ADJUSTABLE SET BACK CONTROL DEVICE

Abstract: A device for adjustably controlling the position and set-back of a piston rod (60) which extends from a damping cyl­ inder (102) used on an airfeed peck drill. The device includes a plurality of ball members (12) which surround the piston rod. The ball members are positioned against the piston rod by a spring biased cone (18) having a tapered internal surface. The balls are further positioned with respect to the cone by a spring biased ball separator (22) which is movable with re­ spect to the cone by air pressure. The travel of the piston rod is made ajustable by a nut member (26) threadably engaged with the housing.

Description: An airfeed peck drill is, in its simplest terms, an air driven rotary drill mounted on the front of an air driven power cylinder. The features of a peck drill which distinguish it from a continuous feed drill are the short period of contact of the drill with the workpiece, the repeated complete extraction of the drill from the hole being drilled and the repeated re-insertion of the drill into the hole being drilled. Drill travel speed into the workpiece is controlled by mechanical contact of the moving drill with the outboard end of a piston rod extending from a very accurate damping cylinder. A controlled leak around the piston in the damping cylinder enables the damping cylinder to provide a continuous moving counter-force to the force used to move the airfeed peck drill into the workpiece. In order for the drill travel speed control to remember the position of the drill while the airfeed peck drill is extracted and returned to the bottom of the hole being drilled, it is necessary to prevent rearward travel of the piston rod while the face of the drill bit is not in contact with the workpiece. The rearward travel of the piston rod is prevented by a device which has become known as a set-back control device. Further explanation of set-back control in airfeed peck drills may be found in U.S. Patent Nos. US-A-4,123,188 and US-A-4,421,441. The accurate control of airfeed peck drill travel is necessary to prevent improper drilling of holes or drill bit damage. Improper hole drilling or damage to drill bits usually occurs when a drill bit reengages material at the bottom of a hole under uncontrolled feed rate conditions. Such uncontrolled feeds normally occur just at the beginning of the compression of the fluid in the damping cylinder and during the period while the drill frame is loaded by thrust from the feed piston. Additionally, as the rotating drill contacts the workpiece, it causes both the fixture and the workpiece to deflect. When the rotating drill moves away from the fixture and the workpiece, the fixture and the workpiece return to an undeflected or relaxed condition. As the rotating drill moves back into the workpiece and contacts the bottom of the holes, the force of the drill tends to cause the fixture and the workpiece to separate. It is necessary that as force is placed against the bottom of the partially drilled hole that this is done under controlled feed conditions. Without adequate set back, this cannot be accomplished. To overcome these conditions, sufficient rearward movement or set back is required to allow the drill feed control mechanism to compensate for the periods of uncontrolled drill feed and fixture and workpiece deflection. US-A-3680970 teaches an airfeed peck drill set back control device which has become a standard in the industry. This airfeed set back control device has two drawbacks. First, it is not adjustable. That is it cannot be made to compensate for differing periods of uncontrolled feed of fixture deflection allowed by the drill feed control mechanism and second it has a large extension which hangs off one side of the piston rod. This extension makes any airfeed peck drill incorporating this device difficult to use in confined areas. There is therefore a need in the art to provide a set back control device which is adjustable and is smaller in size. US-A-3680970 discloses a set back control device for an airfeed peck drill whose drill travel speed is controlled with a damping cylinder having a movable piston rod, the adjustable set-back control device comprising a plurality of ball members positioned to surround and restrict the axial movement of the movable piston rod; means for controlling the axial position of the ball members with respect to the movable piston rod; and means for controlling the radial position of the ball members with respect to the movable piston rod; the means for controlling the axial position of the ball members being positionable by the application of air pressure. According to the present invention, such a device is characterised in that the means for controlling the radial position of the ball members is adjustably positionable so that the travel of the movable piston rod with respect to the damping cylinder is selectively controlled. The airfeed peck drill set back control device of the present invention thus accurately and adjustably controls the travel of the piston rod extending from the drill travel speed control damping cylinder by use of a plurality of ball members which surround and grab the piston rod thus enabling it to remember mechanically where the airfeed peck drill stopped before it was extracted from the workpiece. Additionally, various degrees of rearward travel of the piston rod are allowed by the adjustable positioning of the means for controlling the radial position of the ball members. In one construction of set back control device of the present invention, a cone controls the radial position of the ball members with respect to the piston rod; that is. the cone surrounds the ball members such that their movement either into or out of frictional engagement with the sides of the piston rod is restricted. The axial position of the balls within the cone is controlled by a ball separator. The ball separator determines the position of the balls with respect to the tapered surface on the inside of the cone. This controlling of the position of the ball members with respect to the tapered surface on the inside of the cone controls the amount of force exerted by the ball members when moved into frictional engagement with the side of the piston rod. The ball separator may have a flanged end which sealingly engages a housing so that it may be positioned by air pressure. Movement of the ball separator within the housing causes the ball members to be released from their frictional contact with the sides of the piston rod. An adjustable nut may be located in one end of the housing to limit the travel of the cone. Such limitation on the travel of the cone provides accurate control over the set-back movement of the piston rod. A better understanding of the adjustable set back control device of the present invention may be had by reference to the drawings wherein: Figure 1 is a perspective view of a generic airfeed drill or peck drill with a set-back control mounted thereon; Figure 2 is a sectional view of the set back control device shown in figure 1; Figure 3 is a sectional view of the set back control device of the present invention showing engagement of the ball members with the control rod; Figure 4 is an end view of the set back control device at line 4-4 in Figure 3; Figure 5 is a view similar to Figure 3 with the ball members released from the control rod; Figure 6 is a sectional view taken at line 6-6 of Figure 5; and, Figure 7 is an exploded perspective view showing the assembly of the set back control device. As previously explained, it is necessary to accurately control the speed and depth of forward travel of airfeed peck drills 100. Figure 1 illustrates how accurate control of position is accomplished by mechanical engagement of stop 103 with a travelling piston rod 60 which extends from damping cylinder 102. Air pressure acts on piston in cylinder 106, pushing drill quill 107 forward until stop 103 engages piston rod 60 which extends from damping cylinder 102. The speed of drill travel is governed by damping cylinder 102. Knob 105 is used to adjust the counter force exerted by damping cylinder 102. When the peck cycle is completed, air pushes against the front side of piston in cylinder 106. This pulls drill quill 107 away from the workpiece. Because stop 103 follows drill quill 107, it moves away from the end of piston rod 60. Piston rod 60 would follow stop 103 were it not for set back control device 10 which limits the travel of piston rod 60 and thus remembers the position of piston rod 60 when quill 107 is caused to move rearward during a peck cycle. Drill quill 107 is fully extracted from the workpiece between drilling cycles. There is a small backward movement "A" of piston rod 60 before it is grabbed by setback control device 10. The piston rod 60 dwells during the period that drill quill 107 is withdrawn from the workpiece and reinserted. There is an uncontrolled feed "C" of piston rod 60 caused by mechanically loading the structural components of airfeed peck drill 100, the non-linear travel of piston rod 60 during the initial compression of fluid and the reestablishment of fluid flow within damping cylinder 102 and the deflection of the fixture and the workpiece. The difference between "A" and "C" is a safety margin to assure that drilling does not begin until the drill is under controlled feed. This safety margin is established to be somewhat greater than the fixture and workpiece deflection. If drill 108 is allowed to contact the workpiece during this period of uncontrolled feed, an unsatisfactory hole will be drilled. The set-back control device allows for adjusting the set back of piston rod 60 so that drill 108 does not contact the workpiece during the period "C" of uncontrolled feed. In Figures 2 to 6 it may be seen that the set back control device 10 frictionally engages the sides of piston rod 60 which extends from damping cylinder 102. Specifically, ball members 12 contact the sides of piston rod 60 as shown in Figure 2. As peck drill 100 is feeding towards the workpiece under the force of power cylinder 106, at a rate controlled by the counter force of damping cylinder 102, the bias force of cone spring 14 is greater than the force of separator spring 16. This difference in spring force biases cone 18 in the direction of travel of the forward feed of peck drill 100 into the workpiece. This movement of cone 18 causes tapered internal surface 20 on inside of cone 18 to engage ball members 12, thus causing them to move inwardly into contact with piston rod 60. Separator spring 16 causes ball separator 22 to move opposite the direction of travel further moving ball members 12 into taper 20 on the inside of cone 18. In Figure 3, the set-back control device 10 is shown in the static condition between pecks of peck drill 100. The hydraulic return force of hydraulic cylinder 102 plus the bias of separator spring 16 causes cone 18 to move rearward until it contacts the inside face 24 of adjustable nut 26. Adjustable nut 26 is positioned by inserting a tool (not shown) into holes 28 in its outward face 30 to threadably move it into housing 32. Graduations 27 may be used to mark the position of nut 26. The inward radial force on balls 12 created by cone 18 prevents the continued outward travel of piston rod 60 thus causing the last position of piston rod 60 to be remembered with the addition of a predetermined length of set back. In Figure 5, the release condition is shown. As shown in Figure 3, pressurized air is added through port 34 to enter housing 32. This pressurized air forces ball separator 22 in the direction of the feed of peck drill 100. This movement of ball separator 22 is caused by flanged end 36 of ball separator 22 which acts as a piston within housing 32. The movement of ball separator 22 in the direction of feed of peck drill 100 causes balls 12 to move outwardly within cone 18 thus freeing piston rod 60 for movement back toward the workpiece. Such outward movement of balls 12 is shown in Figure 6. A still better understanding of the construction of the adjustable set back control device 10 may be had by reference to Figure 7. Therein it may be seen that ball separator 22 abuts against disc 38 at the forwardmost point of its travel. Ball separator 22 is sealed within housing 32 by O-ring 40 and within cone 18 by O-ring 42. Similarily, cone 18 is sealed within housing 32 by O-ring 44. Balls 12 ride with ball slots 46 formed in ball separator 22. Housing 32 is sized uniformly about piston rod 60 and includes no unnecessary protuberances or extensions. While a rectangular housing 32 is shown, it is understood that a cylindrical or any convenient shape of housing may be used. As may be seen in Figures 3 and 5, it is possible to adjust the setback position of piston rod 60 by moving adjustable nut 26 in and out of housing 32.

Claims: A set back control device (10) for an airfeed peck drill whose drill travel speed is controlled with a damping cylinder (102) having a movable piston rod (60), the adjustable set-back control device comprising a plurality of ball members (12) positioned to surround and restrict the axial movement of the movable piston rod (60); means (22) for controlling the axial position of the ball members with respect to the movable piston rod; and means (18) for controlling the radial position of the ball members with respect to the movable piston rod; the means (22) for controlling the axial position of the ball members being positionable by the application of air pressure; characterised in that the means (18) for controlling the radial position of the ball members is adjustably positionable so that the travel of the movable piston rod (60) with respect to the damping cylinder (102) is selectively controlled. A device according to claim 1, wherein the means for controlling the radial position of the ball members (12) with respect to the movable piston rod (60) is a cone (18) having a tapered inner surface which causes the ball members to engage the movable piston rod. A device according to claim 2, wherein the cone is positioned in part by a biasing spring (14). A device according to any one of the preceding claims, wherein the ball members (12) are disposed in radial slots (46) in a ball separator (22), which provides the means for controlling the axial position of the ball members. A device according to claim 4, wherein the ball separator (22) is positioned in part by a biasing spring (16). A device according to any one of the preceding claims, further including a housing (32). A device according to claim 6, wherein the means (22) for controlling the axial position of the ball members (12) includes a piston (36) which sealing engages the internal surface of the housing (32). A device according to claim 6 or claim 7, wherein the adjustable positioning of the means for controlling the radial position of the ball members (12) is a screw threaded nut (26) which surrounds the piston rod (60) and engages the housing (32). A device according to claim 8, wherein the position of the nut (26) adjustably limits the travel of the means (22) for controlling the axial position of the ball members (12).

Inventors: STEWART, JOHN, F.
Applicants: COOPER INDUSTRIES INC.
Application year: 1988
Main classification: B23B47
IPC codes: B23Q16, B23Q1, B23B47