Variable lead device for circular scan tracers

An improved circular scan line tracing device for use in connection with flame or plasma cutting machines, as well as other machine tools, is disclosed which features provision for adjustment of the lead of the tracing device while the machine is operating.

FIELD OF THE INVENTION 
This invention relates to the field of photoelectric line or edge tracing 
devices which are used to control flame or plasma cutting torches or other 
machine tools. More specifically, the invention comprises an improvement 
on circular scan tracing devices presently in use. 
BACKGROUND OF THE INVENTION 
For many years it has been common practice in the metal working art to use 
flame or plasma torches to cut out large sections of plate. Such torches 
can be held in the hand of an operator and guided by him; they can be 
mounted on tracks and provided with motors for straight line cutting; they 
can be pivoted about a central point to cut out circles; they can be 
adapted to follow a template cut out in advance; and they can be used on 
machines provided with optical-electronic "tracers" and appropriate 
servomotors and controls for movement of the torch (or torches) according 
to a blueprint or other plan. The invention relates to the last class of 
devices. Several different types of tracers have been made which have been 
of some success. One such is the "circular scan" tracer, an improvement on 
which is the subject of the present application. Such circular scanning 
tracer devices are described at least as early as U.S. Pat. No. 3,004,166; 
more advanced versions of such tracers are described in U.S. Pat. No. 
3,704,372. The present invention may to some extent be construed as an 
improvement on that shown in U.S. Pat. No.3,704,372 and the specification 
of that patent is therefore hereby adopted and made a part of the present 
specification. Where possible reference will be made to such patent in 
order to simplify the description of the present invention. 
One deficiency of the tracer shown in U.S. Pat. No. 3,704,372 is that no 
provision is made for ready adjustment of the "lead" of the tracer. The 
lead of such a tracer is the distance between the point at which a change 
in the line being traced is detected and the point on the line 
corresponding to the actual position of the torch. Some lead is necessary 
because there is an inevitable delay between the tracer's sensing that a 
change of direction of travel is necessary and the actual change of 
direction, mostly due to the momentum of the machine. Momentum being a 
function of velocity, the lead must be varied with the travel speed. If 
too little lead is used the torch tends to overshoot corners, particularly 
if they are sharp, and if too much lead is used corners tend to be rounded 
off. As the same tracers and cutting machines are commonly used with 
oxy-fuel cutting torches, for which travel speeds of approximately 30 to 
100 ipm (inches per minute) (75-250 cm/min) are appropriate, and with 
plasma torches which cut at 200 to 250 ipm (500-625 cm/min), a tracer the 
lead of which is readily adjustable is clearly desirable. 
U.S. Pat. No. 3,704,372, referred to above, describes a tracer having means 
for varying the lead; however, said means are not convenient to use as 
they require at least partial disassembly of the tracing unit and 
replacement of one of its parts with another, in order that the lead may 
be varied. It is therefore, clearly desirable to produce a circular scan 
tracer having its lead readily adjustable; preferably the lead would be 
adjustable even while the machine is running which would simplify the 
adjustment of the lead under given operating conditions. 
OBJECTS OF THE INVENTION 
It is therefore an object of the invention to provide an improved 
photoelectric scanning device. 
Another object of the invention is to provide a circular scanning device 
having adjustable lead. 
Yet another object of the invention is to provide a circular scanning 
tracer device which has a lead which may be varied and adjusted while the 
machine is in operation. 
A final object of the invention is to provide an improved circular tracing 
scanning device having lead which may be varied while the machine is 
operational, which can be manufactured at a reasonable cost. 
SUMMARY OF THE INVENTION 
The present invention satisfied the needs and desires of the art and the 
objects mentioned above by its substitution of a pivoted mirror, the angle 
of which to the rotational axis of the machine can be adjusted, for the 
replaceable, fixed angle mirrors used in the class of tracers exemplified 
by U.S. Pat. No. 3,704,372. By adjusting the angle of the mirror the lead 
of the tracer is adjusted; means are provided whereby the angle can be 
adjusted even while the machine is operating. In a preferred embodiment, 
the angle can be adjusted continuously, thus providing continuously 
variable lead to the tracer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As mentioned above, the present invention can to some extent be construed 
as an improvement on the circular scanning tracing device described and 
claimed in U.S. Pat. No. 3,704,372. The operation of such devices is fully 
described in the specification of that patent and therefore will only be 
summarized here for purposes of simplicity. 
Such circular scanning tracing devices provide a motor on the shaft of 
which is mounted a mirror which is not perpendicular to the axis of the 
motor; that is, it is a "nutating" mirror. A magnet and pickup coils are 
also provided to send a regular sine wave signal to electronic control 
devices which control the servomotors driving the entire flame cutting 
unit along orthogonal axes. Flood lights are provided which light the 
pattern being traced. This light is reflected from the pattern, through a 
lens, off the angled mirror and onto a photocell. Since the angled mirror 
is rotating, the photocell "sees" a spot the location of which rotates on 
the pattern. When light is not reflected from the white section of the 
pattern but is rather absorbed by a black line or an edge the photocell 
output varies, thus producing a signal. By comparing the timing of the 
signal sent by the photocell with that provided by the magnet and coil 
assembly, mentioned above, the logic circuits of the control unit are 
provided with information which is then used to control the servomotors of 
the cutting machine. 
As mentioned above and as well known in the art it is necessary that in 
order for a tracer to properly respond to changes in orientation of the 
line being traced the instructions relating to these changes must be 
received in the control system of the cutting machine some time before the 
change is to be made, in order that the momentum of the machine and any 
delays in the control system do not deform the cut piece. Thus, some 
"lead" must be provided. Furthermore, it is apparent that the lead must be 
varied in accordance with velocity of the machine, since the momentum is a 
direct function of velocity. Clearly, a certain minimum lead is required 
for a given cutting velocity. It is also well known that if too much lead 
is used for a given velocity, the accuracy of the cut will also be 
impaired, because the control system will respond at the same speed it 
would at higher velocity; thus, for example, square corners tend to be 
rounded off when too much lead is used. 
It will therefore be apparent that in a system which is capable of cutting 
at different speeds, depending on, for example, the thickness of plate to 
be cut and the type of torch to be used, it is essential that the lead be 
variable. In the prior art such variable lead was provided by replacement 
of the angled mirror on the motor shaft with a mirror having a different 
angle; such an arrangement is shown in U.S. Pat. No. 3,704,372. While this 
method works well enough, it is inconvenient and time-consuming. Moreover, 
only leads corresponding to the mirror angles supplied are available; the 
lead is only discretely variable. The present invention overcomes these 
difficulties by its provision of a workable mechanism which allows 
adjustment of the mirror and hence of the lead without dismantling the 
machine. In fact, the present invention allows the lead to be adjusted 
while the machine is moving, which is very useful when setting the machine 
up to make a given cut; the operator can simply run the tracer around the 
pattern to be traced without starting the torches and can examine the 
accuracy of the trace, adjusting the lead as required. 
Reference will now be made to the drawings whereby a clear understanding of 
the invention will be obtained. 
FIG. 1 shows a cross section of one embodiment of a tracer according to the 
invention. Motor 31 rotates its shaft 36 to which is attached mirror 
carrier 40 and mirror assembly 41, 45, 46. Because the mirror rotates, the 
light cast by lamps 52 on pattern 53, reflected towards the mirror through 
lens 51, and off mirror 41 onto photocell 50, represents a circular scan 
of pattern 53. As the intensity of the spot of light read by photocell 50 
changes when, for example, the light beam crosses a line of the pattern or 
goes from a dark area into a light area, a signal is put forth by 
photocell 50. Shaft 36 also rotates magnet 34 between coils 35, generating 
a reference signal which can be compared with the signals generated by 
photocell 50, in the manner taught in U.S. patent 3,704,372, and used to 
control the motion of the tracer and associated cutting torch(es) or 
machine tools. 
The improvement made by the present invention is the provision of pivot 45 
and associated adjustment mechanism for mirror 41. As mirror 41 pivots, it 
will be apparent that the lead distance, indicated in FIG. 1, is changed 
as a direct function of the mirror angle, also indicated on FIG. 1. The 
mirror angle is changed by moving rod 37 up or down with respect to motor 
shaft 36. This motion up or down is provided by adjusting knob 54 which is 
threaded onto housing 32. By turning knob 54 with respect to housing 32, 
the cooperating screw threads on these parts cause the knob 54 to push 
down via thrust bearing 55 and flange 56 onto rod 37. This in turn moves 
mirror adjuster 46 down with respect to mirror carrier 40. As the mirror 
41 is pivoted on part 40 (as will perhaps be more clear from an 
examination of FIG. 2), pin 42 pushes down on the mirror, forcing it to 
pivot. Spring 44 and locating pin 43 are provided so that when the mirror 
pivots, thus changing the mirror angle and lead, the mirror is kept in 
contact with pin 42. In this way adjustment of lead may be provided. It 
will be apparent that any desired degree of lead may be provided by this 
mechanism and that the operator is thus not restricted to values set by 
the angles of preset mirrors; therefore, the lead is continuously 
variable. 
Provision must be made in any cutting machine for "kerf compensation"; that 
is, allowance for the width of the cut. Kerf compensation is provided in 
the tracer of the invention in the same way as in the tracer described in 
U.S. Pat. No. 3,704,372; that is, the housing is made in two parts 32 and 
33; by rotating one with respect to the other, kerf adjustment is 
effected. 
A second embodiment of the invention is shown in similar views in FIGS. 3 
and 4. There motor 1 spins assembly 24 on which is carried mirror 11, 
which is pivoted by means of pivot 15, on mirror carrier 10. Rather than 
the threaded knob arrangement shown in FIG. 1, adjustment is provided by 
means of a yoke 13 pivoted on pivots 12 and operated by knob 19 which 
pushes the mirror carrier 10 up or down via cam followers 14 with respect 
to the assembly 24. As the mirror assembly is moved up or down by moving 
the yoke assembly, cam followers 14 push against the mirror carrier, thus 
pushing the mirror against assembly 24 and forcing the angle to change. 
Springs 7 and 16 keep the mirror engaged with assembly 24. The optical, 
kerf correction and control arrangements of the embodiment shown in FIGS. 
3 and 4 are similar to those described above with reference to FIGS. 1 and 
2. 
It will be apparent that other embodiments of the invention are 
conceivable, in addition to the two described above. Therefore, the 
invention should not be construed as limited to the embodiments described 
above, but as more properly defined by the following claims.