Development of workpiece installation position det

Development of workpiece installation position determination system in parallel machine tools

in recent years, the development and application of parallel machine tools are increasing. However, due to its very complex structure and workspace, when using it to process workpieces, it is very likely to cause self-interference between machine parts and interference between machine tools and workpieces. In addition, if, like the parallel machine tool HexaM, the worktable is also equipped with peripheral instruments such as automatic tool change device, it may restrict the installation position of the workpiece when installing the workpiece, that is, there may be interference between the workpiece and the peripheral instruments, and there may also be interference between the machine tool and the peripheral instruments during processing

there have been many studies on the inspection method of self interference of parallel machine tools and the machine tool work to reduce the power supply noise space. Among them, taking the parallel machine tool HexaM of Toyota machine tool company of Japan as a model, the author discussed the inspection method of the parallel machine tool's own interference, and proposed the approximate expression of the workspace. On the other hand, in 5-axis NC machining, there are many ways to check the interference between the tool system (tool, chuck and spindle) and the workpiece system (workpiece and fixture). Among them, Takeuchi and others take several points from the surface of the tool system as interference checkpoints, and the surface of the workpiece is composed of a relatively simple function. When the interference checkpoint is located inside the function, it can be judged that the machine tool and the workpiece interfere; The method proposed by the author is just the opposite, that is, select the interference check point from the surface of the workpiece system to see whether it enters the tool system for interference inspection. Since the peripheral instruments of the NC machine tool are generally not placed on the workbench, the interference between the machine tool and the peripheral instruments in the processing process is not considered in these two inspection methods

the above studies have discussed in detail the self interference of parallel machine tools and the interference inspection and avoidance methods between machine tools and workpieces, but before using parallel machine tools, there are still the following problems: ① for machine tools with obstacles such as peripheral instruments on the workbench, the possible interference between machine tools and peripheral instruments should be considered; ② When checking the interference between the machine tool and the workpiece, we should also consider the possible interference between the spindle splint, splint hinge and connecting rod and the workpiece, but the existing CAM software does not include such interference checking; ③ When there is interference between the machine tool and the workpiece, the interference is usually avoided by adjusting the tool posture. In fact, for parallel machine tools, in addition to adjusting the tool posture, you can also avoid various interferences by adjusting the installation position of the workpiece. Moreover, the biggest advantage of this avoidance method is that it can not change the existing CAM software

based on the tool path output by the general cam software, this paper puts forward the composition of the checkpoint file to shorten the inspection interference time, and avoids all kinds of possible interference by adjusting the installation position of the workpiece, and puts forward the calculation method to check whether the workpiece can be processed and the best installation position of the workpiece when it can be processed

1interference inspection and interference avoidance

the installation diagram of parallel machine tool HexaM, peripheral instruments (tool magazine) and workpiece used in this paper is shown in Figure 1. Therefore, for this machine tool, the interference that may occur in the process of processing and installing the workpiece includes its own interference, the interference between the machine tool and the workpiece, the interference between the machine tool and the surrounding instruments, and the interference between the workpiece and the surrounding instruments

Figure 1 HexaM diagram of parallel machine tool

if there are no peripheral instruments on the workbench of the parallel machine tool or the peripheral instruments are placed far away, which will not affect the installation position of the workpiece or the interference between the machine tool and the peripheral instruments will not occur in the processing process, the interference inspection of the machine tool and the workpiece and the peripheral instruments can be omitted

1) self interference

a. composition of check points in the processing process, all tool positions can be checked whether self interference will occur by using the method described in the literature. If interference occurs, adjust the installation position of the workpiece to avoid interference. After the installation position of the workpiece is changed, the tool position in the processing process will naturally change. Therefore, it is necessary to check again until there is no self-interference. Since all tool positions must be checked at each installation position, the calculation time must be very long. In order to shorten the calculation time, this paper does not check all tool positions, but only extracts a small number of tool positions (hereinafter referred to as checkpoints) for interference inspection. Therefore, it is necessary to extract the checkpoint: if there is no self-interference at the checkpoint, there will be no self-interference at other tool positions. The advantage of setting checkpoints is that the number of checks can be greatly reduced

the farther the tool is from the z-axis, the higher the position and the greater the inclination, the more likely it is to interfere with itself. Therefore, for the workpiece to be processed, the outermost point, the point with higher tool position and the point with larger tool inclination angle are extracted from the tool path as the check points to judge whether their own interference occurs

b. method of checking interference when the tool position and posture are in the straight space circle and the rotary space circle respectively, there must be no self-interference. Therefore, the inspection method of self interference is: ① read all checkpoints and calculate the straight space circle and rotating space circle corresponding to each checkpoint. ② When the tool position and posture are in the corresponding straight space circle and rotary space circle respectively, it is determined that there will be no self-interference at this checkpoint; When there is no interference at all checkpoints, it is determined that there will be no self-interference at the installation position. ③ If the position or posture of a checkpoint exceeds the straight space circle or rotary space circle, the method of checking interference described in the literature is used for re inspection. Since the calculation amount of spatial circle is much smaller than that of general inspection interference, such inspection method can shorten the calculation time of each inspection,

C. The method of avoiding interference is the least prone to self-interference when the tool position is on the Z-axis in the same horizontal plane. Therefore, if self interference occurs at a checkpoint, the line between the checkpoint and the Z axis is taken as the avoidance direction. Then, move the installation position of the workpiece 10 mm in the avoidance direction. If the included angle of the avoidance direction for two consecutive times is greater than 150 °, it can be considered that the avoidance direction has been overturned, and there is no installation position in the horizontal plane, so the installation position of the workpiece can only be raised upward. In this paper, it is increased by 10mm each time. If the self interference is the interference between the main motor and the lower end of the rotary lead screw, it means that the workpiece is too high or the installation position of the workpiece is too high, and this machine tool cannot be used for processing

2) interference between machine tool and workpiece

a. the composition of inspection points is shown in Figure 2. The components that may interfere between machine tool and workpiece include tool, collet, spindle head, spindle collet, collet hinge and connecting rod. The workpiece can be considered to be composed of machined surface and non machined surface. Among them, the machining surface is composed of machining points, and the non machining surface that may interfere with the machine tool is composed of non machining points. Here, the machining points and non machining points are collectively referred to as query points

Fig. 2 inspection method of interference between parallel machine tool and workpiece

the interference between query point and tool, collet, spindle head and spindle collet is called type 1 interference. This kind of interference is only related to the posture of the tool and has nothing to do with the installation position of the workpiece. Therefore, if such interference is detected, it can only be avoided by adjusting the tool posture, which requires modifying the cam software, which is beyond the scope of this article

for any machining point P, calculate the included angle Q between the line PQ between P and all query points Q and the tool axis, the length d of the line segment, and the length h of the projection of the line segment PQ in the direction of the tool axis. Therefore, the calculation method of the installation position of the workpiece that meets the condition Q2

1) the best installation position of the workpiece

this paper takes "the lowest height of the cushion block; the shortest distance from the center of the workbench" as the standard to measure the best installation position of the workpiece. Specifically, first of all, when the bottom surface of the workpiece is placed on the workbench (at this time, the height of the cushion block is 0), and the projection center of the tool path on the XY plane coincides with the center of the workbench (the distance from the center of the workbench is 0), it is the best installation position of the workpiece. If there is interference in this position and it cannot be installed, in order to ensure the minimum height of the cushion block, move the workpiece from inside to outside at the same level to find the installation position where there will be no interference. If the installation position without interference cannot be found in the same horizontal plane, move the workpiece upward gradually (increase the height of the cushion block), and then continue to search in the same horizontal plane until the installation position is found or the conclusion that it cannot be processed on this machine tool is reached

2) calculation method of the best installation position

based on the concept of the best installation position of the workpiece, this paper proposes the calculation steps of the installation position of the workpiece during machining as follows:

a. read the basic data. It includes: the radius and extension length of tools and collets; Query point; Tool path; The position of peripheral instruments and the projection on the XY plane

b. judge whether it can be processed on this machine tool. According to the inquiry point and tool path of the workpiece, the overall dimension of the workpiece in XYZ direction and the moving distance of the tool are calculated. When the overall dimension of the workpiece or the moving distance of the tool is too large, give a warning that the workpiece cannot be processed on the machine tool, and then end the operation of the system

c. generate checkpoint files to check various interferences. Among them: check point documents for self interference, class 1 and class 2 interference between machine tool and workpiece, and interference between machine tool and surrounding instruments

d. check whether there is class 1 interference between the machine tool and the workpiece. If it exists, because the interference cannot be avoided by moving the installation position of the workpiece, the system operation is ended after giving the warning of "there is unavoidable type 1 Interference"

e. set the initial position of tool path center C (x, y, z). In order to meet the definition of the best installation position of the workpiece, the initial setting of C (x, y, z) is: x=0; y=0； z=s。 Where, s is the distance from the center of the tool path to the bottom of the workpiece

f. calculate the position of tool path center C. According to the inspection method and avoidance method of interference, keep moving the installation position of the tool path center until the installation position without interference is found or the warning that the workpiece cannot be processed on this machine tool is given

g. output the installation position of the workpiece. If the position of the tool path center without interference is obtained in (6), the installation position of the workpiece is calculated and output according to this value and the structural size of the workpiece

3 analytical and experimental results

for the overall dimension shown in Figure 4, it is 350mm × 350mm × For 150mm workpiece, when vertical machining is adopted, the general cam software Cayman is used to generate the tool position and tool pose during machining (the same as the normal of the machining point). In the figure, the number of position points in the tool path is 123201 points. The cutter is a ball end milling cutter with a diameter of 10mm and a slave collet