Renewable nut for a lead screw drive and method of making same

In a lead screw drive, such as that used in a lightweight lathe or milling machine for driving the workpiece relative to the cutting tool, the core of a drive nut is made of a material having a relatively low melting temperature, such as babbitt metal or a relatively rigid thermoplastic material. The low melting point material is contained within a nut housing as of steel. The openings in the nut housing for passage of the lead screw are scaled in a fluid type relation by means of elastic sealing members as of silicone rubber having a melting point above that of the relatively low melting point core portion of the nut. An electrical heating cartridge element is contained within the nut housing in good thermally conductive relation with the low melting point core material. When backlash develops due to wear of the core material of the nut, the heating element is energized under control of a controller to melt the core of the nut so that the core material is caused to flow and to reform itself to the thread profile of the lead screw. A thermocouple disposed in thermally conductive relation with the core senses the temperature of the core. A controller responsive to the sensed temperature, causes the nut to cool in accordance with a predetermined sequence of time and temperature and to be run in on the lead screw while lubricant is caused to be sprayed on the lead screw.

BACKGROUND OF THE INVENTION 
The present invention relates in general to lead screw drive mechanisms 
particularly suitable for driving the workpiece carriage of a light weight 
milling machine or lathe, and more particularly, to a method and apparatus 
for renewing the threads of a drive nut so as to eliminate backlash. 
DESCRIPTION OF THE PRIOR ART 
Heretofore, thermoplastic inserts have been employed as locking mechanisms 
for locking a nut to a screw or bolt. Such lock nut inserts have been cast 
in place and have included the use of thermoplastic resins. Examples of 
such prior art plastic inserts are found in the following U.S. Pat. Nos. 
2,506,477; 2,316,338; 2,410,730; 2,983,534; 2,390,759; and 2,421,105. 
It is also known from the prior art of semiconductor wafer probe testers to 
employ a lead screw drive including a drive nut wherein the drive nut is 
made of an epoxy resin cast in place on the lead screw. Such a nut was 
utilized for driving a wafer chuck relative to an array of electrical 
probers for probing integrated circuit devices formed in the wafer. 
Lathes and milling machines conventially employ lead screw drives for 
driving the workpiece carriage relative to the tool. However, with use, 
the threads of the nut and lead screw become worn and develop backlash or 
play which manifest itself as an inaccuracy or slop in the vernier scale 
readings employed by the operator for precisely locating the tool relative 
to the workpiece. These inaccuracies in the vernier scale reading are 
particularly noticeable when the direction of the motion of the workpiece 
relative to the tool is reversed from a previous setting. Heretofore, 
backlash has been remedied by replacing the drive nut and/or lead screw. 
However, this is a relatively costly and time consuming fix. 
SUMMARY OF THE INVENTION 
The principal object of the present invention, is the provision of an 
improved lead screw drive mechanism, and more particularly to an improved 
method and apparatus for renewing the drive nut. 
In one feature of the present invention, the drive nut includes a 
relatively low melting point core material which is renewed by raising the 
temperature of the core material above its melting point to cause it to 
flow and conform to the thread profile of the lead screw and then allowing 
the core material to freeze in the renewed state. 
In another feature of the present invention, the renewable nut includes a 
housing for housing the low melting point core material and a heating 
element is disposed within the housing whereby heating of the core 
material is facilitated in use. 
In another feature of the present invention, the renewable nut housing 
includes sealing members for sealing the nut housing to the lead screw to 
prevent the escape of liquid core material when the nut is being renewed. 
In another feature of the present invention, a heat sensing element senses 
the temperature of the core material when the nut is being renewed. 
In another feature of the present invention, a controller is responsive to 
the sensed temperature of the core of the nut to control the temperature 
cycling of the renewable nut and to actuate a motor drive to cause the 
renewed nut to be run in on the lead screw. 
Other features and advantages of the present invention will become apparent 
upon a perusal of the following specification taken in connection with the 
accompanying drawings wherein:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 there is shown a lead screw drive mechanism 11 
incorporating features of the present invention. The lead screw drive 
mechanism 11 includes a lead screw 12 mounted to a rigid support via a 
pair of thrust ball bearing assemblies 13 and 14 at opposite ends of the 
lead screw 12. A motor 15 is mounted to the support and is coupled to the 
ends of the lead screw 12 for turning same about its axis of revolution. 
A workpiece carriage 16 is mounted for rectilinear translation along the 
longitudinal axis of the lead screw 12. The workpiece carriage 16 is 
coupled to the drive screw 12 via a drive nut 17. 
The drive nut 17 includes a sheet metal housing 18 containing a relatively 
low melting point core material 19 as of babbitt metal alloy. Opposite end 
walls of the housing 18 are apertured at 21 in alignment with the lead 
screw 12 for passage of the lead screw therethrough. Elastic grommets 22, 
as of silicone rubber, are affixed to opposite end walls of the housing 18 
and bear in sealing engagement with the lead screw 12 passing 
therethrough. A cartridge electrical heating element is disposed within 
the housing 18 in coaxial relation with the lead screw 12 for heating the 
core material 19, in use. A pair of nozzles 23 are carried from the 
carriage 16 in positions for directing a spray of lubricant onto the lead 
screw 12 at opposite ends of the housing 18. Lubricant is supplied to the 
nozzles 23 from a reservoir, not shown, via flexible conduits, not shown. 
A thermocouple 24 is embedded in the core material 19 within the housing 
18. The thermocouple 24 is electrically connected to one input of a 
controller 25 via a flexible lead 26. A source of heater power 27 is 
connected to the heating element 20 via a flexible electrical cable 28. 
Referring now to FIG. 2 there is shown a typical thread profile for the 
thread on the drive screw 12. The thread comprises a helical land 29 of a 
predetermined pitch, such land having a depth A for example 0.020 to 0.025 
inch, a tip land width, B, of for example 0.020 to 0.125 inch, a root 
width, C, of 0.040 to 0.025 inch, a face angle .theta. of 5 to 30 degrees 
and a diameter, D, of 0.50 to 2.0 inch. 
In use, the lead screw and drive nut 17 are utilized in relatively light 
duty milling machines or lathes for positioning the workpiece relative to 
the cutting tool. With use, the lead screw 12 and nut 17 will wear 
producing backlash. The backlash is removed by renewing the conformation 
of the threads of the nut to the threads of the lead screw 12. The nut 
renewing cycle is described by reference to FIGS. 1 and 3. When it is 
desired to renew the nut 17, the operator 17 actuates the controller 25 
which includes an internal sequencer which turns on the heater power to 
the heater cartridge 20 at time t.sub.o. The heater power stays on until 
the temperature of the core material 19, as sensed by thermocouple 24, 
reaches a certain maximum temperature T.sub.M. T.sub.M is chosen to be 
above the melting point T.sub.A of the core material 19. When the 
temperature of the core material 19 exceeds the melting point, the core 
material liquifies and flows into conformance with the thread profile of 
the lead screw 12. 
When the maximum temperature T.sub.M is reached, the controller 25, which 
includes an internal comparator circuit, senses that T.sub.M has been 
reached and turns off the heater power. The core material 19 is then 
allowed to cool until such time as the sensed temperature receeds to a 
predetermined moderately hot temperature. The controller 25 then turns on 
the heating element 20 and adjusts its duty cycle so as to maintain the 
temperature of the core material 19 at the annealing temperature T.sub.B 
for a predetermined length of time t.sub.1. After the annealing portion of 
the cycle, the controller 25, again turns off the current to the heating 
element 20 and the core material 19 is allowed to cool from the annealing 
temperature down to a lower temperature T.sub.C which is above room 
temperature T.sub.R. T.sub.C is chosen so that it provides a slight amount 
of softening of the core material of the nut 17. When the temperature 
reaches T.sub.c the controller senses that this threshold has been reached 
by means of an internal comparator and responds by energizing a lubricant 
sprayer for spraying lubricant onto the lead screw slightly ahead of (in 
the direction of advancement) the nut 17 along the lead screw 12. In this 
manner, lubricant is carried into the interface between the threads of the 
core material 19 and the threads of the lead screw for lubricating same. 
Also, the controller 25 turns on the motor current for driving the lead 
screw 12 in the predetermined direction. The controller causes the motor 
15 to run a sufficient length of time until the nut reaches one extreme of 
travel at which point a limit switch is tripped which causes the drive nut 
to be repetitively driven a predetermined number of times to and fro along 
the lead screw 12 while the temperature of the core material 19 is cooling 
toward the room temperature. After a predetermined number of nut run in 
cycles have been performed, the controller ceases operation and the core 
material cools to room temperature. This concludes the renewing cycle of 
the nut 17, and the machine is ready for operation free of backlash. 
The advantage to the renewable drive nut is that backlash can be eliminated 
without time consuming and expensive dismantling and changing of parts 
within the lead screw drive mechanism of the milling machine or lathe or 
other apparatus employing the lead screw drive of the present invention. 
As thus far described, the nut 17 has surrounded the lead screw 12. 
However, this is not a requirement. The nut 17 may only partially surround 
the lead screw 12 in which case the side of the nut housing 17 would be 
open. In such a case, the elastic sealing member 22 would extend along the 
marginal lip of the side opening for sealing the nut housing 18 to the 
lead screw 12.