There are single-line and multi-line threads. The thread formed along one spiral line is called a single-line thread, and the thread formed along two or more spiral lines is called a double-line or multi-line thread. When processing multi-line threads on ordinary lathes, axial line division method and circumferential line division method are often used.
The axial line division method means that after the first spiral line is processed, the screw nut remains connected, and the tool holder is moved forward or backward longitudinally by one pitch to process the second spiral line, the third spiral line... This method requires precise control of the distance the turning tool moves along the axial direction to achieve the purpose of line division. The specific control methods are mainly: (1) small slide scale line division method. Although this method is relatively simple, the line division accuracy is not high. Because of the influence of the small slide screw gap and the subjective estimation error when the pitch is not an integer multiple of the corresponding movement of the scale, it is inevitable that line division errors will occur. (2) Dial indicator and gauge block line division method. Although the line division accuracy is higher, the preparation work is cumbersome, the processing efficiency is low, and it is also easy to produce line division errors.
The circular line division method is based on the characteristic that the spiral lines are evenly distributed on the circumference. That is, after turning a spiral line, the transmission chain between the workpiece and the lead screw is disconnected, the spindle is rotated by an angle α (α=3600/number of threads), and then the transmission chain between the workpiece and the lead screw is connected to turn the next spiral line. The specific processing methods are mainly: (1) using the three-jaw or four-jaw chuck line division method. This method is simple and fast, but the line division accuracy is low and the line division range is narrow. (2) Using the exchange gear line division method. This method has a high line division accuracy, but the operation is troublesome, and it can only be used when the number of teeth of the lathe exchange gear is an integer multiple of the number of thread lines. (3) Using the multi-hole dial line division method. Although the line division accuracy is slightly higher, it requires the addition of a multi-hole dial, and there is a lot of preparation work, low processing efficiency, and high processing cost [1].
From the above, it can be seen that the processing of multi-thread on ordinary lathe is relatively cumbersome, and the spindle speed is limited by the thread lead, so the cutting speed cannot be improved and the processing efficiency is low; in addition, the thread is prone to errors in the process of thread division, and the processing accuracy is low, which will affect the working performance of the thread and reduce its service life.
With the continuous advancement of science and technology, in today's rapid development of information technology in the manufacturing industry, the application of CNC machine tools to process multi-thread can solve many problems caused by ordinary machine tool processing. The processing principle of CNC lathe multi-thread is basically the same as that of ordinary lathe. The processing methods usually include changing the starting angle of thread cutting and changing the starting point of thread axial cutting. In the FANUC system, there are three multi-thread programming function instructions: G32, G92, and G76. Among them, G32 is a single-stroke thread cutting instruction, which has a large programming task and a complex program; instruction G92 can realize a simple thread cutting cycle, which greatly simplifies the program segment, but requires the workpiece blank to be rough-machined in advance; and instruction G76 is a thread cutting compound cycle instruction, which overcomes the shortcomings of instruction G92 and can complete the workpiece from the blank to the finished thread in one go, and the program is simple, which can save programming and processing time.
2 Processing multi-line threads by changing the initial angle of thread cutting 2.1 Method Principle Changing the initial angle of thread cutting to process multi-line threads is to divide along the circumferential direction according to the number of threads. After each thread is processed, the spindle rotates a certain angle, and the axial position of the starting point remains unchanged, and then the next thread is processed. 2.2 Application of G32 instruction to process multi-thread 2.2.1 Instruction format G32X(U)__Z(W)__F__Q__;
Where:
X, Z--the coordinates of the end point of the thread when programming in absolute dimension; U, W--the coordinates of the end point of the thread when programming in incremental dimension; F--thread lead (if it is a single-thread thread, it is the thread pitch); Q--thread starting angle, the value is a non-modal value without a decimal point, that is, the increment is 0.0010; if the starting angle is 1800, it is expressed as Q180000 (single-thread thread can be specified without specifying, in which case the value is zero); the range of the starting angle Q is between 0 and 360000. If a value greater than 360000 is specified, it is calculated as 360000 (3600). 2.2.2 Application example Example 1, use the G32 instruction to compile a thread processing program on the part shown in Figure 1.
Process analysis: The part has a double thread M24X3 (P1.5) -6g, with a pitch of 1.5mm and a lead of 3mm. The programming origin is set at the center of the right end face of the workpiece. Determination of cutting parameters: Check the turning manual and determine the cutting depth (radius value) to be 0.974mm, the number of feeds is 4 times, and the cutting depth (diameter value) of each back of the knife is 0.8mm, 0.6mm, 0.4mm, and 0.16mm respectively. S1 (speed-up feed section length) = 4mm, S2 (speed-down retraction section length) = 2mm. Assuming that the starting angle of the first line of thread is 00, the starting angle of the second line of thread is 3600/2=1800. The reference program is as follows:...; G00X30.0Z4.0; X23.2; G32Z-32.0F3.0Q0; /The first cut of the first line of thread G00X30.0Z4.0; X22.6; G32Z-32.0F3.0Q0; /The second cut of the first line of thread...; G00 X30.0Z4.0; X22.04; G32Z-32.0F3.0Q0;/The 4th cutter of the 1st thread G00X30.0Z4.0;X23.2;G32Z-32.0F3.0Q180000;/The 1st cutter of the 2nd thread G00X30.0Z4.0;X22.6;G32Z-32.0F3.0Q180000;/The 2nd cutter of the 2nd thread……;G00X30.0Z4.0;X22.04;G32Z-32.0F3.0Q180000;/The 4th cutter of the 2nd thread G00X30.0;X100.0Z100.0;M05;M30;2.3 Apply G92 command to process multiple threads2.3.1 Command format G92X(U)__Z(W)__F__Q__; The meaning of each parameter in the formula is the same as 1.2.12.3.2 Application Example 2: Use G92 command to program the thread machining on the part shown in Figure 2[2].
Process analysis:
The part has multiple threads M30X3(P1.5)-6G, with a pitch of 1.5mm and a lead of 3mm. The programming origin is set at the center of the left end face of the workpiece. Cutting parameter determination (omitted) reference procedure is as follows: ...; G92X29.2Z18.5F3.0; /Double-line thread cutting cycle 1, back cutting amount 0.8mmX29.2Q180000; X28.6F3.0; /Double-line thread cutting cycle 2, back cutting amount 0.6mmX28.6Q180000; X28.2F3.0; /Double-line thread cutting cycle 3, back cutting amount 0.4mmX28.2Q180000; X28.04F3.0; /Double-line thread cutting cycle 4, back cutting amount 0.16mmX28.04Q180000; M05; M30;
