Note: Descriptions are shown in the official language in which they were submitted.
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LASER MEASURING SYSTEM AND METHOD FOR TURN_NG MAC~IN
Technical Field
This invention relates to laser measuring systems
- for machine tsols.
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Backgrollnd Art
There are a number of factors wh~ch influence the
dimensional accuracy of a workpiece which can be achieved
in a turni~g operation. Illustrative of such f2ctors,
are: the accuracy of the mechanisms which control
machine tool slide movement; the deformation of the
- 10 machine tool and the workpiece due to cutting force;
the heat generated during cutting; and the extent of
tool wear. The prior art has recognized that the a~ore-
mentioned unpredictable factors are not susceptible to
effective operator monitoring and in response has
provided laser optical devices adapted to generat2 beams
which impir.ge on the peri?hery of a workpiece, sucn as
shown in U.S. Patent Nos. 3,749,500 and 3,812,376 Such
prior art systems have a drawbaek in that they require
positioning units wh~ch can be relatively compl~x. _n
addition, such systems do not offer the many ~dvantagss
o~ laser interfe.rometry.
Currently, som~ numerically controlled ~achine ~ools
employ laser interferome~ry to precisely position the
slides of the machine 1:o obtain su~erior resolution.
Typically such systemC embody a las2r source and one or
more receivers, interrerometer and re~roflector sets.
However, laser interrerometry has not bsen ada?ted to
d~rectly ~easure the d~mensions of a turned workpiece.
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Disclosure of t.~e Invention
n accordance wi.h the invention, las2r interfer-
ometry ~is em?loyed ~o directly measure the in~erior sr
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exterior dimensions of a turned wcrkpiece. The only
positioning apparatus required for a system is that
which is typically incorporated in a two axis lathe
or chucker.
In brief, a lathe or chucker, having a tool
carrying turret mounted upon an X-axis slide movable
perpendicular to the spindle axis, is provided with a
gage head incorporating a reflector adapted to be
positioned for measurement by an indexing rotation of
la the turret to a particular index station. An interfer-
ometer is fixedly mounted upon a Z-axis slide (which
carries the X-axis slide) movable parallel to the spindle
; axis. The interferometer is aligned with the reflector
in the gage head ~hen the turret is in the measurement
index station and is always aligned with a laser source
and receiver mounted upon the machine tool frame or
adjacent thereto. The chuck is encircled by a master
reference collar of known diame-ter (internal as well as
external~ whereby a reference point may be established
2~ when the gage head engages the inner or outer periphery
of the collar.
A ~easuring system of the invention may be u-tili2ed
to determine and compensate for excessive tool wear. For
example, a correction signal could be transmitted to the
slide position control device of the machine tool when
the machining error îs greater than a specifisd dimen-
sional tolerance. In addit.ion, a system of the invention
could be employed to determine size of a part just prior
to or upon the completion of a final finishing operation.
It will of caurse be appreciated that, under all
circumstances, the accuracy of a measuring system of the
invention does not depend upon the precision with which
the machine tool slides are positionea but is independent
thereof.
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Accordingly, it is a primary object of the invention
to provide a system for measuring the dimensions of a
turned workpiece which utilizes laser interferometry.
Another object is to provide a laser interferometer
based system and method for measuring the dimensions of
a workpiece machined upon a two axis turret lathe where-
in the turret thereof includes a gage head incorporating
- a reflector and the axial slide has an interfero~eter
mounted thereupon.
}O These and other objects and advantages of the in-
vention will ~ecome more readily apparent frorn the
following detailed description when taken in conjunction
wîth the accompanying drawings, in which:
BrieI Description of D_aw ngs
FIGUR~ l is a schematic diagram of a preferred
embodiment of a laser measuring system of the inven-tion.
FIGURE 2 is a block diagram depicting the basic
elements of the laser transducer and the receiver of
FIGURE l and their relationship to other components of
the system.
FIGURE 3 .is a schematic view of the lînear inter-
ferometer and retroflector arrangement of FIGURE l
illustrating the beam paths and the principle of
operation.
FIGURE 4 is a 8ide elevational view, par~ly in
section, of the gage head of FIGURE l.
FIGURE 5 is a front elevational view of the gage
head, partly in section, taken substantially along the
line S-5 of FIGURE 4.
FIGURE 6 is a top view of the gage head, taken
substantially along the line 6-6 of FIGURE 4.
FIGURE 7 is a rear elevational view of the gage's
roller slide assembly,per se, taken substantially along
the line 7-7 of FI~URE 4.
~ 7~;5
~ GUR~ 8 is a s~de elevational vie~ partly in
section, of the roller slide assembly, taken substan-
tially along the line 8-8 of FIGURE 7.
FIGURE 9 i~ a sectional vie~ of the roller slide
assembly, taken substantially along the line 9-9 of
FIGURE 7.
FIGURE 10 is a per~pective view showing the engage-
ment between the right side rail and a V-way of FIGURE
9.
Best Mode of Carryin~ Out the Invention
Referring to FIGURE 1, there is shown a numerically
controlled lathe or chuc~er, generally shown at 10,
incorporating a measuring system of the invention. The
particular form of lathe shown em~odies a vertically
indexing turret 12 capable of movement along two axes.
This type of lathe (sometimes termed a universal lathe)
is well-known in the art and an example thereof is
illustrated in U.S. Patent No. 3,1gl,470. It will be
appreciated that althoug~ the invention is specifically
illustrated and described with reference to a machine
tool having a vertically indexing turret, it is also
applicable to o-ther lathes or chuckers.
Tha lathe 10 embodie6 a frame 14, the outline of
which is shown by phantom lines. Mounted upon the frame
for rotation about a horizontal axis is a spindle 16
having a work~iece holding chuck 18 attached thereto.
The chuck also has a master reference collar 20 (dis-
cussed hereinafter) mounted upon the chuck in encircling
relationship thereto. A pair of longitudinally
extending ways 22 and 24 are secured to frame 14 for
guiding the longitudinal motion (z-axis) of a ~-axis
slide 26. Carried by the z-axis slide 26 are ways 28
and 30 upon which is slideably mounted an X-axis slide
32. The X-axis is, of course, perpendicular to both
s~?
the Z-axis and the chuck or workpiece axis. The turret
12 is mounted for rotation upon the X-axis slide 32
to a plurality of discrete index stations about an
axis parallel to the chuck axis. The turret may be
configured to carry a plurality of turning tools, a
plurality of boring tools or both turning tools and
boring tools. Slide positioning and turret indexing
are controlled by a computer numerical control unit
~CNC) 34 which commands all machine ~ool operations.
A gage head, generally indicated at 36, is
fixedly mounted upon the turret 12 at an index station
by means of a bracket 38. The heart of the gage head
is a reflector 40 ~which in this case is a retroflec-
tor~ which is movable with a gage arm 42 having O.D.
COutside diameter) and I~D. (Inside diameter) tips or
probes 44 and 46, respectively. In the measuring index
station depicted in FIGURE l~ the retroflector 40 is
in precise alignment with an interferometer 48 (which
in this case is a linear interferometer~. The inter-
ferometer 48 is rigidly secured to the Z-axis slide 26
by means of an L-shaped mounting arm 50. Hence, any
movement of the Z-axis slide 26 when the turret is at
the measuring index station does not in any manner
affect the alignmen~ of the interferometer and the
reflector 40. In like manner, movement of the X-
axis slide does not alter alignment but only the distance
between the interferometer 48 and t~e reflector 40.
A laser transducer 52 is the source of the laser
beam directed at the interferometer 48, the outgoing
main beam being shown by a dashed line. A portion of
the outgoing main beam Cshown by dashed line~ is
directed by the interferometer to the reflector 40 which
sends a return beam ~solid line) back to the interfer-
ometer in a latterally offset and parallel relationship.
The return beam from the reflector interferes with
another portion o~ the main beam in the interferome~er
~7~
to produce a return main beam (solid line~ directed at
a receiver 54. The return main beam is offset from
and parallel to the outgoing main beam. Tne receiver
54, which senses the main return beam generates an
RF measurement signal which is applied to a pulse
converter 56. Th~ receiver also incorporates A means
to verify proper alignment. The pulse converter 56
also receives an RF reference signal from the laser
transducer 52. The RF measurement and ref~rence signals
are transformed by ~he Pulse converter 56 into dis-
placement information in pulse format (e.g., up pulses
and down pulses~ which can be utilized, for example,
by a reversible counter Cnot shown) in the CNC uni~
34. A power supply 58 functions to provide the laser
transducer 52 with a positive voltage D.C. supply and
a negative voltage D.C. supply.
The basic optical elements of the measuring system
of the invention, i.e., the retroflectors, linear
interferometer 7 Laser tranducer, receiver and pulse
converter units are all commercially available items
manufactured, for example, by the Hewlett Packard
Co. The detailed construction of the aforementioned
elements, of course, forms no part of the present in
vention; and it will ~e understood that other suitable
elements could be employed in a measuring system of the
invention. However, the laser transducer, receiver
linear interferometer and retro~lector will be
cursorily described to ~acilita-te a better understanding
of the present invention~
Turning to FIGURE 2, the Laser transducer 52 and
the receiver 54 are depicted in block diagram form.
T~e laser transducer 52 comprises a low power Helium-
Neon laser 60 which emits a coherent light beam composed
of two slightly different optical frequencies, fl and
f2 of opposite circular polarizations~ After conversion
to orthogonal linear polariza-tions,the beam is expanded
~64~
and collimated at 62 and then directed to the reference
beam splitter 64 where a small fraction of ~oth
frequencies is split o~f. The downwardly directed
portion ~6 of the beam is used ~oth to generate a
S reference frequency and to provide an error signal to
the laser cavity tuning system. Beam portion 66
impinges upon a polarizing ~eam splitter 68 which splits
beam 66 into a portion directed to a photodetector 70
and a portion directed at a mirror 72, which, in turn,
reflects the portion to another photodetector 74. The
output signal of photodetector 7 a is directed to an
input terminal of D~C. amplifier 76. The output signal
of photodetector 74 is directed b~th to another input
term;nal of D.C. amplifier 76 and an AC amplifier 78,
the latter of which generates the reference signal
f1-f2 which is one of the inputs to the pulse converter
56. The output of the D~CO amplifier 76 ~the difference
in the amplitudes of fl and f2~ is applied to a tuning
regulator 80 which is connected to the laser 60.
- 2~ The receiver 54, which senses the main return
beam via a photodetector 82, includes an amplifier ~4,
connected thereto, which produces the measurement
signal fl-f2+~f2~ Relative motion between the linear
interferometer 48 and the retroflector 40 causes a
2S doppler shift (+~22 in the difference frequency
tfl-f2~ measured by the receiver 54. Th;s Doppler
modulated difference frequency tfl-f2~f2) is, of course,
amplified by amplifier 84 to become the measurement
signal.
The pulse converter 56 rece.ives the reference and
measure~ent signals and compares them cycle-by-cycle.
The pulse converter 56 produces an appropriate up or
down output pulse whenever one of the signals is one-
half cycle ahead of or ~ehind the other. Each pulse
3s corresponds to a retroflector movement of one-quarter
wavelength of light. These pulses are directed to
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the computer numerical control for counting therein.
With reference to FIGVRE 3, the linear intexfero-
meter 48 and its relationship to the retroflector 40 are
displayed schematically. The outgoing main beam exiting
from the laser transducer 52 is split into a laser refer
ence beam and a laser measurement beam at the surface of
a polari~ing beam splitter 48a with one frequency (fl)
reflected to a reference cube corner 48b (i.e., a retro-
flector) mounted on ~he interferometer housing 48c. The
other frequency (f2) is transmitted to the retroflector
40 and returned (f2+~f2) parallel to, but displaced from,
the outgoing beam. Both return beams interfere with each
other at point 48d from where both frequencies are direct-
ed back along a common axis to the receiver as the main
return beam. The retroflectors are comprised by high
quality cube~corners which have the property that inci-
dent laser beams are relfected parallel to the incoming
direction within seconds of arc and retain their coherence.
Such retroflectoxs are notably advantageous in that their
alignment during installation is not subjected to critical
tolerances.
The detailed c~nstruction of the gage head 36 is
shown in FIGURES 4-10. The gage head includes a housing
86 which has mounting bracket 38 secured thereto whereby
it may be attached to an index station of the turret 12.
The retroflector 40 is cradled within a support 90 so as
to be upwardly facing to receive and emit light beams
through an aperture 86a in the top of the housing 86.
The support 90 is attached by means of screws 92 to a
slide 94 which is axially movable vertically relative to
the housing 86. To the surface of the slide 94 is attached
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a depending shaft 96 which is secured to the gage arm
42 (which carries probes 44 and 46~ at its lower extremity.
Shaft 96 extends downwardly through an opening 85b in the
lower portion
-8a
of the housing 86 and is encircled by a seal 98 attached
to the base of the housing for preventing the entry of
contaminating fluids, such as cutting coolant. Hence,
it will be noted that the gage arm 42, shaft 96, slide
94, support 90 and retroflec-tor 40 form an integral
structure and are movable in unison relative to housing
86 when the probes 44 and 46 are contacted.
In order to prevent contaminants from antering
aperture 86a when the gage is not ;n use, a cover 100
is pivotally mounted upon a pin 102 supported by a
Bracket 104 attached to the front of the housing. The
bracket lQ4 and the cover 100 incorporate la~terally
projecting pins 106 and 108 for mounting springs which
hold the cover 100 closed or open. A handle 110 is
provided for opening the cover~and a stop 112 7 which is
secured to the housing,limits the ex~ent o~ its opening.
When closed, the cover rests upon a gasket 114 whiah
acts to seal out contaminants.
The slide 94 is mounted for axial sliding movement
upon a base }16 which is securely fastened to the
housing 86 ~y means of screws 118. The slide and base
assembly is shown in detail in FIGURES 7,8,9, and 10.
The slide 94 comprises a ta~le 120 having longitudinal
sides 122 and 124 and end plates 126 and 128 which
together define an open box-like structure. As best
æhown in FlGURES 7 and 8, the end plates include pins
130 and 132 inserted therein which function as mounting
guides for springs 134 and 136. The springs are
mounted in cavities 138 and 140 in the ~ase 116 for
biasing the slide 94 to the neutral or intermediate
position depicted when displaced therefrom. The end
plates also include stop bolts 142 and 144 for limiting
slide travel in both upward and downward directions~
Turning to FIGURE ~, it may be seen that the ~ase
includes two roller strips 146 and 148 attached to the
body thereof By mounting screws 150 and 152. The slide
~'764't5~
--10--
also incorporates a pair of V-ways 154 and 156 secured
to the ta~le 12~ ~y mounting screws similar to those
associated with the roller strips. The sliding inter
engagement ~et~een t~e V-ways and roller strips is
occasioned by respective axial arra~s of crossed roller
bearings 158. As shown in FIGURE 10 the rol.ler bearings
158 in each strip have alternately opposed axes.
Screw 160 allows for preload adjustment. A slide
assembly, as shown, permits superior travel accuracîes
to be attained. The basic el~ments of such a slide
assembly are commercially avail,a~le from Micro Slides
Inc. of Westbury, New York.
In a typical OD operation~ the workpiece is
machined to final rough OD dimensions. The turret 12
is then indexed, Ci.e. ? rotated) to the measuring
station whereby t~e retroflector 40 is aligned with
the linear interferometer 48 and the gage head 36 i.s
also aligned with a diameter of the workpiece, The
, gage head 36 is then moved over the master references
collar 20 ~y moving the Z-axis slide 26 to the left
as viewed in FIGURE 1 until t~e probe 44 on the gage
arm 42 is aligned with the diameter of the collar.
Next, the X-axis slide is moved downwardly until the
pro~e 44 firmly engages the outer periphery of the
collar 20. The cover 100 on the ~ge head 36 is now
opened to expose the retroflector 40. Also, at the
same time, a c~ver ~should one be provided~ on the
linear interferometer is opened. The opening of covers
could ~e performed manually or by automatic means such
as air cylinders or solenoids. Alignment is now
verified by a signal Ce.g., a DC voltage or warning
light~ from the receiver 54 which indicates that the
main return beam is being properly received. The
distance A is subsequently preset into a memory register
in the CNC unit 34 whereupon workpiece measurement:s
are ready to ~e taken.
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The Z^axis sl~de 26 ~s t~en moved to the righ*
until the probe 44 overlies t~at axial station on
the ~o~kpiece ~ere the diameter is to ~e measured.
Next7 the X-axis slide 32 moves downwardly toward the
s workpiece, as vie~ed in ~I~URE 1. During t~is downward`
travel, the distance between the linear interferometer
48 and retroflector 4a is continuously increasing,
thereby causing the pulse converter 56 to issue for-th
a stream of pulses to the CNC unit 34, the pulses func-
tioning to decrement the num~er A stored in the CNC unit.When pulse generation ~y t~e pulse converter 56 term-
inates during X-axis slide movement, the probe 44 has
engaged the ~orkpiece and the num~er in the CNC unit is
an extremely precise mea~urement of a workpiece OD
dimension. Similar OD measurements may be taken at other
axial stations ~y simple slide movement without the need
for placing the pro~e 44 again upon the collar 20.
~uring such latter mentioned movements, the pulse con-
verter will, of course, generate incrementing or decre-
menting pulses to the CNC unit register. Variousmeasurements at different axial stations may be trans-
ferred to respective me~ory locations in the CNC unit.
The gage head is then removed from the workpiece
Ce.g., to a home position~. The part program is then
adjusted for the di~ference in the programmed and
measured dimensions and ~inal workpiece finishing is then
completed. If desired, final workpiece dimensions may
be checked as previously described~ Any excessive
variation between measured diameters and programmed
diameters may indicate excessive tool wear or some other
difficulty and corrective action is accordingly mandated.
- Measurement of internal diameters may ~e made by
using a simi.lar procedure and contacting the inner
periphery of the workpiece with probe 46. It will be
noted that for such measurements, an interior reference
surface could be provided on -the collar, or alterna-tively,
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the distance between the tips of the probes could be
precisely measured.
At the conclusion of a measuring operation, all
covers are closed and the laser transducers and
receivers turned off. Cutting cycles are resumed using
measured values to properly position the X-axis slide.
It will, of course, be understood that all measurements
are predicated on thP continuity of the laser alignm~nt
signal. Loss of this signal during measuring will
render subsequent readings unreliable.
Obviously~ many modifications and variations are
possible in light of the a~ove teachings without
departing from t~e scope or spirit of the invention as
defined in the appended claims. For example, a system
of the invention could ~e associated with a lathe
having a dual or single level turret mounted on the
X-axis slide and rotata~le about an axis parallel to
the X-axis and intersecting the spindle axis in ortho-
ginal or canted relationship thereto. Such a lathe is
shown in U.S~ Patent No. 3,878~742. In addition9 the
invention could be utilized with a lathe as shown in
U.S. Patent No. 3,75a,245. Furthermore it is wi~hin
the ambit of the invention to utilize other forms of
gage heads, re~lectors or inter~erometers with approp-
riate positioning of the receiver and laser transducer.
