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Grinding Advances Power Faster, More Efficient Round Solid-Carbide Toolmaking

Ed Sinkora
By Ed Sinkora Contributing Editor, SME Media

New tool grinders are faster, more accurate, and more capable than ever.

Cutting tool manufacturers are producing more complex, high value tools, like this form tool being ground on a Walter machine from United Grinding.

Hats off to tool manufacturers! They make some of the most geometrically complex products out of the toughest materials and with the tightest tolerances. And yet cutting tools represent only 3% of the total cost of the average metalworking process.

Producing good parts with less labor is one way to drive down costs. But that’s especially difficult when tolerances get down into the tenths and the parts require a five-axis grind in carbide. Yet Pat Boland, co-founder and co-director of ANCA Inc. (Wixom, MI) reported that there’s a general movement to achieve 120 hours of production each week with 80 of it unmanned.

“Two things work together to achieve this. In-machine measurements to make sure you’re producing quality components and machine monitoring to collect and archive those measurement results to track machine performance.”

One of ANCA’s latest advances in this regard is using a laser inside the machine to scan the tool profile. Software then compares the resulting scan to the desired profile and makes automatic adjustments to the grinding program. This is especially important for complex, tight-tolerance profile tools like fir tree cutters (aka turbine root form cutters). It also works on things like a ball end mill profile, compensating for any errors in tangency, explained Boland. And it’s remarkably accurate. He said ANCA’s customers “achieve accuracies within a few microns on a fir tree profile.”

Walter, half of the tool grinding and measuring division of United Grinding Group (Miamisburg, OH), offers a very similar approach but measures outside the machine on a Walter Helicheck unit.

Aside from the inherent difficulty in automating a process involving two machines, Boland pointed to slight variations in temperature between the two machines and the need to unclamp and re-clamp the tool as sources of inaccuracies in such a process. “Our customers maintain these tight tolerances unmanned on an overnight shift.” Score one for ANCA?

Not quite, said Markus Stolmar, United Grinding’s vice president for tool grinding and measuring. “Assuming you have a rigid machine, stable temperature, and so forth, the real key is accurately measuring the grinding wheel. Take profile tools, most of which are ground using a full radius wheel. We have a process in which you grind seven flats on a blank in different radial positions. The machine then probes these flats and automatically adjusts its understanding of the wheel profile. If you do this you’re right on the money.”

And you don’t have to keep scanning the profile, Stolmar explained, thanks to the inherent precision of the machine. You would have to repeat the “seven flats” method to account for wheel wear at some point, but that’s to be expected as all wheels wear. Although Walter has had this feature for several years, Stolmar added that few customers seem to take advantage of it. Now you know!

Before leaving the subject of internal optics, we should add that three major players (ANCA, Rollomatic, and Walter) all now offer non-contact methods for orienting the blank before grinding. This becomes critical when grinding micro tools, because it’s impossible to probe the location of coolant channels that go down to 0.05 mm in diameter.

Then again, probing is also delivering new benefits. For example, Walter’s latest software includes a routine that probes several points along the length of a blank and uses the measurements to calculate the axial runout. Software then adjusts the entire grind relative to the actual center of the tool. Stolmar said it helps on blanks that are wildly out of kilter, but it works best on blanks that are already within about 10 µm of axial runout, which requires both a good blank and a good chuck. In that case it would take runout down to 5 µm or better.

The feature is limited to straight blanks but on new controls adds only 10-15 seconds in cycle time. All three tool grinder manufacturers also all now offer wheel probing to determine wheel size after in-process dressing, but ANCA claims that in addition to length and diameter it can probe a wheel’s radius profile.

Like the products they help produce, cutting tool lot sizes have decreased while complexity has increased, so the ability to automatically choose from a wide variety of grinding wheels has become critical. A notable example is the 15-station wheel changer on Rollomatic’s new GrindSmart 830XW, which switches the wheel set and associated coolant nozzles in under five seconds. And Rollomatic’s claim that it’s “ultra-compact” is no exaggeration since it’s contained within the same cabinet you’d expect if the machine had no such changer. Thus it requires no extra floor space. In fact, the whole machine, including the tool change robot and 10 cassettes for up to 4,500 tools, takes up just 59 sq ft (5.5 sq m).

Battle for Better Precision

ANCA now offers an internal laser that scans a tool profile and corrects for form deviations to automatically maintain tolerances within a few microns.

Walter applies linear motors to tool grinding and they offer several models in which every axis is either a linear motor or a rotary torque motor. The advantages include faster grind times (since the high acceleration shortens the non-grind time), better surface finishes, and higher precision. Interestingly, Walter also offers nearly the same set of machine configurations with ballscrews. That’s because Stolmar and other experts say that ballscrew technology is still sufficiently precise for most applications and it’s a lower cost solution. One thing that’s not open to debate is that linear motors are as precise after years of use as they are when new, whereas ballscrews wear and develop backlash.

ANCA has gone in a somewhat different direction. First, they’ve created their own tubular linear motor called the LinX. Second, ANCA has largely switched to linear motors, the exceptions being the vertical axes in their MX Linear and FX Linear machines and some special purpose machines. According to ANCA, because the LinX magnetic field is circular there is no additional down force on the rails. It also says that LinX motors use less energy than an equivalent flatbed style linear motor, don’t require a chiller and isolate the heat they generate from the rest of the machine.

Stolmar countered that no linear motors “require” chillers but they all deliver more power when chilled, and the difference impacts how quickly you can grind a tool. “For a Helitronic Vision, a powerful machine, you probably want to chill the drives because it doesn’t make sense to minimize the torque capabilities of the motors.”

Regarding the use of ballscrews for the vertical axes of the ANCA MX Linear and FX Linear machines, Boland explained that similarly sized linear motors can’t deliver as much force as a ballscrew and this becomes an issue when you have to overcome gravity. So using a linear motor on a vertical axis requires the extra cost and complexity of a counterweight.

At the same time, he argued, “you’ve always got pre-load on a vertical axis because of gravity. So backlash isn’t a problem. Also, the vertical axis is not doing a lot of work in many applications in tool grinding.” On the other hand, ANCA recently switched to linear motors on all axes in its flagship TX Linear model.

Rollomatic’s new GrindSmart 830XW can switch out a wheel set in under five seconds and the 15 station changer requires no additional floor space.

For years Rollomatic held off on linear motors and touted its hydrostatic ways, which combine minimal friction and maximum longevity with excellent vibration dampening and stiffness. This in turn contributed to high accuracy and an outstanding surface finish. Now it has taken a belt-and-suspenders approach and combined hydrostatic ways with linear motors in the new GrindSmart 830XW. The machine also features a direct drive spindle and a sixth axis that swivels the wheel to maintain a constant contact point.

Put it all together and the machine delivers “mirror surface finishes, radius shape accuracy under five microns and dimensional accuracy within one micron throughout a production run,” said Eric Schwarzenbach, president of Rollomatic USA (Mundelein, IL).

Another thing to note about the machine is its capacity to grind tools up to 1.25″ (31.75 mm) in diameter and 12″ (0.305 m) overall length. (The spindle has 19 hp/14 kW peak power.) Schwarzenbach said the 830XW makes them a stronger player for larger tools.

New Kid is the Fastest?

Vollmer has been building grinding machines for 109 years and has a strong position in CNC saw grinding. It also has a strong position in wire and rotary erosion machines for PCD cutting tools. They will likely grow in solid round carbide tool production with the new VGrind 360.

Andy Allwood, rotary tooling development manager for Vollmer of America (Carnegie, PA) explained that the VGrind combines short axis travels with high rigidity for attractive cycle times. The key is a new kinematic.

The VGrind has two vertically oriented spindles and switching from one to the other is a short move. The spindles are mounted on a swiveling C-axis casting and “the wheels are always positioned over the C-axis pivot point, maximizing the accuracy of the grind on any tangency and decreasing the possibility of digs,” added Allwood.

The rotary C-axis rides on two linear guides at the base plus a third rail mounted to a massive 1,100 lb X-axis wall on the left side of the machine, thereby securing C at both top and bottom.

The Vollmer VGrind is unique in probing the loader gripper to maintain the center line throughout a production run.

Also, the fixed bearing for each spindle is mounted forward at the wheel adapter, while the floating bearings are to the rear, so any thermal expansion is to the rear and doesn’t affect the grind. This contrasts with a side-by-side twin spindle design in which one spindle is susceptible to thermal expansion. The machine can also be equipped with an eight-position wheel changer that also switches out the nozzle manifold.

So how fast is it? Allwood said it has been 20-40% faster than competing machines in every demo they’ve done. In one example, the VGrind produced a 20 mm diameter, four-flute, 2″ LOC, variable helix, corner radius carbide end mill in 20 minutes, versus 28 minutes 37 seconds on a competing linear motor machine. That’s a 21% advantage. In another example, the VGrind produced a batch of 500 ballnose end mills (10 mm diameter) 7% faster than one competing machine and 25% faster than another.

Allwood said Vollmer used the same carbide, feeds, and grinding wheels, plus all dressing and wheel opening was performed before the test. The cycle time advantage can be credited solely to the kinematics of the machine. Equally impressive, the tool’s surface finish ranged from 0.15 to 0.18 µm Ra. Finally, Allwood pointed out the short axis travels result in faster cycle times on small tools too, such as a 3 mm, three-flute carbide end mill in 4.5 minutes versus seven minutes on a competing machine.

Both rotary axes use torque motors while the linear axes are ballscrew driven. Yet tests on a 10 mm diameter ballnose cutter showed profile accuracy of ± 2 µm throughout a run of 60 tools. The spindles can be direct or belt drive. Despite the short axis travels, the machine can grind tools up to 3.9″ (100 mm) in diameter and 14.17″ (360 mm) OAL.

One more key mechanical feature: In addition to probing the grinding wheel, the VGrind uses the same probe to calibrate the loader gripper. That’s critical for today’s ultra-precise workholding, which only opens about four-tenths on diameter. “Any thermal growth in the loader will quickly move the gripper off center,” Allwood pointed out. “The Vollmer design enables us to probe the loader gripper to maintain that center line throughout a production run.”

Infinite Geometric Challenges

Software has probably been the most important consideration in CNC tool grinding in recent years. As Stolmar put it: “There are an infinite number of geometric challenges in the cutting tool world.” Yet everyone seems to agree that the software from all the major players has gotten so good that recent and foreseeable advances will likely be incremental.

ANCA’s unusual LinX tubular linear motors have a circular magnetic field, so there is no additional down force on the rails.

Walter, ANCA and Rollomatic each offer their own tool grinding software, while Vollmer and several other competi-tors use NUM controls and NUMROTO PLUS software. But no matter which you choose, practically anyone can now grind a complex end mill or a high tech drill with very little training. The differences come down to very specific features and the relative ease with which they can be combined. And while more complex tools are more difficult to program, they’re not that hard.

For example, Walter’s Tool Studio software now includes 79 different operations, including 14 different clearance options (e.g., first clearance, secondary clearance, cam relief, and faceted relief), various fluting options, various gashes, K-lands and so on. And they can be combined virtually without limit. You can even vary features from flute to flute or front to back, making it easy to create tools such as variable helix end mills.

Stolmar pointed out that one reason it’s now easy to grind complex geometries is because 3D simulation gives you the ability to see everything you program. “You can play around with it, measure it and make sure everything is perfect before you grind. The differences between what you simulate and what you actually grind are mainly due to differences between the grinding wheel you simulated and the actual grinding wheel.” Another strong vote for getting an accurate measurement of your grinding wheel geometry.

Speaking of wheels, another new feature in Tool Studio that enables both complex tool geometries and faster grinding is the ability make custom wheel shapes to save fluting time. In other words, said Stolmar, you can calculate the optimal shape of the wheel in order to generate the desired flute in one pass, within certain limitations. (TS outputs a DXF which you import into your dresser to profile the wheel before grinding.) In cases in which you might have needed to use several wheels and/or several grinds to make a particular flute shape, you’d now need only one.

Conversely, you can also use standard wheels to generate more complex flute shapes or minimize the number of required standard wheels. If, for example, you have a standard 1V1 with a 30º angle, the software calculates the required moves to generate a given flute shape. Changing between right- and left-hand helices is also no problem.

ANCA recently introduced an interesting new capability it calls “scripting,” which is a simple way to automate tasks within ANCA’s standard iGrind software. For example, if you have a family of drills in which the back-off diameter is always 95% of the tool diameter, you can create a script that automatically makes this input for you.

Or, let’s say you’ve learned that the appropriate feed rate for another family of tools should always be set at a certain ratio relative to the tool diameter. You can create a script that automatically sets the feed rate based on the programmed diameter. So scripting saves setup time, reduces errors, and helps you capture and benefit from process knowledge.

Wheel of Choice

Finally, what about grinding wheels? The Paradigm diamond wheel from Norton | Saint-Gobain (Worcester, MA) remains a favorite for solid round tools. That’s because the wheel delivers everything a tool manufacturer would want: fast cycle times, fine cutting edges, long wheel life, and the ability to dress online for automated operation.

It achieves these benefits thanks to a proprietary patented metal matrix bond with up to 46% porosity and a greater than 2:1 diamond-to-bond ratio. So the wheel presents finer grits and more working particles with higher exposure.

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