Through hole depth measurement method and device

A through-hole depth measurement device includes a cutting force sensor configured and mountable in a position to sense cutting force exerted by a drill bit of a drill against a workpiece or stack of workpieces. A controller is coupled to the force sensor, is coupleable to a workpiece surface position sensor, and is configured to record the through-hole depth of the workpiece or stack of workpieces in response to signals received from the workpiece surface position sensor when the drill bit reaches a drill bit exit point. The cutting force sensor is configured and positionable to sense cutting force transmitted between a drill spindle and a spindle housing.

CROSS-REFERENCES TO RELATED APPLICATIONS

Not Applicable

Not Applicable

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a method and device for measuring the depth of a through-hole formed through one or more workpieces.

The thickness of sheet material such as composite fiber reinforced plastic (CFRP) can vary from nominal engineering values. Consequently, stack-up thicknesses of multiple sheets of such material and, thus, the depth of fastener holes drilled through a stack of such material varies sufficiently that fastener selection can require that individual hole depth measurements be made. It is, therefore, advantageous to be able to quickly and accurately measure the depths of holes that have been formed through sheet material stacks so that respective fasteners of appropriate grip length can be selected for use in the holes. It's also advantageous to be able to determine such hole depths and corresponding fastener grip lengths in advance of fastener selection and installation, and without having to execute a separate measurement step following the drilling of each hole. Accurate individual hole depth measurements can also allow a fastener bill of materials (BOM) to be prepared based on actual rather than nominal hole depths, which would allow fasteners to be delivered and kilted in advance and delivered in proper quantities to an assembly station. However, known through-hole depth measurement methods and devices are unable to sense changes in drill force in a drill that comprises a pressure foot system—such as one driven by an air cylinder—that's configured to exert a constant pressure against the facing surface of a workpiece.

BRIEF SUMMARY OF THE DISCLOSURE

A through-hole depth measurement device is provided for measuring the depth of a through-hole formed through one or more workpieces. The device may include a cutting force sensor configured and mountable in a position to sense cutting force exerted by a drill bit of a drill against a workpiece or stack of workpieces. A controller is coupled to the force sensor, is coupleable to a workpiece surface position sensor, and is configured to record a through-hole depth of a workpiece or stack of workpieces in response to signals received from the workpiece surface position sensor when the drill bit reaches a drill bit exit point. The device may also include a cutting force sensor configured and positionable to sense cutting force transmitted between a drill spindle and a spindle housing. This would allow the device to sense changes in drill force in a drill that comprises a pressure foot system, such as one driven by an air cylinder, that's configured to exert a constant pressure against the facing surface of a workpiece.

The cutting force sensor may be configured to sense axial cutting force transmitted between the drill spindle and the spindle housing. It may also be positioned in direct axial alignment with the direction of drill force application. When axially-aligned in this way, the cutting force sensor is advantageously positioned to sense axial cutting force exerted by a drill bit of a drill against a workpiece.

The cutting force sensor may include a plurality of axial cutting force sensor units. The sensor units may be mounted in respective spaced-apart positions between the spindle housing and a flange engagement plate that transmits axial cutting force from the spindle to the spindle housing

The controller may be configured to record through-hole depth as being the difference between a workpiece near-side surface position and a workpiece far-side surface position when the controller receives a decaying force signal from the force sensor indicative of drill bit breakout.

The workpiece surface position sensor may include a workpiece near-side surface position sensor and a workpiece far-side surface position sensor. The controller may be configured to record through-hole depth as being the difference between a workpiece near-side surface position indicated by the near-side position sensor and a workpiece far-side surface position indicated by the far-side position sensor when the controller receives a decaying force signal from the force sensor indicative of drill bit breakout.

The workpiece near-side surface position sensor may comprise a pressure foot axis position sensor, and the workpiece far-side surface position sensor may comprise a spindle feed axis position sensor. The controller may be configured to record through-hole depth as being the difference between a workpiece near-side surface position indicated by the pressure foot axis position sensor, and a workpiece far-side surface position indicated by the spindle feed axis position sensor when the controller receives a decaying force signal from the force sensor indicative of drill bit breakout.

The controller may be configured to determine, during a gradual and nonlinear reduction in force experienced as a drill bit exits the far-side surface of a workpiece or stack of workpieces, the point at which the distal end of a drill bit has actually breached a far side of a workpiece or stack of workpieces. The controller may also be configured to distinguish between decaying force signals associated with drill bit breakout and decaying force signals associated with peck cycles. The controller may be configured to determine peckless cutting force signals by identifying and removing peck cycle forces from cutting force readings.

Also, a method is provided for measuring the depth of a through-hole formed through one or more workpieces. According to this method, one can measure the depth of such a through-hole by first positioning a drill spindle and attached drill bit of a drill in axial alignment with a desired hole location adjacent a work piece or stack of workpieces, rotating and advancing the drill spindle axially, causing the bit to cut into the workpiece or stack of workpieces, and monitoring cutting force and workpiece surface position sensor readings until decaying cutting force signals indicative of drill bit breakout are detected. Workpiece near-side surface position and workpiece far-side position may be then be determined by polling workpiece surface position sensor readings at the instance of breakout. Hole depth may then be calculated as being the axial distance between the workpiece near-side surface position and the workpiece far-side surface position. Finally, the drill may be polled for coordinates identifying hole location in the workpiece or workpiece stack and the measured hole depth may be recorded and associated with the hole location.

The detection of a decaying cutting force signal indicative of drill bit breakout may include processing the breakout signal to determine the precise instance of breakout. Detection of a decaying cutting force signal may also include determining a peckless cutting force signal by detecting and removing peck cycle forces from cutting force readings. In other words, the detection of a decaying cutting force signal may include the reconstructing of a signal that may include interruptions from peck cycles, and doing so by removing these cycles from the signal, resulting in a signal having a profile similar to one without peck cycles.

The step of determining workpiece near-side surface position and workpiece far-side position may include determining workpiece near-side surface position from pressure foot position along a pressure foot axis and determining workpiece far-side position from drill spindle position along the spindle feed axis. This may be done by polling pressure foot axis and spindle feed axis position sensor readings at the instance of breakout.

DETAILED DESCRIPTION OF INVENTION EMBODIMENT(S)

A device for measuring the depth of a through-hole11formed through one or more workpieces12is generally shown at10in the Figures. The device10may include cutting force sensors, shown at18inFIGS. 2-7, configured and mountable in respective positions to sense cutting force exerted by a drill bit22of a drill (e.g., an autodrill or numerical control (NC) machine), as is generally indicated at20, against a workpiece or stack of workpieces12. The device10may also include a controller, shown at24inFIG. 8, that's coupled to the force sensors18and is coupleable to a workpiece surface position sensor system, such as the system generally indicated at26inFIG. 8. The controller24may be configured to record the through-hole depth of a workpiece or stack of workpieces12(e.g., panels in a panel stack) in response to signals received from the workpiece surface position sensor system26when the drill bit22of the drill20reaches a drill bit exit point32along a spindle feed axis34. The drill bit exit point32is the point (as best shown inFIGS. 5 and 6) where the drill bit22exits a far-side surface16of a workpiece or stack of workpieces12.

As best shown inFIG. 8, the cutting force sensors18may be configured and positionable to sense cutting force transmitted between a drill spindle36and a spindle housing38of the drill20. The drill bit22may be removably fixed to the spindle36by a drill chuck42. The drill spindle36may be supported with the drill bit22on the spindle housing38for rotation relative to the spindle housing38and for reciprocal axial motion with the spindle housing38along the spindle feed axis34of the drill spindle36. Axial cutting force may be transmitted between the spindle36and spindle housing38along the spindle feed axis34via a flange40that's carried by the spindle housing38and that axially engages the spindle36.

The cutting force sensors18may be configured and positioned to sense the cutting force that is transmitted axially between the drill spindle36and the spindle housing38. The cutting force sensors18may be advantageously positioned in direct axial alignment with the direction of drill force application so that the sensors18can sense axial cutting force exerted by a drill bit22of a drill20against a workpiece12. As best shown inFIG. 8, the cutting force sensors18may comprise a plurality (3) of axial cutting force sensor units that are configured and mountable in respective spaced-apart positions between the spindle housing38and a flange engagement plate43that's carried by the spindle housing38and that's connected to and cooperates with the flange40to transmit axial cutting force between the spindle36and the spindle housing38.

The controller24may be configured to record through-hole depth as being the difference between a workpiece near-side surface14position indicated by the workpiece surface position sensor system26and a workpiece far-side surface16position indicated by the workpiece surface position sensor system26when the controller24receives a decaying force signal from the force sensors18indicative of drill bit breakout. Drill bit breakout may be defined as the emergence of a tip of the drill bit22from the far-side surface16of the workpiece or stack of workpieces12.

As shown inFIG. 7, the workpiece surface position sensor system26may include a workpiece near-side surface position sensor28and a workpiece far-side surface position sensor30. The far-side surface position sensor30may include the cutting force sensors18. The controller24may be configured to record through-hole depth as being the difference between a workpiece near-side surface position indicated by the near-side position sensor28and a workpiece far-side surface position indicated by the far-side position sensor30. The controller24may receive far-side surface position information from the far-side surface position sensor30in the form of a decaying force signal that's received from the force sensors18and is indicative of drill bit breakout.

The NC machine20may include a pressure foot44supported for reciprocal motion along a pressure foot axis35that may be parallel to the reciprocal motion of the drill bit22and spindle36along the spindle feed axis34. As best shown inFIG. 7, the pressure foot axis35and the spindle feed axis34may be coaxially disposed.

As best shown inFIG. 7, the workpiece near-side surface position sensor28may comprise the pressure foot44a pressure foot axis position sensor49(e.g., a pressure foot axis encoder of an NC machine or autodrill20), which may be configured to send the controller24a signal indicating the position of an outboard surface45of the pressure foot44when the pressure foot44is resting against the workpiece near-side surface14. The workpiece far-side surface position sensor30may comprise a spindle feed axis position sensor (e.g., a spindle feed axis encoder of an NC machine or autodrill20) configured to send the controller24signals indicating the position of the spindle along the spindle feed axis34. The controller24may be configured to record through-hole depth as being the difference between a workpiece near-side surface position indicated by the pressure foot axis position sensor49, and a workpiece far-side surface16position indicated by the spindle feed axis position sensor30when the controller24receives a decaying force signal from the force sensor18indicative of drill bit breakout. The device10may be used with a drill having no pressure foot, with the controller24being configured to determine workpiece near-side surface position by recognizing the sudden increase in force sensed by the force sensor18when the drill bit engages the near-side surface. However, an advantage of instead relying on pressure foot position reading to establish near-side surface position is that it neglects any deformation of the workpiece that might result from axially-directed pressure applied by the drill bit.

The controller24may be configured to determine the point in time and/or space at which the distal end of the drill bit22has actually breached a far side of a workpiece or stack of workpieces12. The controller24may be configured to make this determination when a gradual and nonlinear reduction in force is experienced as the drill bit22exits the far-side surface16of the workpiece or stack of workpieces12.

The controller24may be configured to distinguish between decaying force signals associated with drill bit breakout, and decaying force signals associated with peck cycles, i.e., drilling and pulling back cycles of an autodrill20when the autodrill is being operated in a “pecking” fashion. An autodrill20is operated in a pecking fashion to intermittently remove accumulated metal chips while drilling through a workpiece or stack of workpieces12. The chips are removed to prevent the chips from interacting with the drill bit22of the autodrill20in a way that causes excessive wear to an inner circumferential surface of the hole being drilled). The controller24may also be configured to determine peckless cutting force signals by identifying and removing peck cycle forces from cutting force readings.

In practice, the depth of a through-hole11formed through one or more workpieces12can be measured by first actuating a drill20such as an autodrill or NC machine to position its drill spindle36and attached drill bit22in axial alignment with a desired hole location adjacent the workpiece or stack of workpieces12as shown in action step52ofFIG. 8. The drill20may then be actuated, as shown in action step54, to rotate and advance its drill spindle36axially, causing its drill bit22to cut into the workpiece or stack of workpieces12while the controller24is monitoring cutting force and workpiece surface position sensor28,30readings. If, as shown in decision step56, the controller24detects one or more peck cycles as the drill is cutting through the workpieces12, the controller24may calculate or otherwise determine a peckless cutting force signal, as shown in action step58, by detecting and removing peck cycle forces from cutting force readings.

As indicated in action step60, the controller24may determine that the drill bit22has reached the far-side surface16of the workpiece or workpieces12by detecting a decaying cutting force signal indicative of drill bit22breakout. The controller24may then determine the precise instance of breakout by processing the breakout signal as shown in action step62. The processing of the breakout signal may include, for example, comparing a decay profile of the cutting force signal to known decay profiles associated with drill bit breakout, and identifying the point in time within the sensed decay profile where breakout has been demonstrated to have occurred.

Once the precise point of drill bit breakout has been determined, the controller24may then poll or review readings taken from the pressure foot axis position sensor49and the spindle feed axis34position sensor30at the instance of breakout to determine workpiece near-side surface14position from the recorded pressure foot position along pressure foot axis35and to determine workpiece far-side position from the recorded drill spindle36position along the spindle feed axis34as shown in action step64.

As indicated in action step66, the controller24may then calculate hole depth as being the axial distance between workpiece near-side surface position and workpiece far-side surface position.

Where the drill20is an NC machine, the controller24may also poll the NC machine for coordinates identifying hole location in the workpiece12or workpiece stack. As shown in action step68, the controller may associate and record the measured hole depth with the location of the measured hole11.

A through-hole depth measurement device constructed as described above is able to measure the depth of a hole as it's cut by a drill, by sensing changes in drill force. By calculating hole depth based on drill bit position at point of drill bit breakout, such a device is able to record an accurate hole depth measurement despite workpiece deformation or displacement that may occur during the drilling process.

This description, rather than describing limitations of an invention, only illustrates an embodiment of the invention recited in the claims. The language of this description is therefore exclusively descriptive and is non-limiting. Obviously, it's possible to modify this invention from what the description teaches. Within the scope of the claims, one may practice the invention other than as described above.