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White Papers
Through-Hole Inserters Use Rugged Slides for Maintenance-Free Operation
By Jim Morey, Engineering Manager, Cambridge Automatic, Natick, MA
Small but rugged linear slides help make a new line of through-hole inserters virtually maintenance-free. The inserters are used to populate printed circuit boards or connectors with a range of ping, terminals and sockets carried on an integral continuous strip. The key design problem in developing these inserters was to find a linear slide to carry the cutting head -- a slide that is small enough to meet tight space restrictions but rugged enough to withstand substantial cutting forces. The machine's developer was able to meet these constraints, and in the process producing a system that has earned tongue-in-check complaints from distributors because of near-zero spare parts sales.
The Model SS425 strip-fed pin-and-terminal inserter from Cambridge Automatic, Natick, Massachusetts, is offered in automatic, semi-automatic and manual configurations. Special features include vacuum pin positioning to hold the part securely in the die after removal from its strip, and sensors to detect missing or bent parts and validate proper insertion depths. Parts can be inserted one at a time at rates up to 120 per minute or in multiples at even higher rates. Custom engineered for single or multiple parts insertion, the Model SS425 standard model accommodates standard industry quick disconnect terminals up to 0.880-in. wide x 1.25-in. tall with 0.001-in. cutoff accuracy and 0.002-in. insertion repeatability in computer-controlled applications. The SS425 can also handle a wide variety of pins presented on carrier or bandolier.
Operating Sequence
Parts are fed from an interleaved reel up to 24-in. in diameter. Adjustable tension is obtained through a brake on the reel hub. the infeed engages a standard 0.040-in. sprocket hole or the part itself. The strip is pulled through a nonabrasive tension device and is fed into the insertion head through separate guide plates to maintain flatness of the part and accurate pin location. Pneumatically-driven insertion tooling powers CAM-driven cutting blades that cut off correctly indexed terminals and position them in the top die assembly where they are captured on three sides and held in by vacuum if required. As the terminals are cut from the integral carrier strip, the waste strip is removed by vacuum and terminal is positioned, ready for insertion. Once the cutoff die completes its stroke, the two-piece top die assembly descends, the fourth side of the die is closed and the terminal is inserted into the waiting substrate.
Cambridge Automatic has always positioned itself to supply the most rugged and durable equipment on the market. The company was determined to maintain that position with the SS425 inserter. But the company faced a serious obstacle; the original concept design positioned the cutting head on a vertical slide that guided it as it moved up and down to sever the continuous strip of components. This meant that the entire 400-pound cutting force would have to be absorbed by the slide. In and of itself this was not difficult, but when combined with the tight envelope restrictions of the slide, it presented the major challenge of the design.
Cambridge engineers evaluated several different slides but found they were not able to absorb the forces on a continuous basis. then they heard about Del-Tron's new series of cross roller slides which, when compared to ball slide products of equal size, offer higher load-carrying capacity and, when operating at high cycle rates or with shock and overhanging loads, improved performance.
Crossed roller slides physically resemble ball slides except for the bearing design.
Crossed roller slides physically resemble ball slides except for the bearing design. Specifically, each slide is comprised of an aluminum carriage straddling an aluminum base. Using a bearing system containing cylindrical steel rollers, the carriage glides, almost friction-free, over the base. The rollers, alternately crisscrossed with each other, move between a set of four, partially flat, parallel, smooth rods on each side of the base. The rollers share a larger contact surface with the rods as compared to the point contact of steel balls. This bearing design allows crossed roller slides to carry larger loads and absorb greater load impacts than equivalent size ball slides. The carriage and base are made of aluminum, while hardened steel rods and rollers and stainless steel end caps are used.
Two years in the Field
A crossed roller slide that fits a 2.500-in. deep by 5.50-in. wide by 8.00-in. high envelope was able to handle this application without difficulty. The head travels at 960-in. per minute and a speed of 120 cycles per minute. Under these conditions, the slides have provided perfect reliability in two years of field applications. Not one single slide has failed in the field and warranty costs for the product have been zero. In addition, the slides don't require any maintenance at all.
Del-Tron crossed roller slides met another important requirement of this application--very high accuracy. They provide straight-line accuracy of 0.001-in. per-in. of travel when measuring the line of travel to a master straightedge, using a gauge or indicator mounded on the slide. Positional repeatability of the slides is 0.0001-in. The coefficient of friction of these devices is only 0.003. At rated load capacity and moderate speeds, expected life is 10 million inches of travel. The expected life at one half the rated load is 100 million inches.
Del-Tron offers over 20 models of crossed roller slides. Load capacities range from 30 lb to 704 lb, with travel from 0.5-in. to 6-in. Installation into a production system is simple. Counterbored holes in the base permit quick attachment to the assembly. Components attach to the carriage in the existing threaded mounting holes. Crossed roller slide rated load capacities may be a mass load on a horizontal slide, or a force load normal to the mounting surface in any position. The rated load must be centered and distributed over the slide, and the base must be supported on a flat mounting surface, while though crossed roller slides cost more than the equivalent size ball slides, their overall performance, especially in applications where heavy loads must be moved in compact assemblies, often justifies selection.
Inserter Reliability
The slides have contributed to the reputation that Cambridge Automatic's inserter has earned for exceptionally high reliability. Customers report that they can run between 12 and 20 million cycles without performing any maintenance at all to the machines. These machines have helped to further burnish Cambridge Automatic's reputation as a high-reliability supplier in the inserter market.
For more information, contact: Del-Tron Precision, Inc. 5 Trowbridge Drive, Bethel, CT 06801.
phone 203-778-2727
fax 203-778-2721
Web: http://www.deltron.com
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