WO2012082543A2

WO2012082543A2 - Method for automatic compensation of thermal distortion in a gantry machine

Info

Publication number: WO2012082543A2
Application number: PCT/US2011/064102
Authority: WO
Inventors: Mark D. Kohring; Donald J. Borisch; Troy D. Wesselman; John L. Sanford
Original assignee: Mag Ias, Llc
Priority date: 2010-12-14
Filing date: 2011-12-09
Publication date: 2012-06-21
Also published as: WO2012082543A3; WO2012082543A4

Abstract

A method for automatically compensating short-term relative thermal transient deformation of the cross rail of a gantry machining device includes the steps of measuring the horizontal expansion of the cross rail, measuring the horizontal expansion of the bed, and determining the difference between the two measurements to determine a compensation amount.

Description

METHOD FOR AUTOMATIC COMPENSATION OF

THERMAL DISTORTION IN A GANTRY MACHINE

Cross-reference to related application

This application claims the priority of U.S. Provisional Application No. 61/422,704 filed December 14, 2010, the entire contents of which are hereby incorporated by reference.

Background

Gantry style machine tools comprising one or more spindles that are supported by a cross rail over a bed on which a workpiece is positioned are well known in the art. The cross rail is usually supported along two ways and as the cross rail of a gantry grows horizontally in Y-axis due to thermal expansion it is constrained by the two ways and deforms. This deformation affects the vertical motion of the cross rail and changes the position of the cutting spindles relative to the bed in varying amounts along the Y-axis. This can affect part dimensional accuracy along the Z-axis depth in several ways, such as by causing the part to be thicker or thinner than desired along the Y-axis depending on the amount of thermal deformation.

Because of the complexity of inter-relationships involving distribution of heat sources, structural stiffness, coefficients of thermal expansion, specific heat capacity (thermal time constant), conductivity, convection and radiation that exist between various structural components— including the machine foundation— a very comprehensive transient thermal model and a large array of thermal sensor input information would be required in order to create a deterministic system model (such as that proposed in U.S. Patent Application 2010/0152881 Al), which is robust enough to predict cross rail motion induced by a multitude of environmental and internal heat sources. On the other hand, a simplified thermal model that includes only limited discrete sensor input information such as cross rail and bed temperatures to apply to a linear thermal expansion model with preset standard temperature (described by the method taught in U.S. Patent 7,266,903 B2) is found to be insufficient to account for longer term thermal stabilization of the other non-sensed structural components such as the foundation or the structural deflection shapes induced by constraints of the component boundary connections. U.S. Patent 5,421,683 discloses a single axis gear shaving machine where a thermal model is used to automatically compensate short-term relative thermal growth once a desired set of absolute positioning results has been obtained about a given operating point.

Summary

According to an embodiment of the disclosure, there is provided a method of automatically compensating thermal transient deformation of a cross rail of a gantry machining device. The method includes measuring a temperature of a cross rail of a gantry machining device using one or more sensors located on the cross rail; calculating a parabolic shape of the cross rail using a processing device based on the measured temperature of the cross rail; calculating a deflection amount along a Z-axis of the cross rail using the processing device based on the calculated parabolic shape of the cross rail; and adjusting the height of one or more spindles in the direction of the Z-axis according to the calculated parabolic shape of the cross rail to compensate for the calculated deflection of the cross rail.

According to another embodiment of the disclosure, there is provided a method of automatically compensating thermal transient deformation of a cross rail of a gantry machining device. The method includes sensing a temperature of a cross rail of a gantry machining device a plurality of times using one or more sensors located on the cross rail; storing the sensed temperatures in a computer-readable medium; calculating a moving average of the sensed cross rail temperatures using a processing device based on the sensed temperatures stored in the computer-readable medium; sensing an ambient temperature at a gantry machining device; calculating a deflection of the cross rail along a Z-axis using the processing device based on the calculated moving average of the measured cross rail temperatures, a current cross rail temperature, the sensed ambient temperature, and one or more sensitivity coefficients describing parabolic deformation; and adjusting a position of one or more spindles according to the calculated deflection of the cross rail.

According to yet another embodiment of the disclosure, there is provided a method of automatically compensating thermal deformation of a cross rail of a gantry machining device. The method includes measuring the temperature of a gantry machining device using temperature sensors mounted at one or more points; measuring one or more dimensions of the gantry machining device at the same time the temperature is measured; storing the measured temperatures and dimensions in a computer-readable medium; calculating a change in the measured dimensions of the gantry machining device based on a plurality of stored measured temperatures and dimensions and one or more sensitivity coefficients describing parabolic deformation using a processing device; determining a position of one or more spindles using an numerical control (NC) processor; and changing the determined position of the one or more spindles based on the calculated change in the measured dimensions of the gantry machining device. Brief Description of the Drawings

Figure 1 shows the deformed shape of a gantry cross rail due to thermal distortion;

Figure 2 shows a gantry cross rail thermal compensation model; and

Figures 3 and 4 show a method of calibrating a gantry cross rail.

Detailed Description

The method and system disclosed herein provides a means of using a simplified thermal system model to automatically compensate for short-term transient relative deformation of the cross rail in a gantry style machine, while also providing a means of periodically checking and/or adjusting for the steady-state absolute deformation resulting from longer term thermal stabilization of the machine. The method proposed herein eliminates the need for a comprehensive system model by providing a means for periodically calibrating the deformation against a given reference location.

Due to thermal expansion, the vertical motion of the cross rail has been found to take on a 2^nd order polynomial (i.e. parabolic) shape due to a bending moment induced by the lateral constraints placed on the cross rail by the X-axis ways fixed to the bed. The machine bed can also experience thermal distortion. Growth of the bed in the vertical direction is of little consequence to work-piece tolerance because the gantry rides upon the bed, resulting in very little relative motion between the spindle and bed. Growth of the bed in the horizontal direction, however, can contribute in an opposite sense to the bending moment imparted on the gantry uprights, thus adding inversely to the thermally-induced parabolic deformation of the cross rail. The net vertical motion of the cross rail can be determined by the difference in horizontal expansion between the cross rail and the bed.

Since the bed of the machine is typically fixed to the foundation through a series of leveling screws, the amount of horizontal bed growth can be constrained by the leveling screws. Due to thermal expansion of the concrete however, horizontal growth of the foundation can in turn influence horizontal bed motion which in turn affects cross rail bow as described above.

The method proposed herein can compensate for the parabolic deformation shape of the cross rail in a gantry machine in the presence of structural constraint forces. Changes in standard temperature can be periodically compensated for by calibration against a pre-qualified reference surface and added to the automatic thermal compensation model. In addition, the proposed method can dynamically account for the parabolic deformation shape of the cross rail and apply a differing amount of compensation at various Y-axis spindle positions at a high processing speed. The proposed method can dynamically adjust for thermally induced cross rail deformations that vary with spindle position about a pre -qualified reference surface, while employing a simplified thermal model that does not require a fully comprehensive structural system model with a full array of strategically placed temperature sensors to account for complex thermal inter-relationships. The polynomial coefficients and sensitivity scaling that describe the gantry cross rail bow due to a steady state environmental change in temperature can be determined by the use of a Finite Element Analysis (FEA) or independent indicator measurement, etc. This simplified steady-state model can then be adapted for response to longer-term transient changes by adjusting the offset with a given amount of cross rail compensation that is automatically determined by periodically checking against a known reference surface.

Turning to Figure 1, it depicts an exaggerated view of the deformed shape of a gantry cross rail style machine tool due to thermal distortion. The machine is designated generally by the reference numeral 10 and comprises a bed 12 that extends along the Y-axis, a pair of uprights 14 that extend along the Z-axis, and a cross rail 16 that extends along the Y-axis. It will be understood that the bed 12 also extends along the X-axis in a direction that is orthogonal to the plane defined by the Y-axis and the Z-axis. The bed 12 is attached to the floor 18 by a series of leveling screws 20. The uprights 14 are supported by and movable on X-axis ways 22 that are mounted along the sides of the bed 12. The uprights 14 support the cross rail 16, and the cross rail 16 supports one or more spindles 24 each of which drive a tool 26. Each spindle 24 is mounted on a saddle 28, and means are provided to drive the saddle to a desired position along the Y-axis on the cross rail 16. Z-axis drives are provided for each spindle 24 to drive the tool 26 into a workpiece that is supported on the bed 12.

Thermal expansion of the cross rail 16 causes it to grow horizontally in Y- axis, and the resulting vertical motion of the cross rail has been found to take on a 2^nd order polynomial (i.e. parabolic) shape due to a bending moment induced by the uprights 14 that are laterally constrained by the X-axis ways 22 that are fixed to the bed 12. This vertical motion of the cross rail 16 affects the position of the cutting spindles 24 relative to the bed 12 in varying amounts along the Y-axis which affects part dimensional accuracy along the Z-axis depth. The machine bed 12 can also experience thermal distortion. Growth of the bed 12 in the vertical direction is of little consequence to work-piece tolerance because the cross rail 16 rides on the X- axis ways 22, resulting in very little relative motion between the spindle 24 and the bed. Growth of the bed 12 in the horizontal direction however can contribute in an opposite sense to the bending moment imparted on the gantry uprights 14 by growth of the cross rail 16, thus adding inversely to the thermally induced parabolic deformation of the cross rail. The net vertical motion of the cross rail 16 can be determined by the difference in horizontal expansion between the cross rail and the bed 12.

A first example of a model or method for automatically compensating short- term relative thermal transient deformation of the cross rail is as follows:

Z_comp = (Toe - Tc) * (A1*Y² + B1*Y + CI) + (Tob - Tb) * (A2*Y² + B2*Y + C2)

where Z _',comp = required Z-axis compensation as a function of Y- position,

Tc = sensed average cross rail temperature,

Toe = cross rail temperature standard temperature,

Tb sensed average bed temperature,

Tob = bed temperature standard temperature,

Y = Y-axis location along cross rail,

[A1.B1. C1] = sensitivity coefficients describing parabolic deformation along Y-axis per deg of cross rail ΔΤ, and, [A2,B2, C2] = sensitivity coefficients describing parabolic deformation along Y-axis per deg of bed ΔΤ.

This model can account for the net vertical motion of the cross rail as a function of Y-axis position as determined by the difference in horizontal expansion between the cross rail 16 and bed 12 around a given operating point. The model uses both cross rail temperature and bed temperature sensor inputs as predictors of relative difference between cross rail and bed growth. The temperature sensors can be implemented using a variety of available hardware, such as thermocouples that generate a voltage signal depending on sensed temperature. An exemplary implementation of this thermocouple is available from Phoenix Contact, part number MCR-TE-JK. The model can be further simplified if it is assumed that the bed contribution is fixed around the given operating point by setting the bed sensitivity coefficients equal to zero. On the other hand, the model could also be expanded to include the effect of thermal expansion of the concrete foundation by adding a third ΔΤ multiplied by a parabolic polynomial at the end of the equation above.

A second example of a model or method for automatic compensation that uses an empirically based thermal input is as follows:

Zcomp = [ !*(T_MA - Tc) + N₂*(Tc - T_ambient)] * (A1*Y² + B1*Y + CI) where: Z_comp = required Z-axis compensation as a function of Y-axis position,

i = percentage of cross rail contribution, 2 = percentage of ambient contribution,

Tc = sensed average cross rail temperature, TMA = moving average of sensed cross rail temperature,

Tambient = sensed ambient temperature,

Y = Y-axis location along cross rail, and

[A1,B1, C1] = sensitivity coefficients describing parabolic deformation along Y-axis per deg of composite ΔΤ.

This model uses a combination of the temperature of the cross rail 16 and ambient air temperature sensor inputs along with a moving average of previous cross rail temperature as predictors of net cross rail deformation around a given operating point. This model may give a better estimate of cross rail motion than the simple temperature difference model as evidenced by increased statistical correlation coefficient. If desired, a similar model can be created that includes the temperature of the bed 12. The percentage of cross rail contribution and the percentage of ambient contribution, i and N₂, can be calculated by measuring the cross rail temperature and the ambient temperature from the gantry machine over some time period (e.g. 2-3 days). Using those measurements statistical analysis or regression analysis can generate values for i and N₂. Examples of software used to perform this analysis are provided by Minitel and/or Matlab. And exemplary values of i and 2 can include a 20% contribution for i and an 80% contribution for N_2. Also, some exemplary values of the sensitivity coefficients [Al, Bl, CI] are [-242e -16, 584e -16, 192e -6].

Typically, the rate of change of structural and ambient temperatures versus time is relatively slow compared to the position control update time of a numerical control (NC) controller using an NC processor. On the other hand, the change in Y- position versus time can be quite fast depending on the commanded machine motions required for machining a part. For this reason, it is convenient for the ΔΤ calculations involving sensed temperatures to be performed at a relatively slower rate outside of the NC controller that can be used to control the gantry machine. The parabolic deformation calculations involving quadratic equations of Y-axis position can be performed at sufficiently high rate by a separate processor that is closely synchronized with the NC processor or by the NC processor itself. This function is typically performed by an NC compile cycle that has an update time similar to that of the NC processor. The close synchronization between a processing device performing deformation calculations along the Y-axis and the NC processor determining the position of spindles helps to ensure that adjustments in spindle positions can be determined at least as fast as the NC processor determines the spindle positions. NC processors are included with commercially available CNC controllers, such as the FANUC 300i and the Siemens 840D. The calculations that can be performed at the comparatively slower rate may be carried out using a personal computer (PC) or even optional modules available with NC controllers such as the FANUC 300L The NC processor can receive temperature data via voltage signals from temperature sensors and (optionally) data can be passed from the PC through an input/output (I/O) device as is known to those skilled in the art. Similarly, it is not uncommon for the bed 12 of a gantry machine to be several hundred feet long in which case there could be a non-uniform temperature distribution along the X-axis. In this case, the ΔΤ temperature calculations can be adapted to allow for averaging or interpolating the sensed temperature of multiple sensors along the bed in order to estimate a temperature for a given X-axis position. These temperature distribution calculations would then be a function of a moving X-axis position along the bed which could also be changing at a fast rate based on direction of the NC controller. In this case, the ΔΤ calculations involving an averaging or interpolation of the sensed temperature(s) need to be performed at the higher rate close to the update time of the NC controller.

A third example of a model or method of compensating steady state deformation resulting from long term thermal stabilization of a gantry machine comprises the following steps:

1) Use a Finite Element Analysis (FEA) or independent indicator measurement, etc. to determine polynomial coefficients and sensitivity scaling that describes the bow of the cross rail 16 due to a steady state environmental change in temperature.

2) Normalize the polynomial coefficients to unity scaling (e.g. 1 in/deg or 1 mm/deg) corresponding to the maximum displacement at the center of the cross rail 16.

3) Set the coordinate offset for Z-axis zero set-point at the center of the cross rail 16, and with the simplified automatic compensation model activated around the given operating condition, use a touch probe mounted on the spindle 24 to measure and record the position of a reference surface on the machine bed 12 relative to the spindle.

4) Perform a regression analysis of measured probe position versus normalized cross rail sensitivity at corresponding Y-axis positions to estimate the amplitude of measured vertical displacement of the cross rail 16 due to cross rail bow.

5) Divide the estimated vertical cross rail bow amplitude by the cross rail sensitivity scaling per degree of input temperature change. This determines the amount of effective input temperature change that would be required to compensate the measured vertical motion. 6) Apply a constant amount of cross rail compensation using the following offset equation:

Z_comp = ATc * (A*Y² + B*Y + C),

where: ATc = effective input temperature change to compensate measured vertical motion,

Y = Y-axis location along cross rail, and,

[A,B, C]= sensitivity coefficients describing parabolic deformation along Y-axis per deg of ΔΤ. If the surface on the bed 12 is known to be bowed (as would be the case if it had been machined with improper thermal compensation applied), a proposed method of correcting the compensation for both the cross rail bow and reference surface deformation combined comprises the following steps in addition to the six steps listed above:

7) Set the coordinate offset for Z-axis zero set-point at the center of the cross rail 16, and use external means (e.g. a Laser Tracker) to independently measure and record the absolute Z-axis profile of a reference surface on the machine bed 12 as a function of Y-axis position.

8) Perform a regression analysis of measured absolute reference surface position versus normalized cross rail sensitivity at corresponding Y-axis positions across the bed 12 to estimate the amplitude of measured bow of the surface.

9) Subtract the estimated bow amplitude of the reference surface on the bed 12 measured by external means from the estimated vertical bow of the cross rail 16 measured by probing to obtain the net absolute vertical deformation amplitude.

10) Divide net absolute vertical deformation amplitude by the cross rail sensitivity scaling per degree of input temperature change. This determines the amount of effective input temperature change that would be required to compensate the measured bow of the reference surface on the bed 12 along with the measured bow of the cross rail 16. 1 1) Apply constant amount of overall cross rail compensation using the following offset equation:

Zcomp = AT_b * (A*Y² + B*Y + Q,

where: ΔΊ = effective input temperature change to compensate net absolute vertical motion,

Y = Y-axis location along cross rail, and,

[A,B, C]= sensitivity coefficients describing parabolic deformation along Y-axis per deg of ΔΤ. A schematic of the proposed gantry cross rail thermal compensation model is shown in Figure 2. All of the elements shown in Figure 2 can be carried out using one or more processing devices that are capable of processing electronic instructions including microprocessors, microcontrollers, host processors, controllers, and application specific integrated circuits (ASICs). The processing device can be a dedicated processor used only for calculating the ΔΤ temperature calculations, the calibration calculations, or both. However, the NC controller discussed above could be called on to perform all of the calculations described herein. The processing device can execute various types of digitally-stored instructions, such as software or firmware programs stored in computer readable memory to carry out the method discussed herein. The processing device can perform the ΔΤ temperature calculations independently and at a relatively slow rate relative to the calculations performed by the NC processor. As discussed above, these slower calculations can be carried out using a PC carrying the processing device that is separate from the NC controller using the NC processor, which can be used to ultimately control the gantry machine. However, it is also possible to use the NC processor to carry out all calculations.

Various combinations of cross rail temperature, bed temperature, ambient temperature, etc. can be implemented depending on the switch settings and scaling factors employed. The ΔΤ temperature calculations are included within the dashed line 30. An exemplary NC processor 32 can perform calculations relating to the deformation equations generally shown within the dashed line using ΔΤ temperature as input. In another example, the calculations shown within the dashed line of NC processor 32 can be carried out as part of a C programming executer including a highspeed processor option available with the FANUC 30i CNC controller. Calibration calculations are performed on probe and laser data to periodically adjust the Z-axis compensation about a given operating condition and are generally shown within the dashed line 34.

The ΔΤ temperature calculations 30 can be carried out using a switch 36 and a switch 38 that can control whether standard temperatures for the cross rail and/or bed are used to calculate ΔΤ or moving averages of the cross rail temperature and bed temperature will be used. For example, if the switch 36 is in the "up" position (as is shown in Fig. 2) and the switch 38 is in the "down" position (not shown), then the ΔΤ calculation can use a standard programmable temperature to calculate the ΔΤ. This can begin by subtracting the measured cross rail temperature (T_c) 40 from the standard cross rail temperature (T₀__c) 42. The result of this operation can then be multiplied by a cross rail scaling factor 44. The cross rail scaling factor 44 can depend on the size and/or geometry of the cross rail 16. In one example, this value can be 0.609. Once the result is multiplied by scaling factor 44, it can then be input into the deformation calculations generally shown within the dashed line of NC processor 32— more specifically, it can then be input into a temperature deformation formula 55 (A1*Y² + B1*Y + CI). Similarly, if the switch 38 is in the "down" position, the bed temperature (T_b) 46 can be subtracted from the standard bed temperature (T_c__b) 48. The result of this operation can then be multiplied by a bed scaling factor 50. The bed scaling factor 50 can depend on the size and/or geometry of the bed 12. Once the result is multiplied by scaling factor 50, it can then be input into the deformation calculations generally shown within the dashed line of NC processor 32— more specifically, a bed temperature interpolation formula 52 T2+(X- X2) *(Ti-T₂)/(Xi-X2), the output of which can then be added to the output of the cross rail scaling factor 44 and passed into the temperature deformation formula 55 (A1*Y² + B1*Y + C1).

Alternately, switch 36 can be moved to the "down" position and switch 38 can be moved to the "up" position. In this configuration, the moving average of temperatures measured at the cross rail 54 (T_c) and the moving average of temperatures measured at the bed 56 can be used along with the ambient temperature 58 to calculate ΔΤ. In that case, the cross rail temperature (T_c) 40 can be subtracted from the cross rail moving average temperature 54 and multiplied by a cross rail percentage multiplier 60 (Xi). The ambient temperature 58 can be subtracted from the cross rail temperature 40 and multiplied by an ambient percentage multiplier 62 (Yi). The outputs from the cross rail percentage multiplier 60 and the ambient percentage multiplier 62 can be summed and multiplied by the cross rail scaling factor 44. The output from the cross rail scaling factor 44 can then be input to the temperature deformation formula 55.

Similarly, if the switch 38 is in the "up" position, the bed temperature (T_b) 46 can be subtracted from the moving average of the measured bed temperature 56. The result of this operation can then be multiplied by a bed percentage scaling factor 64 (Y₂). The bed percentage scaling factor 64 can depend on the size and/or geometry of the bed 12. Simultaneously, the ambient temperature 58 can be subtracted from the bed temperature 46 and multiplied by an ambient bed percentage multiplier 66. The outputs from the bed percentage multiplier 64 and the ambient bed percentage multiplier 66 can be summed and input into the bed temperature interpolation formula 52 T2+(X-X2) *(T]-T2)/(Xi-X2). The result from the bed temperature interpolation formula 52 can be added to the output of the cross rail scaling factor 44 and passed into the temperature deformation formula 55 (A1*Y² + B1*Y + CI).

Depending on whether or not the calibration calculations 34 are used, the output of the temperature deformation formula 55 may ultimately be a thermal compensation 68 (Z Comp). If the calibration calculations 34 are used, then the output of the calibration deformation function 70 (A2*Y² + B2 *Y + C2) may be added to the output of the temperature deformation formula 55 to generate the thermal compensation 68. The calibration calculations 34 can be linked to the NC processor 32 by a switch 72 that engages and disengages the calibration calculations 34 as needed. The calibration calculations 34 include a comparison between probe calibration measurements 74 and surface calibration measurements 76. The comparison can include subtracting the probe calibration measurements 74 from the surface calibration measurements 76 to obtain a calibration ΔΤ 78. The output of the calibration ΔΤ 78 can be multiplied by a calibration scale 80 and input into the calibration deformation function 70. In one example, this value can be 0.00054. To the extent that the cross rail scaling factor 44, the bed scaling factor 50, or the calibration scaling 80 are carried out by a processing device separate from the NC processor 32, each of factors 44, 50, or scale 80 can be passed to the NC processor 32 via an input/output (I/O) device 51 known to those skilled in the art. Further details of the calibration calculations are shown in Figure 3. It is envisioned that measurement of the reference surface using independent external means would be performed once during initial machine installation to verify that the surface is parallel to earth level. Once this reference surface is established, it then becomes the reference to perform touch probe operations against. Periodic checks can then be made against this surface to check for cross rail bow at various operating conditions. The automatic cross rail compensation model can then be "re- zeroed" against this surface by performing the steady state cross rail calibration method described above. This can be automated within the NC processor 32 and performed very quickly and efficiently (e.g. at the beginning of each work shift) to assure that the spindle motion tracks straight along the Y-axis. After steady state cross rail calibration is performed, the spindle will track parallel to this surface along Y-axis.

If the reference surface is known to be bowed, the combined cross rail and reference surface calibration method can be performed. Figures 3 and 4 depict a method of carrying this out. For instance, the calibration can be calculated based on temperature (probe) data and laser tracker bed data that can measure the dimensions of the cross rail. A number of constants can be used including Z₀ as a sensitivity coefficient (degrees), L as the distance from the center of the rail to one end of the cross rail, Y₀ as a coordinate offset for a first spindle head, and Yi as a coordinate offset for a second spindle head.

Zo can be described as a scaling factor that represents the amplitude of a parabola much like "A" represents the amplitude of a sine wave of the form ^sin(cot), but here Z₀ can represent the scaling factor of a parabola that has a fixed "period" of 2L. ZP1 can represent the temperature data for the first spindle head and ZP2 can represent the temperature data for the second spindle head. YM1_P represents the Y position of ZP1 and YM2_P represents the Y position of ZP2. Zc can represent (relative) temperature readings and Zf can represent an independent (absolute) laser measurement of the reference surface.

Using these elements, a number of relationships can be derived. Zc/Zo and Zf/Zo can be vectors that are a function of Y position describing the normalized shapes of the deflection parabolas. These vectors can describe the fundamental shape of the deformation that exists without specific consideration of amplitude. In other words, these parabolic shapes can exist at a temperature differential of exactly 1 degree from the ideal normalized condition. Using this parabolic shape, the deflection on each end of the cross rail can be determined to be 1" higher on the ends than the middle (which would be equal to zero) according to the equation -(l/L²)*[Y-Yo]² where Y=+/-L and Yo=0. It would be similar to defining the scaling factor A=l in the sine equation above in which case the peak amplitude would have a value of 1" or more precisely 1 "/degree. Performing a least square regression can find the ZP and ZF estimates that are multiples of the chosen Zo scaling factor and can determine the amplitude correction. The scaling factors can be mutually dependent and choosing a different Zo reference scaling could cause the ZP and ZF estimates to be scaled accordingly. In one example, the Zo scaling to be the same as the cross rail scaling factor 44 and the calibration scaling 80.

After the combined cross rail and reference surface steady state calibration is performed, the spindle can track parallel and level to earth along Y-axis. Subsequent probe measurements can indicate the actual absolute position of the reference surface. The old surface could then be re -machined to produce a new surface that is once again straight and parallel to earth.

Having thus described the device, various alterations and modifications and alterations will be apparent to those skilled in the art, which alterations and modifications are intended to be within the scope of the invention as defined by the appended claims.

Claims

We Claim:

1. A method of automatically compensating thermal transient deformation of a cross rail of a gantry machining device comprising the steps of:

(a) measuring a temperature of a cross rail of a gantry machining device using one or more sensors located on the cross rail;

(b) calculating a parabolic shape of the cross rail using a processing device based on the measured temperature of the cross rail;

(c) calculating a deflection amount along a Z-axis of the cross rail using the processing device based on the calculated parabolic shape of the cross rail; and

(d) adjusting the height of one or more spindles in the direction of the Z- axis according to the calculated parabolic shape of the cross rail to compensate for the calculated deflection of the cross rail.

2. The method of claim 1 further comprising the steps of:

measuring the cross rail temperature a plurality of times; calculating an average cross rail temperature (Tc) based on the measured cross rail temperatures;

measuring the temperature of a bed (Tb) of a gantry machining device a plurality of times using one or more temperature sensors;

calculating the deflection of the cross rail using the processing device according to the following formula:

Z_comp = (Toe - Tc) * (A1*Y² + B1*Y + CI) + (Tob - Tb) * (A2*Y² + B2*Y + C2)

where: Z_comp is a Z-axis compensation as a function of a Y-axis position,

Tc the average cross rail temperature, Toe is a cross rail temperature standard temperature, Tb is an averaged bed temperature, Tob is a standard bed temperature, Y is a Y-axis location along cross rail, [A1,B1, C1] are sensitivity coefficients describing parabolic deformation along Y-axis per deg of cross rail ΔΤ, and [A2,B2, C2] are sensitivity coefficients describing parabolic deformation along Y-axis per deg of bed ΔΤ.

3. The method of claim 1, further comprising the steps of: measuring an ambient air temperature using one or more temperature sensors and altering the calculated deflection amount based on the measured ambient air temperature.

4. The method of claim 1, further comprising the steps of: measuring a foundation temperature using one or more temperature sensors and altering the calculated deflection amount based on the measured foundation temperature.

5. The method of claim 1, further comprising the steps of measuring the temperature using temperature sensors at two or more points along the length of the cross rail to compensate for a non-uniform temperature distribution along an X-axis.

6. The method of claim 1, further comprising the steps of measuring a temperature of a bed of the gantry machining device using one or more temperature sensors to determine horizontal expansion of the bed and calculating the deflection of the cross rail along the Z-axis based on the measured temperature of the bed.

7. The method of claim 1, further comprising the steps of calculating the deflection amount along a Z-axis of the cross rail using the processing device and adjusting the height of one or more spindles using a numerical control (NC) processor.

8. A method of automatically compensating thermal transient deformation of a cross rail of a gantry machining device comprising the steps of:

(a) sensing a temperature of a cross rail of a gantry machining device a plurality of times using one or more sensors located on the cross rail;

(b) storing the sensed temperatures in a computer-readable medium;

(c) calculating a moving average of the sensed cross rail temperatures using a processing device based on the sensed temperatures stored in the computer-readable medium;

(d) sensing an ambient temperature at a gantry machining device;

(e) calculating a deflection of the cross rail along a Z-axis using the processing device based on the calculated moving average of the measured cross rail temperatures, a current cross rail temperature, the sensed ambient temperature, and one or more sensitivity coefficients describing parabolic deformation; and

(f) adjusting a position of one or more spindles according to the calculated deflection of the cross rail.

9. The method of claim 8 further comprising the step of:

calculating a percentage of cross rail contribution to the Z-axis deflection using the processing device;

calculating a percentage of ambient temperature contribution to the Z-axis deflection using the processing device;

automatically determining an amount of deflection along a Z-axis of the cross rail using the processing device during gantry machining device operation based on the following formula:

Zcomp = [ !*(T_MA - Tc) + N₂*(Tc - T_ambient)] * (A1*Y² + B1*Y + CI), where Z_compis the Z-axis deflection as a function of a Y-axis position, i is the calculated percentage of cross rail contribution, N₂ is the calculated percentage of ambient temperature contribution, Tc is the sensed cross rail temperature, T_MA is the calculated moving average of the sensed cross rail temperatures, T_ambient is the sensed ambient temperature, Y is a Y-axis position along the cross rail, and [A1,B1, C1] are sensitivity coefficients describing a parabolic deformation along Y-axis per degree of a composite ΔΤ.

10. The method of claim 8, further comprising the step of measuring temperature using one or more thermocouples that generate a voltage signal.

1 1. The method of claim 8, further comprising the step of calculating the moving average of the sensed cross rail temperatures using a first processing device and operating the gantry machining device using a second processing device.

12. The method of claim 8, further comprising the step of adjusting a position of one or more spindles using a numerical control (NC) processor.

13. The method of claim 8, further comprising the steps of measuring a temperature of a bed of the gantry machining device using one or more temperature sensors to determine horizontal expansion of the bed and calculating the deflection of the cross rail along the Z-axis based on the measured temperature of the bed.

14. A method for automatically compensating thermal deformation of the cross rail of a gantry machining device comprising the steps of:

(a) measuring the temperature of a gantry machining device using temperature sensors mounted at one or more points;

(b) measuring one or more dimensions of the gantry machining device at the same time the temperature is measured;

(c) storing the measured temperatures and dimensions in a computer- readable medium;

(d) calculating a change in the measured dimensions of the gantry machining device based on a plurality of stored measured temperatures and dimensions and one or more sensitivity coefficients describing parabolic deformation using a processing device;

(d) determining a position of one or more spindles using an numerical control (NC) processor; and

(e) changing the determined position of the one or more spindles based on the calculated change in the measured dimensions of the gantry machining device.

15. The method of claim 14 further comprising the step of changing the determined position of the one or more spindles using the following equation:

Z_comp = ATc * (A*Y² + B*Y + C), where ATc is an input temperature change to compensate for vertical motion, Y is a Y-axis location along a cross rail of the gantry machining device, and A, B, and C are sensitivity coefficients describing parabolic deformation along the Y-axis per degree of ΔΤ.

16. The method of claim 14 further comprising the step of changing the determined position of the one or more spindles using the following equation:

Z_comp = AT_b * (A*Y² + B*Y + C), where AT_b is an input temperature change to compensate for net absolute vertical motion, Y is a Y-axis location along cross rail, and A, B, and C are sensitivity coefficients describing parabolic deformation along Y-axis per degree of ΔΤ.

17. The method of claim 14, further comprising the step of measuring one or more dimensions of the gantry machining device using a laser-generating device and a reference surface on the gantry machining device.

18. The method of claim 14, further comprising the step of changing the determined position of one or more spindles using the NC processor.

19. The method of claim 14, further comprising the steps of measuring a temperature of a bed of the gantry machining device using one or more temperature sensors to determine horizontal expansion of the bed and calculating the change in measured dimensions based on the measured temperature of the bed.