MILLING METHOD

Abstract

The invention relates to a method for machining a workpiece by means of a milling tool arranged on a rotatable spindle, the spindle being moved relative to the workpiece or the workpiece being moved relative to the spindle along a machining path and, at the same time, the spindle rotating about a spindle axis. In said method, an improvement in the surface quality is achieved by controlling the rotational speed and/or the phase position of the rotation of the spindle along the machining path, the machining path comprising linear parallel tracks and the phase position of the spindle along the machining path being substantially the same on adjacent tracks, the phase position being controlled by varying the rotational speed of the spindle and/or the advancing speed of the spindle relative to the workpiece along the machining path.

Claims

1. A method for machining a workpiece with a milling tool arranged on a rotatable spindle, the method comprising: moving the spindle relative to the workpiece or moving the workpiece relative to the spindle along a machining path; rotating the spindle about a spindle axis; and controlling a rotational speed and/or a phase position of the rotation of the spindle along the machining path, wherein the machining path comprises linear parallel paths and the phase position of the spindle along the machining path is substantially the same in adjacent paths, the phase position being controlled by varying the rotational speed of the spindle and/or the feed rate of the spindle relative to the workpiece along the machining path.

2. The method according to claim 1, further comprising: controlling the rotational speed and/or the phase position along the processing path to a predetermined desired value at at least one synchronous point.

3. The method according to claim 2, further comprising: controlling the rotational speed and/or the phase position along the processing path at several synchronous points to a respectively predetermined desired value.

4. The method according to claim 2, further comprising: before reaching a synchronization point at a trigger point, starting the controlling of speed and phase position to the setpoint value.

5. The method according to claim 1, further comprising: controlling the phase position by decreasing or increasing the spindle speed.

6. The method according to claim 1, further comprising: controlling the phase position by decreasing or increasing the feed rate of the spindle with respect to the workpiece along the machining path.

7. The method according to claim 1, further comprising: shifting the phase position of a milling tool with several cutting edges by a multiple of the pitch of the cutting edges.

8. A device for executing the method in claim 1, the device comprising a means for displacing a workpiece relative to a milling tool arranged on a spindle, and means for controlling the speed and phase of rotation of the spindle.

9. The device according to claim 8, wherein the spindle is driven by a position-controlled electric motor.

10. The device according to claim 8, wherein the device further comprises a means for determining the angular position of the spindle.

11. A non-transitory computer readable medium having a computer program stored thereon that when executed by a processor performs the method according to claim 1.

12. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Examples of how the invention was implemented are explained in more detail below using the enclosed drawings. Show thereby:

[0021] FIG. 1 is a sketch of a speed limit profile as a result of the path planning,

[0022] FIG. 2 is a sketch of a speed limit profile as an approximation to the limit profile from below,

[0023] FIG. 3 is a sketch of a sampling of the path velocity profile in the interpolation cycle,

[0024] FIG. 4 is a sketch of joint sampling of the path velocity profile and a spindle velocity profile in the interpolation cycle,

[0025] FIG. 5 is a sketch of spindle phase compensation towards a block limit,

[0026] FIG. 6 is a sketch of phase compensation by temporary reduction of the feed level towards a block limit,

[0027] FIG. 7 is a sketch of the downstream phase compensation in case of dynamically impossible trajectory planning to the programmed block boundary,

[0028] FIG. 8 is a sketch of a surface of a milling workpiece produced with a state-of-the-art milling process.

MODES FOR CARRYING OUT THE INVENTION

[0029] NC (numerical control) programs describe the milling path in a sequence of simple geometric elements. Support points or path support points are the respective boundaries between two subsequent geometry elements. These coordinates are taken from the NC program per line as NC block. The path parameter (also called path length integral) describes exactly one point on the path described in the part program, which may well lie between the support points of the NC blocks

[0030] In the following, a synchronous point is understood to be any point of the path (=path parameter) at which simultaneously applies:

[0031] nSpdl=nprog (the spindle speed is at the value specified in the program)

[0032] νb=νprog (the feed rate is at the value specified in the program)

[0033] φSpdl=φprog (the angular position (phase) of the spindle is at the value specified in the program)

[0034] aSpdl=0 (the spindle speed is constant)

[0035] ab=0 (the spindle is not accelerated in x, y, z direction)

[0036] A trigger point is the start (in time and/or path parameters) for planning a motion profile towards a synchronous point.

[0037] By programming a feed rate and a spindle speed in an NC program of a CNC control, a technologically conditioned tooth feed rate is determined. It is the task of every CNC to maintain this programmed (desired or maximum) feed rate as accurately as possible without exceeding this maximum value, while maintaining the dynamic limit values of the axes involved. In areas of path curvature, the feed level must be lowered in order not to dynamically overload the axis feed drives or to meet certain requirements for path accuracy. This is realized by the pre-calculation of a so-called speed limit profile within the so-called path planning, as shown in FIG. 1.

[0038] In FIG. 1 and in the following text, each program line is designated N10, N20 and so on up to N100 in accordance with the DIN/ISO or G code. For each program line, a state-of-the-art CNC control system determines a maximum feed speed as a speed limit value profile for path planning. The speed limit value profile contains the feed rate or feed speed νlim, .sub.prog programmed in the NC program and, as a result of the path analysis, the dynamically conditioned maximum values for the path speed within the block νlim, .sub.dyn as well as the maximum transition speed per block transition ν.sub.lim.

[0039] The task of the downstream so-called velocity profile generator is to compute a velocity profile over the path parameter (or time, depending on the implementation), which follows the minimum values of the limit profile from below, i.e. from lower values, since the maximum value must not be exceeded. If a representation over time is chosen, the velocity profile results in 2nd order polynomial segments, whose segment boundaries in the most general case neither lie exactly on a sentence boundary nor on a point in time, which is later sampled by the interpolator, as shown in FIG. 2. The speed νB is at any point below the values of ν.sub.lim, prog, ν.sub.lim, dyn and ν.sub.lim, über, shown in FIG. 1 above.

[0040] Finally, the velocity profile thus precalculated is sampled by the so-called sampling interpolator in the cycle time of the so-called interpolation take as IPO sampling points and the position setpoints for all feed axes involved in the movement are calculated. This also means that not every NC support point is output to the axis controller as an exact position setpoint, because the exact NC block limit is generally located between two IPO scan points. The scanning is shown schematically in FIG. 3. With a time interval Ta, position values sb are determined by corresponding position sensors of all axes and transmitted to the controller.

[0041] The determination of the speed profile shown above and the resulting control of the feed rates of all axes are known from the state of the art.

[0042] The speed curve of a position-controlled spindle is determined and controlled by the path of the spindle. In addition, the angle (phase) of the spindle is controlled to a predetermined value at certain points. A graphic representation is based on that of the path speed profile.

[0043] The velocity of a position-controlled spindle can be ingeniously represented as a path velocity profile in the same representation of a velocity plot over the path parameter (or time) as shown in FIG. 4. The cyclical position setpoints can then be sampled from both profiles simultaneously.

[0044] By such a procedure, each interpolated path point is assigned exactly one spindle orientation (i.e. an angle of the spindle relative to a zero point; one phase of the rotation of the spindle). In particular, there is a “quasi-gear synchronicity” between spindle speed and path speed in the areas of constant path feed and constant spindle speed. However, in the state of exact speed coupling between spindle and path for the spindle there is still the degree of freedom of the spindle orientation in relation to the path length integral, which can be mathematically understood as an integration constant. According to the invention, this degree of freedom is provided with target parameters (=programmed) and produced in a time-optimized manner while maintaining the dynamic limits.

[0045] For the supply of the target parameters an exact spindle orientation is assigned at at least one point on the path. According to the invention, this assignment takes place particularly advantageously in the NC program. The programming of such a synchronization condition leads to a fixed spindle orientation at least (FIG. 5) once per adjacent milling path. With given synchronization commands, the task of the procedure according to the invention is first to determine whether trajectory planning is possible while maintaining the dynamic limit values. If trajectory planning is possible, two possible methods for establishing synchronicity are described below. Furthermore, a procedure is given how to deal with the case that trajectory planning is not possible.

[0046] For the following explanations, it is assumed that the system properties of the CNC control system are given (or will be produced first):

[0047] According to the invention, a specific geometric location on the path is assigned a spindle angular position desired at this point. In execution examples of the invention, this is realized by adding an additional NC block base to the part program in the CNC program (possibly redundant to the pure path description). Here is an example of one (of many conceivable) syntax extension of a DIN/ISO-compliant NC program, this is inserted in line N40 with “G119 S77” and explained there as a comment:

[0048] N10 M03 S1000

[0049] N20 G00 X-110 Y0 Z10

[0050] N30 G01 Z-1 F2000

[0051] N40 G01 X-100 G119 S77; at position X-100 the spindle should be at 77°.

[0052] N50 G01 X+100

[0053] The syntax extension provides a G119 command which expects a parameter S with the specification of an angular position of the spindle relative to a zero point (phase position).

[0054] This results in a “quasi-gear synchronism” between the path length integral and the angular position of the milling spindle at constant feed rate and constant spindle speed. In practice, this requirement is probably best met by position-controlled operation of the milling spindle.

[0055] Under the boundary condition that the limits of the three derivatives of the path according to time (vmax, amax, jmax) are kept for all feed axes and the milling spindle, there is a minimum time (and thus also a minimum path on the milling path) required to reach a synchronous point for each movement state of the path axes and the milling spindle. Conversely, this means that it is not physically possible to establish any desired synchronization in any short time or distance. Therefore, the procedure to be described must also explain what happens in such cases.

[0056] In contrast to linear feed axes, the cumulative position of the milling spindle since the beginning of its movement is of no interest for the present task. Only the angular positions within the 0 . . . 360° modulo range are of interest. This means that the maximum phase compensation required for the milling spindle to achieve the synchronization target is

Δφmax=+/−180°/No. of cutting edges

[0057] A further special feature for the position-controlled operation of spindles with setpoint specification in the module area is that each setpoint position can be reached in two ways (forward or backward rotation.) If the information about the direction of rotation is known, the position specification works up to 360°/TA. In other words, with an interpolation cycle of 1 ms, up to 60,000 rpm spindle speed.

[0058] If trajectory planning is possible, two methods can be used.

[0059] In the first process, the spindle performs a phase compensation. The definition of a synchronous point shows that synchronous points can only be located at such geometric locations on the path (path parameters), in which a constant feed rate is achieved. According to the definition of the nestling problem explained above, such synchronous points can lie exactly on a block boundary if the braking or acceleration distance on the path is sufficient to comply with the condition ab=0 at block entry in addition to νb=νprog, and if this braking or acceleration distance or its duration is also sufficient to establish the necessary phase compensation of the spindle.

[0060] Such a case is shown in the following diagram:

[0061] . . . N10 M03 S1000

[0062] :

[0063] N30 G00 X5 Y0 Z-5 F2000

[0064] N40 G00 X10 Y10 Z-1

[0065] N50 G00 X15 Y5

[0066] N60 G01 X85

[0067] N70 G01 X100 Y90 Z-2 F1500 G119 S77; Synchronize spindle to 77

[0068] N80 G01 X150

[0069] The individual program lines are marked as before with N10, N20 and so on. In addition to the path to X=100, Y=90 at a depth position Z=−2 and a feed rate of 2000, program line N70 contains the information that the spindle should assume an angle of 77° there.

[0070] The area created in FIG. 5 is the speed-time integral corresponding to the required phase correction of the spindle position. The respective segment boundaries of the velocity profiles are encircled. In this case, the movement is time-optimized without affecting the feedrate planning and without any negative influence on the total machining time of the part program. To calculate the synchronization process, proceed as follows: After a (new) NC block with orientation conditions for the spindle is found in the look-ahead buffer, the spindle speed profile is recalculated in addition to the path speed profile, similar to the way a “normal” new NC block is entered.

[0071] Since the look-ahead process runs in real time with the speed profile generation in the interpolator, the position setpoint output to the spindle in the last cycle is known, as well as the exact path parameter and thus also the duration (in path travel and time) until the synchronous point is reached. This allows the angular position of the spindle at the synchronous point to be calculated if the machine were to continue without correction of the spindle speed. The required amount of phase compensation is then the difference between these two angular positions in the modulo circle. In the case of multi-blade tools, the required phase compensation can of course also be shortened to 360°/angular positions. With knowledge of the dynamic limit values of the spindle, a spindle speed profile can then be calculated which establishes the synchronization point exactly at the block limit If the path feed (as in our example) is not constant during the phase correction of the spindle, it is important that the synchronization process of the spindle ends exactly at the block limit.

[0072] In a second execution example of the method according to the invention, phase compensation is achieved by lowering the feed level for a limited time, see FIG. 6.

[0073] The method for time-limited reduction of the feed level of the speed profile is particularly efficient for high-speed spindles. The required height and duration of the feed rate reduction is low because the spindle can take up practically any angular position within milliseconds without varying the spindle speed. To calculate the synchronization process, proceed as follows: The required phase correction amount is first determined as in the above procedure. After the calculation of the required phase correction up to the synchronous point, a modified piece of velocity profile is calculated towards the synchronous point in such a way that the movement section requires exactly as much additional time (compared to the original planning) as the spindle needs to pass the required phase correction.

[0074] Trajectory planning is always impossible if there is not enough time or distance to find a trajectory. The procedure according to the invention provides for such a case to determine a downstream synchronous point, which is a certain distance . . . d from the programmed synchronous point (downstream). The selected distance . . . d is an integral multiple of the planned feed rate known from the S and F word. The S word controls the spindle speed, the F word the feed rate. Thus, synchronicity of the tool cutting edge engagement is achieved with a delay, but still defined, to a geometric location programmed in the NC program. With a well-filled look-ahead buffer and NC blocks with synchronous conditions that do not follow each other too closely, this case will probably occur in practice rather rarely or not at all. The easiest way to “provoke” it is to use NC blocks with synchronous conditions that follow each other too closely. The following illustration shows an example of such a case: With a programmed feed level, there is no possibility of changing the orientation of the spindle by 64° within the distance of X 0.1 mm while maintaining the dynamic limit values, see FIG. 7;

[0075] . . . N10 M03 S1000

[0076] :

[0077] N30 G00 X5 Y0 Z-5 F2000

[0078] N40 G00 X10 Y10 Z-1

[0079] N50 G00 X15 Y5

[0080] N60 G01 X85

[0081] N70 G01 X100 Y90 Z-2 F1500 G119 S77; Synchronize spindle to 77

[0082] N80 G01 X100,1 Y90 Z-2 G119 S13; Synchronize spindle to 13

[0083] N90 G01 X150

FORMULA SYMBOLS

[0084] s Location, Position, Path [0085] ν Speed [0086] a Acceleration [0087] j Jerk [0088] φ Angular position [0089] n Speed [0090] TA Interpolation cycle time (IPO clock)

INDEXES

[0091] b Path, values that refer to the NC path [0092] Spdl Spindle, values that refer to the path [0093] prog (in NC program) programmed value