Vibratory finishing’s mass appeal

Vibratory, or mass, finishing is frequently viewed as the “bastard” secondary operation, said Ken Raby, vice president and general manager of Walther Trowal LLC in Byron Center, Michigan.

“That’s what we are,” he said. “We are in the booths in the back, in the corner, in the dark where parts go to get deburred, but nobody wants to go back and work there.”

Raby said a lot of manufacturers of machined parts outsource mass finishing — not just for deburring but for surface finishing and polishing, cleaning, edge radiusing, descaling and pickling — because companies don’t want to deal with the wastewater that the process generates or the labor requirements. Nonetheless, when the applications are appropriate for deburring machined parts in a vibratory tub or bowl and the finishing equipment and consumables are well suited for the types of parts and volumes produced, mass finishing adds significant value.

“If (shops) are looking at a more consistent process and bringing value to the finishing operation, those are the right (shops) to invest in a finishing operation,” said Bernie Kerschbaum, CEO of Battle Creek, Michigan-based Rosler Metal Finishing USA LLC. “If they see mass finishing as a necessary evil because somebody put it in a print, then they might let a job shop handle it.”

Saving Labor

Even with a record number of people becoming unemployed because of the pandemic, manufacturers continue to struggle to attract workers.

“We are constantly trying to find ways to increase automation,” said Sales and Marketing Manager Cole Mathisen at Mass Finishing Inc. in Howard Lake, Minnesota. “That’s a big thing.”

Parts are deburred in a vibratory finishing machine. Image courtesy of Walther Trowal

Part manufacturers also are seeking to retain the workforce already in place by improving working conditions through stress reduction.

“The main focus for a lot of companies is ergonomics,” Mathisen said, “making sure workers are not having to do a lot of lifting and bending.”

Kerschbaum concurred that Rosler Metal Finishing USA is seeing robust demand for automation equipment for finishing operations. Automation is suitable for vibratory bowls and tubs, he noted, but it is more challenging than automating drag finishing equipment in which parts are fixtured.

He said another type of finishing equipment that’s relatively easy to automate is high-energy disc machines. When they are automated, an operator simply brings a bin of unfinished parts to a loading station and removes a full bin of finished, dried parts from the machine.

Even without automation, vibratory finishing machines reduce the amount of labor needed to deburr and otherwise finish parts by hand. Raby said Walther Trowal has a customer in Mexico that employed 90 workers to manually finish plastic parts before purchasing several pieces of mass finishing equipment.

“There the payback was huge,” he said. “All of our machines are more or less sold on a payback.”

Raby said return on investment for finishing machines is typically less than a year.

In addition to reducing the number of workers needed to deburr parts, mass finishing significantly reduces the scrap rate.

“It’s hard to find the lunch pail guys, so to speak, who are willing to clock in at 8 and work until 5 and make every part from part one to part 500 look identical,” Mathisen said. “It’s appealing to have some sort of hands-free process where those pieces are taken care of by equipment.”

He said a high-energy centrifugal barrel finisher is one equipment option that offers benefits, such as quiet operation.

“There’s no vibration,” Mathisen said about high-energy equipment. “It’s all centrifugal force, almost like a Ferris wheel.”

He said nonvibratory equipment is effective for deburring hardened material, sometimes removing burrs in an hour that a vibratory tub was unable to remove.

“It’s extremely high energy,” Mathisen said, “and the G-forces involved are upward of 12 G compared with 1.5 G you see in the vibratory equipment.”

He said centrifugal tumblers also are able to impart a finer surface finish than vibratory tumblers.

Media Choices

When deburring and otherwise finishing parts in a vibratory machine, end users have two basic media materials to choose from: ceramic and plastic. Kerschbaum said ceramic is more economical per running hour and available in a wider range of shapes and sizes, as well as suitable for finishing relatively hard metals, such as carbon steel and stainless steel.

On the other hand, plastic media, which is molded to a specific size and shape, has a lower density than ceramic media and generally is used for finishing aluminum and other nonferrous parts. But Raby said parts made of a sintered ferrous metal can be somewhat delicate and should be treated like nonferrous parts.

A disc machine is displayed operating with plastic media. Image courtesy of Rosler Metal Finishing USA

According to Walther Trowal, pressure deburring, or ball burnishing, also can be performed in small trough vibrators with steel media.

Kerschbaum said chipping is a concern with ceramic media, so it is more prone to cause impingement, especially in part features like blind holes.

“It’s not a problem for 95% of parts,” he said. “For those parts where there is a concern, a plastic media would be the alternative.”

Mathisen said ceramic and plastic media have abrasive material that is baked, or mixed, into a piece.

“The plastic gives a little better surface finish on nonferrous parts,” he said.

In addition to deburring with a low-density media, delicate parts require a higher media-to-part ratio than robust workpieces. Kerschbaum explained that a 3-1 ratio of media to parts is a good starting point for delicate parts while a 1-1 ratio is appropriate for raw forgings and other parts that are not so delicate. For highly delicate parts, such as brass ones with seating surfaces, the ratio might need to be increased to 10-1.