Common problems and solutions in CNC machining process

Based on the actual production, this article summarizes common problems and improvement methods in the CNC machining process for your reference.

Ⅰ. Over-cutting of the workpiece. Over-cutting means that the tool cuts parts that cannot be cut, causing damage to the workpiece.

reason:

1. Problems with the machine tool itself

Problems such as the stiffness, stability and accuracy of machine tools will directly affect the machining accuracy of machine tools and exceed the allowable error of parts.

2. Tool wear

If the tool surface is not smooth or sharp, it will be difficult for the tool to cut into the workpiece surface, resulting in more serious wear and exacerbating the over-cutting phenomenon.

3. Uneven cutting allowance

For example: leave 0.5 on the side of the curved surface and 0.15 on the bottom.

4. Improper cutting speed setting

If the cutting speed is set too high or too low, or the cutting speed is not adjusted reasonably according to the different characteristics of the workpiece material, it may cause the machine tool to overcut. (For example: tolerance is too large, SF setting is too fast, etc.)

improve:

1. Reasonable selection of machine tools

The appropriate machine tool should be selected based on the materials, process requirements, accuracy requirements and other characteristics of the parts to be processed. If the machine tool is too old or poor, it will easily cause overcutting.

2. Principle of using tools: The wear of tools should be checked regularly, and tools with greater wear should be replaced in time to avoid processing tools that have exceeded their service life. The tool path and the programmed path must be set using the tool radius compensation instruction, and the positive or negative of the scalar product of the programmed vector and its corresponding correction vector is judged;

3. Add the corner cleaning procedure, and try to keep the margin as even as possible, so that the margin on the side and the bottom are consistent;

4. Reasonably adjust the cutting parameters, round the corners with large margins, and use the SF function of the machine tool to allow the operator to fine-tune the speed to achieve the best cutting effect of the machine tool.

Ⅱ. The problem of scoring

reason:

1. Equipment aging: The machining center has been used for too long and precision components are severely worn, which may lead to a decrease in machine tool accuracy and coordinate deviation. Failure of electrical components such as the power supply, sensors, and servo systems of the machining center will affect the coordinate accuracy.

2. The surface of the workpiece is uneven, causing the fixed position of the fixture to be unstable or deformed.

3. The centering rod is magnetic.

4. The four sides of the mold are not vertical.

improve:

1. Regular maintenance, replacement of equipment and parts, manual operation should be carried out repeatedly and carefully checked, and the points should be at the same point and the same height as much as possible.

2. Check the surface finish of the workpiece before processing to ensure that the contact surface between the fixture and the workpiece is smooth. Use a whetstone or file to remove burrs around the mold, wipe it clean with a rag, and finally confirm with your hands.

3. Demagnetize the centering rod before centering the mold (ceramic centering rod or other can be used).

4. Calibrate the meter to check whether the four sides of the mold are vertical (if the verticality error is large, please review the plan with the fitter).

Ⅲ. Tool setting problem: establish the workpiece coordinate system for tool setting, and inform the CNC system of the specific location of the workpiece coordinate system. On a horizontal CNC lathe, the positive direction of the Z-axis of the workpiece coordinate system is to the right, and the positive direction of the X-axis is upward or downward (the rear tool holder is upward, and the front tool holder is downward)

Reasons for tool setting problems:

1. Operator’s manual operation is inaccurate.

2. The tool is incorrectly clamped.

3. The blade on the flying knife is wrong (the flying knife itself has certain errors).

4. There is an error between the R knife, the flat knife and the flying knife.

improve:

1. Manual operations should be carefully checked repeatedly, and the tool should be set at the same point as much as possible.

2. When installing the tool, blow it clean with an air gun or wipe it clean with a rag.

3. When the upper blade of the flying knife needs to be measured on the tool bar and the bottom surface is smooth, one blade can be used.

4. A separate tool setting program can avoid errors between the R knife, the flat knife and the flying knife.

Ⅳ. Collision-Programming

reason:

1. The safety height is not enough or not set (the knife or chuck hits the workpiece during rapid feed G00).

2. The tool on the program list and the actual program tool are written incorrectly.

3. The tool length (blade length) and actual processing depth on the program sheet are written incorrectly.

4. The depth Z-axis fetch and the actual Z-axis fetch are written incorrectly on the program sheet.

5. The coordinates are set incorrectly during programming.

improve:

1. Accurately measure the height of the workpiece and ensure that the safe height is above the workpiece.

2. The tools on the program list must be consistent with the actual program tools (try to use automatic program list or use pictures to generate program list).

3. Measure the actual depth of processing on the workpiece, and clearly write the length and blade length of the tool on the program sheet (generally the tool clamp length is 2-3MM higher than the workpiece, and the blade length is 0.5-1.0MM).

4. Take the actual Z-axis number on the workpiece and write it clearly on the program sheet. (This operation is generally written manually and needs to be checked repeatedly).

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Ⅴ. Collision-Operator

reason:

1. Depth Z axis tool setting error.

2. The number of points is hit and the operation is wrong (for example: unilateral fetching without feed radius, etc.).

3. Use the wrong tool (for example: use D4 tool with D10 tool for processing).

4. The program went wrong (for example: A7.NC went to A9.NC).

5. The handwheel rotates in the wrong direction during manual operation.

6. Press the wrong direction during manual rapid traverse (for example: -X press +X).

improve:

1. When performing deep Z-axis tool setting, you must pay attention to where the tool is being set. (Bottom surface, top surface, analysis surface, etc.).

2. Check the number of hits and operations repeatedly after completion.

3. When installing the tool, check it repeatedly with the program sheet and program before installing it.

4. The procedures should be followed one by one in order.

5. When using manual operation, the operator himself must improve his proficiency in operating the machine tool.

6. When manually moving quickly, you can first raise the Z-axis to the workpiece before moving.

Ⅵ.Surface accuracy

reason:

1. The cutting parameters are unreasonable and the workpiece surface is rough.

2. The cutting edge of the tool is not sharp.

3. The tool clamping is too long and the blade clearance is too long.

4. Chip removal, air blowing and oil flushing are not good.

5. Program the cutting method (you can consider down milling as much as possible).

6. The workpiece has burrs.

improve:

1. Cutting parameters, tolerances, allowances, speed and feed settings must be reasonable.

2. The tool requires the operator to check and replace it from time to time.

3. When clamping the tool, the operator is required to keep the clamp as short as possible, and the blade should not be too long to avoid the air.

4. For downcutting with flat knives, R knives, and round nose knives, the speed and feed settings must be reasonable.

5. The workpiece has burrs: it is directly related to our machine tools, cutting tools, and tool feeding methods. Therefore, we need to understand the performance of the machine tool and make up for the edges with burrs.

Ⅶ.Blade chipping

Reasons and improvements:

1. Feed too fast

Slow down to a suitable feed speed

2. Feed too fast at the beginning of cutting

Slow down the feed speed when cutting starts

3. Loose clamping (tool)

Clamping

4. Loose clamping (workpiece)

Clamping

5. Insufficient rigidity (tool)

Use the shortest knife allowed, clamp the handle deeper, and try down milling.

6. The cutting edge of the tool is too sharp

Change the fragile cutting edge angle, primary edge

7. Insufficient rigidity of the machine tool and tool holder

Use rigid machine tools and tool holders

Ⅷ.Wear

Reasons and improvements:

1. The machine rotates too fast

Slow down and add enough coolant

2. Hardened material

Use advanced cutting tools and tool materials to increase surface treatment methods

3. Chip adhesion

Change the feed speed, chip size or use cooling oil or air gun to clean the chips

4. Improper feed speed (too low)

Increase the feed speed and try down milling

5. Improper cutting angle

Change to appropriate cutting angle

6. The primary clearance angle of the tool is too small

Change to a larger rear angle

Ⅸ.Destruction

Reasons and improvements:

1. Feed too fast

Slow down the feed speed

2. The cutting amount is too large

Use smaller cutting amount per edge

3. Blade length and overall length are too large

Clamp the handle deeper, use a shorter knife, and try down milling

4. Excessive wear and tear

Regrind in the initial stage

Ⅹ.Vibration pattern

Reasons and improvements:

1. Feed and cutting speed are too fast

Correct feed and cutting speed

2. Insufficient rigidity (machine tool and tool holder)

Use better machine tools and tool holders or change cutting conditions

3. The rear corner is too large

Change to a smaller relief angle and process the edge (use a whetstone to sharpen the edge once)

4. Clamping is loose

Clamp workpiece