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Grinding controls go CNC.

After conquering turning and milling, CNC technology is now entering the more challenging world of grinding. Until recently, grinding automation consisted of customized solutions by grinding manufacturers for each of their unique machines. Now, it's possible for the grinding manufacturer or rebuilder to buy a CNC control specifically designed for grinding and adapt it to his machine, regardless of whether that machine is a cylindrical ID/OD grinder, surface grinder, centerless grinder, gear grinder, or eight-axis, multiwheel behemoth.

What has taken so long? Part of the problem is grinding's complexity, things like constantly changing cutting-tool (wheel) geometry and the need for reciprocating cutting patterns to cover up wheel tracks on the finished part. These are not problems in turning and milling, and thus, the marriage between control and grinder requires a much longer courtship (and an extra 10% in control-hardware costs). Without the right grinding-specific functions, interfacing, or programming potential, that off-the-shelf grinding control may still prove an inferior choice over a given grinding-machine builder's older, time-proven proprietary control.

The demand for grinding sophistication is certainly growing daily. Just as the grinding-machine builder cannot afford to sit tight with an obsolete control, neither can you, the user, ignore competitors who have mastered the new levels of grinding precision that sophisticated controls bring to the forming and finishing processes.

Some key differences

For starters, here are some specific technological functions that grinding machines require over and above normal CNC turning and milling functions:

* Mandatory in-process measurement--feedback from measurement of part size to gage the effects of constantly changing wheel geometry.

* Inclined in-feed axes--the ability to adapt to a formed wheel attacking the part from an oblique angle.

* Axis-specific input/output functions--the ability to tailor the movement of one or more axes independently of normal composite vectoring movement.

* Manual override in automatic--enabling the operator to step in and modify one or more infeeds manually with a handwheel without interrupting the program.

* Constant wheel-surface-speed modification--monitoring wheel speed and changing it constantly, sometimes in extremely fine increments (in continuous dressing, for example, with each revolution of the grinding wheel). Despite constantly changing, dimensions, the grinding wheel is kept at a constant surface speed and monitored for minimum wheel diameter, minimal width, maximum permissible speed, and maximum surface speed.

Linking control to machine

In operation, the control must link dimensional data on the grinder's three basic machine elements: grinding wheel, workpiece, and dressing-system. Each time you grind a part, you must:

Input grinding-wheel data. Wheel type is selected from a menu and its specific dimensions entered, distances from a selected reference point are established, dressing and wheel-wear data are entered, and the data to be monitored are defined. If the grinding wheel has two reference points (i.e.; left and right) the second reference point is similarly described. The wheel's profile is generated, either by retrieving standard profiles from memory and merging them, or by creating a unique contour in a separate profiling subroutine.

Input dressing data. Dressing data is also entered via graphics support. Various wheel types require different dressers, available individually or in combination on the given grinding machine. These can include a single-point diamond (left or right), a two-point diamond, a dressing wheel, or a single-point diamond used as a profile dresser with a preprogrammed contour subroutine.

Setup the machine. This will be required whenever a wheel is changed or a wheel profile changed to machine a different part geometry. This, also, will often be menu-driven. The main areas of concern here are acquiring a definition of dresser position, profiling the new wheel geometry, zeroing on the workpiece, and acquiring longitudinal positioning. These data are optimized at the machine by scratching--either moving in to touch wheel to part manually with the handwheel, or via an automatic routine if a proximity sensor is available. After this procedure is done axially and radially, the control can establish the relationships between wheel, dresser, and workpiece.

The control side

To get some idea of the kinds of magic tricks they are putting in these complex control boxes, we talked with Siemen's Reinhard Koller, an applications engineer specializing in grinding. Siemens has come a long way from its first grinding control, the Primo SG introduced in 1982 with its small control panel and keypad, three-line LCD display, and ability to control up to two axes and a spindle. Next came their 3G CNC five years ago, their 16-bit 810G in '88, and the high-end 32-bit 880G introduced this spring.

The Siemens 810G provides for entering as many as 1000 machine and program variables to be used within the control--freely assignable variables to use for calculations, to store results, and such things to be set like maximum speed, acceleration, deceleration and definitions of machine axes, spindles, and other machine mechanics.

Measuring the in-process part is usually based on a caliper measurement of part diameter or some other form of in-process gaging, but not merely feedback of wheel slides or positioning devices. When the desired tolerance is reached, a signal is sent to the control to abort the feed and retract the wheel from the part.

The machine automatically converts an inclined infeed axis into Cartesian coordinates. Normal interpolation, the integrating of two or more axes of movement into a single oblique vector, may or may not be the desirable. The Siemens control gives you the option of programming vectored movement or breaking that up into non-interpolated axes movement to achieve a more desirable grinding pattern. It's called "simultaneous transversing of up to four axes without interpolation linkage" It means the ability to program a feedrate for each axis independently.

This is more in line with how you want the grinding wheel to traverse the part, explains Siemens' Koller. "I can program four different feedrates for four different axes and run them all at once. You could not do that with a milling control. Those three of four axes in a milling program run only at a vectored feedrate."

Manual means

The ability to manually override programmed feeds is also significant, says Koller. "In the 810G, you can program a G code to activate the handwheel as a feed override--turning it counterclockwise to reduce feedrate and clockwise to increase it. This gives the operator more control without stopping the program and taking over the whole process manually."

Another key issue is maintaining constant surface speed as the wheel shrinks. "The 810G is constantly monitoring wheel speed and adjusting surface speed, based on what was removed in the previous dressing routine. You tell the control the starting diameter and what the dressing cycle removed, and it then compensates accordingly. This is also based on knowing dressing-tool location exatly--touching the diamond to set its coordinates either manually or via an automated subroutine. Depending on the machine design, this can be detected by a strain gage in the dressing tool holder."

How does the operator input a whell profile into the control? "We provide a graphical representation on a screen of various profiles, and you select from a number of standard profiles and add parameters, putting standard shapes together to create a profile (within some limitations). Or a profile program can be developed off-line on some type of CAD system."

Speed needs

It's hard to picture one grinding control covering the full range of grinding machines, from a cnterless grinder to a complex multi-wheel ID/OD grinding center, but the 810G can be set up for from one to four axes, and one to two spindles, says Koller. "It contains a basic grinding intelligence that applies to any grinding situation, and is very flexible and adaptable to different machine types."

What is the basic difference between the 880G and the 810G, other than the 880's 32-bit technology verus 16-bit? "Higher updating speed," replies Koller. "it is a higher-end control and is more accurate. The speed advantage is most noticeable in servo updating and positioning. "It's geared for more axes, a more powerful PLC, and larger memories."

Most machines employ some form of in-process measuring capability, usually mechanical calipers measuring part size. The 810G offers a host of external inputs from gages or sensing devices to influence program execution. "Its really up to the OEM," says Koller. "what he wants to measure, how fancy he wants to get with these input devices, and what machine performance benefits he can derive. If an OEM wants to do these things, the control can incorporate them."

Machine builder viewpoint

For the view from the side of the grinding-machine builder, we talked to Pat Harrington, Controls Manager, Bryant Grinder, Springfield, VT. Bryant has been successfully applying Siemens controls to their ID/OD grinders for many years, along with comparable Fanuc controls, and their own control. "We have predominantly used other people's controls for a variety of reasons," he explains, "the main one being customer acceptance and availability of support. We've been very successful with Siemens controls, yet when we're really pushing the state-of-the-art, we still use our own control. Depending on the grinding process and the tolerances to be held--when we're asked to hold 0.0001" tolerances or measure in millionths--the critical factors are servo performance, speed of the processor within the control, how you integrate these things, and what software features are available for us to use. Even with state-of-the-art 32-bit-processor controls, sometimes we cannot get the same servo or total-package performance that we can with our own control."

Can you explain that? "Well, no," he laughs. "because that's our niche. I would say that for 95% of all applications, we could take a Siemens control with their servos and do the job just fine. The same goes for the Fanuc controls we've used. There are things we do in our software and servos--how we use available information--that we can't do with their products."

That's why Bryant is so busy right now, he explains, "We're doing things that others can't do."

The challenge of implementing

It sounds like the marriage between these controls and your machine is a very complex and difficult task? "Yes it is. Implementing someone else's control is a big chore for us to do, and do successfully."

Grinder builders have already made decisions on what they want their machines to do. The question is will the control let them do it and more fully utilize the contrl's capability? "Yes, there are always things in the control geared for other applications that we won't use.

"For example, we will sometimes elect to use our own oscillation package because it meets our criteria better. With someone else's control, you never get complete access. I would love to use someone else's hardware exclusively--it would save me a big headache. But my competition dictates that I have to be able to do things quicker, faster, and better. What I would like is a little more open architecture that allows us to get in and provide more capabilities--open things up so that we can do things with their software. But, of course, that's proprietary, and they're reluctant to do that." Just as Bryant is about their proprietary edge.

Dept of CNC use

From Bryant's viewpoint, all grinders are now CNC and have been since the early '80s. But that's because it's on the high end of the market. Only in the last few years has the rest of grinding (even surface grinders) caught up to CNC. "Yes, everybody's doing it now," says Harrington. "They have to, and you will see more and more of the grinding function automated. The axes count will continue to go up and up."

Harrington is a believer in manual overrides. "We do this in our control, and others are allowing users this capability--a manual or teach mode. Rather than total automation, where the operator is considered a dummy who just pushes the go button, our concept is to allow the operator the capability of teaching the process by answering simple questions. We have manual control of all the servo axes on all our machines, whether commercially controlled or our own. We have handwheel incrementing between a very fine and a course feed. Sometimes, we will even implement some pushbuttons to manually move things around. People want to do a whole series of parts--from 2" to 8" diameters--and they want a quick changeover and the ability to move things around with the servo in a learning mode."

How much speed do you need?

Are controls finally fast enough to do anything you would want your machine to do today? "For a while, 1 millisec update was considered the greatest in the world, yet they're down to one quarter of that now. The problem is really in the tolerancing. When you get down to millionths, and talking about holding size, then, for servo considerations alone, the faster the better."

But some claim 32-bit control speeds are hardly ever needed? "It depends on what you're doing, and how you're doing it. In our ID / OD grinding applications for some of the bearing companies, we need that capability."

But, in the more typical application? "No," he laughs. "There, the 16-bit processor will be fine forever. The 32-bit control has become a buzzword. In most cases, multiprocessing with 16-bit processors would be fine for 99.9% of all applications. For example, we have a machine that uses one 16-bit processor to handle everything. I'll match it against almost anybody's 32-bit processor for performance. But, we do have some applications where we're devloping new designs that will need faster update to get the necessary servo performance."

32-bits for graphics?

Bryant has had its own 16-bit processor since 1980, and has recently done some 32-bit work on a few jobs where servo update was becoming too slow and limiting things such as grinding-oscillation options. "You give your servo positioning the highest priority, and lower-priority things like screen updates can become a little slow. Because of a delay between the process and its graphic representation, it became more or less a cosmetic thing to get screens updated more quickly. If your positioning screen doesn't update quickly enough, you can give the user the wrong idea that the whole processor is too slow. A 32-bit processor can speed these things up a little bit."
COPYRIGHT 1991 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1991 Gale, Cengage Learning. All rights reserved.

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Title Annotation:computer numerical control
Author:Sprow, Eugene
Publication:Tooling & Production
Date:May 1, 1991
Words:2363
Previous Article:Multiaxis EDM makes large dies.
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