by Kip Hanson
The call for flexible five axis workholding comes in loud and clear
Tooling up a vertical machining centre is easy. Just bolt a pair of 6 in. machinist vises on the table and call ‘er done. And while this path-of-least-resistance approach may be limited, it’s also fast, easy and cheap. Unfortunately for the owners of five axis vertical machining centres, machinist vises don’t cut the mustard.
Five axis machining can be broken down into two distinct processes. The first involves simultaneous movement of up to three linear axes (X, Y and Z) together with the machine’s two rotary axes, typically labeled A and B. This process is used to produce sculpted, three dimensional surfaces such as knee replacements, pump impellers, mag wheels or a mould for the next greatest motorcycle helmet.
The other type is called 3+2, where the rotary axes are used to index and position the workpiece, enabling five-sided machining of manifold blocks, valve bodies, and a host of other parts that, prior to five axis machines, would have been “tumbled” through multiple operations on a three axis machining centere. Today, five axis machines eliminate the handling and need for multiple fixtures, greatly reducing part lead-time while improving accuracy.
There’s more to five axis machining than spinning a part. Programming, process planning, and the machine tool itself are all more complicated than what’s needed for traditional three axis work. One aspect of this is how to best grip the workpiece. No matter what you’re making on a five axis machine, the parts must be positioned farther from the table, sometimes much farther, than on a three axis machine. That’s because, when the part is tipped on its side or rotated at some extreme angle, the cutters and toolholders need room to clear the machine table. If the part is snugged down close to the table, as with a machinist vise, interference is as sure as a February snowstorm in Winnipeg.
Workholding providers have responded with a variety of solutions. Mittmann Industrial Equipment, Rigaud, QC, distributes the Kipp five axis compact clamping system from German manufacturer Heinrich Kipp Werk KG. President Thomas Mittmann says five axis machining is prevalent in the aerospace, automotive and die/mould industries. “It’s especially useful for machining low volume, complex parts. You can do the whole part in a single clamping on these machines.”
Mittmann says the idea behind five axis workholding is to make the workpiece rigid “like it’s welded to the machine table,” but still provide sufficient clearance. “The machine spindle has to be able to go around the workpiece, while keeping the cutting tool itself as short as possible. This avoids vibration and improves both tool life and part quality.”
Kipp has two systems for this—the first uses a fixed jaw on one side of the vise, the other is a self-centering version. Both are mechanical systems, and rely on a centre-mounted spindle to exert downward clamping force. Like many five axis workholding systems, multiple units can be used to grip large workpieces. “Many shops use two or more Kipp vises side by side to increase clamping width, which is either 90 mm (3.54 in.) or 125 mm (4.92 in.), depending on the vise model.”
The Kipp also offers two jaw configurations, with either a smooth-faced jaw for gripping finished surfaces, or one with a series of pins that dig into the workpiece. This approach is often necessary with 5-axis clamping systems, which typically grab a very small amount of material across the bottom of the workpiece blank.
Ontario’s Machine Tool Solutions Ltd. is another company offering workholding with a bite. It’s Lang Technik brand of five axis clamping systems from Holzmaden, Germany, which uses an offline stamping unit to create zipper-like indentations 0.1 mm deep (0.004 in.) on the workpiece gripping surface. This allows Lang’s Makro-Grip self-centreing vise to hang on to the part with only 3 mm (0.118 in.”) of material.
As Lino Libertella, president of Machine Tool Solutions explains, this pre-stamping process reduces the amount of clamping pressure compared to systems that rely on brute force for gripping. “However, it’s important to point out that pre-stamping is only required on parts with hardness greater than 34-45 HRC, says Libertella. “Below this, the Makro-Grip is able to make its own indentations.”
The Makro-Grip comes in 46, 77, and 125 mm widths (1.8, 3.03”, and 4.92 in.), and clamping lengths up to 355 mm (13.97 in.). Like the Kipp, several jaw configurations are possible, including a pin-style jaw and one with a tungsten carbide coating for non-slip “6th side” gripping of finished workpieces. Libertella says all of Lang’s five axis solutions can be equipped with zero-point style clamping. This makes for fast part swaps and rapid job changeovers. This is important on a five axis machine, because unlike a three axis mill with its gaggle of vises and the possibility of multi-part pallet systems, five axis machining is often performed one part at a time.
Gerard Vacio, Unilock product specialist at BIG Kaiser Precision Tooling Inc., Hoffman Estates, IL, says five axis machines cost more money to operate, and for this reason it’s especially important to keep the door closed and the machine running. “Most shops with five axis machine tools are interested in ways to shorten the downtime for fixture changeover.”
Vacio agrees there are a number of good workholding systems out there: self-centering vises, dovetail-style clamps, adjustable and twin vises, even lathe-style chucks designed specifically for holding round parts on five axis machines. What shops need to focus on, is how to make the best use of the machine table.
BIG Kaiser accomplishes this by replacing the platen—the big, round chunk of steel that the workholding gets bolted to—with one designed for flexibility.
“Often what happens is that shops bolt an adapter plate onto the existing t-slot table, and then bolt the workholding to that. But if you can replace the stock t-slot platen with one that accepts both standard tooling as well as quick-change, you gain Z axis travel and make the workholding more flexible.”
Vacio has a good point. Machine travels can become a limiting factor with large workpieces, and raising the workpiece to achieve clearance is less necessary. Also, as parts get farther away from rotational centreline, accuracy suffers somewhat due to axial error stacking. Finally, there’s plumbing to consider. Many five axis workholding systems can be actuated via hydraulics or pneumatics. If the stock platen isn’t equipped for this, adapting it may mean adding yet another plate, driving the workpiece even farther from centreline. It’s a Catch-22 situation, one that can be alleviated by clever workholding.
Whether you agree with this approach or not, none would argue that getting parts in and out of the machine quickly is important.
Kaiser and other tooling manufacturers offer zero-point systems that integrate tightly with theirs or other workholding solutions. One of these is Schunk Intec Corp., Mississauga, ON. Brad Evans, a product manager based in North Carolina, says zero-point brings speed and flexibility to five axis machining centres. “You can quickly switch between a vise, a magnetic chuck, or a collet, for example. It lets you do your part clamping offline, and get the next setup ready while the machine is busy making parts.”
Evans says there are way too many shops that spend $250,000 on a five axis machine, and then invest a few hundred bucks on a vise. “Customers don’t look at the importance of workholding like they should. Getting it done quicker and more accurately, with more access to the part—that’s every bit as important as the machine itself.”
Another thing that’s frequently overlooked is increasing workpiece density. Running one part per cycle is, at least some of the time, the equivalent of tossing a pair of machinist’s vises on a three axis machining centre. By equipping a five axis machine with workholding that presents multiple parts to the spindle, you improve machine efficiency.
“If you can’t increase the density of your table, you’re wasting your time,” Evans says. Because of this, Schunk is one toolmaker offering multi-part modules for its workholding system, allowing four or more parts per cycle. “This is ideal on a five axis because you can use the same program and the same tools to machine all four parts. In many cases, this lets shops run unattended overnight. The next morning, you can reload the fixture, or just take the whole thing off and do other kinds of work.”
Shops the world over are discovering the benefits of five axis machining. Even for those that aren’t making propellers, hip replacements and other complex, multi-axis parts, five axis machines reduce work in process and eliminate secondary and tertiary operations. Because those operations are no longer necessary, tooling costs go down and turnaround times become days rather than weeks.
If your shop is looking at a five axis machine, or if you’ve already bought one and aren’t sure you’re getting the most out of it, consider the options outlined here. SMT
Kip Hanson is a contributing editor. [email protected]