WO2012021429A1

WO2012021429A1 - Load cell for chuck with jaw for workpiece having constant holding force

Info

Publication number: WO2012021429A1
Application number: PCT/US2011/046890
Authority: WO
Inventors: Jyi-Jiin Luo; Madhav Puppala
Original assignee: Illinois Tool Works Inc.
Priority date: 2010-08-10
Filing date: 2011-08-08
Publication date: 2012-02-16
Also published as: EP2603341A1

Abstract

A chuck for securing a workpiece in a machine provides a variable clamping force on the workpiece. The chuck includes a base element having a longitudinal center, and a plurality of jaw elements movable to clamp the workpiece. The jaw elements exert a measurable clamping force on the workpiece. One or more load cells are operably connected to the jaw elements to measure the clamping force exerted on the workpiece. The load cells are positioned within the jaw. A drive moves the jaw elements to vary the clamping force exerted on the workpiece and a control system includes a comparator to compare the measured clamping force to a predetermined clamping force. The jaw elements are movable, as the chuck is rotating, to adjust the clamping force on the workpiece relative to the predetermined clamping force. A jaw element includes an integrated load cell.

Description

TITLE

LOAD CELL FOR CHUCK WITH JAW FOR WORKPIECE HAVING CONSTANT

HOLDING FORCE

BACKGROUND

[0001] The present invention is directed to a jaw for use in a machine operating on a workpiece. More particularly, the present invention is directed to a load cell for use with a chuck with a jaw for use in a machine in which a jaw exerts a constant holding force on the workpiece.

[0002] In machines that operate on a rotating workpiece, such as lathes and the like, typically, the workpiece is held in a chuck to rotate the workpiece relative to a tool (such as a blade) so that the tool can operate on the workpiece. The chuck, which is comprised of multiple moveable or adjustable jaws (often three jaws), exerts a force on the workpiece to secure or clamp the workpiece in the chuck between the jaws.

[0003] There are three commonly used chuck arrangements. The first is an arrangement in which the jaws grip the workpiece on an outer surface such that the gripping force is an inwardly exerted force. That is, the jaws move inward, toward the workpiece to effect the grip. This is referred to as an external grip chuck.

[0004] The second arrangement is one in which the jaws grip an internal surface of the workpiece, such as that which may be used to grip a hollow shaft or a bushing for machining the outer surface of the shaft or bushing. In this arrangement, the jaws gripping the interior surface exert an outward force, that is they move outwardly, toward the workpiece to effect the grip.

[0005] The third arrangement is one in which the workpiece is gripped in an axial direction, as when an end of the workpiece is gripped. This third arrangement is referred to as axial gripping.

[0006] In the external grip chuck arrangement, the workpiece is typically clamped by the jaws at a very high initial clamping force. With an exterior grip chuck, this compensates for the loss of clamping force as the chuck rotates. The clamping force decreases with increased rotational speed of the chuck. Because the clamping force decreases with increased rotational speed, the maximum velocity of the chuck (and thus, the workpiece) is limited.

[0007] The initial clamping force is, however, limited by the materials and structure of the chuck and by the need to preclude permanent distortion of the workpiece. Thus, there is a balance between the upper end of the initial clamping force that can be exerted on the workpiece and the maximum rotational or operating speed of the chuck and machine tool.

[0008] With the internal grip arrangement, the workpiece is initially gripped at a low clamping force to compensate for the increased clamping force as the chuck rotates. This limits the maximum velocity of the chuck as the clamping load continuously increases as the speed of the chuck increases. Here, permanent distortion of the workpiece can occur if the rotational speed of the chuck is too high due to the increase in the clamping force.

[0009] In the axial grip arrangement, the jaws tend to pull back toward the body of the chuck (outward) as the speed of the chuck increases. Excessive axial gripping force on the workpiece can result in distortion of the workpiece, which can cause poor machining quality. In contrast, insufficient axial force on the workpiece can allow the workpiece to move during machining also causing poor quality machining.

[0010] One solution to balancing the load even at high chuck rotational speeds has been disclosed in copending, Puppala, U.S. Patent application Serial No. 12/831 ,887, the disclosure of which is incorporated herein by reference. In this application, a load cell is used at the clamping location to measure the load on the workpiece and provide a signal for the clamping force to increase or decrease depending upon the load sensed at the workpiece.

[0011] Accordingly, there is a need for a chuck jaw with an integrated load cell.

SUMMARY

[0012] A chuck for securing a workpiece in a machine provides a variable clamping force on the workpiece. The chuck includes a base element having a longitudinal center and a plurality of jaw elements movable to clamp the workpiece. The jaw elements exert a measurable clamping force on the workpiece.

[0013] The chuck includes means for measuring the clamping force exerted on the workpiece, the means being positioned within the jaw. In one

embodiment, the means for measuring the clamping force includes one or more load cells.

[0014] The chuck also includes means for moving the jaw elements to vary the clamping force exerted on the workpiece. Such means for moving the jaws includes, for example, a hydraulic drive system, a mechanical drive system, an electromechanical drive system, an electronic drive system, and the like.

[0015] Comparison means for compares the measured clamping force to a predetermined clamping force. Such comparison means includes, for example, a control system that includes a comparator for comparing the measured force to the predetermined force acting on the workpiece.

[0016] The chuck is configured such that the jaw elements are move, as the chuck rotates, to adjust the clamping force on the workpiece relative to the predetermined clamping force.

[0017] In a present chuck, the one or more load cells are disposed in the jaw. The load cells can include one or more strain gauges disposed in the jaw.

[0018] A present chuck includes a transmitter operably connected to the measuring means. A preferred transmitter is a wireless transmitter that provides a signal to the comparison means to determine the difference between the clamping force and the predetermined clamping force.

[0019] A jaw element for use in the chuck includes a load cell integrated therewith. The jaw element includes one or more strain gauges and a transmitter, preferably a wireless transmitter, is operably connected to the one or more load cells.

[0020] The one or more loads cell include one or more strain gauges and the wireless transmitter provides a signal to the control system to determine a difference between the clamping force and the predetermined clamping force.

BRIEF DESCRIPTION OF THE DRAWINGS [0021] The benefits and advantages of the present invention will become more readily apparent to those of ordinary skill in the relevant art after reviewing the following detailed description and accompanying drawings, wherein:

[0022] FIG. 1 is an illustration of a chuck configured with an external gripping arrangement, with a jaw arrangement in which the jaws exert a constant clamping force, the jaw including an integrated load cell;

[0023] FIG. 2 is an exploded view of the jaw with an integrated load cell, the illustrated jaw being a three piece jaw;

[0024] FIG. 3 is a view of a two piece jaw with an integrated load cell;

[0025] FIG. 4 is a view of the load cell showing the strain gage locations for load measurement; and

[0026] FIG. 5 is an exemplary Wheatstone bridge for the strain gauges.

DETAILED DESCRIPTION

[0027] While the present invention is susceptible of embodiment in various forms, there is shown in the drawings and will hereinafter be described a presently preferred embodiment with the understanding that the present disclosure is to be considered an exemplification of the invention and is not intended to limit the invention to the specific embodiment illustrated.

[0028] Referring to figures, and in particular to FIG 1, there is shown an embodiment of a chuck 10 with a jaw arrangement 12 for exerting a constant holding force on the workpiece W. The chuck 10 is shown without any machine tools, drives, or the like for ease of illustration. However, it will be appreciated that typically, the chuck 10 is used to hold a workpiece W that is rotated by a drive and is being operated on by the machine tool. The illustrated chuck 10 is configured for external gripping of the workpiece W - that is, gripping the workpiece W on an outer surface and exerting an inward clamping force.

[0029] The chuck 10 includes a base element 14 (also referred to herein as

"base") that is mounted for rotation by the drive (not shown). Jaw elements 16 (also referred to herein as "jaw(s)") are mounted to the base that move radially toward and away (as indicated by the arrow at 18) from the longitudinal center C to clamp and release the workpiece W. The jaws 16 typically include a contact point or surface 20 that contacts the workpiece W. The jaws are maintained in a track or guide 22 to assure smooth movement to clamp and release the workpiece W.

[0030] The system includes means for moving the jaws relative to the center. In the illustrated arrangement, the jaws are controlled (moved) by a hydraulic system illustrated diagrammatically at H, which will be readily understood by those skilled in the art. Other means includes mechanical drives, electro-mechanical drives, e.g., servomotors, and the like.

[0031] Strain gauges or load cells 24 are located on each of the jaw elements 16. The strain is transmitted to a receiver/reader 26. Preferably, transmission is by a wireless transmitter 28 to preclude hardwiring the system and to eliminate wires extending from the jaws 16. Wireless technology can be, for example, transponder (active or passive RFID technology) or like wireless protocols that will be understood by those skilled in the art.

[0032] The control system 30 (also referred to herein as the "controller", which may include the reader/receiver) can include, for example, an analog to digital (A/D) converter, EEPROM, and a microprocessor (shown at 30) to process the signal and convert it to an equivalent jaw clamping force. Software will compare the measured jaw 16 clamping force to a required clamping force and signal the controller 30 to increase or decrease the hydraulic pressure (to increase or decrease the clamping force) as necessary. Continuous monitoring of the clamping force and adjustment of the hydraulic system H pressure enhances the safety of the chuck 10, and the quality and productivity of the control system 30.

[0033] In addition, the control system 30 will determine (calculate) the theoretical required clamping load based on the speed of the chuck 10, and the mass and center of gravity of the jaws 16. The control system 30 will use the higher of the clamping force (measured vs. calculated) as the required clamping force applied. It will be appreciated that this arrangement (the self-contained jaw elements 16) provides self- identification (each jaw element 16 can be uniquely identified), and is self-calibrating and field programmable/reprogrammable, and has wireless connectivity in a compact readily installed/replaceable assembly 12. [0034] In that the strain gauges 24 and the transmitters/transponders 28 are located in the jaws 16, and are all preferably wireless, the jaws 16 can be readily replaced without affecting the functionality of the chuck 10 and control system 30.

Because each set of jaws 16 will have a unique identifier associated with it, the mass, center of gravity, and other pertinent information about each set of jaws 16 can be stored for easy retrieval. Power to the transmitter 28 can be provided by a rotary generator (not shown), or a power source can be provided though an electrical induction

arrangement/circuit. Alternately, a remote power system with, for example, a small rechargeable battery can be used.

[0035] As seen in FIG. 2, the jaw element includes an integrated load cell assembly 32. The jaw element illustrated is a three piece element including a base portion 34, a load cell 36 and a workpiece contact element 38. The base portion 34 carries the load cell 36, while the load cell 36 is positioned between the base 34 and the workpiece contact element 38. In this manner, a force exerted by the jaw 16 on the workpiece W is transmitted through the load cell assembly 32.

[0036] Alternately, as shown in FIG. 3, the jaw element 116 can be a two piece element with a base 134 and the load cell and workpiece contact element integrated into a single element 137. The pieces, two piece (134, 137) or three piece (34, 36, 38), are bolted or otherwise mounted to one another. Preferably, the electronics are located at the rear 39 of the jaw 16.

[0037] Strain gauges 40a,b, 42a,b are positioned within the load cell 36 or load cell/workpiece contact element 137. The strain gauges 40a,b, 42a,b function by generating a signal as a result of deformation of the gauge, for example, due to a force exerted on the gauge. The strain gauges 40a,b, 42a,b are located on the face 41 of the load cell 36, 137.

[0038] A typical strain gauge 40a,b, 42a,b functions by deforming an electrical conductor that is shaped in a desired pattern to change the geometry of the shaped conductor. The deformation changes the electrical resistance of the shaped conductor through an electrical circuit, such as a Wheatstone bridge.

[0039] The strain-gage based load cell 32, as shown as the middle piece in

FIG. 2, responds to the clamping load by deforming elastically like a three-point beam, where the four bolted holes 44 at the sides anchor the supports and the load is applied through the middle. The strain gage Wheatstone bridge circuit and gauge 40a,b, 42a,b locations (CI, C2, Tl, T2) are illustrated in FIG. 4. Due to the unique geometrical design, the load cell experiences a bending deformation under loading, regardless of the clamping location of the front piece 38. This leads to an output that is sensitive to the load, but independent of the clamping location.

[0040] The unique male and female patterns between the three pieces 34,

36, 38 are designed for positive connection so that it will not be knocked loose during machine operation.

[0041] If desired, the three-piece design (FIG. 2) can be readily converted into the two-piece design (FIG. 3) by combining the load cell 36 and front piece 38 of FIG. 2, into a single integrated element 137 (FIG. 3).

[0042] If desired, a different connection method, instead of bolting, can be used. For example, a quick change connection might be preferred in situation where the front piece 38 is considered a consumable. These connection methods will be recognized by those skilled in the art.

[0043] Minor modifications to the geometry or strain gage locations can be made while maintaining the bending nature of the design and capturing the spirit of the present invention.

[0044] All patents referred to herein, are hereby incorporated herein by reference, whether or not specifically done so within the text of this disclosure.

[0045] In the present disclosure, the words "a" or "an" are to be taken to include both the singular and the plural. Conversely, any reference to plural items shall, where appropriate, include the singular.

[0046] From the foregoing it will be observed that numerous

modifications and variations can be effectuated without departing from the true spirit and scope of the novel concepts of the present disclosure. It is to be understood that no limitation with respect to the specific embodiments illustrated is intended or should be inferred. The disclosure is intended to cover all such modifications as fall within its scope.

Claims

CLAIMS What is claimed is:

1. A rotatable chuck for securing a workpiece in a machine, comprising: a base element having a longitudinal center;

a plurality of jaw elements movable to clamp the workpiece, the jaw elements exerting a measurable clamping force on the workpiece;

means for measuring the clamping force exerted on the workpiece, the means being positioned within the jaw;

means for moving the jaw elements to vary the clamping force exerted on the workpiece; and

comparison means for comparing the measured clamping force to a predetermined clamping force,

wherein the jaw elements are movable, as the chuck is rotating, to adjust the clamping force on the workpiece relative to the predetermined clamping force.

2. The rotatable chuck in accordance with claim 1 wherein the means for measuring the clamping force is a load cell disposed in the jaw.

3. The rotatable chuck in accordance with claim 2 wherein load cell includes one or more strain gauges disposed in the jaw.

4. The rotatable chuck in accordance with claim 1 wherein the means for moving the chuck is a hydraulic system.

5. The rotatable chuck in accordance with claim 1 including a transmitter operably connected to the means for measuring the clamping force.

6. The rotatable chuck in accordance with claim 5 wherein the transmitter is a wireless transmitter.

7. The rotatable chuck in accordance with claim 6 wherein the wireless transmitter provides a signal to the comparison means for determining a difference between the clamping force and the predetermined clamping force.

8. A rotatable chuck for securing a workpiece in a machine, comprising: a base element having a longitudinal center;

one or more load cells operably connected to the jaw elements to measure the clamping force exerted on the workpiece, the one or more load cells being positioned within the jaw;

a drive for moving the jaw elements to vary the clamping force exerted on the workpiece; and

a control system including a comparator to compare the measured clamping force to a predetermined clamping force,

9. The rotatable chuck in accordance with claim 8 wherein the drive is a hydraulic system.

10. The rotatable chuck in accordance with claim 8 including a transmitter operably connected to the one or more load cells.

11. The rotatable chuck in accordance with claim 10 wherein the transmitter is a wireless transmitter.

12. The rotatable chuck in accordance with claim 8 wherein the load cell includes one or more strain gauges.

13. The rotatable chuck in accordance with claim 11 wherein the wireless transmitter provides a signal to the control system to determine a difference between the clamping force and the predetermined clamping force.

14. A jaw element for use in a chuck, the jaw element including a load cell integrated therewith.

15. The jaw element in accordance with claim 14 wherein the load cell includes one or more strain gauges.

16. The jaw element in accordance with claim 14 including a transmitter operably connected to the one or more load cells.

17. The jaw element in accordance with claim 16 wherein the transmitter is a wireless transmitter.

18. The jaw element in accordance with claim 8 wherein the one or more loads cell include one or more strain gauges.

19. The rotatable chuck in accordance with claim 11 wherein the wireless transmitter provides a signal to the control system to determine a difference between the clamping force and the predetermined clamping force.