Improvements relating to screw threads

Abstract: A hydrostatic nut, which may for example be used on a lead screw in a machine tool, is produced with a female Acme thread 18 extending through the nut. Formed along the middle of at least one flank of the thread is a shallow groove 20, 21 extending the full length of the thread. Each end of the groove is closed by a plug or pin 44, 48 which is preferably sealingly secured in a reamed hole 40, 46 passing through a profile 42, 48 in the thread at an orientation parallel to the central axis of the thread. The thread may then be finish machined to remove any end parts of the pins protruding from the flanks of the thread.

Description: Subject of the Invention This invention relates to screw threads and more particularly to the manufacture of a screw thread for a hydrostatic nut. Background to the Invention Hydrostatic nuts are used in various engineering applications where an accurate movement is required with minimum backlash and wear. One particular application is in the field of machine tools where a hydrostatic nut, when mounted on a complementary screw threaded shaft, can be used as a lead screw for producing relative movement between a tool and a workpiece, eg in a grinding machine. The manufacture of the female screw thread for a hydrostatic nut presents technical problems: thus the basic thread form, which will usually have straight flanks (often referred to as an Acme thread), must extend all the way through the nut, and an additional thread or groove for the hydraulic oil can only extend along each flank within the central part of the nut, so that the oil does not leak to any extent out at the ends of the groove. Difficulties have therefore been experienced in manufacturing such a nut with a concealed oil groove in each flank. A method according to the preamble of claim 1 and a hydrostatic nut according to the preamble of claim 9 is known from GB-A- 1132927. It is an object of the present invention to provide a new method of machining such a nut, and to provide an improved form of such a nut. Summary of the Invention According to one aspect of the present invention there is provided a method of producing a hydrostatic nut comprising the steps of forming a female screw thread so as to include a groove which extends along the full length of at least one flank of the thread, and thereafter closing each end of the groove by means of a plug. A groove may be formed in each flank of the thread, and if so, plugs are used to close off the ends of each such groove. The flanks of the thread may be rectilinear in longitudinal section, in the form of an Acme thread. Advantageously each end of a groove is closed by drilling and reaming a hole into the end of each groove, and sealingly .securing a plug therein. The plug is conveniently in the form of a pin. The axis of each drilled hole is preferably parallel to the central axis of the thread. The method preferably comprises the further step of finishing off the basic thread profile closure of the groove or grooves by the insertion of the plugs to achieve this. The invention also extends to a hydrostatic nut when produced in accordance with the method as aforesaid. According to another aspect of the invention there is provided a hydrostatic nut having a female thread, a groove extending along the full length of at least one flank of the thread, and a separate closure at each end of the groove, to prevent oil loss therefrom and the closure comprising a pin or plug sealing enganged in a hole formed in the groove. Preferably each flank of the thread is formed with a groove. A preferred closure comprises a pin sealingly engaged in a hole formed in the groove. Typically each such hole is formed with its axis parallel to the central axis of the thread. Brief Description of Drawings The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an end view of a hydrostatic nut constructed in accordance with the invention; Figures 2 to 4 are sectional views taken on the lines II-II, III-III and IV-IV respectively of Figure 1. Detailed Description Referring first to Figures 1 and 2, the hydrostatic nut is rough machined on an NC lathe using a pre-formed insert. Typically an aluminium bronze alloy is used, and machining the nut comprises a main body 10 and a flange 12 at one end. Flats 14 are machined at the top and bottom of the flange, and six counter-bored fixing holes 16 are formed in the remainder of the flange. Formed inside the nut, by a method to be described below, is a female Acme thread 18 typically of 25mm pitch and 54mm pitch circle diameter. The flanks of the thread are rectilinear in longitudinal section, as seen in the sectional view of Figure 2. Extending along the middle of each flank, at a position corresponding to the pitch circle diameter, is a shallow groove of semi-circular section and approximately 1mm deep, which in use is fed with hydraulic oil under appropriate pressure. As viewed in Figure 2, the left-hand facing groove is identified by reference numeral 20 and the right-hand facing groove by 21. As shown in Figures 2 to 4, drillings 22, 24 and 26 connect the groove 20 to a tapped oil port 28; and drillings 30, 32 and 36 connect groove 21 with tapped oil ports 34 and 38. Referring again to Figure 2, an axial drilling 40 joins grooves 20 and 21 across one of the thread profile 42, the drilling being closed, by a tightly fitting pin 44. The pin may be made from an aluminium bronze alloy. A similar drilling 46 between the oil grooves 20, 21 across a profile 48 at the other end of the thread is closed by a second pin 50 in a similar manner. The detailed operations in the manufacture of the nut will now be described. A block of aluminium bronze alloy is faced and rough machined on an N.C. lathe tool. The basic Acme screw thread 18 and oil grooves 20, 21 are also rough turned, using a pre-formed tool insert to generate the required profiles. In accordance with the invention, the grooves 20, 21 are formed along the full length of the thread from one end to the other. The various ports and passages are then drilled. The ports are tapped, and the two holes 40 and 46 for the pins 44 and 50 respectively are then drilled and reamed. Typically the holes are of 4mm diameter. The whole of the partly finished hydrostatic nut is then cleaned and flushed out, after which the two pins 44, 50 are securely fitted into their respective holes 40 and 46 by first applying an adhesive such Loctite (R.T.M) and then pushing each pin into its hole. opposite ends of the pin 44 protrude into the grooves 20, 21 so as to prevent oil from leaking to the left-hand end of the thread 18 beyond the pin 44, thereby blocking off the left-hand ends of the two oil grooves 20 and 21. In a similar way the pin 50 serves to prevent oil from leaking to the right hand end of the thread 18 beyond the protruding ends of pin 50, which close off the grooves 20, 21 at that end of the assembly. Finally, the Acme thread profile of the nut is finish machined on an accurate thread cutting machine, so that when fitted to an appropriately machined male thread a clearance of the order of 5 microns is achieved between the corresponding faces of the male and female threads. During this finish machining step the ends of the pins 40 which protrude beyond the flanks are also machined so as to be flush with the respective flanks. In this way the ends of both grooves 20 are completely blocked off by the pin ends, which as a result of the finish machining do not interfere with the engagement of the male and female threads.

Claims: A method of producing a hydrostatic nut comprising the steps of forming a female screw thread (18) so as to include a groove (20,21) which extends along the full length of at least one flank of the thread (18), characterised in thereafter closing each end of the groove (20,21) by means of a plug (44,48). A method according to claim 1 in which a groove (20,21) is formed in each flank of the thread (18), and plugs (44,48) are used to close off the ends of each such groove (20,21). A method according to claim 1 or claim 2 in which the flanks of the thread (18) are rectilinear in longitudinal section, in the form of an Acme thread. A method according to any one of claims 1 to 3 in which each end of the groove (20,21) is closed by drilling and reaming a hole (40,46) into the end of each groove, and sealingly securing a plug therein. A method according to any one of claims 1 to 4 in which the plug (44,48) is in the form of a pin. A method according to any one of claim 4 or claim 5 in which the axis of each drilled hole (40,46) is parallel to the central axis of the thread (18). A method according to any one of preceding claims in which, prior to closing each end of the groove (20,21), passages are drilled in the nut to connect the exterior thereof with the groove, so that in use hydraulic oil can be fed to the groove. A method according to any one preceding claims comprising the further step of finishing off the basic thread profile after closure of the groove (20,21) or grooves by the insertion of the plugs (44,48). A hydrostatic nut having a female thread (18), a groove (20, 21) extending along the full length of at least one flank of the thread(18), characterised by a separate closure at each end of the groove (20,21), to prevent oil loss therefrom, the closure comprising a pin (44,48) or plug sealing engaged in a hole (40,46) formed in the groove (20,21). A hydrostatic nut according to claim 9, in which each flank of the thread is formed with a groove. A hydrostatic nut according to claim 9 or 10 in which each such hole is formed with its axis parallel to the central axis of the thread.

Inventors: RILEY MARTIN JOHN, RILEY, MARTIN JOHN
Applicants: UNOVA UK LTD, UNOVA U.K. LIMITED
Application year: 1998
Main classification: B23G1
IPC codes: B23G1