Tech Talk
How has "Optical Form Grinding" aided in grinding perfect carbide tools with imperfect wheels?
Making carbide punches and dies is a task that pushes technology and operator skill to the limit. Surface grinding carbide is a constant struggle with wheel-breakdown; optical profile grinding is tedious and challenging; wire EDM is slow and leaves a damaged surface, undercutting the iron or cobalt binder of the carbide and leaving weakly-supported carbide particles standing on the cut surface.
It looks like an opportunity for the application of some new technology or clever idea. The EDM people have worked on it mightily, and so have the CNC grinder manufacturers, producing the seesaw of progress between the two competing methods.
One emerging approach to grinding has upped the ante in the high-precision category, which is where the punch-and-diemaking action is. It combines CNC and optical profile grinding, using the visual, optical side to "teach" the number-crunching, CNC side. The key to the method's success is that it produces sub-tenths accuracy in the finished part even when the diamond grinding wheel has lost its shape - which it inevitably does when grinding carbide. To make such an imperfect wheel grind perfect arcs requires a very un-arclike path in the program. This is where the CNC's number-crunching power makes itself useful. Fortunately, it's not something the operator has to do much about, except to make some judgments about how many data points the computer will need to make a smooth arc using a worn wheel.
"Using the optical-projection/CNC teaching method, we can grind arcs to +/-50 millionths accuracy" says Bob Hamada, Wasino Corp USA's grinding product manager. "The CNC may have to blend a lot of small arcs to produce one continuous one, if the wheel is far out of shape, but that's what all of that computer power is for".
The operator creates the program visually. Watching the wheel's relationship to a Mylar drawing on an optical-projection screen at up to 50 times magnification, he uses the machine's handwheels to move the wheel to the edge of the drawing's lines. He has a set of "soft" keys that he punches to identify the beginning and end of a line or arc; for the latter, he needs to identify one more point on the arc so the CNC can caluculate its radius and center. For roughing and semi-finishing, each arc requires only those three points. So far, the method is standard "teaching mode" programming, which has been in use on varoius types of machine tools for years. To get fine accuracy with worn grinding wheel, two extra steps are required. First, instead of defining a single arc, the operator defines multiple arcs along the path, which the machine blends into a continuous contour. The geometry involved in making an out-of-shape wheel, with two different edge radii and a flat or two, tangent to a circular arc at all points can be very complex. But that's the CNC's problem. To the operator, it's just a series of points that the computer has to figure out how to connect.
The second extra step requires setting up a test workpiece of some easily-ground material, such as graphite, and taking "divots" with the wheel as you define the picup points. This is required because the wheel's edge isn't seen clearly enough on the projection screen to ensure accuracy in the sub-tenths range. The test piece projects an extremely sharp, high-contrast edge.
There are some fine points to getting subtenths accuracy with a minimum number of operations but the important point is that we can do it without taking the part off the machine for intermediate inspections, and without redressing the wheel. Re-dressing diamond wheels is always problematic. Taking this approach, wheel breakdown is no big deal. It just means we have to identify some more arcs along the grinding path.
Typical applications are punches and dies for use on high-speed pressures - electronics component dies, for example. The computer-chip leadframe dies that have to be ground to +50 millionths are the target application that both EDM and grinder manufacturers shoot for. Inside corners often have radii of 0.003 in. or even 0.002 in.; well shaped wheels are saved for the finishing steps in these applications. Producing a finished punch or die requires changing wheels on the job, unless roughing has been done on another machine: a CNC surface grinder, often, or a manual optical profile grinder. Wasino's applications for their CNC optical profile grinders often involve finishing parts that were roughed on wire EDMs. We can write macros and write complex programs that anticipate wheel wear but that's the hard way. Nothing beats actually watching the wheel, and letting CNC do the hard work.
