Z Axis Opens New Doors For Jig Grinding

With the addition of full three-axis contouring capabilities, jig grinding is poised to take on new 3D applications.

Article From: 12/1/1996 Modern Machine Shop,

If your machining work is focused on 3D contouring, chances are pretty good you don't think much about jig grinding. After all, jig grinders are mostly for finishing holes, aren't they? Even with the addition of CNC, they're still limited to grinding straight-sided features, although now they can follow 2D contours.

The only problem with that notion is that some jig grinders today can do much more. A few companies have recently introduced models that include a programmable Z axis and that are capable of full three-axis contouring motions. That lifts the lid off many of the old limitations of jig grinding, rendering the machines able to handle all sorts of complex surfaces that simply weren't practical to accurately grind with any kind of prior technology. So jig grinders may now become the right tool for precision finishing of mold cavities, as well as for accurately generating a range of other complex 3D contours.

How accurate? Consider that while most molds have historically been finished by hand, jig grinders have for years been holding tolerances easily measured in ten thousandths of an inch. While it is still unclear just how precise these machines will be in full 3D applications, there are indications that they'll be able to far surpass the tools and techniques previously available for final size and surface finishing.

Of course, having this capability is one thing; putting it to good use is quite another. And the technology is so new that there isn't a whole lot of application data available yet to get a definitive handle on just when and where 3D jig grinding will make the most sense. Still, if you require precision finishing of complex surfaces, this technology is worth consideration. Here is an early look into its workings and potential.

How It Works

Conventional jig grinders use a simple vertical reciprocating motion to produce straight-sided workpiece features. Some mechanical means is usually incorporated to accommodate a small amount of taper--at most a few degrees. But large tapers and non-straight vertical features are either difficult or impossible to grind.

With a full 3D jig grinder, the simple vertical reciprocating axis is replaced by a servo-controlled Z axis with resolution and speed capabilities similar to the machine's other axes. In the case of the Moore 450 CPZ machine, for example, the Z axis can operate in three modes--reciprocation, positioning and interpolation.

In the reciprocation mode, the machine operates similarly to conventional jig grinding systems, except that the reciprocating endpoints and speeds are fully programmable. In operation, the endpoints are also more accurately maintained with the Z axis now incorporating precise position feedback as well as look-ahead capabilities in the control that help determine proper deceleration points so that any motion does not over-travel the programmed endpoint.

In the positioning mode, the Z axis can be programmed, moved via manual data input or moved with the manual pulse generator (electronic handwheel) to any location in the Z axis travel.

But it is the interpolation mode that offers the greatest potential in terms of broadening the application range of the jig grinder. Here, just like a vertical milling machine, the Z axis moves simultaneously with any combination of the other axes. The resulting tool motions can be linear (moving from point-to-point in a milling-like part program), circular, or a combination of both. To accurately generate some geometric forms, additional interpolation modes include polar, involute, exponential, spline, hypothetical and cylindrical.

Unlike milling machines--but just like earlier generations of jig grinders--the machine also has a C (normalcy) axis and a U (outfeed) axis. Both had their origins in the early days of hole finishing, and were necessary to precisely orient the grinding wheel and to establish and maintain size. The U axis allowed the tool center line to be fed laterally relative to the spindle center line--not unlike the outfeed of a boring head. As the grinding wheel wore, the U axis would be used to re-establish the proper size. And the C axis allowed for the rotary orientation of the spindle so that this outfeed could be positioned normal to the vertical workpiece surface. (Remember that the traditional cutting action was an up-and-down reciprocating motion.) The C axis initially functioned as a simple feed for driving the wheel around the circumference of a hole, but took on a more critical function as NC technology enabled conventional jig grinders to follow 2D contours (in the X-Y plane).

With 3D jig grinders, the U and C axes still play a role, but in somewhat different ways. For example, the U axis can still be used to compensate for variations in wheel size, but the CNC's cutter compensation feature may become even more useful for some 3D work. Contoured three-dimensional shapes, either male or female, can be produced by simultaneously interpolated combinations of the X, Y and Z axes. But the addition of the U and C axes greatly facilitate generating some forms. Tapers of any angle may be created by the simultaneous motions of the X, Y, C and Z axes. Internal and external tapers of any cross section can be produced as well.

With a fully programmable Z axis, the combination of Z-X or Z-Y motions provides a means to either create or refresh a form in a dressable wheel. The dressing program can be a macro in the part program and, with operator-controlled parameters, can be set up to dress as often and as deeply as required to do the job. Following dressing, an automatic sizing capability (more on this in a moment) is used to locate the surface of the wheel and to apply the appropriate cutter compensation or workpiece location offsets.

But it's not just grinding wheels that are located now. Three-dimensional work can involve a variety of workpiece materials and thus require a range of other kinds of tooling. Mills, burrs and small flycutters find frequent use in aluminum, soft steel and other ferrous and nonferrous materials. To accommodate these tools, a wide range of spindle speeds are required. By drawing from a selection of air turbine and electric grinding heads, spindle speeds of up to 175,000 rpm can be achieved.

Where Does It Fit?

While the first applications of 3D jig grinders are likely to be aimed at smaller workpieces (Moore's first machine has an X-Y-Z travel range of 18 by 11 by 5.5 inches), the base technology is capable of addressing much larger work. Whatever the size, these machines potentially hold a broad processing capability, which continues to be explored.

In steel mold work, the mold cores and cavities as well as the tapered core pin holes can all be ground on one machine. The mold form may utilize a number of combinations of interpolated axes, and the core pin holes can be of any cross section including elliptical. Core pin tapers and end features are also ground utilizing the X, Y, C and Z axes.

EDM electrodes make a good application. Electrodes are produced with the same kinds of motions as grinding applications, but burrs may be used instead of abrasive wheels.

Optical applications for complex surfaces include molds, laps and "one-up" pieces. Simple spherical and aspheric (non-circular, but axis-symmetric) surfaces can be created with conventional jig grinders by using X, Y and C motions in combination with a spinning table on which the workpiece is mounted. However, non-symmetric surfaces including "anamorphs" (totally without symmetry) require a third orthogonal axis--the Z--to produce their surfaces.

Ceramic pieces, particularly those with complex or multiple holes and features that must be held to close tolerances, are easily ground with diamond wheels.

An "interpolatable" Z axis also becomes invaluable for often required but awkward shapes to grind--for example, the very accurate contouring of a tiny land deep in a small hole. Without a controlled Z axis, the only approach is to attempt to wipe-grind the surface with a formed wheel, an approach that frequently results in lands with poor geometry and surface finish. In contrast, the Z axis, in combination with the other axes, moves a known part of the grinding wheel along a spiral path to create the land feature.

Controlled Automatic Operation

A particularly good aspect of 3D CNC jig grinding is that it provides a highly controlled environment in which to execute a finishing process that heretofore has strictly been the domain of hand tools. Required accuracies and high finishes can be produced right on the machine, eliminating hours of polishing. Recognize that we're talking about machine tools with X and Y positioning accuracy in the range of 2 microns (80 millionths of an inch) and Z accuracy of 3 microns. There are many other machine and process specific components to maintaining accuracy, of course, but it's still a far more accurate means to controlling both size and surface quality than other finishing techniques as they are commonly applied today.

As for the workpiece programming, the X, Y and Z positioning commands can be generated with common 3D CAM systems, since these programs are essentially similar to those generated for a machining center. As die and mold makers will know, these programs can be quite large, so they may require the additional memory of an auxiliary buffer to feed the program a portion at a time. Also, a postprocessor can be used to convert the point-to-point contouring commands (the typical output of CAM systems) to a series of arcs joined end-to-end, which are then executed on the machine with circular interpolation. Because a considerably longer portion of a curving form can be represented with an arc than a line segment (with the same accuracy relative to the base geometry), the arc-fitting method will result in substantially smaller NC programs (since fewer program blocks are necessary to describe an acceptable tool path) and a generally smoother cut as well.

Productive use and control of the U and C axes are a function of software in the CNC. For example, the automatic wheel sizing system locates a surface on the grinding wheel relative to the machine coordinate system. In short, a piezio electric sensor is attached to a metallic blade mounted on the table of the grinder. At grinding speed, the wheel is gently fed toward the blade until it makes contact, at which point the sensor sends a signal back to the control, and then the true wheel position is automatically "comped." This routine is repeated periodically throughout the program to ensure wheel size is continually updated.

In conventional jig grinders, such a wheel sizing routine need only be executed in the X-Y plane. With a three-axis jig grinder, however, there is also the need to locate the lower edge, bottom or some other feature of the grinding wheel. But this can be accomplished with the same basic automatic sizing system.

In the cut, the control system also has the ability to detect wheel-to-workpiece contact, and thus can automatically increase or decrease the feed to minimize grinding time in roughing operations.

These and other capabilities, in combination with full three-axis contouring control, are taking jig grinding into new applications every day. Only time will tell how far it can go.

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