Servos Revolutionize High-Speed Assembly
The origin of an innovative, new high-speed assembly system from Canadian systems integrator Transformix Engineering Inc. came not from a project to assemble small parts in high volumes, but rather a need to plant sugarcane.
Since Transformix opened its doors in 1995, the company has designed special machines for a wide range of industries. It has built machines for reworking nuclear fuel rods and machines for cleaning newly pilgered metal tubes. The company has designed machines to assemble small products, such as IV sets and aerosol valves, and large products, such as train axles.
So when the company was asked to design a high-speed machine to produce sugarcane billets for planting, it was just another day at the office.
Although sugarcane does produce seeds, it’s typically grown by planting short lengths, called billets, of the cane itself. Not any part of the stalk will do, however. Each billet must contain at least one bud from which a fresh stalk can sprout. Since buds appear at random intervals along a length of cane, mechanizing the process of producing billets was no easy task. The process had to be fast and accurate. It was also important to minimize damage to the billets.
Transformix’s engineers solved the problem with a vision system and a high-speed servo, says Larry Allingham, vice president of operations for Transformix.
Razor-sharp blades are mounted to the shaft of the motor. The cane is fed into the machine lengthwise. The stalks pass beneath a camera, and the vision system locates areas with buds. It then signals the motor to speed up or slow down to cut the billets at just the right time. The billets are deposited into one bin, while the waste cane is deposited into another. The entire process happens in the blink of an eye.
Continuous Motion Assembly
Transformix’s experience with the cane-cutting machine inspired a new approach to the design of continuous motion assembly systems. To understand just how new that approach is, it’s useful to look back on where the technology has been.
Depending on the system and the product, rotary indexing dials or pallet-transfer systems can produce 15 to 250 assemblies per minute. In contrast, continuous motion assembly systems can routinely produce 400 to 800 parts per minute, and some machines can produce more than 1,800 parts per minute.
With intermittent motion systems, the assemblies must pause periodically so that some action can be performed. The assembly tools, such as screwdrivers and pick-and-place units, are stationary. With continuous motion systems, the assemblies never stop moving. Assembly operations are carried out as the parts are moving, and the tools travel with the work in process.
The heart of a continuous motion assembly system is a cylindrically shaped rotating turret. The turret consists of a round dial with a series of tools, called pins, arrayed around its circumference. The size of the turret and the number of pins around it varies.
Parts enter the system single file on tracks from feeders or upstream operations. A star wheel, moving synchronously with the turret, strips off parts one after another and transfers them into nests spaced evenly around the dial. As the turret rotates, each pin is actuated in turn until the process is complete and the assembly is unloaded. Cams govern the motion of the pins.
Continuous motion systems are used to assemble small, high-volume products, such as aerosol valves, bottle caps, syringes, electrical connectors, tube assemblies, lipstick tubes, ballpoint pens, irrigation components, spray valves for household cleaners, and foam generators for beer cans. The parts assembled on continuous motion systems are typically round and symmetrical, but they don’t have to be.
That said, continuous motion assembly systems work best with parts that can be snapped or pushed together quickly and easily. If springs are part of the assembly, they should have closed coils at the ends, so they’re easier to feed. If one part will be inserted into another, a chamfer in the base part will help compensate for any misalignment during assembly.
Continuous motion systems can accommodate most assembly processes, as long as they can be done quickly and from a vertical direction. These include crimping, dispensing, curing, ink-jet printing, labeling, laser marking, riveting, screwdriving, slitting, ultrasonic welding, vision inspection, leak and flow testing, and functional testing.
When Transformix was asked to design a standard, cam-driven continuous motion assembly system for a consumer product, the company’s engineers questioned some basic assumptions about the technology:
Why do continuous motion assembly systems need to have so much tooling?
Why can’t continuous motion dials turn faster than 15 to 30 rpm?
Are cams really necessary?
Could the tooling be programmable?
Their answers to these questions led to CNC Assembly, a new automated assembly platform that combines the speed of continuous motion assembly systems with the flexibility of robots.
“Manufacturing plants demand greater output per unit of time and space,” says Ken Nicholson, director of technology at Transformix. “These objectives have been tough to achieve with assembly machines, since they are typically customized to specific products. This means companies usually have relatively low speed, specialized machines spread across their factories, consuming a lot of expensive floor space.
“If they do install high-speed machines, they are invariably for high-volume, long-running products, since high rates have always been incompatible with flexibility and adaptability. Our goal in developing CNC Assembly was to give manufacturers a standard machine that could run at high rates—200 to 600 parts per minute—but that also had the quick-change versatility to run a range of different products.”
To achieve this combination of speed and flexibility, the CNC Assembly platform relies on high-speed servomotors, rather than cams, to control and coordinate individual assembly operations.
“We use continuous motion techniques to deliver parts to the assembly tooling at high rates and then employ robotic, servo-controlled motions to carry out the operations,” explains Nicholson.
The CNC Assembly platform consists of a family of standard, plug-and-play modules, each designed to accommodate a set of common assembly tasks.
“We call it CNC Assembly because it is directly analogous to CNC machining,” says Nicholson. “Just like CNC machining centers, CNC Assembly machines are general-purpose tools that can be configured and programmed to run a wide range of products. And just like CNC mills are different from CNC lathes, different CNC Assembly machines are intended for different functions.”
The base of a CNC Assembly system is the Rotating Tooling Platform (RTP), the equivalent of the turret in a conventional continuous motion system. In typical applications, the RTP is equipped with one to four tooling stations, and it rotates between 75 and 200 rpm. Each tooling station is actuated by one or more linear motors rather than cams.
Transformix has designed a series of servo-controlled Rapid Speed Matching (RSM) devices to deliver parts to and from the RTP.
For example, the RSM feed screw is a subturn feed screw that handles only one part at a time. (In contrast to conventional, variable-pitch feed screws that contain multiple parts within them as they turn.) With each turn, it picks off a part from a linear track. The screw’s servo is electronically cammed to the servo turning the RTP. As a nest or pocket on the RTP approaches, the screw’s servo accelerates so the part can match the speed and position of the pocket—just like the servo that cut billets in the cane processing machine.
This design enables the RTP to capture parts at a much faster rate than a conventional continuous motion machine—and with 80 percent to 95 percent less tooling, says Allingham.
Other devices in the RSM line include a rotating disk and “linear runners.” The disk has one or more pockets machined into its circumference to carry parts. The linear runner is a large, servo-driven X-Y positioning stage. A rotary actuator equipped with a gripper can be mounted to the platform to reach into an RTP to deliver or retrieve a part. Like the feed screw, the motion of the disk and runner are electronically cammed to match the motion of other parts of the system.
The CNC Assembly system is already being used by several manufacturers. For example, Transformix recently designed a system to assemble a small product made from bent wire. The system feeds, cuts, bends and wraps the wire. It then solders the wire to another part. The system operates at a rate of 200 parts per minute with just one tooling set.
Although it was designed for high-speed operation, CNC Assembly can be economically deployed for jobs requiring rates of 60 parts per minute or less—particularly when flexibility is required, says Allingham.
For more information about CNC Assembly, call 800-639-5644 or visit www.transformix.com.