U.S. patent application number 11/984851 was filed with the patent office on 2008-07-03 for monitoring method and system for a tool-holding spindle.
This patent application is currently assigned to VIGEL S.P.A.. Invention is credited to Andrea Gay, Gian Luca Giovanelli.
Application Number | 20080159822 11/984851 |
Document ID | / |
Family ID | 38016630 |
Filed Date | 2008-07-03 |
United States Patent
Application |
20080159822 |
Kind Code |
A1 |
Giovanelli; Gian Luca ; et
al. |
July 3, 2008 |
MONITORING METHOD AND SYSTEM FOR A TOOL-HOLDING SPINDLE
Abstract
A sensor detects an axial position of a rod controlling a
tool-gripping collet in a tool-holding spindle whenever a tool is
picked during a working session. The current obtained rod position
value is compared with previous rod position value, which has been
obtained and stored during the previous tool-picking and, if their
deviation is larger than a predetermined threshold, the collet
closure is notified as defective. The rod position is obtained as
an average of a plurality of closely spaced readings, which are
taken after a predetermined delay from the collet closure.
Inventors: |
Giovanelli; Gian Luca;
(Castagneto, IT) ; Gay; Andrea; (Torino,
IT) |
Correspondence
Address: |
MODIANO & ASSOCIATI
Via Meravigli, 16
MILANO
20123
omitted
|
Assignee: |
VIGEL S.P.A.
|
Family ID: |
38016630 |
Appl. No.: |
11/984851 |
Filed: |
November 23, 2007 |
Current U.S.
Class: |
409/131 ; 408/1R;
409/232; 82/1.11; 82/146 |
Current CPC
Class: |
Y10T 409/309464
20150115; B23Q 17/003 20130101; Y10T 82/2561 20150115; Y10T 82/10
20150115; Y10T 409/309408 20150115; Y10T 408/03 20150115; Y10T
409/303752 20150115; Y10T 483/14 20150115; Y10T 483/10
20150115 |
Class at
Publication: |
409/131 ;
408/1.R; 409/232; 82/1.11; 82/146 |
International
Class: |
B23Q 1/00 20060101
B23Q001/00; B23B 19/02 20060101 B23B019/02; B23Q 1/25 20060101
B23Q001/25 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 29, 2006 |
EP |
06425871.8 |
Claims
1. A monitoring method for a tool-holding spindle having a
tool-gripping collet driven by an axially shiftable control rod,
and a sensor positioned for sensing an axial position of the rod
and delivering an output signal as a function of said position,
comprising, whenever each tool is initially picked from a tool crib
in a working session, the steps of: (a) after checking that the
collet is closed, taking a predetermined number of readings of a
position of the control rod (P.sub.c1, P.sub.c2, . . . , P.sub.cn)
by means of said sensor, at predetermined intervals starting from a
predetermined delay (T) after the closure of the collet; (b)
computing a current average of said readings (P.sub.c1, P.sub.c2, .
. . , P.sub.cn) whereby a value for the current position of the rod
(P.sub.c) is obtained; (c) notifying that the collet closure is
regular and storing said current position value (P.sub.c) as
previous position value (P.sub.m); and further comprising the
steps, to be carried out when subsequently picking the respective
tool within the same working session, of: (d) taking said
predetermined number of rod position readings (P.sub.c1, P.sub.c2,
. . . , P.sub.cn) by means of said sensor, at said predetermined
intervals starting from a predetermined delay (T) after the closure
of the collet; (e) computing the current average of said readings
(P.sub.c1, P.sub.c2, . . . , P.sub.cn) whereby a fresh value for
the current rod position (P.sub.c) is obtained; (f) comparing the
current rod position value (P.sub.c) with the corresponding stored
previous rod position value (P.sub.m) and, if their deviation is
less than a predetermined threshold (S), notifying that the collet
closure is regular, and otherwise notifying that the collet closure
is defective; (g) if the collet closure is regular, storing said
current rod position value (P.sub.c) as previous rod position value
(P.sub.m).
2. The spindle-monitoring method of claim 1, wherein the following
cycle of rescue steps is repeated at least once whenever the collet
closure is defective: (h) actuating in succession a release of the
spindle collet, a disengagement of the spindle from the tool crib,
a re-engagement of the spindle with the tool crib, and a
re-clamping of the collet; (i) repeating steps (d), (e), (f), (g);
(j) if, at the end of the above cycle, the collet closure is
ascertained to be defective, stopping the operation of the tool
machine and notifying an alarm signal to an operator.
3. The spindle-monitoring method of claim 2, wherein said cycle of
rescue steps is repeated three times before stopping the tool
machine.
4. The spindle-monitoring method of claim 1, wherein said
predetermined number of rod position readings (P.sub.c1, P.sub.c2,
. . . , P.sub.cn) is 5.
5. The spindle-monitoring method of claim 1, wherein said
predetermined delay (T) is about 100 msec.
6. The spindle-monitoring method of claim 1, wherein said
predetermined deviation threshold (S) is about 25 mV.
7. A tool machine system having a tool-carrying spindle with a
tool-gripping collet driven by the axial shift of a control rod and
a sensor arranged for detecting an axial position of the rod and
delivering an output signal as a function of said position, and an
electronic control unit connected for controlling said collet and
for reading an output of said sensor, wherein the control unit is
programmed for executing the monitoring method of claim 1.
Description
[0001] This invention is concerned with a method and a system for
monitoring a tool-holding spindle in a tool machine, in order to
determine whether a tool picked by the spindle from a tool crib in
preparation to a machining operation has been properly gripped in
its collet.
BACKGROUND OF THE INVENTION
[0002] When a machine cycle is to be repeatedly carried out with
different tools, tool machines are programmed for operating a
spindle through a working cycle such that the spindle will pick a
chosen tool from a tool crib, will clamp its collet on the tool
shank, will carry out the programmed operation on the workpiece and
finally will return the tool to the tool crib. Two facing, annular
registering surfaces on the spindle and the tool, respectively,
axially abut with each other to insure that the tool is properly
coupled with the spindle, so that a good degree of accuracy and
repeatability is achieved.
[0003] However, minute chips may deposit on the registering surface
during machining, which often stick to it, due to the capillarity
of the cooling/lubricating fluid that is always present in the
area. If the chip fragment is very small, of the order of a few
hundredths of a millimeter (a few tens of microns) thickness, the
compressed air usually blown over the spindle before picking the
tool may be unable to remove it, and the chip will therefore be
pinched between the abutted registering surfaces, thereby
preventing the collet from closing, or, alternatively, causing a
positioning error, generally involving a lack of coplanarity of the
registering surfaces, whereby the tool is inclined with respect to
the axis of the spindle.
[0004] It is known to provide the spindle head with a proximity
sensor arranged for detecting the axial position of a control rod
controlling the opening and closing of the collet, and for
notifying a collet-clamping error in case of an incomplete stroke
of the rod. However, such sensor is only able to notify coarse
errors, due to the unavoidable variation in the nominal end of
stroke of the control rod in different tools. Consequently, the
above sensor is unable to detect very small fragments, which,
however, as stated above, do cause a deviation of the tool axis
from the spindle axis, with consequent linear errors amounting up
to several tenths of a millimeter at the tool tip.
SUMMARY OF THE INVENTION
[0005] The aim of the present invention is to provide a method and
a system for monitoring a tool-holding spindle in a tool machine,
in order to determine whether a tool has been properly gripped in
the spindle collet without any foreign particles coming in between,
however small.
[0006] The above aim, as well as other objects and advantages such
as will appear from the following disclosure, is attained by the
invention by a method for monitoring tool-holding spindles, having
the features recited in claim 1, the claims dependent from it
reciting other advantageous though secondary features. The
invention also concerns a system for carrying out the above method,
as recited in claim 7.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The invention will now be described in more detail below,
with reference to a preferred embodiment, shown by way of
nonlimiting example in the attached drawings, wherein:
[0008] FIG. 1 is a view in axial cross-section, partly schematic,
of a tool-holding spindle belonging to the system of the invention,
holding no tool;
[0009] FIG. 2 is a view similar to FIG. 1, showing the spindle with
unclamped tool;
[0010] FIG. 3 is a view similar to FIG. 1, showing the spindle with
clamped tool;
[0011] FIG. 4 is a flowchart showing the monitoring method
according to a preferred embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0012] With reference to FIGS. 1-3, a spindle 10 is journaled in a
tool machine 12 through ball bearings 14. Spindle 10 is hollow and
has, at its operating end, a frusto-conical socket 16 and a coaxial
flat, annular registering surface 18 which can receive a
frusto-conical shank 20 of a tool (FIGS. 2 e 3). Shank 20 has an
axial, shaped recess 22, which, also in a way known per se, can be
engaged by a clamping member or collet comprising a plurality of
tongues or reeds 24, which can be radially withdrawn or expanded by
the longitudinal shifting of a mushroom-shaped head 26 carried at
the end of a control rod 28, which is axially shiftable within the
spindle. A compression spring 30 biases rod 28 to a backward
position in which head 26 expands the collet outwardly. A hydraulic
drive (not shown) is able to push rod 28 forward, as known per se,
against the bias of spring 30, thereby withdrawing reeds 24 to a
contracted condition which frees tool 20. An analog proximity
sensor 36, which may be capacitive or inductive, is mounted in a
stationary position in the tool machine, so that it will interact
with a conical surface 38 that is coaxial and integral with control
rod 28. The sensor can therefore sense the axial position of the
rod by measuring the distance from the conical surface.
[0013] In order to load a tool, the spindle, with spread-out collet
as shown on FIG. 1, moves forward in front of the shank 20 of a
desired tool carried in a tool crib (not shown), and the control
rod 28 is then pushed forward by the hydraulical drive in order to
withdraw the locking reeds 24 and thereby allow the shank (as shown
on FIG. 2) to move into the frusto-conical socket 16, until the
annular surface 18 of the spindle abuts against a corresponding
annular surface on the shank. Control rod 28 is then released and
is shifted by spring 30 (FIG. 3) in order to spread the collet
reeds and clamp the tool.
[0014] If a chip fragment of about a tenth of a millimeter
thickness is pinched between the two facing annular surfaces, the
fragment will cause an axial rod position error that is generally
smaller than the error threshold measurable by sensor 36, although
the error will affect the machining accuracy, because the tool, due
to the chip, will take a slightly inclined position with respect to
the spindle axis, and the linear error at the tip of the tool will
be amplified.
[0015] The method of the invention is illustrated in the flowchart
of FIG. 4. Whenever a given tool is picked from a tool crib (not
shown), the tool is allowed to be clamped, and the current position
P.sub.c of the control rod is then measured by the sensor 36.
Preferably, a measurement of the current position P.sub.c is
obtained by taking a set of successive readings P.sub.c1, P.sub.c2,
. . . , P.sub.cn, at close time intervals, e.g. five readings (not
shown in detail in FIG. 4 for simplicity), starting with a
predetermined delay T after checking that the closure of the collet
has been completed, and then by assuming the average of the
readings as measurement P.sub.c. The number of readings is chosen
as an acceptable compromise between the time required to perform
the measurement and the accuracy of the measurement. The interval
between any two successive readings is chosen, e.g., as 10-15 msec,
and the delay time T is chosen such that the transients are masked
and the sensor output is stabilized, typically as 100 msec.
[0016] The value of the current position P.sub.c is then compared
with the position P.sub.m that was obtained and stored for the same
tool in the previous cycle. If the two values P.sub.m and P.sub.c
differ by less than a predetermined deviation S, the collet closure
is regarded as regular and the work cycle is allowed to proceed,
while the current value P.sub.c is substituted for the stored value
P.sub.m. If, on the other hand, the difference between the two
values P.sub.m and P.sub.c is larger than the deviation threshold
S, the closure is considered to be anomalous, the tool machine is
stopped, and an alarm signal for operator is emitted.
[0017] Alternatively, and in fact preferably, as shown in FIG. 4,
whenever the above comparison detects a defective closure, rather
than immediately stopping the tool machine, the collet is released,
the spindle is backed off from the tool crib and the collet is
again clamped, the above described position measurement is
repeated, and the stored position P.sub.m is again compared with
current position P.sub.c. This attempt is repeated a few times,
e.g. four times as shown, and if the comparison continues to have
an anomalous issue, the tool machine is stopped at the fourth
failure, and the operator is warned by means of an alarm signal. It
has in fact been found that a repeated release and clamping of the
collet is often sufficient to remove or crumble the fragment,
thereby detaching it from the surface to which it was clinging.
[0018] At the beginning of each working session, when a previous
position has not yet been stored for a tool, the value for the
previous position P.sub.m is set to zero, and it is therefore
impossible to make a comparison. Accordingly, when first picking a
tool the collet closure is always assumed to be regular, and the
current rod position P.sub.c is stored directly as a value for the
previous rod position P.sub.m. This step is equivalent to abstain
from monitoring the first tool picking, and may give rise not only
to a defective tool alignment in the first machining operation
performed, but also to an erroneous measurement in the second
operation that will be carried out by the same tool. Such a
circumstance is highly unlikely, and will be corrected, in any
case, starting with the third operation.
[0019] In a typical case given by way of example, the proximity
sensor has a sensitivity of a 10 mV variation of the generated
voltage for each 10 .mu.m variation in the distance from the
observed surface. The threshold S is chosen at 25 mV, which
nominally correspond to the ability to sense a thickness of about
25 .mu.m, although in practice only foreign particles of at least
40 .mu.m thickness can be guaranteed to be detected.
[0020] It is understood that changes can be made in the preferred
embodiment of the method as described above, particularly with
respect to the values of the threshold S and the delay T, as well
as in the number of readings taken by the sensor for computing an
average, and also in the number of tool-picking attempts made.
Moreover, it should be noted that, for the sake of simplicity, no
mention has been made of a number of options and functions that are
normally available in the numeric control of tool machines, such as
displays, possibility for the operator to activate or suppress
desired functions, change parameters such as S or T, and the
like.
[0021] The disclosures in European Patent Application No.
06425871.8 from which this application claims priority are
incorporated herein by reference.
* * * * *