KR100842474B1 - System and method for measuring bore erosion in a fluid dynamic motor, and method for aligning an axis of motion - Google Patents

Abstract

A method and system are disclosed for measuring and characterizing fluid dynamic grooves produced by an ECM process. The method includes the process of aligning a workpiece with a measuring device and a technique for quickly, accurately and reliably measuring erosion patterns. The present invention also provides a system for measuring and characterizing the erosion of such grooves.

Description

SYSTEM AND METHOD FOR MEASURING BORE EROSION IN A FLUID DYNAMIC MOTOR, AND METHOD FOR ALIGNING AN AXIS OF MOTION

This application claims the priority of US Provisional Patent Application No. 60 / 401,796, filed August 6, 2002.

FIELD OF THE INVENTION The present invention relates generally to fluid dynamic bearings and, more particularly, to etching grooves used in the spindle motors of disk drives to form such bearings.

Conventional disk drives use magnetic properties to store and recover data. Typically, disc drives are incorporated into electronic equipment such as computer systems and home entertainment equipment to store large amounts of data in a form that can be recovered quickly and reliably. The main components of the disk drive include magnetic media, read-write heads, motors and software. The motor used to rotate the medium at thousands of revolutions per minute is configured to rotate in end point vibrations, and to be reliable and effective. One way to do this is to ensure proper lubrication of critically moving parts in the motor with oil. Proper lubrication of the motor is typically achieved by integrating the grooves in the bore and shaft through a cutting process such as an electro-erosive machining (ECM) process. Because such grooves are important for maintaining proper oil circulation, erosion of the grooves causes improper oil circulation due to lockup, resulting in motor failure. Therefore, measuring and understanding the erosion of grooves in the bores and shafts is important to make the motor strong enough for the hard drive.

The cutting process can be performed in any of a variety of electroerosion processing modes. In electrical discharge machining (EDM), the cutting liquid is a dielectric liquid, for example deionized water, and the operating electric current is supplied in the form of combustible electric pulses. In electrochemical processing (ECM), the cutting medium is a liquid electrolyte, for example an aqueous electrolyte solution, and the processing current is a high amperage continuous or pulsed current. In electrochemical discharge machining (ECDM), the liquid medium has both electrolytic and dielectric properties and the processing current is preferably applied in the form of pulses that facilitate the production of an electrical discharge through the conductive liquid medium.

The work piece may be placed in a bath of the cutting liquid medium so that the cutting area is submerged. However, more typically, the cutting zone is disposed in an air or atmospheric environment. Advantageously, one or two nozzles of conventional design are disposed on either side or on one side of the workpiece to deliver the cutting liquid medium to a cutting area that is submerged in or disposed in the air. The cutting liquid medium is conveniently water as described above, which is deionized or ionized to varying sizes to function as the desired electroeroded cutting medium.

Since modern hard drives require smaller and faster motors with more sophisticated critical features, there are practical challenges to manufacturing and measuring more sophisticated features using this ECM process and the like. For example, smaller motors have corresponding smaller and more sophisticated grooves that are made in the bore and shaft than conventional ones. ECM processes are generally known in the art. However, ECM processes have a need to place grooves precisely and simultaneously on surfaces across gaps that must be precisely measured. Defects in mechanical tolerances can cause misalignment of electrodes and workpieces, resulting in nonuniform gaps and correspondingly nonuniform depths of fluid dynamic grooves. Precise measurement of these grooves requires understanding the wear pattern and ultimate design to make a better motor. Since conventional methods used to measure part wear in motors have been developed for the measurement of larger features, they are inadequate to measure to find smaller size features in current bores and shaft grooves.

Accordingly, what is needed is a system and method that can overcome these deficiencies and measure finer features such as grooves on the motor and grooves on the shaft.

The present invention provides a system and method for specifying and measuring fluid dynamic grooves produced by an ECM process. In addition, the present invention provides a system and method for specifying and measuring erosion of such grooves.

Methods for measuring bore erosion include aligning the stylus and gauge pins, covering the length of the journal, moving the stylus to its apex position, positioning it in the lowest groove by rotating the hub, and securing the hub. Rotating to position, scanning between the first and second endpoints collecting data during the scan, analyzing the data by fitting the data into a line, placing the lowest peak in the groove, and calculating erosion It includes a step. The process then repeats after rotating the workpiece to a new position. Typically three such measurements occur with each measurement occurring after the workpiece has been rotated by 120 degrees.

Systems for measuring bore erosion include gauge pins for alignment, theta chucks for supporting the workpiece, theta chucks for rotating theta chuck and workpiece about the axis of rotation, while the workpiece is rotating A stylus pin for probing topography on the workpiece, a stylus for supporting the stylus pin, and a surface scanner for controlling and moving the stylus along the direction of stylus movement by measuring the response of the stylus to the surface form of the workpiece. It includes. The gauge pin may have a size substantially the same as the size of the bore diameter being measured, the gauge pin having an area along a line with a maximum change of 30 microns in height over a length of about 20 mm.
The method of aligning the axis of movement with respect to the axis of the workpiece according to the invention comprises the steps of providing a gauge pin having a size substantially equal to the size of the bore diameter being measured; Scanning the gauge pin over a length of 10 mm to 30 mm; And scanning the gauge pin along a line with endpoints such that the maximum height deviation between the endpoints is less than 30 microns.
Scanning the gauge pin preferably includes scanning the gauge pin over a length of about 20 mm.

1 is a flow chart showing the preferred steps used to measure land erosion of a fluid dynamic motor having grooves in the bore and shaft in accordance with one embodiment of the present invention;

2 is a block diagram illustrating a groove measuring system according to an exemplary embodiment of the present invention.

3 shows a scan line of a typical measurement performed on a fluid dynamic motor bore with grooves, FIG.

4 shows a typical profile of a groove of a bore including radial erosion, peaks, original diameter and lowest peak,

5 is a plot showing a typical scan of a fluid dynamic motor bore with grooves,

6 is a plot showing a typical scan showing the results and analysis of outer bore erosion pattern measurements.

The present invention provides a system and method for measuring and specifying fluid dynamic grooves produced by an ECM process. The present invention also provides a system and method for measuring and characterizing the erosion of the groove.

1 is a flow chart showing the preferred steps used to measure land erosion of a fluid dynamic motor with grooves in the shaft and bore in accordance with an embodiment of the present invention. First, in step 105, the rotary fixture is aligned such that the direction of rotation of the stylus and the axis of rotation coincide. Then, in step 110, the stylus movement endpoints during the measurement are defined by moving the stylus from the first journal point of the stylus to the second journal point for the groove. The first journal point and the second journal point of the stylus for the groove are typically set to 11.8 mm.

Then, in step 115, the stylus is moved to the vertex position determined by the motor structure. In step 120, the position of the lowest groove MIN is set by keeping the stylus stationary and rotating the hub. In step 125, after the position of the MIN is set, the hub is further rotated by theta angle, whereby the position of the stylus becomes MIN + Θ angle. The value of Θ is typically set to 12, but this is determined based on the number of grooves.

Then, in step 130, the stylus moves across the fixture from the first journal point to the second journal point, scanning the surface and creating a profile of the surface as shown in the accompanying drawings. In step 135, a best fit line of intrinsic diameter and vertex region is calculated using a least square fit algorithm. Next, at step 140, the lowest peak is positioned and the distance from the least squared fitting line to the lowest peak is calculated. In step 145, the radial erosion (R _i) for such a hub angle is calculated. Typically, three radial erosions (R _i ) are calculated, one for each hub angle offset 120 ° from the previous angle, and three values, R ₁ , R ₂ , R _3, are calculated.

Then, in step 150, the hub is rotated by 120 ° to the position of MIN + Θ + 120 °. In step 155, it is determined whether the hub has been rotated to a position of MIN + Θ + 360 ° or more. If it is determined in step 155 that the position of the hub is not greater than MIN + Θ + 360 °, steps 130 to 155 are repeated. Typically, step 155 performs three scans and calculates three erosion values R ₁ , R ₂ , and R ₃ at three different angles, as described in step 145. In step 150 the hub is rotated 120 °, but it may be rotated at any angle, for example 30 ° or 60 °. There is no restriction on the amount of rotation. If in step 155 the position of the hub is determined to be greater than MIN + Θ + 360 °, then step 160 is executed.

In step 160, the total erosion is calculated by averaging the three measured erosion values R ₁ , R ₂ , R ₃ and multiplying the average value by two. If N scans are performed, not just three, as described above with reference to steps 145-155, then the total erosion is determined by averaging all measured erosion values and multiplying the average by two (i.e., total erosion = ∑ (R _i ) _{I = 1 TO N} / N). Finally, in step 165 the fixture is removed.

2 shows workpiece 210, theta chuck 215, theta stage 220, axis of rotation 225, stylus tip 230, stylus 235, gauge head 237, surface scanner 240. And a groove measuring system according to an embodiment of the present invention including a direction of movement of the stylus 245. Typically, the workpiece 210 is a motor shaft with grooves therein or a motor sleeve with grooves or gauge pins for calibration. Theta chuck 215 is a conventional chuck used to firmly hold and hold workpiece 210 during profiling. Theta stage 220 rotates the workpiece 210 to a specific profiling position, and typically includes a stepper or servo motor capable of rotating the workpiece 210 0 to 360 ° with a resolution of 0.1 °. do. Theta stage 220 rotates the workpiece 210 about an axis of rotation 225 that is typically set to coincide with the axis of symmetry of the workpiece 210. Typically, theta stage 220 moves the workpiece in three different orientations (0 °, 120 °, 240 °), and scanning is performed as described above with reference to FIG.

By moving the stylus 235 along the same direction as the rotation axis 225, the stylus tip 230 is moved over the workpiece 210. When the stylus tip 230 moves over the workpiece 210, the stylus 235 moves up and down according to the appearance of the workpiece 210. The movement of the stylus 235 is detected by the gauge head 237, which generates an electrical signal in response to the movement of the stylus 235 to mimic the change in appearance of the workpiece 210. The gauge head 237 measures the mechanical motion of the stylus 235 using a piezo, measures the capacitance difference between the stylus tip 230 and the workpiece 210 or the stylus tip 230 and the workpiece 210. As with measuring electron tunneling of the liver, electrical signals can be generated by means known in the art. Stylus tip 230 is mounted to stylus 235, which not only holds and drives stylus tip 230, but also provides coupling to head gauge 237. Surface scanner 240 is a conventional contact surface profiler used to move stylus 235 and stylus tip 230 as well as to record and analyze data generated by electronics within the surface scanner. The surface scanner 240 drives the stylus 235 and the stylus tip 230 in the direction of stylus movement 245 generally parallel to the axis of rotation 225. The stylus tip is preferably made of carbon.

FIG. 3 shows scan lines typically measured in a fluid dynamic motor bore with grooves. Although only three grooves are shown in FIG. 3, there is no restriction on the number of grooves. Typically, the actual number of grooves may be 10-20. The scan direction is from left to right as indicated by the direction of the scan line 330. See FIG. 4 below for a more detailed description of the scan. Scan line 330 is in a direction in which stylus 235 moves and corresponds to stylus movement 245.

4 shows a general profile of the grooves in the bore including the initial diameter 410, the lowest peak 415, the vertex 420, the least squared line 425, and the ECM radiation erosion. The initial diameter 410 is data generated by the scanner 240 and represents the surface shape of the bore groove erosion pattern. The lowest peak 415 represents the lowest portion of the erosion pattern and is used to determine the ECM radiation erosion 430. Peak 420 represents the highest point of the erosion pattern and is used to determine ECM radiation erosion 430. Least squares line 425 is calculated using a least squares fitting algorithm known in the art. The least squares line 425 is calculated using the vertex and initial diameter 410 (quiet zone).

ECM radiation erosion 430 is defined as the distance between the smallest square line 425 passing through the vertex and the initial diameter (quiet zone) and the lowest peak of the ECM bore on a given journal. ECM radiation erosion 430 is determined using the following equation.                 

Ri = radial erosion obtained from the i th scan

N = number of scans

R _mean = ∑ (Ri) _{i = 1 to N} / N

Land erosion = 2 * R _mean

5 is a diagram illustrating a general scan of a fluid dynamic motor bore with grooves. The x-axis of FIG. 5 shows a scan length of 11.8 mm. The scan length is generally set in the range of 5 mm to 20 mm. The scan length is chosen to optimize the measurement speed and resolution. The longer the scan length, the longer the measurement will take and vice versa. The y-axis represents the depth profile of the erosion pattern such that the combination of scan length and depth profile provides accurate observation of the grooves along the direction of movement of the stylus 235.

6 is another embodiment of a general scan showing the analysis and results of external bore erosion pattern measurements. The scan shown in FIG. 6 is obtained in accordance with the present invention by first aligning the surface scanner 240 with the axis of rotation 225 using a workpiece 210 which is a gauge pin having the same size as the bore diameter. Scanning of the gauge pin over 20 mm performs this alignment. The scanned profile should be straight with the maximum height difference between the ends of 30 microns or less. Alignment about the (theta) axis (run out) is performed by positioning the stylus on top of the gauge pin and rotating the pin a full revolution. The reading of the stylus should be constant through rotation. When the alignment is performed, the θ stage 220 is taken to the home position. A portion of the workpiece to be measured is mounted on the chuck 215 so that the land of the workpiece to be measured is above and parallel to the upper edge of the stylus 235. The stylus 235 is then moved inside the bore so that there is a travel distance of about 11.9 mm between the start points of the edge edge cans of the bore. The stylus then comes into contact with the workpiece 210. The UCS of X and Z is set to zero and the workpiece is scanned. The position of the stylus 235 is set at the vertex of the portion (eg 2.5 mm position) and the Y stage is manually moved to the lowest point of the bore. The chuck 215 is rotated at an angle at which the stylus indicator indicates the lowest value indicating the presence of the groove. The chuck 215 is rotated about 12 degrees. The position is labeled 0 ° relative to the rest and serves as a reference point.

At this time, UCS X is set to 11.8 and Z is set to 0 as shown in FIG. 5. Once the reference point is set, the scan is performed and the surface profile is measured. Profile data is shown in FIG. 6. Once the surface scan is complete, the data is analyzed by manually finding the highest point on the scan, which is defined as the shortest distance from Z = 0. Delta z shown in FIG. 6 is an R1 outer bore. θ stage 220 is rotated 120 ° and the same measurement is performed to obtain a second measurement at a second position, referred to as the R2 outer bore. θ stage 220 is again rotated 120 ° and another measurement is taken to obtain a third measurement R3 outer bore. Finally, bore erosion is calculated using the equation

R _mean outer bore = (R1 outer bore + R2 outer bore + R3 outer bore) / 3.

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Outer bore erosion = R _mean outer bore * 2.

Those skilled in the art will recognize that the present invention has been described above by way of the preferred embodiment, but not limited thereto. The various features and aspects of the invention described above may be used individually or in combination. In addition, while the present invention has been described in connection with its implementation in a particular environment and with respect to a particular field, those skilled in the art will recognize that the usefulness of the present invention may be used in a number of environments and practices, without being limited thereto.

Claims (16)

As a method for measuring bore erosion, a) providing a workpiece comprising a journal; b) aligning a stylus to the workpiece; c) defining a length of the journal; d) moving the stylus to a vertex position; e) positioning the lowest groove by rotating the hub; f) rotating the hub to a fixed position; g) scanning between a first end point and a second end point for the movement of the stylus and collecting data during the scanning; h) analyzing the data by fitting the data to a line; i) positioning the lowest peak in the groove; And j) calculating erosion Bore erosion measurement method comprising a. The method of claim 1, Rotating the workpiece to the second position by an amount; And And repeating said steps a) to j) at said second position. The method of claim 2, And said predetermined amount of rotation is 120 [deg.] From the first measurement position. The method of claim 2, Rotating the workpiece to a third position by a second predetermined amount; And And repeating step a) to step h) at the third position. The method of claim 4, wherein And said second predetermined amount of rotation is 120 [deg.] From the second measurement position. The method of claim 1, Fitting the data to a line comprises using a least square fit. The method of claim 1, Aligning the stylus to the workpiece comprises measuring a gauge pin having a size equal to the size of the bore diameter being measured. The method of claim 7, wherein Measuring the gauge pin comprises scanning the gauge pin over a length of 10 mm to 30 mm. The method of claim 7, wherein Measuring the gauge pin comprises scanning the gauge pin over a length of 15 mm to 25 mm. The method of claim 7, wherein Measuring the gauge pin comprises scanning the gauge pin over a length of 20 mm. The method of claim 7, wherein Measuring the gauge pin comprises scanning the gauge pin along a line with a maximum height deviation between the ends of less than 30 microns. A method of aligning a moving axis with respect to an axis of a workpiece, Providing a gauge pin having a size equal to the size of the bore diameter being measured; Scanning the gauge pin over a length of 10 mm to 30 mm; And Scanning the gauge pin along a line with endpoints such that the maximum height deviation between endpoints is less than 30 microns Alignment method of the moving axis comprising a. The method of claim 12, And scanning the gauge pin comprises scanning the gauge pin over a length of 20 mm. delete delete delete

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