THE WALL OF A PASSAGE

This invention relates to a method and apparatus for forming a longitudinal groove in the wall of a passage. In the preferred embodiment, the invention provides a method and apparatus for forming a longitudinal groove of varying depth in the wall of the passage defined by a hollow tubular body.

The invention is particularly useful in manufacturing the cylinder member of a damper in which one or more grooves in the cylinder wall permit passage of working fluid past the piston of the damper. The invention will be particularly described with reference to this application. It should be appreciated, however, that the invention is generally applicable to any machining operation in which a longitudinal groove is required in the wall of a passage of the work piece.

A common method for damping movement of one member relative to another is to couple a cylinder to one of the members and to couple a piston, working in the cylinder, to the other of the members. A suitable working fluid, for example an oil, partially or fully fills the cylinder and a passage is provided which permits restricted flow of the working fluid past the piston so that movement of the two members relative to each other is damped. One possible means of providing the passage for fluid flow is to provide one or more grooves in the internal wall of the

cylinder. By varying the cross-sectional area of the or each groove along the length of the cylinder the damping characteristics of the damper can be varied according to the position of the piston within the cylinder. Heretofore forming longitudinal grooves of varying cross-section within the cylinder has involved relatively slow and expensive machining techniques and has not, in any event, been possible for relatively small diameter cylinders.

The preferred embodiment of the present invention provides a method by which a longitudinal groove of varying cross-section can be formed accurately and rapidly within relatively small diameter tubes suitable for use as the cylinders of relatively small and inexpensive dampers.

According to one aspect of the present invention a method of forming a longitudinal groove in the wall of a passage comprises inserting a mandrel into the passage to engage the wall of the passage, said mandrel including a working surface which, when the mandrel engages the wall of the passage, is located facing, and in a predetermined spatial relationship with, the portion of the wall in which the groove is to be formed; and forcing a carriage to run through said passage along the working surface of the mandrel, the carriage carrying a forming member such that as the carriage moves along the working surface the forming member forms a groove in the wall of the passage with the base of the groove parallel to the working surface of the mandrel.

If a groove of constant depth is required the mandrel working surface will be parallel to the portion of the wall in which the groove is to be formed so that as the carriage moves along the working surface the forming member forms a groove of constant depth. If a groove of varying depth is required then the working surface of the mandrel is machined to converge and/or diverge from the portion of the wall in which the passage is to be formed so that as

the carriage moves along the working surface a groove of varying depth is produced. If, for example, it is desired to produce a groove which is of constant depth for half the length of the work piece and then progressively increases in depth the working surface of the mandrel will be machined to be parallel to the corresponding portion of the wall of the passage over the zone in which the constant depth groove is required and will be machined to converge with the portion of the wall of the passage in which the progressively deeper groove is required.

In a particularly preferred embodiment of the invention the method additionally comprises the step of withdrawing the mandrel from the passage in a manner which causes a profiling member provided on the mandrel distal of the working surface to run along the wall of the passage in which the groove is formed in order to re-form the wall to a desired profile. By this means, any local deformation of the portions of the wall adjacent the groove may be corrected.

According to another aspect of the present invention there is provided apparatus for forming a longitudinal groove in the wall of a passage, the apparatus comprising a mandrel for entering the passage, the mandrel having a working surface thereon which, when the mandrel is within the passage and engaging the wall of the passage faces, and has a predetermined spatial relationship with, the portion of the wall of the passage in which the groove is to be formed; and a carriage for movement along the working surface, the carriage comprising a forming member which, as the carriage moves along the working surface, forms the required groove in the wall of the passage.

Preferably, the mandrel incorporates a profiling member which, as the mandrel is withdrawn from the working passage, runs along the portion of the wall of the passage adjacent the groove to re-form the wall of the passage to a

desired profile. In a particularly preferred embodiment of the invention the forming member comprises a roller mounted on the carriage and the profiling member comprises a ball¬ bearing mounted within a socket provided in the distal end of the mandrel.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawing wherein the single figure shows, schematically, the formation of a groove in the interior wall of a hollow cylindrical tube.

Referring to the drawing there is illustrated apparatus for forming a groove in the interior wall 1 of a round tube 2. The tube 2 is intended to form the body of a damper in which a piston is slidably mounted within the tube. The damper may, for example, be a combined damper and gas spring.

In order to form the required groove a mandrel 3 is inserted through the tube so that the distal end 4 of the mandrel is clear of the tube. The mandrel, when viewed in transverse cross-section, has an exterior part cylindrical surface which is a clearance fit within the tube 2. A working surface 5 is machined on the mandrel. In the illustrated embodiment the working surface 5 is the base of a channel 6 which is formed along the relevant portion of the mandrel. The arrangement is such that when the lower portion of the tube (as viewed in the drawing) rests snugly against the underside of the mandrel the working surface 5 faces, and is in a predetermined spatial relationship with, the interior upper surface of the tube. In the illustrated embodiment of the invention, in which it is desired to produce a groove of progressively increasing cross-section from approximately the point A to approximately the point B the working surface 5 converges with the opposite interior wall of the tube 2 from

approximately the point C to approximately the point D. In the illustrated embodiment the convergence is approximately 0.25mm.

In order to form the required groove a carriage 7 is moved along the working surface 5. The carriage runs on suitable bearings 8 which engage the working surface 5 and carries a forming roller 9 which has a forming profile on the periphery thereof to form the required shape of groove. The carriage is moved along the working surface by means of a member 10 which is coupled to a suitable power source, for example a hydraulic ram or lead screw arrangement. As the carriage 7 moves from the position illustrated in the drawing towards the distal end 4 of the mandrel the roller 9 forms the required groove. After formation, the carriage 8 is withdrawn to the illustrated position relative to the mandrel. The mandrel may then be withdrawn from the tube. In the preferred embodiment of the invention the mandrel carries a hardened steel ball 11 in a pocket 12 formed in the distal end of the mandrel. The ball is positioned such that as the mandrel is withdrawn the ball will roll along the interior surface of the tube 2 in the zone of the groove in order to roll out any excess material movement generated by the groove rolling process.

It will be appreciated by those skilled in the art that the above described process is readily susceptible to automation in order to provide the required grooves in pre-cut tubes. However, it should be appreciated that the invention is more generally applicable than in the mass production of tubes having grooved interior surfaces.