Causes and Prevention of Collisions in CNC Lathes

Key words: CNC lathes way to prevent collisions reasons

CNC lathes are expensive. In the practice teaching, if the machine tool collides due to student programming, operation, etc., it will damage the workpiece and damage the tool. If it is heavy, the machine tool will be partially damaged, the precision will be reduced, the machine tool will be scrapped, and even endangered. The safety of the operator's life is very serious. After many years of careful study and practice teaching practice, I have summarized the following:

One, programming zero error

Those who are engaged in CNC lathe programming know that programming the zero point is the decision of the workpiece size in numerical control programming.

The basis is that if the programming zero error occurs, then the possibility of a collision on the CNC lathe is very high. In the workpiece shown in Figure 1 below:

The procedure is:

N0000 G54 G92 Z70.0;

If G54 is mistakenly written as G57 or G58, the two offset sizes (specified by G57 or G58, G92) from the same group and the offset size (specified by G92) finally called are valid according to the CNC lathe programming. Its program becomes:

N0000 G57 G92 Z70.0;

The above result will be:

That is, after the zero offset, only 3.8 mm of the pawl is left. In the programming, if the axial dimension is less than 3.8 mm, there will be a collision between the turning tool and the claw.

For this reason, it is necessary to understand the detailed meaning of the zero-offset command and correctly use the zero-offset command to prevent the CNC lathe from colliding.

Second, the program has data beyond the jaw size

For example, Figure 1 above, the programmed zero point to the jaw face size is 50mm, if the axial dimension is from the right

The end face distance exceeds 50mm. If it is 51, the turning tool collides with the claw 1mm. Therefore, after programming is completed, do not rush to process. Carefully check that the value of all axial dimensions Z exceeds 50 mm.

Third, when the machine tool is idle, there is no open jaws to keep the tool holder away from the chuck and tailstock

When the machine tool is running in an automatic cycle, if the jaws do not open, the knife holder will be far away from the chuck and tailstock. If there is any motion interference, the claw or tailstock may collide. If you open the jaws, check the coordinate value of the program in time, or move the tool holder between the tailstock and the chuck on the CNC lathe with the tool path simulation function, and lock it in the premise of ensuring the safe position of the toolholder. If the machine tool is used for automatic air running, it can also roughly determine whether the turning tool collides with the machine tool and the workpiece. After confirming that there is no problem, the cutting machining can be performed.

Fourth, improper programming, collision

Due to the special shape of the workpiece, the advance and retraction of the tool must be single-coordinate when programming.

In the workpiece shown in Figure 3:

When boring a hole, if you use the G00 command to move the tool directly to the target point, the programming is as follows:

N0050 G00 X19.0 Z-25.0;

In this way, it is bound to collide with the workpiece as shown by the solid line;

If the single-coordinate step moves to the target point, this is programmed:

N0050 G00 Z2.0;

N0055 X19.0;

N0060 Z-25.0;

As shown by the dashed lines, collisions are effectively avoided.

For example, in the workpiece shown in Fig. 4, when the grooving is completed, it needs to be quickly returned to

Ф 100mm and 100mm from the right end of the workpiece, as programmed:

N0080 G00 X100.0 Z100.0;

The grooving knife will collide with the step of the part due to walking slashes. Even in most CNC systems, after executing the G00 instruction, the actual trajectory of the tool is the broken line, and because G00 is the fast point positioning, the fastest tool is the machine tool. The speed of the tool moves to the target point. The actual trajectory of the turning tool is the dotted line shown in Fig. 5, that is, the broken line, which also collides with the workpiece.

The correct programming method is as follows:

N0080 G00 X100.0;

N0085 Z100.0;

The trajectory of the tool is shown in the figure below:

That is to say, the grooving tool can be withdrawn radially from the workpiece first, and then moved axially to the end surface of 100mm, which can effectively avoid collision.

Fifth, the programming data is wrong or does not meet the requirements, and the resulting collision

Some CNC lathes are programmed with decimal points or have been programmed for decimal point, as shown in Figure 6 below

In the case of:

The procedure is:

N0100 G00 X62.0 Z-35.0;

Even if the tool goes off line, it will not collide with the workpiece. However, due to carelessness or other reasons, the program was mistakenly written as follows:

N0100 G00 X62 Z-35.0;

or

N0100 G00 X62 Z-35;

The numerical control system will divide the 62 and -35 after the coordinate word by 1000 to become:

N 0100 G00 X0.062 Z-35.0;

or

N0100 G00 X0.062 Z-0.035;

Obviously, the grooving knife should collide with the outer circle or the right end face of the workpiece.

In order to avoid this kind of collision, the best way is to use multiple simulations, multiple inspections, and be careful, careful and timely, to find out clearly wrong or inconsistent data, and correct it in time.

Sixth, unfamiliar with the use of programming instructions, resulting in the collision of the turning tool and the workpiece

In the FANUC 0I system, multiple composite fixed cycles of G71 (G73) and G70 rough and finish car part profiles, especially when the G73 command is combined with the G70, is shown in Fig. 7:

At the end of the cycle, the starting point of the return point is not properly selected (see point A, low), causing a collision with the workpiece during tool return. To prevent this from happening, simply raise the starting point of the return point to B point.

Seven, the specified tool change point is not suitable and a collision occurs

The tool change point is too close to the workpiece or too close to the tailstock or too close to other parts of the machine tool.

In the indexing, it is easy to hit the workpiece or the machine. To avoid this happening, you must follow the selection principle of the tool change point, that is, the tool change point must be between the workpiece and the tailstock, as close as possible to the workpiece, and the tool holder must not touch the workpiece, tailstock, and machine tool during indexing. Any part of it.

Eight, return to reference point selection error, causing collision

Some students, when returning to the reference point, do not look at the operation panel when selecting a working method.

The switch is selected in the manual (JOG) mode, not the reference (REF) mode, and does not look at the monitor screen. It does not result in an overtravel alarm. If the reference is returned, the sequence of the axes is incorrect, which can easily cause collisions. Tailstock phenomenon occurs. In order to avoid this happening, as long as the students are serious, careful, not sloppy, the machine can be operated according to regulations.

Nine, did not return to the reference point caused collision

The CNC lathe encoder has two kinds of relative encoders and absolute encoders.

The CNC lathe will lose the memory of the reference point when the machine tool is powered on or powered off or reset after troubleshooting. At this time, if the reference point is not operated, tool setting or automatic processing will occur, which will result in data. Read and write errors, which can lead to a collision between the turning tool and the workpiece or the machine tool.

In order to avoid collisions in this situation, simply return to the reference point as required and perform related operations.

In short, there are many reasons for collisions in CNC lathes.

It will not be repeated one by one. In order to effectively avoid collisions, in the programming and operation, students are required to be familiar with the NC machining process, understand the instructions, familiar with the machine operation rules, and strictly program, carefully check, and safe operation of the machine according to the provisions, must not act rashly.

Reference books:

1. "CNC Lathe Training" Yuan Feng Machinery Industry Press

2, "CNC machine programming and operation" (CNC lathes volume)

The Office of the Ministry of Labor and Social Security Textbook Office Organized and Prepared China Labor Social Security Publishing House

3. "Introduction to CNC Lathe Operation" Cheng Meiling Anhui Science and Technology Press

4, "CNC application of key technologies" Hao Rui Electronics Industry Press

5, "CNC machine tooling direct programming technology" Sun Demao Machinery Industry Press

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