Parts Feeding: The Black Art of Tooling Feeder Bowls
Ronald J. Ruhl, sales and marketing manager at Automation Devices Inc. (Fairview, PA), can predict how parts will behave in a vibratory feeder bowl by dropping a handful on a table and analyzing their positions.
"We can't be sure what a part will do, especially under vibration, without knowing its center of gravity, and we can't tell that from a drawing," says Ruhl. "One way to find that out is to drop the parts on a tabletop and look at their natural orientations."
Comparing a bowl builder to a voodoo priest is an apt metaphor. Tooling a vibratory feeder bowl is truly a black art. Indeed, two bowl makers working side-by-side on bowls for the same part may devise two completely different solutions.
"Bowl toolers are artists," says John F. Firlik, systems design engineer at Spirol International Corp. (Danielson, CT). "Every bowl tooler has his favorite techniques for welding and polishing steel and for selecting parts. Creativity plays an important role."
Experience is critical, and the competition among bowl builders for skilled craftsmen can be intense, says Larry Freiherr, sales manager at Hendricks Engineering Inc. (Indianapolis). He says it takes 4 to 5 years of on-the-job training for someone with basic welding skills to learn the bowl trade.
Start With the PartThe first step in building a bowl is to analyze the geometry of the part. The builder needs to know all the potential positions the part might take in the bowl. The builder also looks for dimensional features that can be used to orient the part.
This process doesn't have to be complicated. Ruhl can learn a lot about a part simply by holding it between the palms of his hands. "If the part fits in my hand, I know we can make a bowl large enough to feed it," he explains. "If I can't tell one end of the part from the other, I know it will be difficult to orient."
Terry Herbel, proposals manager at California Vibratory Feeders Inc. (Anaheim, CA), starts with the required orientation of the part and works backwards. "Can two or three parts be stacked together without interlocking? Is there a good surface for the parts to push against each other? If not, it's going to be difficult," he says.
Another early step is to put several parts in a generic bowl with a flat track to see how they react under vibration. The goal of this exercise is to discover the part's natural orientation and to see how many parts accept that orientation. For example, a pen cap will likely feed with the open end trailing, because more weight is at the closed end.
"If 80 percent of the parts come up in a certain position--even if it's the wrong position--you have to start with that position and look for ways to change it," Herbel says. "You don't want to fight it and get a small percentage of parts to work with from the beginning."
Once the builder has an idea of the features and behavior of the part, the tooling process can begin, says Darin Diehl, applications engineer at FTI International Inc. (Rockford, IL). Some parts of the bowl can be designed and simulated with CAD software; others by cutting them out of pattern paper and attaching them to the bowl with masking tape. Often, the bowl builder will tool the bowl in stages, installing and testing one section of track before constructing the next one.
The first section of track singulates the parts. Builders get the parts in a single line simply by narrowing the track. A cam or wiper gets them into a single layer. "We have to be able to accept or reject the parts one at a time," says Ruhl.
Subsequent sections of track are responsible for getting the part in the correct orientation. These sections have two tasks: to save parts that are slightly misaligned and to reject those that can't be saved. To maximize the feed rate, bowl builders try to save as many parts as possible.
Bowl builders have several options for orienting a part, says Freiherr. A vertical step or a ramp can turn a part 90 degrees. A sweep or blade can rotate a part or catch and hold a protruding feature. The track itself can be shaped to fit a particular part contour, or the track can be angled to fan out the parts and take advantage of the part's center of gravity.
Another option is to cut a gap into the wall or floor of the track. Correctly oriented parts will glide over the gap, but misoriented parts will fall through.
The last section of track contains the parts and feeds them to an in-line feeder or a pick-and-place device. Often, this section will contain one final screening device for misaligned parts. Then, the parts will be tightly confined by rails, or the section will be enclosed. "Once we have the part in the right orientation, we want to make sure that we don't lose it," says Ruhl.
Once the bowl builder gets the parts, it usually takes 4 to 8 weeks to deliver the bowl, says Firlik. Actual build time might be as little as 8 hours for a symmetrical, cylindrical part with a simple orientation, or as much as 300 hours for a multitrack bowl with interchangeable tooling that can feed more than part. An average bowl takes 40 hours to craft.
The Devil in the DetailsVibratory feeder bowls are usually made from 300 series stainless steel, because it's durable and easier to weld than other stainless alloys, says Freiherr. Standard bowls are made from 304 stainless, while bowls for food and medical applications are made of 316 stainless.
Various coatings can be applied to the bowl to protect fragile parts, reduce noise, improve part traction, and increase the durability of the bowl, says Randy D. Roering, president of Vibra-Flight Systems Inc. (Milwaukee). "Feeder bowls get smoothed down with use, so parts have less traction. A coating will increase the life of the bowl," he says.
The size of the bowl depends on the size of the part, but a good starting point is what bowl builders call "the rule of seven."
"Measure the largest dimension of the part, multiply that by seven, and that will give you the minimum diameter of the bowl," explains Firlik.
From there, the size of the bowl can be modified depending on the feed rate and the space limitations of the assembly system, Roering says. If the feed rate is high, the bowl will have to be enlarged to accommodate more parts and more lanes. Alternatively, the assembler can add a hopper feeder that replenishes the bowl whenever the level of parts reaches a minimum level.
The width of the track is also a function of part size. If the part is 2 inches wide, the track also has to be 2 inches wide. If the part is long and slender, the track may have to be wider.
The pitch of the track--how steeply it rises out of the bottom of the bowl--varies with the maximum height of the part. The bowl builder has to provide enough clearance so that two or three parts stacked on top of each other won't jam between the tracks. A flat part can be fed in a low-profile, low-pitch bowl. If the part must stand on end or drop into a parallel track, the bowl will have to be made taller to accommodate that.
"The key is not to get the climb rate too steep, because you have to tune the bowl very high to push the parts up that hill," says Herbel. "You also have to think about what will happen when the parts come out of the bowl and start going downhill. If they go too fast, they'll vibrate out of control and you'll lose the finesse needed for handling critical parts."
Bowl Building RequirementsTo design and build a vibratory feeder bowl, the supplier will need some help from the assembler. Part drawings, including dimensional tolerances, are useful. Assemblers should also provide an honest estimate of the desired feed rate.
"The higher the feed rate, the more costly the bowl, so we encourage our customers to request a realistic feed rate," says Firlik. "As a safety factor, some customers ask for a much higher feed rate than they really need. But, bowl toolers always exceed the requested rate anyway, so those customers are just needlessly increasing their costs."
The biggest need, of course, is the part itself. This may seem obvious, but it's a frequent trouble spot in the bowl building process. A prototype part can be used to issue a quote on a bowl or determine its "feedability," but production parts are mandatory for building the bowl, says FTI's Diehl. Similarly, bowl makers say it's not uncommon for customers to change part suppliers while the bowl is being built. If these "new" parts differ only slightly from the parts used to design the bowl, the feeder might not work as expected.
A few parts are usually enough for a bowl builder to determine if the part can be fed and to provide a quote on the project. But, to build and test the bowl, the manufacturer will need a bulk supply of the parts. The amount of parts needed varies with the part size and feed rate, but bowl manufacturers have some rules of thumb. Most builders want at least enough parts to fill the bowl with two or three layers of parts.
"We want to simulate the weight that will be in the bowl, especially with heavy parts, because as the bowl runs dry and the weight gets lighter, the bowl will vibrate harder," Herbel explains. "We want to find the tuning range that works when the bowl is full as well as when it is almost empty."
Diehl likes to get enough parts to run a bowl for 1 hour. "If the feed rate is 20 parts per minute, we would need a minimum of 1,200 parts," he says.
Bowl builders will need extra parts if the feeder will be working in conjunction with a supplementary hopper. In addition, if the parts are plastic, the bowl builder may need more samples than if the parts are metal.
"Plastic deteriorates after a period of recirculating through the bowl in our shop environment," Herbel says. "With all the welding and grinding dust in the air, the parts get dirty, and dirty plastic parts run better than new, clean parts. So, we'll keep a batch of clean parts in storage until the bowl is done, and we'll run those when we're ready for final acceptance."
Assemblers can help bowl builders in other ways besides providing parts. For example, assemblers are well-advised to contact their bowl suppliers early in the design process--as soon as they have a rough idea of how the part will be configured. "In many cases, we may be able to suggest they add a small feature or a little bulk at one end of the part to help us differentiate it," says Ruhl.
Firlik agrees. "It may cost a few pennies more per part to add a feature, but it could save thousands of dollars on the bowl and increase the feed rate," he says.
Freiherr always asks his customers about conditions on the shop floor. "We need to know what the parts will exit into, so we can anticipate problems with back-pressure," he says. "It's also good to know about sound level requirements or if there is airborne oil or water, which could affect the performance of the feeder."
Assemblers can also help by being flexible. For example, bowl builders may ask assemblers to adjust part tolerances or alter the part orientation to ensure optimal feeding. "You can't feed a pin on its point," Herbel says, "but if you can lay it down, it's simple to feed. Rather than ask for an impossible orientation, it's better to find the most reliable position to feed the part and design the assembly machine from there."
Sidebar: Coating Quiets Feeder Bowl at TRWThere's a good reason why vibratory feeder bowls are the most common technology for presenting parts to automated assembly systems: They work.
However, feeder bowls are not without disadvantages, and one of the biggest is that they are noisy. More than an annoyance, noise can be a genuine safety hazard. Noise increases fatigue and shortens attention spans. If it's loud enough, it can cause permanent hearing damage.
Feeder bowl noise was a concern recently at TRW Inc.'s air bag assembly plant in Queen Creek, AZ. The bowl was approximately 30 inches in diameter and constructed from stainless steel that ranged in thickness from 0.08 to 0.125 inch. When it was running, the bowl produced an average noise level of 101 decibels. That's equivalent to a circular saw cutting through lumber.
The bowl feeds igniter bases for the air bags. These 2-inch aluminum disks are dropped into the bowl in bulk. The bowl then vibrates the parts, moving them up a ramp in single file to be picked up by a conveyor. If the part is not oriented correctly, it falls back to the bottom of the bowl, creating another source of noise.
To solve the problem, TRW engineers used Noisekiller, a sound-reducing coating produced by Industrial Sound Dampening (Tempe, AZ). The water-based coating can be used to deaden sound produced by vibratory feeders, motor cabinets and other machines. It can also be applied to cars, trucks, buses, recreational vehicles and airplanes.
The coating reduces noise by converting vibrational energy to thermal energy. It can be sprayed or applied with a brush or roller. The coating is dry to the touch in less than 30 minutes, but it requires at least 24 hours to fully cure. Once cured, the coating is waterproof and oil- and gas-resistant.
The bowl's thickest metal is on the bottom, so four coats of the material were applied for a total thickness of 0.06 inch. The remaining areas were given three coats for a total thickness of 0.045 inch. Including prep work and a 25-minute waiting period between coats, the total time needed to apply the material was 4 hours.
With the coating, the bowl produced a noise level of 89 decibels, a reduction of 12 decibels. Because sound measurement is logarithmic rather than linear, this represents 16 times less sound than before. Moreover, the coating did not reduce the bowl's feed rate.
To further lower the noise level, TRW engineers constructed a metal enclosure for the bowl and lined the walls with sound-absorbing material. Now the bowl produces a tolerable 85 decibels, or approximately the noise level inside a typical passenger car. Hearing protection used to be required in the room with the assembly system, but now workers can enter the room without restriction.
For more information on sound-dampening coatings for feeder bowls, call Industrial Sound Dampening at 480-804-1124, visit www.noisekiller.com, or Circle 20.
Sidebar: Robot Works With Conveyor to Orient PartsResearchers at Northwestern University are developing inexpensive, conveyor-based methods to feed and align polygonal parts.
In one method, a sequence of fixed fences is placed above a conveyor belt that moves at a constant speed. The fences are analogous to the overhangs used to topple parts in bowl feeders. A part traveling on the conveyor hits the first fence, aligns to it, drifts off, hits the second fence, aligns and drifts off again. This sequence continues until the part is in the desired orientation.
Another method being studied replaces the sequence of fences with a single rotating fence--a one-joint robot, or 1JOC for "one joint over conveyor." The 1JOC alternates between pushing, or sweeping, the part upstream and letting it drift with the conveyor. By executing an open-loop series of motions designed for the specific part, the 1JOC brings it to a unique configuration.
A third technique, called 2JOC, adds a vertical axis to the 1JOC robot. The second joint enables the robot to lift and topple a part over an edge and onto a new support face.
For more information, visit http://lims.mech.nwu.edu/~lynch/research/2JOC.