The secret to success: elevate part, allow access, hold rigidly and engage tool.
The rules of thumb for shops experienced in five-axis machining aren’t any different from those for first- time users of this sophisticated machining process. The difference is that the experienced shop has already traveled the learning curve for five-axis machining.
“The first thing the first-time buyer of five-axis workholding has to do is change his mindset from conventional clamping to what is required in five-axis machining,” said Eric Nekich, operations manager, Lang Technovation Inc. (Hartland, WI). “A lot of people coming from four-axis rotary work are already familiar with the challenge of getting access to machine all five sides without sacrificing holding power or rigidity,” he said.
What may be new to them is the emphasis on learning the subtleties of modular build-up of five-axis workholding. They include ways to elevate the workpiece off the table, provide access to all five sides without interference from the spindle, housing or workholding devices. Finally, there has to be a plan for removal of that last bit of material or the workholding device that provided the stability to hold the part.
Fortunately, there’s plenty of help in getting up to speed with five-axis. The machining process itself isn’t new. What is new is the availability of the latest technology, technical and product support and process information from machine tool builders and distributors as well as workholding, cutting tool and CAD/CAM software suppliers. Nothing has to be left to chance and users can download CAD models that can be used to build up the necessary workholding setups—often modular for flexibility—and prove them out through simulation. Avoided are wasted time and possible catastrophic machine crashes, as spindles and workpieces may all be in motion at the same time as in simultaneous five-axis machining.
“Efficient five-axis workholding solutions locate and stabilize the workpieces without obstructing access to the top and sides of the part, and they also allow a clamped workpiece to be flipped into new orientations for subsequent machining operations without unclamping from the workholding,” explained John Zaya, product manager for workholding, BIG Kaiser Precision Tooling Inc. (Hoffman Estates, IL).
BIG Kaiser’s multi-axis workholding products provide flexibility and functionality for five-axis machining and five-sided machining via multiple part transfers. To get started, the first-time user of five-axis machining needs to communicate basic machining requirements.
According to Zaya, “There are two basic things we look at for five-axis machining. Is the machine a trunnion-style machine that tilts and rotates the platter or a gantry-style machine where the workpiece is stationary and the tool and spindle move completely around the workpiece?”
The two scenarios pose similar challenges with some differences. “With the trunnion-style machine the part has to rotate and move at the same time to allow for full simultaneous machining. You have to be more cognizant of clearance problems with the machine’s spindle housing and the machine’s table, especially at full tilt when the table is perpendicular,” Zaya pointed out.
“There is a natural limit of how close you can get down to the table. The options are to make the tools really long or elevate the workpiece off the table,” he said. “Since the general rule of thumb is that it’s better to use a short tool to minimize chatter and maximize tool life and accuracy, shops lean toward elevating the workpiece off the table to get the spindle closer to the workpiece.”
For gantry-style machines where the workpiece is stationary and the tool moves, many of the same concepts apply. Zaya noted: “Here, we’re dealing with workpieces where the mass of the workpiece may be considerably larger for aerospace structural workpieces or molds for the automotive industry, for example. We spread the system out, based on the footprint of the machine. The spindle can only come down so close to the table so the workpiece is elevated off the table to allow the spindle access.”
Holding the workpiece can be accomplished in a number of ways. “If we just take a standard vise and elevate it off the table, then the problem becomes the vise generating an offset for the center of the round workpiece. Although most modern five-axis machines can handle dynamic offsets, you create a clearance problem with the front end of the vise or the spindle end of the vise with a huge mass of material that is not really helping you in any way but can possibly cause interference or crashes with the spindle and housing. That’s where a lot of companies will go to a self-centering vise where both jaws move uniformly to minimize the amount of dynamic offset they have to use.”
Getting a Grip
In addition to vises, holding parts is achieved in a number of ways, including modular workholding systems with dove-tails. The excess material has serrations to grab onto that will be machined off in subsequent operations. In some applications where the part can’t be grabbed from the outside of the workpiece, you have to go completely underneath the part so that you can access the entire outside surface.
Carr Lane Manufacturing Co. (St. Louis, MO) is known for the number, sizes and variety of the standard components it offers, said Aaron Hull, senior manufacturing engineer.
“When a shop is looking for a solution to five-axis workholding, I like to think of an out-of-the-box solution using our standard components for versatility and replacement. The challenge in five-axis workholding is generally being able to hold the part and access all the sides you need to machine. Complicating factors are that you want to hold it nice and high so that the tool has access and there’s no interference from large, bulky clamps along the sides of the workpiece. Also, depending on how small or large or delicate the part, you don’t want to make parts too oversized for machining off,” Hull explained.
Holding the part concentrically allows knowing where the center is when a new piece of stock is loaded on the table so that variation in material size won’t affect it. Carr Lane offers several solutions.
“One is a concentric five-axis vise that starts at about 40 mm up to larger size vises up to 5″ (127 mm),” said Hull. “Another way is a concentric fixture clamp that centers the part. It’s similar to a vise but it’s a powered version. We also have tiny vises that are very low-profile clamps that don’t hold the parts concentrically, rather they push the part against stops and then hold them with downward force as well as horizontal force.”
Carr Lane’s CL5 five-axis quick-change workholding fixtures include a wide range of solutions for all sizes of rotary tables. They can be classified in three general areas, making use of either a fixture plate mounted on a rotary table, a Quintus quick-change riser mounted on a rotary table, or a subplate mounted on a rotary table. Models of the part and fixturing are readily downloaded for simulation.
These days, much of the talk about workholding focuses on lights-out machining. “The buzz term right now is process reliability in untended operations,” said Eric Nekich of Lang Technovation. “How well do you sleep at night knowing that a robot is loading your machine tools with nobody around?”
The answer to that question is, “if you google five-axis vise, you’ll find that everybody manufactures a serrated tooth, self-centering vise to bite into the material. We do it differently. We use Form Closure Technology for our Makro-Grip five-axis vises to manufacture serrations in such a way that we’re clamping on less than 3 mm of material. That leaves a high proportion of the part open for machining without interference from the tool and spindle,” he said.
“Once we hit a certain hardness with stainless, Inconel or titanium, that’s where the Lang system takes off. Typically, hard material is held with dovetail fixtures cutting a contour like a flat and 45o angle for the whole workpiece to be held on dovetail. As a result, the whole workpiece sits above the dovetail. The problem with dovetail is that it requires a machine tool and operator to cut it into the workpiece, and the amount of workpiece prep can get out of hand.”
Lang Technovation’s Makro-Grip stamping unit for its Form Closure Technology basically allows users to avoid having to dovetail. The 20-ton hydraulic press prestamps the contour of the tooth serration into the workpiece in three to five seconds. When the part is loaded into the vise, the vise’s tooth serrations sit on the prestamped form.
“Normal self-centering five-axis vises with tooth serrations have to do double duty,” Nekich pointed out. “They have to cut to penetrate the workpiece and then hold. With harder material, as they cut and hold and cut and hold, wear occurs. With the Lang Makro-Grip stamping system, all the vise has to do is hold and the teeth on the Lang vise don’t wear out. They just have to hold and sit in the prestamped groove.”
This basically covers the first operation, prepping the raw material, loading it into the fixture and machining the five sides. For first piece or last piece inspection, Lang Technovation’s zero point system allows moving the whole vise out and loading it in a CMM or other inspection device.
“All of our vises are integrated with pull studs that are located and fixtured on the machine table without having to take offsets or turn capscrews,” said Nekich. He added that first-time five-axis shops should “look for modular solutions and don’t get painted into a corner.”
Tim Wachs, president, Hainbuch America Corp. (Germantown, WI) advises putting workholding for
five-axis machining high on your list of priorities if you’re considering getting into five-axis machining.
Hainbuch offers workholding solutions that address the most common challenges posed by five-axis machining.
“For example, you can’t just put a standard vise on a five-axis machining center,” said Wachs. “There are interference problems left and right. And because the part sits up high, you have to build up your fixture trying to put something in the jaws, which may be very small.”
In addition, there are clearance issues all around when the table is moving in five axes. “Tool length can be a problem when you are trying to reach the workpiece. Five-axis machining requires holding the part so that you can reach all the sides and still be able to hold it rigidly.”
That’s where specialized workholding comes into play. Hainbuch offers several styles for clamping depending on holding an ID, including expanding mandrels. The Mando G211 adapter, for example, is designed to provide a lot of clearance in gear cutting and also works for five-axis machining. “As a result, when you’re turning the table in different directions you have plenty of room for the tool to get down to the part and you can grab it very rigidly to eliminate vibrations. Typically, five-axis machining is low volume with a need for quick changeover. Not having to remove a stationary clamp on the table that indicated in at zero makes it very easy to change over with a clamping head or a mandrel,” said Wachs.
“What we need to know to help the first-time five-axis shop is all the features that are being machined. Is there a qualified surface to clamp on prior to machining?” he continued. “What machine model is being used? What is the best place to clamp on the part? A lot of the parts I see that are five-axis require clamping on a qualified surface. You may be machining out of a solid block and you have to grab that somewhere. Most of the time there is some type of prequalified surface that you are able to either ID clamp on or OD with qualified clamp for rigidity.”
Quick Change Pallet
At IMTS 2018 in Chicago, Jergens Inc. (Cleveland, OH) introduced a new Kwik-Lok pallet changer with one screw actuation and tight tolerances that is similar to its Drop&Lock pallet changer and pull studs. According to Tim Easton, national sales manager, this is the latest development in Jergens’ Fixture-Pro multi-axis and five-axis modular workholding that includes all the needed components, from subplate to top tool clamping.
The Drop&Lock and Kwik-Lok pallet changer systems, available in round or square patterns, serve the same function as a fixture plate by not requiring the top tooling to be locked down with four set screws. Instead, it is installed with pull studs dropping into the bottom of the fixture or vise to literally drop into the pallet changer.
“The challenge in five-axis workholding is to get the part up off the table, locate it accurately and hold it securely so that parts are manufactured in the least number of setups possible. What we need to know from a first-time five-axis machining operation is the machine model table size and all the pertinent information about the part, including material, the envelope of the part, what types of cuts are going to be made, how aggressive the machining is likely to be and how much force will be needed to hold the part. We have to really understand whether a vise with a dovetail is the best way to go or is there a void in the part where we can put a ZPS [zero point mounting system] stud in.”
Jergens’ modular ZPS zero point mounting system consists of a module and a stud with a pin that allows it to be used at the bottom of a vise, bottom of the part or the bottom or back of a fixture plate. The company says the ZPS system cuts setup time up to 90% by combining fixuring, positioning and clamping in a single operation. Available with either pneumatic or hydraulic release, the positive locking modules allow operators to quickly change out large and small machine fixtures with accuracy and minimal effort.
Easton pointed out that with the ZPS system, the operator not only can easily change out the part or the workholding, he can also take it right to the CMM and check for first article inspection without unclamping. Another advantage is the ability to do lights out automation with multi-pallet stations.
Jergens also has Ball Lock Quick Change subplates for large and small table machines; for example, the Haas 50 vertical machining center, as well as other popular machines with 40″ x 20″ tables that can be equipped with trunnions or indexers. Easton’s advice: start thinking about workholding early in the five-axis machining process.