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Centerless grinders adjust to small lots.

Centerless grinders adjust to small lots

Engineers usually choose centerless grinders because of the high metal-removal rates, precise rounding action, and tolerances in the 0.0002" to 0.0007" range. But they think of the machines only for long runs because of lengthy setup times involved with hydraulic drives that control feed and other movements.

Setup is almost a black art, depending on a machinist's years of cut-and-try knowledge to properly set the workrest or work-support blade, wheel positions, wheel speeds and feeds, and dresser speeds. Then, he must constantly tweak the temperamental hydraulic systems to cope with speed variations compounded by the machine's varying temperatures throughout the shift.

But today's all-electric servodrives have improved the picture. They can feed in very fine increments--and do it consistently. The new servos also make centerless grinders easier to set up and more affordable for many more shops than ever before.

Centered versus centerless

In center-type cylindrical grinding, the workpiece is supported on centers and rotated about a fixed axis by a power drive while the rotating grinding wheel advances until it contacts the workpiece. The diameter of the ground surface is determined by the distance from the face of the grinding wheel to the centerline of the workpiece. But this method is vulnerable to deflection between the centers, because the grinding wheel applies force perpendicular to the center axis as it grinds the workpiece.

In centerless grinding, there are no centers--and consequently no fixed axis. The workpiece simply lies on a workrest blade and is supported by a regulating wheel. While the workpiece is being ground, its center is free to float as it rotates. Unlike center-type grinding, there is no deflection problem because the opposing force of the regulating wheel does not allow it.

A basic principle of conventional centerless grinding is that workpiece diameter is determined by the distance between faces of the grinding and regulating wheels. The regulating wheel, usually made of rubber-bonded abrasive, serves as both a driving and braking element.

Workpieces are held in firm contact with the regulating wheel and workrest blade by forces generated in the grinding process. Workpiece friction against the workrest blade and the regulating wheel is greater than its friction against the grinding wheel--which prevents the workpiece from spinning out of the machine.

Although the workpiece lies between the two wheels, it can turn only as fast as the peripheral speed of the regulating wheel, which turns at a speed much slower than that of the grinding wheel. The workrest blade usually has a 30-degree angle on the top, which aids the rounding action as the workpiece rotates between the wheels.

Centerless grinding is especially useful for machining cylindrical workpieces ground over the entire length and having no drive means or other projections. Many variations on this basic shape, from a tiny 0.060"-dia needle roller bearing to an enormous 1400-lb freight-car axle, are being successfully centerless ground on a production basis.

More than shafts

Three centerless methods collectively can grind a wide variety of cylindrically shaped parts. Through-feed grinding, the most common and productive centerless technique, processes those parts having a continuous diameter and no shoulders or projections. Workpieces are drawn between the two wheels by an axial tilt of the regulating wheelhead (up to 5 degrees on a Landis(R) 8RE-2(TM) grinder).

The second most common method is infeed grinding to handle parts having a shoulder or projection. An end stop laterally positions the part in reference to the grinding-wheel edge to keep shoulders or projections from contacting the wheel.

The third method is known as endfeed grinding and is used for machining parts with a tapered end.

Centerless grinding usually removes stock faster than center-type equipment, though the rate of stock removal depends on the workpiece. In throughfeed grinding, there is no lost cutting time for load and unload operations because the setup allows a continuous stream of workpieces to flow between the wheels. Centerless grinding also eliminates the cost of machining workpiece centers and the lost time spent setting up chuck jaws or attaching a drive dog to each workpiece.

Electric advantages

The technology that led us away from hydraulic drives also gave us a number of sophisticated electronic capabilities that foster high accuracy and time savings. On hydraulic machines, wheel-feed rates and distances must be adjusted manually. The operator usually employs his eyesight and a stopwatch, adjusting hydraulic valves to set feed rates.

One of the best shortcuts comes from advanced motion-control systems. For example, an operator can set up an all-electric grinder right at the machine-control panel (except for regulating-wheel tilt and workrest and dresser adjustment).

With the Landis 8RE, the feed rates, distances, dwells, and regulating-wheel rpm are all programmed at the control panel. Here, the operator also can program the speed and end points of wheel-dresser advance and retract, as well as diamond-infeed distance and compensation. Regulating-wheel speed can be programmed in 1-rpm increments from 5 to 100 rpm for grinding--and up to 300 rpm for dressing.

Some other points to look for in new controls include battery backup of memory to protect data against power failure. Also, look for universal ports (RS-232 and RS-422) to link machines to other computers or interfaces. A good link enables a machine to load programs developed off-line, and upload or download its own programs.

Electric controls also boost accuracy and save time by precise readout. When infeed is complete, for example, the DRO at an operator's station reads zero.

Don't forget mechanics

Mechanical innovations, too, have improved the accuracy of centerless grinders. For instance, Landis hydrodynamic Microsphere(TM) spindle bearings are almost free of wear and friction. Running on pressurized oil, the bearings hold spindle alignment, and, in fact, compensate for inherent misalignments of bores in spindle housings. Furthermore, the bearings are more rigid, eliminating the need for balancing grinding wheels in most applications.

Other advancements, such as electronic gaging and automatic handling, help make the centerless grinder truly cost effective to many more shops than ever before. Setup is so simple and reliable that less-skilled operators now can do what previously was done only by skilled machinists.

Use of these grinders can eliminate premachining operations, provide faster processing, and ensure better rounding action than cylindrical grinders. Best of all, runs as small as ten parts are practical.

PHOTO : Basics of center-type grinding. Distance R from grinding-wheel face to workpiece centerline determines diameter of ground surface.

PHOTO : Centerless grinding. In horizontal configuration, workpiece simply rests on work-support blade or workrest. Blade's 30-degree angle aids rounding action; regulating wheel prevents deflection.

PHOTO : Handwheel on Landis 8RE centerless grinder sets regulating-wheel tilt from 0 to 5 degrees. Tilt draws straight-shaft parts between regulating wheel and grinding wheel in throughfeed mode.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Sheets, Harvey
Publication:Tooling & Production
Date:May 1, 1989
Previous Article:Controls shape grinding performance.
Next Article:Managing job-shop data.

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