US3825363A

US3825363A - Center drilling hold-down device

Info

Publication number: US3825363A
Application number: US00317628A
Authority: US
Inventors: R Lanman
Original assignee: HADADY MACHINING CO Inc
Current assignee: HADADY MACHINING CO Inc
Priority date: 1972-12-21
Filing date: 1972-12-21
Publication date: 1974-07-23
Anticipated expiration: 1991-07-23
Also published as: CA985532A; GB1396257A

Abstract

An apparatus for automatically aligning a hydraulic drill with the longitudinal axis of a piece of stock material so that a centering hole may be drilled therein to facilitate subsequent machining of the stock is disclosed. More particularly, the apparatus includes a linkage comprising a pair of horizontally displaced bars coupled to a substantially flat frame member so that each is pivotable in a vertical plane normal to the frame. A second, shorter pair of pivoted bars is similarly positioned directly below the first pair. The free-moving end of each short bar, in turn, is coupled to an intermediate point on the corresponding by a vertical bar so that the linkage forms a pair of parallelograms. A hold-down means, attached to the free-moving ends of the longer outwardly extending bars, is positioned to contact the upper surface of the stock as it rests in a V-shaped stock support so that the stock is secured in position for drilling with its end surface parallel to the frame. A drill is mounted between the vertical bars so that it is normal to the end of the stock. Accordingly, when the holddown contacts the top surface of the stock, the drill is automatically aligned with the longitudinal axis of the stock so that a centering hole can be drilled.

Description

United States Patent 91 Lanman CENTER DRILLING HOLD-DOWN DEVICE Inventor: Robert W. Lanman, Munster, Ind. I

Assignee: Hadady Machining Co., Inc.,

Lansing, Ill.

Dec. 21, 1972 Filed:

Appl. No.:

US. Cl. 408/104, 408/234 Int. Cl B23b 45/14 Field of Search 408/105, 106, 104, 103,

[5 6] References Cited UNITED STATES PATENTS l/l890 Minnich 408/106 3/1910 Merrill 408/107 X l/1949 Bowen 408/105 X Primary Examiner-Andrew R. Juhasz Assistant Examiner-Z. R. Bilinsky Attorney, Agent, or F irm-Merriam, Marshall, Shapiro & Klose [57] ABSTRACT a An apparatus for automatically aligning a hydraulic July 23, 1974 drill with the longitudinal axis of a piece of stock material so that a centering hole may be drilled therein to facilitate subsequent machining of the stock is disclosed. More particularly, the apparatus includes a linkage comprising a pair of horizontally displaced bars coupled to a substantially flat frame member so that each is pivotable in a vertical plane normal to the frame. A second, shorter pair of pivoted bars is similarly positioned directly below the first pair. The freemoving end of each short bar, in turn, is coupled to an intermediate point on the corresponding by a vertical bar so that the linkage forms a pair of parallelograms. A hold-down means, attached to the free-moving ends of the longer outwardly extending bars, is positioned to contact the upper surface of the stock as it rests in a V-shaped stoclc support so that the stock is secured in position for drilling with its end surface parallel to the frame. A drill is mounted between the vertical bars so that it is normal to the end of the stock. Accordingly, when the holddown contacts the top surface of the stock, the drill is automatically aligned with the longitudinal axis of the stock so that a centering hole can be drilled.

17 Claims, 7 Drawing Figures PATENIEDJULBWH 3.825.363

sumzum I HIM flai ni 'PATENTEU LZ 3.825.363 SHEET 30E 4 FIG. 6

CENTER DRILLING HOLD-DOWN DEVICE BACKGROUND OF THE INVENTION The present invention relates generally to a device for determining the center of a symmetric cross sectional area and more particularly to a device for simultaneously holding stock material of symmetric cross sectional area in a stationary position and aligning a drilling apparatus to be coincident with its longitudinal axis so that a centering hole may be drilled therein, facilitating subsequent machining of the stock.

Reference may be made to the following U.S. Pat. Nos. 2,810,310; 2,779,040; 2,678,572; 1,292,620.

When a piece of stock material is to be machined on a lathe, centering holes are commonly provided at each end coincident with its longitudinal axis. The centering holes perform a twofold function. That is, the turning pins of the lathe are inserted into the centering holes to support the stock as it is rotated by the lathe. Secondly, accurate placement of the centering holes insures that the mass of the stocks piece will not be disproportionately distributed around the turning axis determined by the centering holes, and consequently, vibrations will be minimized. If, on the other hand, the holes are not accurately centered, it follows that the resulting vibrations make machining more difficult and, if continued, are likely to damage the lathes bearings. Accordingly, the centering holes should be determined with as much accuracy as possible.

When only stock pieces having identical cross sections are to be center drilled, the holesmay be placed with consistant accuracy by clamping the stock in the same position for each piece, aligning a drilling apparatus with the longitudinal axis and limiting the drills movement to that axis. Where stock of different diameter is intermixed, however, the drill must be realigned with the longitudinal axis of each stock piece. Of course, this requires additional time to determine the exact center of the stock. Accordingly, machines have been developed which clamp the stock in position and automatically align a drilling apparatus with the longitudinal, or center axis of the stock.

One such prior art machine. utilizes a pair of V- shaped jaws; one jaw providing a stationary base for supporting the stock in a horizontal position and a second, movable, jaw for contacting the top of the stock to clamp it in position for drilling. A pair of vertical columns, positioned on opposite sides of the stock, are connected to the movable jaw; and a threaded nut assembly is affixed between the bottom ends of the vertical columns. The nut, in turn, engages a stationary, but rotatable, threaded rod which controls the vertical movement of the nut and, hence, the movable jaw. Thus, as the threaded rod is manually rotated, the nut is translated along the threaded rod until the movable jaw contacts the top of the stock. The nut is also linked to one end of a lever, the opposite end of which is similarly linked to a fixed point on the machine frame. A vertical bar is coupled to the middle of the lever and a belt-driven drill mounted thereon. Accordingly, any movement of the top, or movable, jaw is translated through the vertical columns and the nut assembly to the lever so that the drill is moved one-half the distance that the movable jaw travels. Thus, if the drill is initially positioned midway between the jaws, it will be properly repositioned when the top jaw is moved to accommodate stock of different diameters.

In a prior art machine of this type, however, lateral movement of the vertical bar supporting the drill must be minimized to prevent vertical displacement of the drill from the longitudinal axis. Thus, the vertical columns are constrained to move only in the vertical direction so that the nut assembly can not be twisted and thereby impart lateral movement to the lever and the vertical bar. To provide the requisite structural rigidity, the vertical columns are passed through bores, or guides, machined to close tolerances for preventing lateral movement of the vertical columns. Accordingly, the bores must be periodically lubricated to prevent friction wear.

Another disadvantage is that the vertical columns, nut assembly, lever, vertical bar, etc., must meet precise physical specifications if the desired ratio between the displacement of the movable jaw and the positioning of the drilling apparatus is to be preserved. Any deviation sufficient to change the ratio will cause the centering holes to be displaced from the longitudinal axis with expected results. Of course, as greater precision is required, the cost of attempting to achieve and maintain same also increases.

The prior art machine also utilizes a belt-driven drilling apparatus which is highly susceptible to producing vibrations than can damage the drill bearings and, as expected, adversely affect the useful life and accuracy of the machine. Also, vibrations create chatter marks in the conical center pocket being drilled in the stock, which adversely affects the subsequent machining of the stock.

SUMMARY OF THE INVENTION The center drilling apparatus of the present invention includes means comprising a parallelogram linkage having a pair of vertically displaced members pivotally coupled to a frame and constrained to move within a vertical plane normal thereto. A support member, coupled between the two members, is parallel to the surface of the frame as well as the end of the stock. Holddown means secure the stock in position during drilling by contacting the upper surface of the stock as it lies horizontally in the plane of the parallelogram linkage. More particularly, the hold-down means is attached to the end of the top member which extends beyond the support member for a predetermined distance. Accordingly, drill means, mounted on the support member normal to the end of the stock, are automatically aligned with the longitudinal axis of the stock whenever the hold-down means is in contact with its upper surface.

As a further feature of the invention, means are provided for advancing the drill means toward the end surface of the stock to facilitate drilling therein. Also, bydraulic means are attached to the end of the top outwardly-extending member so that proper contact between the hold-down means and the surface of the stock is accomplished. ,An additional feature is the provision of drill means powered by a hydraulic motor thereby reducing the vibration commonly associated with belt-drive motors.

I BRIEF DESCRIPTION OF THE DRAWINGS The features of this invention which are believed to be novel are set forth with particularity, in the appended claims. The invention together with its further objects and advantages thereof, may be best understood, however, by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements in the several figures and in which:

FIG. 1 is a view in perspective of a center drilling hold down device in accordance with a preferred embodiment of the invention;

FIG. 2 is a side elevational view of the center drilling hold down device of the present invention;

FIG. 3 is an end view of the center drilling hold down device of the present invention;

FIG. 4 is another side elevational view of the center drilling hold down device as it appears when-it has changed position to center drill small diameter stock;

PREFERRED EMBODIMENT OF THE INVENTION view of the pivot taken along line Referring now to FIG. 1, there is shown a center drilling apparatus in accordance with the present invention. More particularly, the machine includes a vertical frame member 11 having a pair of base members 12 affixed thereto and extending outwardly parallel to one another in the horizontal plane. Several cross members 13 are attached to the base members 12, lying perpendicularly across the base members 12 to provide a rigid platform, or base, upon which a tongued plate 14 is mounted-A V-shaped stock support 15 having a corresponding groove for slidably interlocking the tongued plate 14 is especially well suited to stably support any stock of symmetric cross sectional area, e.g., round, square, hexagonal. An auxiliary V-shaped stock support 16 may likewise be slidably fitted on tongued plate 14 at a point distant from support 15 so that stock material of any desired length maybe accommodated. A lip 17 is provided on support 15 to provide a stop against which the stock material may be butted during the drilling operation. Accordingly, the cross sectional surface of the stock to be center drilled may be maintained in a horizontally stable position. v

A mounting plate 18 is affixed to the frame 11 for mounting the parallelogram linkage and drilling assembly of the present invention. More particularly, the plate 18 has four mounting-flanges 19, each located at a respective corner of an imaginary rectangle drawn on the face of mounting plate 18. A pair of bars 20 are coupled to the upper-most two flanges 19 by pivots 21 so that the bars 20 are pivotable within an imaginary plane extending outwardly from the vertical frame member 11 through the base members 12. The bars 20 are joined at their free-moving ends by a spacing bar 22 affixed to the bars 20 by pivots 23 about which the spacing bar 22 may rotate. A hold-down plate 24 is fastened to the underside of spacing bar 22 by means of a set screw 25. Accordingly, if pieces of stock having different diameters are to be drilled, the hold-down plate 24 is free to pivot so that it may freely contact the top surfaceof the stock to be drilled at any particular time.

The free-moving ends of bars 20 are, in turn, coupled to a corresponding pair of hydraulic cylinders 26 through a pair of connecting rods 27 each of which passes thru alateral stabilizer bracket 30. One end of each connecting rod 27 ends in a U-shaped bracket 28 which further accepts a pin 29 passing through a hole in the end of rod 20. Although a gravity operated holddown arrangement could be used, the hydraulic cylinders 26 are provided to insure that the hold-down plate 24 continually contacts the top surface of the stock material in each instance so that the center hole may be accurately placed. An incidental advantage is also derived in that the hydraulically positioned hold-down plate 24 further serves to insure that the stock material is securely held in place during the drilling operation.

A pair of shorter outwardly extending bars 31 are, in turn, coupled to the bottom pair of flanges 19 by pivots 21. Further, two corresponding vertical bars 32 are positioned so that one end of each bar 32 is coupled to an intermediate point on the corresponding bar 20 by pivot 33 while its other end is coupled to the end of bar 31 by pivot 33. The vertical bars 32 are placed so that they lie in a vertical plane lying parallel to the frame 1 1. Accordingly, the

bars

20, 31 and 32 combine with the frame 1 1 to provide a parallelogram linkage comprising a pair of parallelograms, each having twocorners permanently fixed in a stationary position.

A plate, or web member, 34 extending between th vertical bars 32 accommodates a hydraulic drill 35 which may be slidably fitted through an aperture in plate 34 so that the drill bit is coincident with the vertical plane passing thru the longitudinal axis of the stock. A slotted rack 36 is affixed to the bottom of the hydraulic drill 35 so that it is in contact with a pinion-gear 37 mounted on an axle 38. In turn, the axle 38 is mountedthrough holes in one of the vertical bars 32 and a flange 39 extending downwardly from plate34. Consequently, the rack 36 and hence the hydraulic drill 35 may be moved in the plane of the longitudinal axis of the stock resting in the V-shaped support 15 by turning a wheel 40 having a handle 41 that is attached to axle 38 for rotation pinion 37.

During the drilling of center holes, the hydraulic drill 35 would be susceptible to twisting within the aperture of plate 34 as the bit comes in contact with the stock. Accordingly, a pair of plates 42 are affixed to the hydraulic drill 35 at points on either side of the plate 34. The two plates 42, are in turn, connected by rods 43 passing through holes in plate 34 so that the hydraulic drill 35 is not rotatable within its aperture. The holes are dimensioned such that the rods 43 pass freely the rethrough. A center drill bit 44 is mounted on the hydraulic drill 35 and rotated thereby so that it is effective to center drill a hole at the center of the cross sectional area of stock material held in the V-shaped support 15.

bar 45 by pivot 47 and to a flange 48 on the spacing bar 22 by pivots 47. Accordingly, a second parallelogram arrangement is provided for maintaining the hold-down plate in a perfectly horizontal position no matter what are the free-moving ends of rods 20 may be moved through.

The operation of the center-drilling apparatus may be more easily understood by reference to FIG. 2. There it may be seen that the hydraulic drill 35 is mounted between the vertical bars 32 in spatial relationship with the bottom surface of hold-down 24 so that the center drill bit 44 is aligned with the longitudinal axis of the stock when the hold-down 24 contacts the top surface thereof. Accordingly, when the center hole is to be drilled, the rack 36 and pinion 37 are actuated to move the hydraulic drill 35 toward the stock coincident with its longitudinal axis so that a center hole can be drilled. Once the center hole is drilled, the combined action of the rack 36 and the pinion 37 withdraws the center drill bit 44 and the hydraulic drill assembly 35 so that the stock may be removed and the next piece placed in the stock support for center drilling.

The lateral displacement, or positioning, of the vertical bars 32 along the bars is mathematically determined so that the center drill bit 44 remains aligned with the longitudinal axis of the piece as the diameter of the stock changes. FIG. 3 is an end view of the center drilling hold-down device as it appears in FIG. 2. There a piece of round stock material having'a radius R is shown resting in the V-shaped stock support 15. Referenced to the base of the V groove, which is identified generally as reference, it may be seen that the distance between the center, or longitudinal, axis of the stock and reference is a distance equal to R (2)" and accordingly, the distance to the top of the stock material is R [1 (2)" Thus in FIG. 2 when the parallelogram linkage is rotated about pivots 21 so that hold-down 24 contacts the top surface of stock, it may be seen that the pivots 23 define an are at a constant radius (A B) about pivots 21 and that pivots 33 likewise define an are about pivot 21 having a radius equal to B. Accordingly, regardless of variations in the diameter of the stock material the hold-down 24 will always be at a distance of R,[l (2)] from reference, and the center drill bit 44 should bear the relationship R ,(2)" to the reference." Consequently, the optimum lateral displacement of the vertical bars 32 along the bars 20 may be determined by the ratio:

where AR R,,R, R and R being the respective radii of two pieces of stock.

Accordingly, the distance (A) at which the pivots 33 are displaced from pivots 21 should be:

Thus, when the hold-down 24 is moved away from the reference, the hydraulic drill 35 will likewise be repositioned to the center point, or longitudinal axis, of the stock. Although the free-moving ends of bars 20 define arcs around the pivots 21, it should be noted that the vertical bars 32, because of the parallelogram linkage, are always in the vertical plane. Hence, the hydraulic drill 35 is always normal to the end of the stock.

FIG. 4 illustrates the center drilling apparatus as it appears when a center hole is to be drilled in stock having a small diameter. A pair of slots 49 are provided in the V-shaped stock support 15 to accommodate the spacing bar 22 so that the hold-down plate 7 24 can contact the upper surface of the stock positioned therein. Altematively, the hold-down 24 may be extended downward from the spacing bar 22 for some distance so that extremely small diameter material can be center drilled even though the spacing bar 22 is limited to movement above the top of stock support 15.

Referring now .to FIG. 6, there is shown a sectional view of the pivot 33 shown in FIG. 5. As shown, the pivot assembly 33 utilizes tapered roller bearings 50 to provide a nearly frictionless rolling surface a pivot vertical bar 32 relative to bar 20 with great rigidity and stability. The tapered roller bearings 50 are placed between an inner cone 51 and an outer cone 52 which are in contact with a bolt 53 and bar 20, respectively. Thus,

view taken along line 7-7 in FIG. 5. An important advantage realized by utilizing tapered roller bearings over any other form of roller bearing lies in the ready provision for minimizing the effect of wear which could decrease lateral rigidity of the parallelogram'linkage.

Accordingly, there has been shown a center drilling apparatus in accordance with the present invention which may be utilized for drilling centering holes in stock of different size and symmetric cross-sectional shape which are intermixed with one another. Thus, there is no need to readjust the machine to the center point, or longitudinal axis, of the stock each time stock of different size or shape is to be drilled. Rather, the machine of the present invention automatically aligns its drilling apparatus with the longitudinal axis of the stock whenever the hold-down means is placed in contact with the upper longitudinal surface of the stock to secure it for drilling. Further, the center drilling machine of the present invention utilizes a hydraulic motor to power the drill bit thereby reducing the vibrations commonly associated with the prior art belt driven drilling machines. Since the parallelogram linkage of the present invention is inherently more rigid than the prior art machines previously discussed and since tapered roller bearings are used at the pivot points to increase the rigidity thereof, vertical guides are not required to insure that the drill bit is correctly aligned with the longitudinal axis of the stock. Rather, in the present invention, the hydraulic vertical guides are used merely to insure that the hold-down device is clamped on to the stock. Consequently, the vertical guides need not be machined to close tolerances or lubricated thereby reducing the maintenance cost of the machine. I

While a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention in its broader aspects. For example, the apparatus need not necessarily be disposed or employed in a generally horizontal drilling position but may be used in any drilling plane. Accordingly, the aim in the appended claims is to cover all such changes and modifications as should fall within the true spirit and scope of the invention.

I claim:

1. An apparatus for drilling a centering hole in the end surface ofa piece of stock coincident with the longitudinal axis of said stock, said apparatus comprising in combination:

a stationary frame member;

means including a parallelogram linkage pivotably connected to said frame member and constrained to move in a plane coincident with said longitudinal axis, said linkage comprising a pair of parallel aligned members pivotably connected to said frame member and a support member pivotably connected between said parallel members, said support member being intermediate and parallel to said frame member and said stock end surface, one of said parallel members extending from said frame member beyond said support member for a predetermined length to contact said stock piece immediately adjacent said end surface;

drill means mounted on said support member normal to said stock end surface for drilling therein, said drill means being automatically aligned with said longitudinal axis when said extended parallel member contacts said stock piece immediately adjacent said end surface.

2. A center drilling apparatus in accordance with 7 claim 1 including hydraulic hold-down means coupled to said parallelogram linkage for pressing said extended parallel member against said stock to securely position said stock and align said drill means with said longitudinal axis during drilling.

3. A center drilling apparatus in accordance with claim 1 including stock support means for positioning said stock in alignment with said parallelogram linkage so that said end surface is normal to said drill means and at a predetermined distance from said frame member so that said extended parallel member contacts said stock immediately adjacent said end surface.

4. A center drilling apparatus in accordance with claim 3 wherein said stock support means comprises a V-shaped base having a lip at one end, said stock resting in a substantially horizontal position with said end surface being butted against said lip.

5. A center drilling apparatus in accordance with claim 3 including auxiliary stock support means positioned in alignment with said stock support means and at a distance therefrom to permit longer pieces of said stock to be stably positioned for drilling by supporting that portion of said stock extending from said stock support means.

6. A center drilling apparatus in accordance with claim 1 wherein said drill means includes a hydraulic drill and means for advancing said hydraulic drill along said longitudinal axis of said stock to drill a centering hole in said stock end surface, said means withdrawing said hydraulic drill from said stock when center drilling is completed.

7. A center drilling apparatus in accordance with claim 1 wherein said extended parallel member is substantially l (2) times longer than the other of said parallel members.

8. An apparatus for drilling a centering hole in the end surface of a piece of stock coincident with the longitudinal axis of said stock, said apparatus comprising in combination:

a frame member;

means including a pivoted parallelogram linkage comprising:

a. a first pair of horizontally displaced bars, each of said first bars having one end pivotably connected to said frame member and having a free-moving end;

b. a second pair of shorter horizontally displaced bars, each of said second bars having one end pivotably connected to said frame member and having a free-moving end, each of said second bars being positioned below said first bars in vertical alignment with a corresponding one of said first bars; and

c. a pair of vertical bars, each of said vertical bars having a first end pivotably connected to said freemoving end of the corresponding one of said second bars and a second end pivotably connected to the corresponding one of said first bars intermediate said pivoted end and said free-moving end thereof, said vertical bars being intermediate and parallel to said frame member and said stock piece end surface, said parallelogram linkage being constrained to move in a plane substantially coincident with said longitudinal axis;

hold-down means for interconnecting said freemoving ends of said first bars; and

drill means mounted between said vertical bars normal to said stock piece end surface for drilling therein, said drill means being aligned with said longitudinal axis when said hold-down means is positioned on said stock piece immediately adjacent said stock piece end surface.

9. A center drilling'apparatus in accordance with claim 8 including hydraulic means attached adjacent said free-moving ends of said first bars to move said hold-down means into contact with said stock immediately adjacent said end surface for securing said stock in position forv drilling and aligning said drill means with said longitudinal axis.

10. A center drilling apparatus in accordance with claim 8 including stock support means for positioning said stock in alignment with said parallelogram linkage so that said end surface is normal to said drill means and at a predetermined distance from said frame member so that said hold-down means contacts said stock immediately adjacent said end surface.

11. A center drilling apparatus in accordance with claim 10 wherein said stock support means comprises a V-shaped base having a lip at one end, said stock resting in a substantially horizontal position with said end surface being butted against said lip.

12. A center drilling apparatus in accordance with claim 11 wherein said V-shaped base includes slots therein aligned with said hold-down means for accommodating said hold-down means when said stock is of a small diameter.

13. A center drilling apparatus in accordance with claim 8 including a substantially flat plate affixed between said vertical bars for mounting said drill means normal to said stock end surface and in a plane coincident with said longitudinal axis of said stock, said plate having an aperture through which said drill means is slidably mounted.

14. A center drilling apparatus in accordance with claim 13 including means for preventing said drill means from twisting within said aperture when said claim 8 including second linkage means for maintaining said hold-down means substantially parallel to the upper surface of said stock when said hold-down means is not in contact with said stock.

17. A center drilling machine in accordance with claim 8 including pivot means comprising tapered roller bearings for pivotably interconnecting said first bars, said shorter second bars, said vertical bars and said frame member.

-'P0-1 9 UNITED STATES PATENT OFFICE (s/ss) i CERTIFICATE OF CORRECTION latent No., 5 363 Dated Julv 23 1 974 Inventofls) Robert W. Lanman It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

FXB STRACT [57] righthand column, Line 11, after "corresponding" insert longer bar-; Line 14, insert hyphen between --outwardly extending;

Column 3, Line 18, "views" should be -view-- Celumn 6, Line 11, change "a pivot" to to pivot---;

Column 7, Line 62, Claim 7,' formula should read -l+ (2) Signed and sealed this 8th day of Cctober (SEAL) Attest. v

McCOY M; GIBSON JR. c. MARSH ALL DANN Attesting Officer Commissioner of Patents

Claims

1. An apparatus for drilling a centering hole in the end surface of a piece of stock coincident with the longitudinal axis of said stock, said apparatus comprising in combination: a stationary frame member; means including a parallelogram linkage pivotably connected to said frame member and constrained to move in a plane coincident with said longitudinal axis, said linkage comprising a pair of parallel aligned members pivotably connected to said frame member and a support member pivotably connected between said parallel members, said support member being intermediate and parallel to said frame member and said stock end surface, one of said parallel members extending from said frame member beyond said support member for a predetermined length to contact said stock piece immediately adjacent said end surface; drill means mounted on said support member normal to said stock end surface for drilling therein, said drill means being automatically aligned with said longitudinal axis when said extended parallel member contacts said stock piece immediately adjacent said end surface.

2. A center drilling apparatus in accordance with claim 1 including hydraulic hold-down means coupled to said parallelogram linkage for pressing said extended parallel member against said stock to securely position said stock and align said drill means with said longitudinal axis during drilling.

7. A center drilling apparatus in accordance with claim 1 wherein said extended parallel member is substantially 1 + (2)1/2 times longer than the other of said parallel members.

8. An apparatus for drilling a centering hole in the end surface of a piece of stock coincident with the longitudinal axis of said stock, said apparatus comprising in combination: a frame member; means including a pivoted parallelogram linkage comprising: a. a first pair of horizontally displaced bars, each of said first bars having one end pivotably connected to said frame member and having a free-moving end; b. a second pair of shorter horizontally displaced bars, each of said second bars having one end pivotably connected to said frame member and having a free-moving end, each of said second bars being positioned below said first bars in vertical alignment with a corresponding one of said first bars; and c. a pair of vertical bars, each of said vertical bars having a first end pivotably connected to said free-moving end of the corresponding one of said second bars and a second end pivotably connected to the corresponding one of said first bars intermediate said pivoted end and said free-moving end thereof, said vertical bars being intermediate and parallel to said frame member and said stock piece end surface, said parallelogram linkage being constrained to move in a plane substantially coincident with said longitudinal axis; hold-down means for interconnecting said free-moving ends of said first bars; and drill means mounted between said vertical bars normal to said stock piece end surface for drilling therein, said drill means being aligned with said longitudinal axis when said hold-down means is positioned on said stock piece immediately adjacent said stock piece end surface.

9. A center drilling apparatus in accordance with claim 8 including hydraulic means attached adjacent said free-moving ends of said first bars to move said hold-down means into contact with said stock immediately adjacent said end surface for securing said stock in position for drilling and aligning said drill means with said longitudinal axis.

14. A center drilling apparatus in accordance with claim 13 including means for preventing said drill means from twisting within said aperture when said drill means engages said end surface.

15. A center drilling apparatus in accordance with claim 8 wherein said drill means includes a hydraulic drill having a rack gear attached thereto and a pinion gear mounted on said parallelogram linkage, said pinion gear engaging said rack gear so that rotation of said pinion gear selectively advances and withdraws said hydraulic drill with respect to said stock end surface.

16. A center drilling apparatus in accordance with claim 8 including second linkage means for maintaining said hold-down means substantially parallel to the upper surface of said stock when said hold-down means is not in contact with said stock.