The present disclosure is generally directed to machine tools and more particularly to runout testing for spindles of machine tools using hydrodynamic bearings.
Spindles used in machine tools, for example, lathes and drill presses often use either ball bearings or hydrostatic bearings for rotatably mounting the spindle. These bearings are active all the time and bearing runout, that is, movement off a center axis of the spindle, can be measured while the tool is rotated very slowly—well below operating speeds. It is common for runout in these tools to be measured using a metallic element mounted to the spindle and a capacitive sensor element put in close proximity to the spindle-mounted ceramic element. As the spindle is rotated, a change in capacitance is measured. The change in capacitance is translated to a runout measurement for the spindle. The distance between the spindle-mounted element and the sensor element must be quite small, on the order of 100 micrometers (μm). Such measurement instruments are commercially available from, for example, IBS Precision Engineering of Stuttgart, Germany.
Some precision grinding or machining applications, where very high tolerance parts are made, use hydrodynamic bearings for the spindle mount. Shafts or journals using hydrodynamic bearings, unlike ball bearings or hydrostatic bearings, are only centered after the shaft or journal has reached a minimum operating speed. When at rest, the shaft may droop several millimeters off center. However, while operating at speeds of tens of thousands of rpms up to even 100,000 rpms, the shaft may have a runout on the order of only a few microns. Because the capacitive sensors must be within about 100 μm of the spindle mounted element but the spindle with hydrodynamic bearings begins off center as much as several millimeters, the capacitive sensors would be damaged as the spindle spins up and reaches operating speed. Attempts to position the capacitive sensors after the spindle is operating at speed are risky because any contact between the spindle-mounted ceramic element and the capacitive sensor can destroy the sensor and damage the spindle. Such a placement process is further hampered by difficulty in maneuvering the capacitive sensor assembly and associated cabling inside the work area of the machine tool, which typically has covers and interlocks to prevent work chamber access when the spindle is operating. Therefore, these capacitive runout sensors cannot be used to map spindle runout for hydrodynamic bearings.
Another type of runout measurement tools are optical in basis. Such optical runout measurement tools use mirrors mounted on a shaft loaded in the chuck of the spindle. These sensors also cannot be operated at speed because the retro reflectors cannot compensate for the change in position as fast as is needed when the spindle is operating at speed.
Because of the difficulty in measuring runout in spindles using hydrodynamic bearings, machine tools so equipped are mostly just monitored for generating work product that is out of specification. This is both wasteful and time consuming and also foregoes early detection of bearing problems that can lead to costly equipment failures.
In an aspect of the disclosure, a runout calibrator adapted for use in a machine tool having a spindle with a chuck that rotates includes a shaft adapted at a first end for mounting in the chuck, the shaft having a centerline at a longitudinal axis of rotation, and a head mounted to a second end of the shaft. The head has at least one line of symmetry that is coincident with the centerline of the shaft. The head also has a reflective surface. The runout calibrator includes a sensor assembly with at least one sensor, the at least one sensor having light source and a light detector. The sensor assembly can be mounted to measure a distance between the at least one sensor and the reflective surface of the head.
In another aspect of the disclosure, a method of testing runout in a machine tool having a spindle with a chuck where the spindle is coupled to the machine tool via a hydrodynamic bearing includes mounting a head assembly into the chuck and disposing a sensor assembly proximate the head assembly, the sensor assembly has one or more sensors that measure a distance to a remote object. The head assembly can include a shaft having a centerline at a longitudinal axis of rotation and has a first end and a second end with the first end adapted for mounting in the chuck. The head assembly also includes a head mounted to the second end of the shaft, the head having at least one line of symmetry coincident with the centerline of the shaft. The head also has a reflective surface. The method continues with activating the machine tool to rotate the spindle and the head assembly at an operating speed, the operating speed being a rotational speed of the spindle at which the machine tool is operated for beneficial use. After the spindle and the head assembly reaches the operating speed, the method may continue by sampling a plurality of distances between each of the one or more sensors in the sensor assembly and the head and for each of the one or more sensors, calculating a difference between a maximum distance and a minimum distance of the plurality of distances to develop a runout value for the head at each sensor of the one or more sensors. The method may continue by determining when any the runout value for any of the one or more sensors exceeds a predetermined limit to indicate that the machine tool requires maintenance.
In yet another aspect of the disclosure, a runout calibrator for use in measuring runout of a spindle of a machine tool using at least one hydrodynamic bearing includes a head assembly with a shaft adapted at a first end for mounting in a chuck of the machine tool, the shaft having a centerline at a longitudinal axis of rotation and a head mounted to a second end of the shaft, the head having at least one line of symmetry coincident with the centerline of the shaft and a reflective surface disposed on all exposed areas of the head. The runout calibrator may also include a sensor assembly including three optical sensors mounted orthogonal to each other.
These and other aspects and features will be more readily understood when reading the following detailed description when taken in conjunction with the accompanying drawings.
Referring now to the drawings, and with specific reference to
A representative runout map 120 for the spindle 108 illustrates that the work tip 118 moves significantly off-center as the spindle 108 is rotated. Runout has components in three dimensions, that is, both vertically and horizontally as well as axially and is almost exclusively influenced by the bearing 114 and any other support bearings or end bearings holding the spindle 108. As discussed above, hydrostatic bearings using a pressurized oil source, or ball bearings are active at all times and runout can be measured simply by low-speed rotation of the spindle.
When a bearing is a hydrodynamic bearing 114, the spindle 108 will droop considerably when the spindle 108 is at rest. As shown in
Referring now to
Because the sensors 136, 138, 140, can be located so far from the head 132, there is no difficulty in positioning the sensor assembly 134 proximate the head 132 while the machine tool 100 is still at rest. After the spindle reaches an operating speed, in many cases 30,000 RPMs or more, the spindle 108 will come back on center and the runout measurement may be safely taken.
A controller 142 may be coupled to each of the sensors 136, 138, 140 via a cable 144 or a wireless network 146 that carry a signal with distance information from each sensor 136, 138, 140. Using either communication type, the sensor assembly 134 may be safely placed while the machine tool 100 is at rest and all the interlocks kept fully functional during the runout testing process. That is, the cable 144 may be brought out along a seal in cover 106 or the measurement data may be transmitted wirelessly using the wireless network 146 so that the cover 106 may remain in place and operator safety is not compromised during the test.
The head 132 may be a sphere covered with a reflective surface. Spheres with an accuracy of 10-20 nanometers are commercially available from a number of suppliers. Because the head 132 is so highly symmetric, the difference between a maximum distance measurement for any individual sensor 136 and a minimum distance measurement for the same sensor 136 can be attributed solely to runout of the spindle 108. To ensure that sufficient light is reflected from the head 132 back to any respective sensor 136, 138, or 140, the head 132 may be made from a reflective surface. These surfaces may include a precision honed or lapped carbide ball or an Inconel ball. Other surface finishes may include polished stainless steel or plated with chrome, silver, gold or another reflective coating. In some embodiments, it may be desirable for the head 132 to be corrosion resistant, for example, when corrosive substances may be used in the machining process.
Another embodiment of the head assembly 133 is depicted in
Yet another embodiment of the runout calibrator 130 is set forth in
In general, the present disclosure can find industrial applicability in a number of different settings. For example, the present disclosure may be employed in manufacturing tools used to make parts for a variety of machines, such as but not limited to, engines, transmissions and actuators. Such machines may be employed in many different end products, such as, but not limited to those used in the earth-moving, construction, mining, agriculture, transportation, and marine industries.
At block 204, a sensor assembly 134 may be disposed proximate the head assembly 133. The sensor assembly 134 may include three sensors 136, 138, 140, arranged on orthogonal axes. In other embodiments, the sensor assembly 134 may include more or fewer than three sensors, for example in the embodiment shown in
The machine tool 100 may be activated to rotate the spindle 108 and the head assembly 133 at an operating speed as shown at block 206. In an embodiment, the operating speed may be that at which the machine tool 100 is operated for beneficial use, that is, a speed typical during machining operations. For some machine tools 100 this speed may be 30,000 revolutions per minute or more.
At block 208, after the spindle 108 and the head assembly 133 reaches its operating speed, a plurality of distances may be sampled between one or more sensors 136, 138, 140 in the sensor assembly 134 and the head 132. That is, each sensor 136, 138, 140 may separately report a continuous measurement of distance between that individual sensor and the head 132. The controller 142 may log these distances as they are reported.
For each sensor 136, 138, 140 a maximum distance between each individual sensor and the head 132 may be identified along with a minimum distance between each individual sensor and the head 132 as shown shown at block 210. The difference between the maximum distance in the minimum distance is the runout value. These individual runout values may be compared to a specified maximum allowable runout value for the particular machine tool 100. In other embodiments, the sensors 136, 138, 140 may internally calculate a running difference between a maximum and minimum measurement for that sensor.
At block 212, each calculated runout value may be compared to a predetermined limit for runout value to determine that the machine tool 100 requires maintenance due to excessive runout. In an embodiment, the controller 142 may post a message or turn on alarm as an indicator that the runout test has failed.
Use of the apparatus and method discussed above for determining runout in a machine tool 100 using hydrodynamic bearings 114 is a significant advantage to machine tool 100 operators in that runout calibration can now be performed quickly and safely at high operating speeds. Using this diagnostic capability significantly reduces the chance of creating parts that fail to meet their tolerance specifications. Beyond that, this testing allows early diagnosis of bearing 114 problems before the bearing degenerates to a point that a very costly spindle failure occurs.