Deep hole boring head and deep hole boring method for boring a production piece

A deep hole boring head is provided which is capable of rotating about a central axis, a body whose front part has at least one cutting edge followed by a recess which leads to a duct inside the body, this duct allowing the chips formed during the boring to be cleared away rearwards, and includes at a front end thereof and at a center portion thereof a pilot drill bit capable of guiding the head during boring. Use for the boring of a production piece is made of titanium, the ratio L/D between the depth L of the hole bored and the diameter D of the latter being greater than or equal to 10, and the diameter D being greater than or equal to 65 mm.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved deep hole boring head and a deep hole boring method for boring a production piece, and more particularly a production piece made of titanium.

2. Discussion of the Background

Deep hole boring differs from shallow hole boring, or from drilling, by the L/D ratio between the length L and the diameter D of the hole to be made. Thus, deep hole boring in the present specification means a boring method used to make holes having an L/D ratio greater than or equal to 10. With such an L/D ratio, it is particularly necessary to clear away the chips cut from the production piece during boring.

Also, it will be noted that the invention is more particularly intended for boring large diameter holes, usually greater than 40 mm and, preferably, greater than or equal to 65 mm.

Deep hole boring tools are already used for boring steel alloy pieces, such as 300M and 4340M alloy steels. These tools are formed of a rotary drive shaft, at the front end of which is mounted a deep hole boring head which comprises a body whose front face has at least one cutting edge followed by a recess which leads to a duct inside the said body, this duct allowing the chips formed during boring to be cleared away rearwards.

To make it easier to clear away the chips, limit the heating and reduce the friction between the boring head and the bottom of the hole made, a boring liquid, usually undiluted oil or oil that is soluble between 10 and 15%, is used.

The general shapes of the front and rear parts of the boring head have a symmetry of revolution relative to the central axis about which the head and the tool are rotated.

The rear part of the boring head and the drive shaft are hollow cylinders, the diameter of the drive shaft being less than or equal to the diameter of the rear part of the boring head. The latter diameter is furthermore less than the maximum diameter of the front part of the boring head, so that, during boring, there remains an annular space between the rear part of the boring head, the drive shaft, and the walls of the hole bored. This annular space, necessary for limiting the friction between the tool and the walls of the hole, is also used for bringing the boring liquid to the front part of the boring head. The liquid chip-laden is then cleared away via the internal duct passing through the rear part of the boring head and the drive shaft.

Because of this annular space, it is necessary to guide the boring head when it moves forward. For this purpose, it is a known practice to use three guide pads usually coated with carbide or Celoron (registered trademark) pads, that are evenly distributed and protrude from the outer lateral periphery of the rear part of the head, and that are intended to press against the walls of the hole formed during boring.

However, the boring heads known to date are unsuitable for boring titanium (Ti) pieces. Specifically, the titanium adheres to the guide pads, which causes the boring head to seize and it then becomes virtually impossible with the machines currently used in the boring field to develop a sufficiently high boring torque to continue rotating the head. Furthermore, premature wear of the guide pads is noted, which necessitates frequent replacement of the pads, while their price is high due to their specialist nature and the materials they are made of. Finally, because of these unsatisfactory boring conditions, the geometry of the hole made is so bad (in particular, significant deviations are observed relative to the boring axis) that it is very difficult, even impossible, to rectify it by machining. In addition, this subsequent machining step is made difficult by the presence of the carbide particles originating from the guide pads.

SUMMARY OF THE INVENTION

The object of the invention is to improve this prior art by providing a deep hole boring head particularly suitable for deep hole boring production pieces made of hard materials such as titanium.

To achieve this object, the subject of the invention is to provide a deep hole boring head capable of rotating about a central axis, comprising a body whose front part has at least one cutting edge followed by a recess which leads to a duct inside the said body, this duct allowing the chips formed during boring to be cleared away rearwards, and which comprises at a front end thereof and at a center portion thereof a pilot drill bit capable of guiding the head during boring to make the head follow the boring axis, which is usually the axis of the drive shaft of the means for rotating the boring head.

The presence of the pilot drill bit, or centering bit, makes it possible to dispense with the presence of the guide pads and the problems associated with the titanium sticking to these pads. Consequently, the boring torque necessary to rotate the head during boring is sufficiently low so as to be able to be developed by machines widely used for boring materials other than titanium. In addition, the guidance obtained with the aid of the bit makes it possible to obtain good geometry of the hole and in particular to limit the deviations relative to the boring axis.

According to a particular embodiment of the invention, the deep hole boring head of the invention also comprises pads that are integral with the body and that protrude from the outer lateral periphery of the latter, the distance between the outer face of these pads and the central axis being less than the distance between the central axis and the outer lateral edge of the cutting edge furthest from this axis.

These pads should not be confused with the guide pads used in the prior art, because these pads in no way guide the boring head, the latter function rather being entirely performed by the pilot drill bit. The pads mounted on the boring head of the invention are useful once boring is complete, as the boring head is being removed from the hole made, and prevent the outer lateral edge of the cutting edge furthest from the central axis of the boring head from deeply scoring the walls of this hole. For this purpose, these pads are positioned on the body of the boring head so that this head can press on at least one of these pads as it is being removed. The weight of the boring head may therefore be distributed between the pad and the cutting edge, so that any pressure exerted by the cutting edge on the walls of the hole is lessened and the score marks made may be easily rectified during a subsequent machining step. These pads may also be useful in the event of breakage of one of the elements of the cutting head (usually one of the cutting elements forming the said cutting edges), to limit the zones of contact between the head and the walls of the hole bored and thus protect these walls.

This difference of functionality between the pads of the invention and the known guide pads also means that different materials are used and that the pads of the invention protrude only by a small distance from the periphery of the body of the boring head, which makes it possible to prevent any sticking phenomenon. The pads of the invention are made for example of bronze, a low cost material so that their replacement does not generate major expenditure.

A further subject of the invention is a deep hole boring method for boring a production piece, which comprises a boring head of the type previously described, a rotary drive shaft attached to the said boring head, and means for rotating the drive shaft.

This method can be used to bore a production piece made of titanium, with a ratio L/D between the depth L of the hole made in this piece and the diameter D of the latter, greater than or equal to 10, the diameter D preferably being greater than or equal to 65 mm. For example, it has been possible to bore in a titanium workpiece a hole 180 mm in diameter D and 3400 mm in length, making an ratio L/D greater than 18.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIGS. 1,2,2aand3, a description will now be given of an example of a boring head according to the invention.

According to this particular embodiment, the boring head comprises a body2which has a front part4and a rear part6. This boring head is intended to be rotated about the central axis A-A. The weight of the boring head is distributed so that the centre of gravity of this piece corresponds to the central axis A-A of rotation, which prevents any imbalance.

The general shape of the body2has a symmetry of revolution relative to the central axis A-A. More precisely, the front part4of the body2has a generally frustoconical shape widening towards the front, and the rear part6has the general shape of a hollow cylinder of external diameter E.

The front face4aof the front part4usually has at least one cutting edge formed by at least one cutting tip mounted on the front part4. In the example, the front face4ahas four cutting edges8a,8b,8cand8d. These cutting edges are substantially perpendicular to the central axis A-A and situated at staggered distances from this axis (naturally it is radial distances that are meant here) and such that the zones swept by these cutting edges during the rotation of the body2partly overlap. In order of distance from the central axis A-A, the cutting edges are8a,8b,8cand8d.

Each cutting edge is formed by two removable cutting tips7that are adjacent but not necessarily contiguous. More precisely, in the example, each pair of cutting tips7is mounted on a removable support, hereinafter called a cassette9, mounted on the front part4. These cassettes9may be mounted on the front part4at different heights or different degrees of inclination. After having adjusted the positioning of the cassettes, no additional adjustment is necessary to mount the cutting tips7on these cassettes9, because the housings made in the cassettes9and intended to receive the said tips7are adjusted to the dimensions of the latter. The cassettes and the tips are attached to one another and to the boring head by screwing or by any other means. The choice of the type of cutting tip9(shape, material, etc) to be used is based mainly on the material of the piece to be bored and is within the capabilities of those skilled in the art. For the deep hole boring of titanium, carbide cutting tips are preferably chosen.

It will be noted that the tangents to the cutting edges may be slightly inclined relative to the plane perpendicular to the central axis A-A to make boring easier and, in particular, that the tangent to the cutting edge8cmay be inclined at an angle “x” of between 0 and 5°, preferably between 1 and 3° relative to the tangent to the cutting edge8a, which makes it easier to clear away the chips.

In the example, the cutting edges8aand8bare situated substantially in one and the same plane P-P containing the central axis A-A shown inFIG. 3, while the cutting edges8cand8dare situated substantially in another plane P′-P′ also containing the central axis A-A, which generally forms with the plane P-P an angle “a” of between 5 and 30°, and preferably between 10 and 20°, the value 15° having been adopted for a boring head used to bore a 185 mm diameter hole in a titanium piece. It has been found that this range of values of the angle “a” makes boring easier. In particular, it is noted that the clearing of the chips is made easier, the boring speed is higher and that there is less wear of the cutting edges.

As can be seen inFIGS. 1,2and3, the cutting edges8ato8dare followed by recesses10aand10bwhich lead to a duct12inside the body2which is formed in the front part4and which extends into the rear part6. This duct12is used to clear away rearwards the chips formed during boring.

A description will now be given of the pilot drill bit20situated at the center of the front face4aof the deep hole boring head. This pilot drill bit is removable and the deep hole boring head of the invention comprises means of preventing this pilot drill bit20from turning, and also comprises means of adjusting the distance d′ by which the pilot drill bit20protrudes. The distance of protrusion d′ being the distance by which the drill bit20protrudes ahead of the cutting head furthest forward of the body2.

The drill bit20is housed in a housing21provided for this purpose at the center of the front face4aof the head2. It is held in this housing21with the aid of the attachment means which include a clamping screw22housed inside a bore (preferably transverse to the housing21) which leads from the said housing21to the outer lateral periphery of the front part4of the body2, thus forming an opening24making it possible to gain access to the head of the screw22. The bore has, in the vicinity of the housing21, an internal thread into which it is possible to screw the screw22. Tightening the screw22tightens the drill bit20between the screw22and the wall of the housing21being situated facing this screw, in order to prevent it from turning. To facilitate fixing of the screw22in position, the drill bit20may have a flat23. It will be noted that the aforementioned turning-prevention means also serve as means for attaching the screw22to the body2. Any other means of preventing turning, such as a pin or a groove/rib system, could be envisaged.

The means of adjusting the protrusion distance d′ of the pilot drill bit20comprise for their part, in the example shown, a set screw26. This screw26is screwed into an internal thread28made in a bore arranged in the front part4of the head2and leading from the bottom of the housing21to the duct12inside the body2. The pilot drill bit20rests on the end of the stem of the screw26, this end protruding at the bottom of the housing21. The tightening/loosening of the screw26is thus used to vary the distance d′, before the bit is tightened using the screw22.

It is advantageous to have the pilot drill bit protrude ahead of the cutting edge furthest forward of the body2, by a distance d′ of between 1% and 20% of the distance d between the central axis and the outer lateral edge of the cutting edge furthest from this axis. This distance d′ by which the outer part of the pilot drill bit extends must be sufficient to ensure a good guidance and must not be too great so as to limit the risks of the drill bit buckling or breaking. Thus, for a boring diameter D of 185 mm, a distance d′ of for example between 8 and 10 mm should be chosen, that is approximately between 8 and 11% of the distance d between the central axis and the outer lateral edge of the cutting edge furthest from this axis, here equal to 185/2=92.5 mm.

Furthermore, advantageously, the radius of the pilot drill bit20is at most equal to a fifth (b20%) of the distance d between the central axis A-A and the outer lateral edge of the cutting edge furthest from this axis. The reason for this is that, the diameter of the pilot drill bit20must ensure a sufficient bearing surface of the bit to ensure the guidance of the boring head, while remaining sufficiently small to limit the zone of action of the drill bit, the latter mainly playing a role of guiding the head and not of boring (although it participates therein). For a distance d of 92.5 mm, 15 mm may be chosen for example as the radius of the drill bit20.

The front end of the pilot drill bit may have various shapes. The literature on the various possible shapes of pilot drill bits in the boring field is abundant, so those skilled in the art may refer thereto. In the present case, it has been found that a of a simple shape front end, substantially pointed, gives good guidance results and easily penetrates into the production piece.

According to a particular embodiment of the pilot drill bit, the latter may have at least one channel24which traverses it longitudinally and inside which a cutting fluid will be able to flow. The drill bit20shown inFIG. 2has two channels24which lead to the front and rear ends of the bit and which are substantially symmetrical relative to the central axis A-A which is, of course, also the axis of rotation of the bit20.

The structure of the deep hole boring head2of the invention being well understood, a description will now be given of the operation of a deep hole boring tool fitted with such a head, with reference toFIGS. 4 and 5.

Irrespective of the chosen embodiment, a deep hole boring tool according to the invention usually comprises a rotary drive shaft, attached to the boring head, and means for rotating the said drive shaft. The boring head2may be mounted on the shaft40by any appropriate means. For example, the end of the shaft40may have an outer thread capable of interacting by screwing with an internal thread inside the rear part6of the body2.

Naturally, the means of rotating the shaft40must be capable of developing a sufficient torque to carry out the deep hole boring.

To clear away the chips cut from the production piece, a boring fluid is used, preferably a fluid of the undiluted oil type or oil that is soluble between 10 and 15%. This boring fluid is injected so that it reaches the bottom of the bored hole. As shown inFIGS. 4 and 5, it is possible to envisage two possible injection methods.

With reference toFIG. 4, the boring fluid may be injected into an injection chamber50formed between a sleeve54and the shaft40or the boring head. The sleeve54is placed around the shaft40(or the boring head) and in contact with the production piece60. Leakage between the chamber50and the exterior is prevented by being sealed by seals52placed between the sleeve54and the piece60and between the sleeve54and the shaft40. Since the maximum external diameter of the front part4of the body2is greater than that of the rear part6of the body2, and greater than that of the drive shaft40, there remains an annular space58delimited on one side by the outer wall of the rear part6and the shaft40and on the other by the inner wall of the hole formed. This annular space58communicates with the injection chamber50so that the injected fluid passes along the annular space58to reach the bottom of the bored hole. The chip-laden fluid is then cleared away through the duct12inside the body2and through the duct44inside the hollow drive shaft40. Openings31may be made in the outer wall of the body to facilitate the circulation of the fluid.

According to a second embodiment, with reference toFIG. 5, the drive shaft40is formed of two concentric hollow cylinders41aand41b. The “incoming” boring fluid is then injected between the inner cylinder41band outer cylinder41aand the “outgoing” fluid is cleared away with the chips through the inside of the inner cylinder41b. To limit turbulence at the boring head, it is possible to make the front end of the inner cylinder41bflared and protruding beyond the front end of the outer cylinder41a, so as to guide the “incoming” fluid towards the openings32that are made in the outer wall of the body2of the boring head and that lead into the annular space58.

The deep hole boring of the piece60takes place as follows. First, the tool is positioned relative to the production piece. For this, a pilot hole intended to receive, at least partially, the pilot drill bit20may be used. Use may also be made of a guide ring61attached to the piece60which will be used to guide the tool at the beginning of boring, and if necessary thereafter, as an addition to the guidance given by the pilot drill bit. Such a ring61is shown inFIG. 5and comprises an annular piece62which surrounds the tool and which has fins62adistributed regularly over its inner face and on which the tool will be able to bear.

Then, the tool is rotated. The pilot drill bit20will then be the first to penetrate the piece60to establish a secondary hole of reduced diameter in the center of the primary hole that the user requires to bore. This secondary hole is used to guide the deep hole boring head during its forward travel, while ensuring that its central axis A-A does not deviate from the boring axis. In addition, the pilot drill bit20, once it has entered the production piece60, steadies the boring head and prevents, for example, any shaking of the rotary driving means from being transferred to the boring head. Thus, thanks to the invention, deviations relative to the boring axis and more generally faults in the geometry of the hole made can be significantly reduced.

Once boring is finished, the tool is withdrawn from the hole. To prevent the outer edge of the cutting edge8dfrom scoring the walls of this hole, bronze pads30are distributed on the outside of the front part4of the body2. These pads8dprotrude slightly outside the body2so that the body can rest on the latter during withdrawal. The weight of the body2is not therefore fully transferred to the cutting edge8d, which limits the depth of the score marks that this cutting edge may make on the walls of the hole. The shallow score marks that may nevertheless result from the removal of the tool may be rectified easily during a subsequent machining step. It will be noted that the pads are sufficiently recessed relative to the wall of the formed hole not to enter into contact with this wall during boring.