Brake Lathe With Electronic Feed Control

The present invention relates to brake lathes for truing brake rotors.

Brake lathes for truing brake rotors have long been known in the art. Generally, in these devices, a rotor is mounted upon an arbor so that the arbor and rotor may be rotated together about a common axis by an alternating current motor. A tool holder is provided for holding cutting tools in working engagement with a braking surface of the rotor. The tool holder is mounted upon a track which allows it to move translationally to draw the cutting tool progressively across the braking surface of the rotor as the rotor rotates with the arbor during a truing operation. In devices of the prior art, movement of the tool holder has been accomplished by a mechanical drive, such as a worm drive, which is linked to the arbor and alternating current motor by a drive train which may include gears, clutches and the like. While these devices of the prior art have provided for selection of the rate of progression of the cutting tool across the rotor surface by changing gear ratios in the drive train between the arbor drive tool holder drive, the selection of cutting rate has been limited to a small number of fixed rates corresponding to the specific gear ratio combinations made available within the drive train. The gear pairs and clutches of these drive trains of the prior art lathes produce vibration and noise and transmit noise from the arbor to the tool drive which increases the difficulty of achieving a satisfactorily smooth surface during the truing process. In recent years, brake rotors have been made of increasingly diverse materials, making it necessary to provide many different tool progression cutting rates if a lathe is to achieve an acceptable finish on the braking surface of all commercially available rotor products.

It is an object of the present invention to provide a brake lathe with a broad range of continuously selectable tool progression cutting rates to allow a high quality surface to be achieved during truing operations on rotors fabricated of a wide variety of materials.

It is a further object of the present invention to provide a brake lathe which produces minimum noise and vibration at the tool-rotor interface.

It is also an object of the present invention to provide a brake lathe which allows a constant cutting width to be maintained as the cutting tool progresses radially across the braking surface of a rotor during a truing operation.

In keeping with the above objectives, a brake lathe according to the present invention includes a direct current motor and a drive train connecting the direct current motor to a tool holder such that a tool held in the tool holder moves radially away from the arbor axis in direct proportion to rotation of the drive shaft of the direct current motor. An electronic tool feed power control circuit is provided for maintaining the speed of rotation of the direct current motor drive shaft at a constant selected speed regardless of radial forces acting upon the cutting tool.

The tool feed power control circuit of the present invention includes a converter/filter circuit, a regulator circuit, a sensing circuit, and a speed control circuit. The converter/filter circuit converts alternating current to direct current and the regulator circuit provides a selected constant direct current output voltage. A speed selection control includes a rheostat and provides a reference voltage corresponding to a selected speed. The sensing circuit compares the absolute value of a voltage drop across the direct current motor to the reference voltage and produces an error voltage output presentative of the difference between the absolute values. The speed control circuit provides a direct current power voltage to the direct current motor and monitors the error output voltage to adjust the power voltage in accordance with variation in motor speed.

FIG. 1 is a plan view of a brake lathe comprising an embodiment of the present invention.

FIG. 2 is a front plan view of a brake lathe comprising a preferred embodiment of the present invention.

FIG. 3 ice a schematic illustration of an electronic power circuit of a brake lathe comprising a preferred embodiment of the present invention.

As shown in FIGS. 1 and 2, brake lathe 10 comprising an exemplary embodiment of the present invention includes arbor 12 and tool holder 14. Arbor 12 is connected to alternating current motor 16 by a drive train, including shaft 17, to rotate arbor 12 at constant speed. The drive train connecting alternating current motor 16 to arbor 12 may also include a clutch and multiple gear pairs to allow arbor 12 to be rotated at different selected constant speeds by alternating current motor 16.

Tool holder 14 is mounted upon a track which allows tool holder 14 to move translationally in a direction perpendicular to the axis of arbor 12, as shown by the doubled headed arrow A in FIG. 1. Direct current motor 18 may drive tool holder 14 outward along path A by means of a tool feed drive train including drive shaft 19, a clutch, and threaded shaft 20, such that tool holder 14 moves outward a distance directly proportional to the number of revolutions turned by drive shaft 19. The speed at which drive shaft 19 of motor 18 turns, and tool holder 14 is driven away from arbor 12, is determined by setting speed control knob 30.

During a rotor truing operation, a brake rotor to be trued is mounted upon arbor 12 as shown in phantom in FIG. 1. The clutch of the feed drive train is disengaged and tool holder 14 is moved inwardly towards arbor 12 by use of crank 21 on the end of threaded shaft 20. A cutting tool, also shown in phantom in FIG. 1, is set at a desired cutting depth to the inside of the breaking surface to be trued. The desired feed rate is set by means of speed control 30 with the assistance of index 31. Alternating current motor 16 is then energized to rotate arbor 12 and the rotor, together, at a constant speed. Direct current motor 18 is then energized to move tool holder 14 and the cutting tool held thereby, away from arbor 12 at constant speed, thus maintaining a constant radial width of cut during the truing operation.

Tool feed power circuit 40 of break lathe 10, shown in FIG. 3, assures that direct current motor 18 continues to operate at a constant speed, in accordance with the setting of speed control knob 30, independent of the magnitude of any radial forces exerted on the cutting tool throughout the truing process. The resistance and capacitance of each of the numbered resistor and capacitor elements of tool feed power circuit 40 is indicated in Table I or II below.

Tool feed power circuit 40 generally includes four distinguishable subcircuits, as indicated by the dotted boxes of FIG. 3; direct current converter/filter circuit 50, regulator circuit 60, sensing circuit 70, and speed control circuit 80.

Converter/filter circuit 50 of the exemplary embodiment changes incoming alternating current voltage at 51 into a filtered direct current voltage of about 44 volts at 52. Regulator circuit 60 turns the 44 volt direct current input at 52 into a constant 32 volt direct current power source at 61.

Sensing circuit 70 compares a voltage difference, V.sub.6 minus V.sub.5, across direct current motor 18 with a reference voltage V4 present at 34, which is set by rheostat 32 connected to control knob 30, and which is representative of the speed at which the operator wishes tool holder 14 to advance. In sensor circuit 70, voltage V.sub.5, present at 71 and 72, is inverted, or made negative, and then added to V.sub.6, present at 73 and 74. The resulting voltage is summed with reference voltage V.sub.4 to produce an error indicative voltage VE at 78. As the operator wants direct current motor 18 to run at a speed proportional to V.sub.4, according to the setting of rheostat 32 by control knob 30, and the motor is actually running a speed proportional to V.sub.5 minus V.sub.6, VE is representative of any speed error.

Speed control circuit 80 provides the power voltage to run direct current motor 18 and adjusts the power voltage to compensate for varying loads and maintain constant speed of direct current motor 18. Regulator 82 is a switching regulator with corrective feedback which provides the power voltage to direct current motor 18. Regulator 82 utilizes a pulse width modulation technique to increase or decrease the average power voltage provided to current motor 18 depending upon the voltage applied at pin 6, which is connected to the error voltage output V.sub.E of sensing circuit 70. When error voltage V.sub.E is 0, the output voltage provided by regulator 82 at pin 1 remains constant. If V.sub.E, at pin 6, varies from 0, the modulated average voltage provided by regulator 82, at pin 1, is varied accordingly to correct for the variation and maintain operation of direct current motor 18 at constant speed.

Potentiometers R.sub.11 and R.sub.14 are provided for feedback adjustment to increase or decrease the sensitivity of tool feed power control circuit 40 to variation in the speed of direct current motor 18, and to dampen the response of tool feed power circuit 40 to changes in the speed of direct current motor 18, respectively. Also, within the limited adjustment permitted by R.sub.11 and R.sub.14, and dependent upon the constant speed of arbor 12, the tool feed rate corresponding to settings of control knob 30 relative to index 31 may be calibrated by adjustment of potentiometers R.sub.11 and R.sub.14.