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Machine-tool thermal compensation.

Machine-tool thermal compensation Here's a technique for automatically correcting thermal errors in NC machine tools, using software compensation. Thermally caused deflections can be predicted, based on input from sensors placed on various structural components.

As a machine tool operates, the temperature of major components will change, influencing machine accuracy and altering dimensions of finished workpieces. Ambient temperatures, duty cycles, loading factors, wear, and other considerations all come into play. Resulting errors can be several thousandths of an inch.

Two methods can reduce the effect of thermal changes on machine accuracies: error avoidance and error compensation.

Error avoidance

Often, it's better to prevent a problem than to correct it after the fact. Error-avoidance techniques normally are implemented at the machine-design stage to minimize effects of heat generation. Engineers develop symmetrical structural designs and choose thermally stable materials to prevent or negate the effects of heat buildup. Also, lubrication systems and oil chillers can help stabilize the running temperature of a machine. These steps will not eliminate heat buildup, but will minimize it.

Error compensation

We can offset thermal distortion by a software correction applied to the affected machine axis or axes. For example, error compensation already has been implemented through the Giddings & Lewis NumeriPath [R] CNC 8000 control. Here's how it works.

Thermal errors are created by differential expansion of a machine's structural elements. This is a result of heat generated at several sources, such as bearings and hydraulic systems. If we can find a reasonable correlation between the temperature at or near a heat source and the machine's thermal distortion, we can compute a correction and drive an axis servo to offset the error.

As a function of temperature, machine thermal errors have a lagging characteristic. For example, when a machine spindle is turned on, the spindle-bearing temperature will rise quickly as a function of time, but machine thermal error will not occur as quickly. As the heat begins to diffuse into the machine structure, however, error accumulates.

This situation approximates an exponential function where thermal error lags behind temperature. As time goes on, the system approaches a steady-state condition. The relationship found suitable for this condition is expressed as: E = K x L x [Delta] T x (1 - [e.sup.-t/C]) Where

E = Thermal error (mm)

K = Coefficient of expansion

(mm/mm/deg C)

L = Expansion length (mm)

[Delta] T = Step temperature change

(deg C)

t = Time (sec)

C = Time constant (sec) This equation computes an axis servo correction E, based on a temperature sensor input [Delta] T. The parameters K, L, and C are machine constants. A machine may have several temperature sensors and one or more compensated axes.

Measuring thermal distortion

Machine-tool thermal distortion is measured by noncontact displacement transducers that sense the position of a spindle-mounted test arbor located close to the sensors. Three transducers mount on the worktable of a vertical machining center (VMC) to measure distortion on the X, Y, and Z axes.

To determine thermal-distortion behavior, we cycle the machine under several operating conditions, recording distortion as a function of time. If we don't use axes motion, the test arbor remains positioned at the transducers during the entire test period. If we do use axes motion, we must program the machine to a "home" position to place the test arbor at the transducers each time we take a set of measurements.

Horizontal application

Let's apply this formula to a four-axis horizontal machining center (HMC). The third (Z) axis is created by an axially movable spindle. Because of large Z-axis travel, the spindle is long, and thermal expansion causes thermal growth out from the headstock. This Z-axis growth is large relative to thermal distortion on the X and Y axes.

The graphs in this article show distortions for several operations: constant spindle speed with no axes motion; variable spindle speed between 0 rpm and maximum throughout the test period with no axes motion; and repeat execution of a CNC program containing most machine functions, complete spindle-speed range, and full axes travels. We plotted distortion in terms of a ratio of measured distortion divided by maximum value recorded for all conditions tested.

Compensation must be implemented on this machine to reduce Z-axis growth to a level similar to that on the X and Y axes. A one-axis, two-temperature-input compensation serves, where one temperature sensor is at the outer race of a spindle sleeve bearing, and a second sensor is placed at the outer race of a ram bearing.

Application to VMCs

For our second example, we used a three-axis, 4000-rpm vertical-spindle machining center with a headstock and column unit that moves on the X and Y axes (stationary worktable). A fixture on each end of the worktable holds four workpieces (eight total).

In each of the eight parts, the setup drills and taps six holes, and drills and reams three more, holding location of the reamed holes relative to the center bore within tight tolerances.

In operation, the system first pilots the center hole of each part on the fixture, so all eight pilot diameters are at a fixed, known location relative to each other. At the start of a workday, the machine spindle is referenced on the X and Y axes at one pilot diameter. After this reference is completed, parts are made continually, four at a time. Of course, parts can be loaded or unloaded at one fixture while machining takes place at the other.

During the workday, the machine gradually heats up, creating thermal distortion on the X and Y axes, affecting the dimensional relationship between spindle and pilot diameters. We measured this distortion and plotted the results in a graph of thermal error versus time history.

In the first 12-hr period, we cycled the equipment to simulate part machining, starting with the machine tool at ambient temperature. During this time, the X axis drifted very little as the machine heated up. However, Y-axis drift gradually increased to 0.0025", which would cause part tolerances to be exceeded.

After the initial 12-hr test period, we "parked" the test arbor at the measurement transducers. Then we stopped program cycling and spindle rotation, leaving the machine turned on overnight with full power, hydraulics, and lubrication systems on.

We did this with the expectation that the machine would remain at a thermally stable condition. However, this was not the case. The Y axis gradually returned to a point close to the zero line on the graph during the 12-hr overnight period. At this point we restarted program cycling and again simulated part machining. Just as before, Y-axis drift gradually increased.

These results illustrate that, in addition to leaving the hydraulic and lubrication systems on overnight, we must also cycle the machine and spindle throughout the period to maintain a thermally stable condition.

An alternative to overnight operation is compensation for the Y-axis distortion. We did this using a single temperature sensor at a spindle bearing. We repeated the distortion test and noted an overall reduction in drift for the entire test period. More importantly, drift was reduced to very small values during the time the part-simulation program was cycling.

Additional comments

In any system for temperature-input compensation, sensor location is critical. Whenever possible, the sensor should be placed directly on the structural element undergoing thermal expansion and contraction. However, in the case of the horizontal-spindle machining center, the structural element undergoing thermal expansion is the rotating spindle, making direct placement difficult.

To solve this problem, we positioned the sensor as close as possible to the spindle (bearing outer race), and used the time-contant feature of the compensating algorithm to account for thermal delay between bearing temperature and spindle-temperature growth.

Another feature of this compensation system is its ability to keep up with fast-changing conditions. Thermal distortion normally is discussed in terms of gradual errors that tend to reach a steady-state value as time progresses. However, this was not the case for the horizontal-spindle machining center, where spindle growth fluctuates as spindle speeds change, even after may hours of operation. Fortunately, the compensation keeps up with fast heating and cooling.

Good response results from the fast update time used in the compensating software. A complete set of compensation corrections is applied every 10 sec. This frequency ensures that each value is small, providing a smooth and continuous mode of operation, with no adverse effect on part finish.

The locations and number of temperature sensors used can vary with different machine types. Some machines will use multiple sensors to compensate one axis, while another might have one sensor compensating multiple axes. Each machine style has its own temperature characteristics, so, for our research, we monitored each machine's temperature characteristics to produce its own algorithm for compensation.

PHOTO : Three transducers mounted to worktable of vertical machining center measure distortion in

PHOTO : X, Y, and Z axes.

PHOTO : Horizontal machining center (HMC) has axially movable spindle for Z axis.

PHOTO : Vertical-spindle machining center. Headstock and column movement provide X and Y axes, and

PHOTO : worktable is stationary. Close-up shows four-workpiece fixture with parts in place on end

PHOTO : of table. Another fixture mounts on other end.
COPYRIGHT 1989 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Janeczko, John T.
Publication:Tooling & Production
Date:Aug 1, 1989
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