Applications Determine Workholding Solutions
Every turning application has a number of factors that should be considered when making the decision between a collet or a three-jaw chuck.
A man down the street owns two vehicles. One is a heavy-duty pickup that he uses for hauling equipment or other large items. This truck gets poor gas mileage. The other vehicle is a four-door economy car that he drives on an everyday basis. Usually, the well-defined purpose of each vehicle provides the man with a clear choice of what to drive for what occasion. Sometimes, though, the lines are vague. If he has to haul something a long distance, is it worth the savings in gas mileage to find a way to stuff it in the back of the car and tie the trunk down? If he’s only running to the store, but his truck is blocking his car in the driveway, should he take the truck?
In turning operations, workholding options often present the same type of dilemma. While the three-jaw chuck is the obvious choice for certain larger parts, and collet-nose chucks are the clear choice for most low-volume, high-tolerance work, the spectrum of turning jobs is far too broad to be able to chart the best workholding choice for each application. Most shops find that the majority of their work calls for one or the other, but rarely can a company count on either the three-jaw chuck or the collet nose to cover its whole lineup. Each situation should be examined to determine the most appropriate workholding option.
Firstar Precision Corp. (Cleveland, Ohio) is one such shop that faces the continuing challenge of workholding selection for their jobs. Started in March of 2000 with six employees, the company has grown to adequately fill its 12,000-square-foot facility with 34 employees producing precision, low-volume, high-tolerance parts. Dave Tenny, company vice president, says, “Our company is a CNC contract job shop specializing in quick turnaround. We are more oriented towards delivery and part quality rather than cost. We try to focus on the types of work that cannot be transferred to China because of volume and difficulty.”
Firstar specializes in parts for air or pneumatic motors for several different industries, including automotive, food and aerospace. The company is also involved in the hydraulics industry, producing exterior manifolds and the internal components—multi-featured items requiring turning and milling operations tied together. According to President Jack Horstman, “The high tolerances we’re able to maintain are what keep us in business.”
Along with three vertical and three horizontal mills, the company has implemented eight lathes, seven of which use collet closures. As much as 90 percent of the work is done with collet systems. While the company definitely makes use of chucks, the tolerances they require more often call for the collets. “It’s more of an industry thing,” Mr. Tenny says. “Some businesses may not have any collet systems at all. Where they do a lot of chuck work, the tolerances typically aren’t as critical. It’s bigger work.” A chuck will hold larger parts a little more solidly because the envelope on the collets only goes so far. The shorter actuation stroke of the collet allows it to open and close more quickly, but also limits the range of workpiece sizes it can accommodate.
The majority of the chuck work that Firstar performs is on larger parts, three or more inches in diameter, and on parts where tolerances are not as critical. When parts get to a certain size, step collets are required and the benefits of the collet start to diminish. The company then will examine the given application and determine which route to take, but according to Jack West, plant manager, “It really boils down to what you’re trying to achieve as to what you’re going to use between chucks and collets. We’re running one current job in a chuck, but we have the ability to put the collet nose on and run it there as well. We actually switch back and forth on this job without issue.”
However, most of the company’s work is small. Mr. West says, “All of our parts are pretty much one-hand parts. We don’t make parts that you can’t lift with only one hand.” Because of the size of the parts the company runs, Mr. West feels collets are the best workholding option the majority of the time. The main benefits he sees in collets include less runout, better concentricity, more versatility and the ease of changing from one collet to the next. The difference in mass between collets and chucks also comes into play.
Collet systems weigh considerably less than chucks. Bringing a chuck up to speed on a machine and slowing it down again takes longer with more mass, and, therefore, affects cycle time. “As long as the collet can handle the part, we tend to go that route whenever possible,” Mr. West says. He also points out the significance of centrifugal force as rpm increases. “When you start getting a chuck going at 4,000, 5,000 or 6,000 rpm, you have a lot of mass going, and centrifugal force starts working against you, pulling the jaws apart. The collet does not have that issue because the collet is contained inside the closure. No matter how fast the collet spins, centrifugal force does not affect it.”
Finally, Mr. West views the concentricity provided by the collets as a big part of what makes his company successful. “We hold concentricity all day long of a thousandth or less.” The company often will initially run parts on the bar fed machine, and then move them over to a small collet machine for secondary ops. It is critical for these parts that the runout from one side to the other is minimal. The runouts are much tighter with the collets than what they can maintain with the chuck. The chuck is not as accurate as the collet in concentricity.
The right choice for how to hold a part is highly dependent on many factors. As Firstar demonstrates, many operations can be handled by either a chuck or collet system, but most of the time, one or the other is the better option. Shops need to evaluate the requirements of the parts being produced and determine what will suit its needs best.
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