Thread chaser mechanism for a lathe

A thread chaser for a lathe comprising a pair of detectors which signal a selected rotational position of the lead screw and the spindle of the lathe. The particular positions of both the lead screw and the spindle are transformed into a signal which is transformed into an alarm or alerting mechanism.

BACKGROUND OF THE INVENTION 
The present invention relates to a novel thread chaser for a conventional 
lathe which permits the cutting of threads in either the metric or 
American system of measurement. 
Threads are generally measured by the U.S. standard or the metric standard. 
The existent dual system of measurement necessitates that a lathe be 
equipped to cut both U.S. standard and metric threads. Many lathes are now 
equipped to perform this function since they have a dual purpose gear box. 
However, dual cutting lathes still have only a single lead screw for the 
carriage which carries the cutting tool. Thus, it is very difficult to cut 
a metric thread with a U.S. standard lead screw since most threads must be 
repeatedly cut until a finsihed product is obtained. This entails 
returning the cutting tool to exactly the same cutting path of the prior 
cut. 
The usual threading procedure now employed is to cause the half-nuts to 
engage the lead screw of the carriage and to begin the first cut. At the 
end of a threading pass the lathe is reversed without disengaging the 
half-nuts and returned to the starting position. The lathe is then 
restarted and the subsequent cut of the thread takes place. This process 
is repeated until a proper thread is obtained in the work piece. As may be 
surmised, this is a slow and tedious process. In certain cases cutting a 
metric thread in this manner is not possible. Specifically, where the work 
piece has a shoulder the lathe may not be stopped in time (because of 
inertia), to prevent the cutting tool from damaging the shoulder. The 
existent thread chasers only work where the lead screw and the cuts being 
performed on the work piece are multiples of each other. 
There is a need for a thread chaser which may be used to accurately locate 
a cutting path for use on multiple cuts of a work piece where the lead 
screw of a lathe and the piece being cut are not necessarily threaded 
according to the same standard of measurement. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, a novel and useful thread chaser 
for lathes is provided. 
The thread chaser of the present invention employs means for detecting and 
signalling a selected rotational position of the lead screw of the lathe 
and the selected rotational position of the spindle of the lathe. Each 
detecting and signalling means may take the form of a switch which is 
activated during a selected rotational position of either the lead screw 
or the spindle. For example, each switch may include a cam mounted to the 
shaft of either the lead screw or the spindle which operates in 
conjunction with a cam follower which travels along the surface of the 
cam. The follower will then indicate any selected position on the surface 
of the cam by the placement of an irregularity or discontinuity thereupon. 
The movement of the cam follower may be translated into an electrical 
signal or pulse. 
The thread chaser of the present invention also includes means for 
indicating coincidental occurrence of the lead screw and the spindle. For 
example, when the detecting means of the lead screw or the spindle emits a 
pulse, as described above, a circuit may be employed to inform the lathe 
operator of this occurrence. This function may be accomplished by an 
electrical circuit which includes placing both switches in series with the 
gate of a silicon controlled rectifier to forward bias the same such that 
a relay is activated sending electrical current to an alarm device such as 
a light, horn, stopwatch, and the like. 
In addition, the thread chaser of the present invention may also include 
means for indicating the time period just prior to the coincidental 
occurence of the selected rotational positions of both the lead screw and 
the spindle. Such a feature may include the placement of another alarm to 
the relay associated with the first alarm indicating activation of both 
switches to the relay operated by the rectifier. In such a case, 
deactivation of the rectifier would switch the relay contact from the 
alarm indicating coincidental activation of both switches to the alarm 
indicating that the system is in the ready mode or in a condition just 
prior to the activation of both switches. Thus, an operator may be made 
aware of the fact that as the lathe turns he should be ready to engage the 
cutting apparatus or that he should immediately engage the cutting 
apparatus. It is contemplated that the present invention also includes 
means for automatically engaging the cutting apparatus upon receipt of a 
signal from the activation of both switches. In such a case, any human 
reaction time would be eliminated from the system. 
It may be apparent that a novel and useful thread chaser for a lathe has 
been described. 
It is therefore an object of the present invention to provide a thread 
chaser for a lathe which permits the cutting and recutting of metric 
threads with a lathe having a U.S. standard lead screw. 
It is another object of the present invention to provide a thread chaser 
for a lathe which eliminates the need for maintaining engagement of the 
carriage half-nuts, stopping the forward movement of the carriage, 
reversing the carriage, and starting the carriage again in a forward 
direction to obtain multiple passes in the cutting of a thread. 
It is yet another object of the present invention to provide a thread 
chaser for a lathe which eliminates the need for constructing a lathe with 
dual lead screws, one having a U.S. standard thread and one having a 
metric thread. 
It is still another object of the present invention to provide a thread 
chaser for a lathe which permits the operator of the lathe to manually 
engage the cutting tool at the precise moment necessary to effect a thread 
cutting path which is identical to a previous thead cutting path on a work 
piece. 
It is another object of the present invention to provide a thread chaser 
for a lathe which is compatible with automatically initiated cutting 
operations for production of a thread via multiple cuts. 
The invention possesses other objects and advantages especially as concerns 
particular characteristics and features thereof, which will become 
apparent as the specification continues.

For a better understanding of the invention, reference in made to the 
following detailed description. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Various aspects of the present invention will evolve from the following 
detailed description of the preferred embodiments thereof, which should be 
taken in conjunction with the heretofore described drawings. 
With reference to the drawings, the invention as a whole is depicted by 
reference character 10. With reference to FIG. 1, a lathe 12 of typical 
construction is shown. Lathe 12 includes a carriage 14 which rides on lead 
screw 16. The work piece is placed on spindle 18 and tailstock 20. A gear 
box 22 provides the proper lead screw speed for the particular threads 
being cut on a work piece. Lead screw 16 is engaged by half-nuts (not 
shown), associated with the carriage 14 such that carriage 14 and tool 
post 24 move along the work piece being cut in the conventional manner. 
Thus, to cut a thread on a work piece the operator selects the proper 
speed to turn spindle 18 and engages lead screw 16 to move carriage 14, 
normally from right to left as shown on FIG. 1. Thus, the cutting tool on 
tool post 24 moves from right left. As heretofore described, most threads 
require multiple passes which necessitates synchronizing or following 
exactly the same cutting path as the prior cut on the work piece. 
Turning to FIG. 2, it may be seen that lead screw 16 includes a shaft 26 
which extends beyond the confines of lathe 12 proper. The device of the 
present invention includes means 28 for detecting and signalling a 
selected rotational position of shaft 26 and therefore lead screw 16 of 
lathe 12. Means 28 may include a first switch 30 and means 32 for 
activating switch 30 during a selected rotational position of shaft 26 and 
therefor lead screw 16. Means 32 for activating switch 30 may include a 
cam 34 which is mounted to shaft 26 for rotation therewith. The specific 
embodiment of cam 34 shown in FIG. 2 includes a flattened portion 36 which 
may have a length of approximately six millimeters. Cam 34 may be fixed to 
shaft 26 in any known manner such as fastening the same with screws or 
bolts, gluing, or integrally forming cam 34 with the shaft 26. 
Switch 30 includes a cam follower 38 which rides on surface 40 of cam 34. 
Cam follower 38 pivots around pin 42 and is fixed to spring 44 by bracket 
46. Bolt 48 holds bracket 46 to mounting plate 48 and to the body of lathe 
12. Spring 44 is metallic and therefor is a conductor of electrical 
current. Contact points 50 and 52 touch one another when cam follower 38 
travels over flattened portion 36 of cam 34. Contact point 52 is connected 
to electrically conductive arm 54 which is fixed to mounting plate 56 on 
the body of lathe 12. It should be noted that mounting plates 48 and 56 
are constructed of electrically insulative material such that an 
electrical current may flow from bracket 46, through spring 44, between 
contact points 50 and 52, and arm 54. It has been found that where shaft 
26 and cam 34 rotate at approximately 250 rpms, contact points 50 and 52 
will maintain contact for about 10 milliseconds. 
Returning to FIG. 1 it may be seen that spindle 18 includes a shaft 58 
which includes means 60 for detecting and signalling a selected rotational 
position of the spindle 18 of lathe 12. Shaft 58, the embodiment shown on 
the drawings, would include an arrangement identical to that shown in FIG. 
2. In other words, means 28 for detecting and signalling a selected 
rotational position of lead screw 16 is identical to means 60 for 
detecting and signalling a selected rotational position of spindle 18. 
Means 60 is shown in phantom on FIG. 1. 
Turning to FIG. 3, it may be seen that cam 34 and cam 62, attach to spindle 
18 and lead screw 16 respectfully for rotation therewith, are depicted 
schematically. Switch 30 and a switch 64 associated with cam 62, identical 
in configuration to switch 30, are connected in series. Arm 54 and bracket 
46 are also depicted schematically as legs on either side of switch 30. An 
electrical source 66, such as 12 volts of D.C. electrical power is 
provided by any known means. For example, power supply 66 may be a dry 
cell battery, a wet cell battery, and the like. Switch 68 connects to 
power supply 66 and is normally in a closed position as shown. Resistors 
70 and 72 flank either side of switches 30 and 64 which as heretofore 
explained, are connected in series. Rectifier 74 includes a gate 76 which 
is connected to node 78 via leg 80. Leg 82 from resistor 72 and leg 84 
from the cathode of rectifier 74 lead to ground 86. The anode of rectifier 
74 passes to a single pole double throw relay 88. Relay 88 switches 
between contact points 90 and 92 which connect to alarm means 94 and 96 
via legs 98 and 100. Leg 102 connects alarm means 94 or 96 to switch means 
68. Leg 104 essentially connects pole 106 of relay 88 to ground 86. 
The following is a table of selected values of components of the circuitry 
shown in FIG. 3. It should be noted that values contained hereinbelow are 
inserted for the purposes of making a complete disclosure of the 
embodiment and are not intended to be restrictive of the invention 
contained herein: 
COMPONENT TABLE 
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Switch 68 125 VAC, 6 amp 
Switch 30 Automobile dist. point 
Switch 64 Automobile dist. point 
Resistor 70 12 K ohm, 1/2 watt 
Resistor 72 620 ohm, 1/2 watt 
Resistors, Alarm 
means 94 and 96 47 ohm, 1/2 watt 
Rectifier 74 SCR, Sylvania ECG5404 
Lamps, Alarm means 
94 and 96 6 volt, 150 milliamp 
Relay 88 SPDT 12 VDC, 1 amp, 950 ohms 
Power Supply 66 Radio Shack, 12V DC 
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In operation, lathe 12 is activated in the proper gear and a tool is 
selected for cutting the desired thread on a work piece. The operator may 
easily view device 10 mounted on top of lathe 12, FIG. 1. Power supply 66 
normally will activate alarm means 94, a light, since pole 106 of relay 88 
engages contact 90. Alarm means 94 would indicate that the operator is 
"ready" to cut the thread on the work piece. At this point, cams 34 and 62 
begin to turn and turn on and off switches 30 and 64. At some point during 
the turning of spindle 58 and lead screw 16, switches 30 and 64 will be on 
at the same time. At this point electrical current will flow through 
resistor 70, switches 30 and 64, to node 78, and from there to gate 76 of 
rectifier 74 via leg 80. Electrical current also flows through resistor 72 
to rectifier 74. This action forward biases the gate of rectifier 74 to be 
on or conducting mode. Such forward biasing will activate relay 88 and 
switch pole 106 from contact 90 to 92. Current will then flow to alarm 
means 96 in preference to alarm means 94. At this point, the operator of 
the lathe should "engage" carriage 14 to lead screw 16 and begin the 
cutting operation of the work piece. After the first pass on the work 
piece the Examiner may disengage carriage 14 from lead screw 16 and return 
carriage 14 to begin the second pass. Switch 68 is opened which will again 
activate alarm means 94 for the "ready" mode. Since carriage 14 must be 
returned to exactly the same position at the start of each cutting pass, 
the operator should wait for alarm means 96 to activate again before he 
begins the second pass. When switches 30 and 64 close at the same time, 
indicating coincidental occurrence of the particular rotational positions 
of spindle 58 and lead screw 16 where the first pass began, the operator 
again engages carriage 14 to lead screw 16. Thus, the second pass follows 
exactly the same path as the first pass on the work piece to produce the 
proper thread. It is contemplated that alarm means 96 may be used in 
conjunction with a timing device or an automatic mechanism to initiate 
cutting of the work piece. In this manner any human reaction time is 
eliminated. 
It should be apparent that metric threads may be cut from a U.S. standard 
lead screw employing the thread chaser of the present application. The 
thread chaser of the present application may also be applied to lathes 
having a dual purpose gear box equipped with a metric lead screw. With 
this arrangement the thread chasing device of the present application 
would be used to cut U.S. standard threads. 
While in the foregoing specification embodiments of the invention have been 
set forth in considerable detail for the purposes of making a complete 
disclosure of the invention, it will be apparent to those of ordinary 
skill in the art that numerous changes may be made in such details without 
departing from the spirit and principles of the invention.