Honing machine

A honing machine in which a micro-processor controls reciprocation with respect to a work-piece of a spindle which carries honing stones, the micro-processor being associated with a memory unit which holds an acceleration profile in terms of position/time and being programmed to control reciprocation in accordance with the required acceleration characteristics. Variable stroke parameters and required mid-stroke speed are input from a console and the system ensures symmetrical honing under controlled acceleration regardless of any conflict between selected stroke length and mid-stroke speed.

The invention relates to a honing machine. Honing machines are used for 
fine-finishing cylindrical bores in work-pieces such as automobile 
cylinder blocks and hydraulic and pneumatic components of all kinds. 
In general, a honing machine has a rotatable spindle which carries at its 
end abrasive stones mounted in carriers which may be expanded radially. In 
a honing operation the spindle enters the bore in the work-piece and is 
rotated as the work-piece and spindle are reciprocated with respect to 
each other. Radial pressure is applied to urge the stones against the wall 
of the bore. Forward movement of the spindle causes the stones to trace a 
helical path in the bore in one sense and reverse movement of the spindle 
causes the stones to trace a helical path in the opposite sense. 
Consequently, a cross-hatched honing pattern is made in the bore. For 
optimum finishing it is found to be important to control the cross-hatch 
pattern accurately. 
The cross-hatch angle is a function of the stroke velocity in relation to 
the spindle speed. Spindle speed can effectively be maintained constant by 
controlling a continuously rotating spindle motor of significant inertia. 
The problem of cross-hatch pattern control resolves itself, therefore, to 
control of stroke velocity. However, because of the reciprocating 
movement, the spindle must be repetitively accelerated and decelerated 
having regard to its position with respect to the work-piece. 
In order to control stroke reversal in honing machines it has been the 
practice to use limit switches or mechanical trips for valves etc., near 
each end of the stroke. These may be micro-switches, proximity sensors, or 
the like which, when activated, reverse the drive to the stroke mechanism. 
The use of limit switches gives an inherently asymmetrical stroke velocity 
characteristic, and while offering repeatability, does not allow precision 
control. Often, the reciprocating mechanism is hydraulic. While this is a 
convenient way of driving a system for a large-scale machine with a high 
power requirement, it again does not lend itself to precision control. The 
characteristics of the hydraulic system change with temperature, for 
example. The present invention seeks to provide a honing machine offering 
very precise control, and while the principles are applicable to 
large-scale machines, the principal application for the invention is found 
in small-piece work where precision is particularly important. 
According to one aspect of the invention there is provided a honing machine 
comprising a spindle; an expandable honing stone carrier at the end of the 
spindle; a drive motor for rotating the spindle; a direct-current stroke 
motor for reciprocating the spindle with respect to a work-piece; a 
position transducer arrangement for giving a digital output in accordance 
with the linear stroke position of the spindle with respect to the 
work-piece; a micro-processor; a memory unit for the micro-processor, 
which memory unit holds a predetermined acceleration profile look-up 
table; and an input console for manually applying input parameters, the 
micro-processor being programmed (a) to accept inputs from the console 
from which can be derived at least the required start and end positions of 
the stroke and the required mid-stroke speed of the spindle, (b) to sample 
the input from the position transducer arrangement periodically, (c) to 
compare the actual position of the spindle with the look-up table to drive 
a position error signal whereby the stroke motor is driven to correct the 
error and cause the spindle to follow the acceleration profile alternately 
in acceleration and deceleration modes, (d) to cause the spindle to 
continue at the mid-stroke speed between acceleration and deceleration 
when the mid-stroke speed is reached, and (e) to cause switching at the 
mid-stroke position from acceleration to deceleration mode if the required 
mid-stroke speed has not been reached. 
This arrangement ensures symmetrical acceleration and deceleration of the 
spindle even if the required mid-stroke speed is not reached. This is 
important when the honing machine is to provide for a wide range of bore 
lengths, since for short bores and for short-stroke local honing the 
inertia of the system may prevent the required mid-stroke steady speed 
being met. The arrangement described ensures that the acceleration and 
deceleration modes have priority over the mid-stroke constant velocity 
mode. It is found that this ensures accuracy in achieving the end points 
of the bore which is again particularly important for small-piece work. 
For example, it may be derived not only to set the stroke length manually 
but also to allow for automatic re-setting in response to gauging 
information, to correct perhaps for detected bore tape. Also, it is 
frequently necessary to include cycles of short-stroke honing within a 
main honing cycle, particularly for blind bores. With the arrangement in 
accordance with the invention, a required honing speed can be set and 
whereas this will apply to limit the speed for long strokes, the 
requirement will be automatically over-ridden when necessary for shorter 
strokes. This helps ensure the positional accuracy which is a feature of 
the preferred positional control system. 
In the reciprocating motion there are six phases: forward acceleration; 
forward steady speed; forward deceleration; backward acceleration; 
backward steady speed; and backward deceleration. A preferred feature of 
the present invention is to provide a micro-processor program which cycles 
successively between these six phases and which skips and steady speed 
phases if necessary. Furthermore, a single acceleration curve, preferably 
stored in position/time form can be used for all accelerations and 
decelerations of a cylce, regardless of stroke length, by reading the 
curve in the appropriate direction according to the 
acceleration/deceleration phase concerned. Thus, symmetry of 
acceleration/deceleration is ensured and it is not necessary to store a 
full position/time or velocity/ time curve in the computer memory. 
Since the spindle velocity slows at the ends of the stroke and since it is 
impracticable to change the rotational speed rapidly, the cross-hatch 
pattern at the ends of the stroke will be different from that in the 
middle. To achieve an optimum pattern in the middle for as great a length 
as possible it should be ensured that the velocity during the middle of 
the stroke is constant at the required value and that acceleration and 
deceleration are as rapid as possible Maximum acceleration is governed by 
the torque of the stroke motor, the inertia of the stroke mechanism, and 
frictional resistance, due largely to the frictional honing force. 
Generally, therefore, the acceleration profile will be calculated to give 
substantially the maximum acceleration and deceleration available, having 
regard to the parameters of the system. However, excessive accelerations 
and decelerations may cause impact damage, particularly with small 
work-pieces. A different shaped profile will then be appropriate. The 
memory unit may include a family of acceleration position/time profiles, 
means being provided to select the appropriate one for a particular 
purpose. Alternatively, different profiles may be contained in different 
plug-in ROM chips which are interchangeable. In another arrangement the 
micro-processor may be programmed to calculate a profile table at the 
start of a honing operation and to place the table in RAM. 
In order to achieve a required cross-hatch pattern the velocity during the 
constant velocity period in the middle of the stroke should have a 
particular relation to the circumferential velocity of the bore. This 
depends on the diameter of the bore and the rotational spindle speed. For 
a given bore diameter the stroke velocity should therefore be adjusted in 
accordance with spindle speed. For many purposes there is a cross-hatch 
angle (i.e. angle of stone traverse with respect to the axis of the bore) 
which is optional. Typically, the optional angle may be a particular angle 
between 40.degree. and 65.degree.. The micro-processor could therefore be 
pre-set to achieve any required cross-hatch angle in the middle of the 
stroke for all bores. Alternatively, however, a required cross-hatch angle 
input can be made at the console, whereby the microprocessor will modify 
the correction output to produce a different constant velocity. 
In order to calculate the correction output, the micro-processor requires 
the rotary spindle speed. This can be derived from a tacho-generator 
coupled to the spindle motor. Alternatively, spindle speed can be set from 
the console, and the input to the micro-processor can be derived from the 
speed setting. 
The micro-processor program may be modified to provide exaggerated local 
honing within the main honing cycle. For example, it may be desired to 
effect local honing of the end of a blind bore, either to compensate for 
under-honing or even to provide a tapered bore. This facility may also be 
effective to correct tapered bores. For this purpose the facility may be 
provided to specify a short-stroke cycle which has its own restricted end 
points, presettable, and a given number of strokes. Thus, for example, it 
may be specified that in each main honing cycle, five short strokes are 
executed over the innermost 10% of the stroke length. 
The invention is applicable to an arrangement in which the spindle is fixed 
relative to a bed and the work-piece is reciprocated by the stroke motor. 
However, it is preferred to provide that the work-piece is fixed and the 
spindle is reciprocated. Drive for the reciprocating spindle could be 
given through splines from a fixed spindle motor. However, it is a 
preferred feature of the invention that the spindle motor is mounted to be 
reciprocated with the spindle, the whole assembly being driven by the 
stroke motor. This has the advantage that the weight of the reciprocating 
assembly is fixed, regardless of the weight of the work-piece, and spline 
friction has no effect. Thus, the acceleration characteristics of the 
assembly are largely predictable and can be embodied in the position/time 
profile table without giving rise to significant errors. 
The spindle motor/spindle assembly can be mounted on a sled which can be 
driven by the stroke motor through a local screw or by means of a chain 
drive. Naturally, there should be as little backlash in the system as 
possible. 
The position transducer arrangement can comprise a linear transducer fixed 
to the bed of the machine, for example a linear inductoryn or optical 
grating. In a preferred embodiment of the invention, however, the 
transducer arrangement comprises a rotary digital encoder fixed to the 
spindle of the stroke motor. It is found that this gives a high degree of 
accuracy, particularly when used in conjunction with a lead screw drive.

Referring to FIG. 1 there is shown a honing machine for honing a work-piece 
mounted on a table fixture F. A honing spindle 2 carries honing stones at 
its end which engage the wall of the bore in the work-piece to effect 
honing. The spindle is rotated by a spindle motor (not shown in FIG. 1) 
which is mounted on a carriage which reciprocates vertically on runners 
under control of a stroke motor mounted in housing H. Control of the 
machine is effected by a micro-processor housed in a housing M. A console 
24 allows an operator to set various parameters for controlling the 
machine. 
FIG. 2 shows the structure of the machine in more detail. The machine has a 
frame 60 which supports the table fixture F where work-pieces are clamped. 
The frame provides vertical runners 62 on which is guided a carriage 63 
which has rollers 64. Linear bearings may alternatively be used. A 
lead-screw is mounted vertically and has a drive pulley 65 at its upper 
end. Pulley 65 is coupled by a toothed belt 61 to a pulley 66 on a motor 
67. The spindle motor is shown at 5 mounted on the carriage and power is 
conveyed to it by a flexible cable 68. 
A counter-balance arrangement comprises a hydraulic cylinder 69 having a 
piston which is coupled to the carriage by a chain and pulley arrangement 
70, 71, 72. Pressure in the cylinder is controlled to provide a force 
which precisely counterbalances the weight of the carriage and spindle 
motor. This arrangment has less inertia than a counter-weight would have. 
The machine of FIGS. 1 and 2 is a vertical honing machine in which carriage 
63 reciprocates vertically, with a counter-balance system as described. 
However, the principles of the machine apply equally if the carriage is 
arranged to reciprocate at angles with respect to the vertical, with 
suitable adjustment of the couterbalance forces. If the carriage 
reciprocates horizontally, no counterbalancing is required. FIG. 3 
illustrates the principles of the machine in a horizontal arrangement. The 
work-piece being honed is shown at 1 and is fixed. The honing spindle 2 
carries the honing stones 3 at its end which engage the wall of the bore 4 
in work-piece 1 to effect honing. The spindle motor 5 is a 3-phase a.c. 
motor which comprises a stator 6 and a rotor 7. Rotor 7 has a hollow core 
which accommodates the hollow spindle 2. 
The carriage 63 which rides on runners 62 and linear reciprocating movement 
is imparted to the carriage by a lead screw 10 which runs in a nut 9 in 
the carriage. The lead screw is turned by the direct-current stroke motor 
67 via toothed belt 61. 
Mounted on stator 6 is a wedge control system 12 which comprises a 
direct-current wedge motor 13 which drives a lead screw 14 via a toothed 
belt and pulley system 15. A nut 16 runs on screw 14 and this is coupled 
to wedge drive strips 17. At the other end of strips 17 is a wedge 
expansion system which expands the stones 3 outwardly in response to 
forward axial movement of the strips. The stones are thus urged against 
the wall of the bore. 
The use of a rotor/stator assembly with a hollow rota mounted to 
reciprocate on the carriage allows the wedge control system to be bolted 
directly to the stator and the mechanical linkage from the wedge motor to 
the wedge strips is thus direct without the necessity to by-pass a pulley 
drive or splined drive system. This enhances the wedge control accuracy. 
Control of the system is effected by a micro-processor 18. The 
micro-processor receives an input on line 19 from a rotary position 
encoder 20 fixed to the shaft of the stroke motor 11. This input 
represents the position of the carriage 63. Another input is received on 
line 21 from a rotary encoder 22 fixed to the lead screw 14. This input 
represents the radial position of the stones 3. A further input is 
received on line 23 from a drive amplifier (not shown) for the spindle 
motor. This is a frequency input representative of the speed of the 
spindle motor. Further inputs are applied to the micro-processor from a 
console 24 which has manually operable push-buttons whereby the operator 
may set all functions and requirements for the honing operation. 
The micro-processor applies control outputs to a stroke drive unit 25 which 
drives the stroke motor; a spindle drive unit 26 which drives the spindle 
motor; and a wedge drive unit 27 which drives the wedge motor. 
FIG. 4 shows the layout of the console 24. The console allows machining and 
wedge parameters to be put into the micro-processor. Two liquid-crystal 
displays 28 and 29 allow the parameters to be displayed as they are input. 
The inputs may be recalled at will. A machine parameter key-pad 30 has 
keys for setting the following parameters: hone park (p); start point (q); 
end point (r); required spindle velocity (s) (mid point); rotary spindle 
speed (t); stroke length (u); short stroke length (w); and number of short 
strokes (x). In order to set a figure for one of the parameters the 
numerical key-pad 31 is used after depression of the appropriate parameter 
key. Also, there is a test mode facility on key-pad 30. 
A wedge parameter key-pad 32 has keys for setting the following parameters; 
stone pressure (a); wedge angle (b); stones worn (c); maximum stone 
pressure (d); retract distance (e); stones in contact (f); approach or 
initial feed speed (g); and honing, or final feed speed (h). 
A sizing key-pad 33 has keys for setting the following sizing parameters: 
plug mode; timer mode; timer set; stone wear set; manual compensate; 
automatic compensate; tolerance; diametric clearance and match gauge. 
A fixture key-pad 34, which relates to control of the work-piece fixture F, 
has keys for the following functions: manual; step; auto-load; and manual 
clamp. 
There are manual controls for adjusting the position of the spindle. Key 35 
moves the spindle forwards, key 36 moves it back and "fast" and "inch" 
keys 37 and 38 control the speed of adjustment. Finally there are start 
and stop cycle keys 39, 40, an emergency stop key 41 and a "controls on" 
and a datum key 42 and 43 respectively. 
FIG. 5 is a block diagram of the drive circuit for the stroke motor 11. The 
micro-processor is shown at 18 and has associated with it a memory unit 
44. Unit 44 carries a set of look-up tables LT1, LT2 . . . etc. Each table 
is a table of values relating distance to time as a position/time profile 
for required acceleration and deceleration of the carriage 63. The 
profiles are constructed from the known characteristics of inertia of the 
stroke mechanism, frictional resistance and motor torque and in this 
embodiment are stored permanently in ROM. The different tables represent 
different respective accelerations and the table used for a particular 
honing operation is selected by means of the acceleration input (v). In 
the absence of a selected input the table giving the highest acceleration 
is chosen. In the particular machine described the highest acceleration is 
1.2g. Acceleration is capable of being set in steps of 0.2g from 0.2g to 
1.2g. In some embodiments of the invention this upper limit may be higher. 
The micro-processor receives input from the console 24, which defines at 
least the end of stroke positions required. There are different possible 
ways of inputting this information. For example, the two stroke end 
positions may be defined, or one end position and the stroke length or the 
mid-stroke position and the stroke length, for example. Also derived from 
console 24 is an input representative of the set spindle speed. An input 
of position is derived from the position encoder 20 of the stroke motor 11 
via a pulse counter PC. This is sampled by the micro-processor regularly, 
typically every 2 m.Sec. Having regard to the position/time table in 
memory unit 44 snd the inputs applied from console 24, the micro-processor 
produces a correction output on line 45. 
The correction output is applied to a digital-to-analogue converter 46 
which produces a speed input signal to a summing circuit 47. The output 
from circuit 47 is applied to a amplifier 48 to drive the stroke motor 11. 
A pulse rate detector 49 gives an output proportional to the speed of the 
motor and this is fed back to the summing circuit 47 to be subtracted from 
the speed input signal from converter 46. 
Referring now to FIG. 6 there is shown at (a) look-up table LT1 of FIG. 5 
represented as a graph of position against time. In the memory unit this 
is stored in digital form as a series of positions at spaced intervals of 
time--in this embodiment the intervals are 2 msec. The table is 
constructed from the motion equation p =1/2 at.sup.2, where p is position, 
a is position and t is time. The control system drives the carriage 63 to 
follow the position curve C until the input mid-stroke speed is reached. 
Two speeds are shown at S1 and S2 in FIG. 6(a). 
FIG. 6(b) shows part of the position/time curve for a control cycle with 
the mid-stroke velocity set to S1 and the end of the stroke position set 
to P1, the beginning of the stroke being at PO. At the beginning of the 
stroke the carriage follows the acceleration curve C. When the speed 
reaches S1 at time t1, acceleration ceases and the carriage is controlled 
to proceed at speed S1. Deceleration starts at an equal distance from the 
mid-stroke position that acceleration ceased. To determine deceleration, 
the curve C is read from the look-up table taking the position as the 
distance to the end of stroke position. When speed S1 is reached in the 
reverse direction, acceleration in the reverse direction ceases at time 
t2, and so on. 
FIG. 6(c) is a velocity/time curve corresponding to the position/curve of 
FIG. 6(b). It will be seen that the curve of FIG. 6(c) is trapezoidal, 
having flat regions where the carriage speed is constant at S1. 
FIG. 6(d) is a position/time curve in which the stoke end position has been 
set at p2 for a short stroke and the mid-stroke speed has been set at S2. 
However, the carriage cannot achieve speed S2 before the mid-stroke 
position. Under these circumstances the control system operates to 
over-ride the velocity demand and simultaneously cease acceleration and 
start deceleration at the mid-stroke position (time t1). Similarly, at 
time t2 and t3 there are sudden reversals of acceleration. The 
corresponding velocity/time curve is shown in FIG. 6(e). It will be seen 
that this has a triangular shape and speed S2 is never reached. 
FIG. 7 is a diagram showing the flow-chart for the micro-processor program 
which achieves the characteristics described. The cycle of movement has 
six sub-cycles, SC1 to SC6. These are selected in sequence according to 
the prevailing time and position of the carriage. The demand signals 
generated are position demands which cause the control circuit to issue 
velocity control signals to the carriage motor. The sub-cycles merge 
sequentially with one another. 
In the diagram, the current time is t; the current position of the spindle 
(derived from the encoder) is P; the current look-up table position value 
is T; the next table position is TN; the start position of the stroke is 
PO; the end position of the stroke is P1; the mid-point of the stroke is 
M; the required mid-stroke speed is S; the table-end position (dT/dt =S) 
is TE; the table start position (dT/dt =0) is TS; and the demand position 
applied to the control circuit is D. 
The functions of the respective sub-cycles are: 
SC1 - forward acceleration 
SC2 - forward steady speed 
SC3 - forward deceleration 
SC4 - backward acceleration 
SC5 - backward steady speed 
SC6 - backward deceleration 
It will be seen that sub-cycles SC2 and SC5 may be skipped if the set-in 
steady speed is not achieved.