.' The first part of this operation consists of two parallel lines to assemble the top and bottom sections of the right-hand and left-hand latches. The first four stations on each leg of this line consist of indexing tables containing a dozen fixtures that pass through a number of manual loading stations, a gaging station, and a multipoint orbital headforming machine supplied by Taumel Assembly Systems, Patterson, NY.
With Jack van Beurden, production supervisor as guide, let's take a close look at the operations taking place on this line. The part being assembled is the right-hand seat-back latch assembly used on 1984 Ford automobiles.
A closer look
At the first indexing table, operators place a side plate and two rivets on one end of the fixture. A handle assembly consisting of one handle, one lever, and two retainers is built up on the other end of the fixture. At the gaging station, a total of five probes are used to ascertain that the plate, two rivets, and two retainers are present and in proper position. A separate height sensor is tripped if the assembly is too high--indicating that the fixtured parts have not been placed accurately or assembled properly.
If the parts pass the gage test, they are indexed through two additional positions and under the orbital headformer. If the parts fail the gage test, they are indexed to the next position and the operation is automatically shut down until an operator corrects the problem and pushes a reset button. Panel lights at the gage station indicate which part or parts tested "missing,' causing the shutdown.
When the parts reach the orbital headforming station, the unit descends, forming all four heads in a single cycle. The table then indexes to the unload station where the parts are removed manually. The handle assembly is placed on a belt conveyor that takes it to table four. The plate/rivet part is placed in the feed tray for station two.
At the second station, the top half of the latch assembly is completed. Starting with the plate/rivet part from station one, operators add 10 internal parts and a cover plate. The gaging station monitors seven different parts. At the riveting station, a multipoint orbital headformer is used to form heads on each of the two rivets. This locks the top half of the latch assembly together.
At the third indexing table, four rivets are attached to one side plate for the bottom half of the latch. At the same time, a quadrant piece is assembled to a pin. The gage station checks for all four rivets and the quadrant piece. Five heads are formed at the orbital headforming station--four rivets and the pin.
This station also has a date-stamping station at which a Julian date is stamped on the side plate. All assembly by Canadian ASE are date stamped. This enables the tracing of any part in the assembly should a problem appear in the future. Complete records are kept on all parts used in the assembly, including production data such as material lot and heat treatment.
At the fourth indexing station, the plate from station three is placed on a fixture. Then it is indexed to the next station where it receives a controlled pattern application of grease. After six parts are added, the assembly passes through another greasing station. Then a cover plate is placed in position. A sensor verifies the presence of all four rivets and measures the total height of the assembly. The riveting station forms the heads on all four rivets at one time with a unit identical to those on the three preceding stations.
Completing the job
As the completed bottom subassemblies leave station four, they join the stream of completed top subassemblies that have been conveyed from station two. Both pass through a demagnetizer to ensure that moving parts within the latch assembly will move freely and not be restricted by induced magnetism picked up during assembly. At this point, the units are almost complete.
The top and bottom portions now are joined. At the next station, a power spring is inserted. This spring returns the seat to its upright position from a reclining position when the handle is actuated. The completed unit now moves to a series of stations where it is tested to be sure it is 100 percent functional. If it passes all tests, it is labeled and shipped.
In addition to the quality aspects of orbital headforming, Torok points out that it is a low cost way to do the job from a capital equipment investment standpoint. He also emphasizes that product design changes do not pose a major problem with the multipoint orbital heading units because there is considerable flexibility in adapting tooling to other configurations, at minimum cost.
About the process
Orbital headforming uses a special tool to displace material with a rolling action--not unlike a baker rolling out fresh dough. The forming tool is mounted off-center in a revolving spindle and is inclined at a slight angle so that the axis of the tool intersects the true centerline of the spindle at the working end of the tool. The spindle rotates, but the tool in the orbital head or chuck is free to rotate on its bearings.
As the spindle advances, the tool comes into contact with the workpiece and forms the head as the tool exerts pressure during the downward stroke. Application of force is along a line that sweeps around the head at about 1700 rpm. This action consists of line contact between the tool and the workpiece, and is similar to the line sweep you see on a radar screen.
The tool does not revolve relative to the part, so on each turn of the spindle a given point on the tool comes into contact with the same point on the work. The length of the downward stroke, the amount of pressure applied, and the amount of time used to form the head are variables that can be adjusted to optimize the headforming process for each part based on its size, shape, metallurgical properties, and other physical parameters.
Orbital riverting has a number of characteristics that often make it more desirable than other forms of riveting and related fastening processes, including impact heading, flanging, flaring, and swaging. Here is a summary of some advantages as they apply to various aspects of the assembly process.
Noise reduction--an important property of orbital forming. Depending on the type of machine--pneumatic or hydraulic--and the load rating of the machine (up to 30,000 lb), the noise level of orbital heading machines will be in the range of 50 to 65 dBA.
Cycle time--a consideration in any manufacturing operation. Orbital heading on solid-steel rivets takes from 0.3 sec to 2.0 sec. This includes fast approach, normal heading feed, and fast retraction. The more malleable a material is, and the smaller its diameter, the shorter the cycle time.
Material suitability--a wider latitude than with most competing methods. Generally, any malleable material with a hardness up to 35 Rc can be formed. Because material displacement per tool revolution is very small and virtually friction-free, the surface intergrity of many types of coatings usually is unaffected by forming. Plated surfaces are sometimes brighter after headforming.
Heading capacity--mild steel pins, rivest, and studs between 0.010 and 1.500 can be headed with a fraction of the force required for staking or impact riveting.
Fixturing--easy to design for orbital heading. Fixtured parts can usually be left freestanding. No spinning force is transferred to parts so heading can be accomplished without clamping.
Multiple-point forming--standard heads can be supplied with in-line, random pattern, or with two, three, and four variable center-distance spindles to fit most standard machines. Heads also can be tooled to work at different levels on a workpiece with an offset plate. Machines can perform fixed pattern multiple headforming on center distances as close as 3/16. Changing a drive plate and toolholder plate converts the unit from one heading pattern to another.
Double-end heading--provides real savings in raw material cost, handling time, and assembly time. It lets you use cut rods or tubing for rivets, which is less costly than preformed solid or tubular rivets.
Nonround headforming--easy to do with orbital forming. Single-or double-D solid shafts, square, rectangular, and oval shafts can all be headed on an orbital former.
No material damage--microphotographs show the molecular structure of metals is not disrupted by orbital heading. In fact, orbital headforming usually increases the strength as the head is formed. Compression of the grain structure work-hardens the material to make a stronger connection in the case of rivets or a harder contact surface in the case of flared, flanged, swaged, and similar parts.
Tool life--exceptionally long with orbital heading. The process results in the contact surface becoming highly polished and work-hardened for extra strength. On abrasive materials, such as case-hardened rivets, some dressing is occasionally required unless tungsten-carbide tools are used. Generally, HSS tools provide long life without special attention.
Operator skill--not needed. Once the machines are set up, they can be operated by anyone, skilled or unskilled, with the same results. Setup usually requires just a matter of minutes using trial-and-error tests until the operation is as required.
The orbital headforming process and the way it has been adapted to the assembly lines at Canadian ASE may have suggested a number of ways in which orbital headforming could be applied in your assembly area. For more information on orbital headforming, circle E2.
Photo: Right-hand, seat-back latch assembly for the Ford Thunderbird.
Photo: The gaging station at assembly table three checks for the presence of four rivets and a quadrant piece. Indicator lights alert the operator to any missing components before the assembly is indexed to the headforming station.
Photo: At Canadian ASE, Ford seat-back latches are assembled on two parallel lines; one for right-hand and one for left-hand latches.
Photo: Five heads are formed in one operation at this orbital headforming station on table three.
Photo: This microphotograph of the cross section of a rivet formed on an orbital headformer shows the smooth material grain flow structure characteristic of this process.
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|Author:||Dobbins, Donald B.|
|Publication:||Tooling & Production|
|Date:||Oct 1, 1984|
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