Get a Grip on Five

Jun 11, 2023

There's more than one way to hold a part: all around, underneath or on top

The pros easily outweigh the cons when it comes to choosing five-axis machining of workpieces. Some parts are intricate and small, while others are large and complex—and most are difficult to hold. Setups have to fit in a machine envelope with sufficient clearances for spindle, workpiece, and workholding.

Five-axis machining opens up new business opportunities in machining parts for aerospace/defense, automotive, medical, energy, and general engineering. But it's in operations and machining productivity where five-axis machining really shines. The technology improves processes and throughput—in an addition by subtraction manner. Adding one versatile machine tool can eliminate less capable machines, reduce the number of setups, and increase accuracy by minimizing the sources of error, human and otherwise.

So, is five-axis for everyone? Is it right for your shop? Maybe. But it takes a systematic analysis of the hows and the whys of workholding to make an informed decision. Here's how some leading suppliers suggest shops should proceed.

By now most everyone has heard about five-axis machining, but you probably aren't familiar with all the issues and complications that come with this highly productive machining technology. That knowledge only comes through hands-on experience and the conditions inherent with the process, according to John Zaya, technical specialist, BIG Daishowa Inc., Hoffman Estates, Ill.

Zaya's steps to the successful design and implementation of five-axis workholding include:

"First, I try to understand the size and mass of the parts and fixtures that are being handled," Zaya explained. "The mass and momentum of a moving part can be a major variable affecting the stability of the system.

"The next considerations are the type of machine, table size and different mounting requirements for the workholding. There is a big difference between a gantry-type machine where the part is stationary on the floor or table and the whole machine moves around the workpiece," Zaya continued. "That is vastly different compared to a trunnion machine with either a vertical or horizontal spindle where the trunnion rotates and pivots the entire table, [where the] fixture and part move in highly dynamic motion. The momentum and loads that result have to be absorbed by both the machine as well as the workholding to keep the part from moving around."

The types of cutting operations being performed also need to be taken into account, Zaya said. This includes drilling size, milling (light finishing, heavy roughing, or both), threading (tapping or thread milling), and boring (heavy roughing, light finishing, or both). The cutting loads, the direction the loads are being applied, and the position at which they are applied all factor into the proper sizing of the workholding system, he added.

"With most horizontal and vertical machining centers, the loads are usually in line with the cutting-tool axis or are able to be absorbed by the machine. However, when it comes to five-axis machines or workholding there is a lot more flexibility as to the direction at which cutting operations occur," Zaya noted. For example, he said, drilling and tapping holes in a simple block involve axial cutting loads that are all in line with the spindle against a flat face that is supported by a table.

"In a vertical machine there are no issues. In a horizontal machine even if the part is overhanging on the side of the tombstone, you can generally resist cutting moves that an axial load like that can generate," Zaya explained. "With a five-axis machine that same axial load could be applied at 30°, 45°, or at any angle."

Conventional clamping, such as vises or fixtures, all have one basic issue, clearance, that needs to be addressed for five-axis machining.

"They usually create obstructions of some kind that prevent the cutting tool from reaching a feature or a face of the part," Zaya said. "Five-axis vises usually minimize this with a low-profile clamping edge of 3 mm and compact footprints. Fixtures can be designed to allow full access to five out of six faces of a prismatic part, but that means you need a fixture for each part or family of parts. The other major clearance that needs to be accounted for is the housing of the spindle to the table. With tables capable of tilting up to 90o or more this creates an interference area that cannot be overlooked."

BIG Daishowa's UNILOCK 5-Axis Zero Point system is designed to provide solutions to these five-axis process requirements. The primary consideration the product resolves is the clearance of the machine spindle to table, according to Zaya, who noted it can be sized to match the part size and weight.

Three knob sizes (System25, System40 and System68), each of which can handle increasing amounts of weight load, momentum, and cutting loads are offered. For example, the UNILOCK 5-Axis System40 is capable of withstanding side loads of 13-28 kN (1,325-2,855 kg) per stack. Adding multiple stacks under the part just adds more side load capacity.

The clamping system also applies 11 kN (1,122 kg) of retention force per clamping knob and combined with large, high-quality fasteners provides a very rigid holding of the part. The access to all faces comes via the use of reductions that transition from 80 mm down to 50 mm, which allows the machine to have clearance to cut the full face of the part and, if needed, cut into the reduction as they are offered in both the hardened and soft state.

The system is modular and can be stacked in various combinations to accommodate the part, fixture, and machine as needed. The bases come in three types with different base profiles and three heights: 75, 100 and 125 mm. The extensions are also offered in sizes of 75, 100 and 125 mm, while reductions are available in 25-, 50- and 75-mm heights. CAD models are available.

"With any workholding system the repeatability of modular components is critical," Zaya emphasized. "Workpiece probing is still an option that many users forego, stating that it's a non-cutting operation and thus has no value. For those situations they rely on the system to have the repeatability, or force the setup person to indicate the part."

Zaya believes that probing should always be used even with accurate fixtures. The UNILOCK system has repeatability of 5 µm which, when combined with a part probing, allows for accurate and repeatable loading and location.

Getting started in designing a workholding solution for five-axis workcenters begins with analyzing the feasibility of the job, according to Colin Frost, chief operating officer of St. Louis-based Carr Lane Manufacturing Co. He noted there are a number of items to consider, including:

"As the five-axis workcenter is likely to be the highest cost piece of equipment in an operation," Frost said, "careful consideration must be given to the workholding, especially if it requires workpiece preparation, like machining a dovetail, or ‘imprinting,’ as these operations do contain costs. Once it is determined that making the part using the five-axis workcenter will result in a lower cost only then should the job proceed."

Carr Lane offers a variety of products designed for five-axis workcenters, including both mechanical and powered versions for low- to high-quantity production and everything from zero-point systems to low-profile edge clamps and vises.

The vises, which are made in Germany by Roemheld, feature an all-steel construction, unlike most other five-axis vises on the market. "Steel, in this application, is much more rigid than aluminum," Frost explained. "This not only facilitates the ability to raise the workpiece farther from the table, but will result in shorter machining cycles where more material can be removed without chatter, poor surface finish or tool breakage."

Many of Carr Lane's products are modular, such as the Drop Zero Mechanical Zero Point Solution, and further with the CL5 range of products that includes risers, vises and plates. They are able to be built up on downloadable CAD model software and proved out through simulation. Digital twins exist for all CLM products at carrlane.com.

There are three primary considerations for five-axis setups that are different from traditional workholding, according to Frost. The first factor, he said, is does the workholding lift the workpiece off the table sufficiently to allow for access to the workpiece by the spindle?

Second, the area where the workholding contacts the workpiece cannot be machined—does this impact the product? If so, what other decisions will be taken to resolve this?

The third question, Frost noted, is can the operator repeatedly position the workpiece appropriately? "Unlike traditional workcenters, five-axis workcenters typically use the center of the table as the origin or 0 point. Aligning the center of the workpiece to the center of the table eliminates oversized material waste, workpiece probing, or both. Each of which adds cost," he pointed out.

Workcenters come in two very different configurations: trunnion- and gantry-style machines. "Machines with trunnion tables or rotary trunnions typically have a much smaller machining envelope than a gantry style machine," Frost said. "This is especially true if the gantry-style machine has a rotating head. Vises are a common solution for flexible workholding in trunnion-style machines."

Jobs likely to repeat usually benefit significantly from the creation of a simple fixture. "What, after all, is a vise but a clamp moving against a fixed stop?" Frost mused. "Gantry-style machines, typically used with very large workpieces, need a different solution as a vise would need to be huge. For these large workcenters, Carr Lane's Drop Zero is a modular, manual, workholding solution designed to provide complete access to all five sides of the workpiece with no workholding interference," Frost said.

Another important consideration for five-axis workholding is its scalability for automation. "Even with a quick-change fixture solution, any opportunity to automate the load/unload of the workcenter is a significant opportunity to control cost (and drive profitability) and increase throughput," Frost said.

"Purchasing an ecosystem of products, like risers, quick-change sub-plates, and vises that are only offered in a mechanical operation will necessitate the repurchase of workholding to support automation," Frost continued. "CLM vises and clamps are available in both mechanical and powered (ready for automation) versions. Some, like the new Pivoting Edge Clamp family, can be upgraded to powered without changing the workholding. And because all Roemheld five-axis vises are available in powered versions of the same size there is no need to remake vise jaws, or even change sub-plates or risers."

"Generally, customers buy based on today's part. What we tell customers is try to put as much flexibility into their application for the day when a part leaves or another part or part family comes in," said Larry Robbins, president commercial division, SMW Autoblok, Wheeling, Ill.

"When compared with conventional three-axis machining, five-axis machining is faster, more precise, and suited for a specific application. Out-of-the-box workholding solutions are sufficient for probably 50 to 60 percent of the applications. Where workholding partners like us shine is designing flexible workholding solutions that maximize production of the workcenter, whether a gantry or trunnion style machine," Robbins said.

Finding the right workholding solution depends on the part configuration and machine configuration. For gantry-style machines, the Z height and table size are the limitation for part and tool clearance, according to Robbins. With a five-axis trunnion machine, he noted, users have to be concerned with "tooling and part clearance and the ability of the workholding and trunnion to rotate completely without hitting an intrusion or the machine itself."

When you look at a five-axis gantry machine, it's a vertical machine with a stationary workpiece. The horizontal machine presents a different set of challenges for five-axis machining where a part is being held on a tombstone or laying flat on a table on a palletized quick changeover system like SMW Autoblok's WPS/APS Workpiece Positioning System. The APS system features a manual Zero Point clamping system that is used in a variety of machining applications including turning, milling, inspection, and finishing operations.

"For five-axis machining applications, the manual WPS and WPS-PPD Workpiece Positioning System is flexible and easy to use," said Robbins, adding that it provides up to 90 percent setup time reduction with no conventional clamping devices required. It also provides greater machine accessibility with a range of accessories and adapters without interfering with machines or contours.

SMW Autoblok provides a wide range of workholding systems, from a full hydraulic or pneumatic setup all the way down to a manual clamping application. The company's new mechanical, hydraulic and pneumatic vises for stationary workholding are "completely configurable in the field," Robbins said, adding that they include the IMG vise, Sintex vises, and the SP52G vise, and pneumatic and hydraulic power vises.

"The SP5 2G system gives us the ability to put pins in a part, a vise, or a fixture and mount those parts on and off different machines over and over again with the same base setup within 5 tenths of where you were part to part, machine to machine," Robbins said. "We also have form-style holding clamps that allow you to take a collet style material, mount it to a base that gives you repeatability of position and to machine the component in a pie jaw. It's almost like you prepare your machine to capture the part on three sides so you don't have to worry about flipping except for your fourth side.

"Because of the design of a five-axis vise you never grip a part like a standard vise. You clamp on the sides and you flip a part to the other side or you go to a different configuration. With a five-axis vise in the system we use something called SinterGrip in our jaws, which is a patented insert that fits in the jaw for our workholding and that of our competitors’ workholding. It allows you to grip on a 3-mm ledge or edge and get just as much rigidity as full surface contact clamping," Robbins said.

SMW Autoblok's SINTEX 100 and 125 vise series offer clamping ranges from 155 to 250 mm and include the company's premium SinterGrip insert technology providing 3.5 mm clamping range (versus 30 mm standard for other industry models) and no dovetailing or pre-machining of the part. At IMTS, SMW Autoblok will introduce its new line of electronically actuated chucks, clamps, and vises to fully automate manufacturing processes.

Designing a five-axis workholding solution begins with gathering information about the part type, its critical features, and any tooling challenges, according to Braden Damman, applications engineer, Vektek LLC., Emporia, Kan. The next steps are determining the intended process with regard to operator loading or robot loading of a pallet or table, knowing the size of the machining envelope, and how many pieces are going to be loaded and unloaded.

"The difference between conventional and five-axis workholding applications depends on how holding the parts rigidly for accurate five-axis machining is going to be done," Damman said. "In a five-axis application, the goal is to reduce as many operations as possible. That may require changing the conventional three-seat principle to clamp the part in a different manner so you can reach as many as the features as possible.

"The 3-2-1 method of fixturing requires three points to define the plane of part location. When machining, a part often requires additional support in the Z plane. A floating support, such as a TuffGrip work support, provides a solution that can be used anywhere a jack screw is used. In applications where part distortion, chatter, ringing, or poor surface finish is a result of part movement or vibration, a work support can decrease or eliminate the problem," Damman explained.

Clearance is a major problem for the two different workcenter styles. Vektek's TuffCam swing clamps and TuffGrip work supports, for example, offer solutions for meeting clearance requirements in both the trunnion and gantry-style machines. "A lot of those aspects are done by the designers, but we help them get started in the right direction. Gantry-style machines typically for larger parts require clamping that is done by work support to reduce vibration. Trunnion workcenters involve smaller workpieces and dynamic motion of the trunnion mounted workpiece must be considered."

Damman explained that Vektek clamping products are universal, meaning they are specific to the part machining application. Vektek also has smaller clamps that lend themselves to five-axis applications.

"Pull cylinders can be paired with a wedge-style clamp or an expanded mandrill or something that would be underneath the part," Damman said. "We also have an Eccentric and Concentric hold down clamps that are designed to clamp inside a smaller hole. These are very handy for five-axis type machining because you are clamping underneath the part and now all sides can be machined and the top side as well," he said.

All of Vektek's CAD models are specific to a model number and all are downloadable from its web site. CAD formats include STEP, IGES, SolidWorks, Creo/ProEngineer PRT, DWG, DXF, and pdf layouts that give dimensions of the part.

Critical to designing an effective five-axis workholding solution is understanding the process flow through the workcenter, whether automated or precision clamping is required, according to Damman. "If so, additional power systems might be needed, such as high-pressure, low-pressure on-demand hydraulic pumps or on the machine side additional M codes or PLC programming that is needed for the automated clamping cycle."

Connect With Us