Common Problems And Improvement Methods In CNC Machining

Starting from the actual production, this article summarizes the common problems and improvement methods of CNC machining process, as well as how to choose the three important factors of speed, feed rate and cutting depth in different application areas, for your reference.

The Workpiece Is Overcut

Reasons:

1. Bounce the knife, the tool strength is not too long or too small, causing the tool to bounce.
2. Improper operation by the operator.
3. Uneven cutting allowance (for example: 0.5 on the side of the curved surface and 0.15 on the bottom)
4. Improper cutting parameters (such as: tolerance is too large, SF setting is too fast, etc.)

Improvement

1. The principle of knife use: can be big but not small, can be short but not long.
2. Add the corner cleaning program, and keep the margin as even as possible (the margin on the side and bottom is the same).
3. Reasonably adjust cutting parameters, and round corners with large margins.
4. Using the SF function of the machine tool, the operator fine-tunes the speed to achieve the best cutting effect.

Scoring Problem

The Reasons:

1. Inaccurate during manual operation by the operator.
2. There are burrs around the mold.
3. The center rod has magnetism.
4. The four sides of the mold are not vertical.

Improvement

1. Manual operation should be carefully checked repeatedly, and the points should be at the same height as possible.
2. Deburr the periphery of the mold with oilstone or a file, wipe it clean with a rag, and finally confirm it by hand.
3. Demagnetize the centering rod before centering the mold (ceramic centering rod or others can be used).
4. Calibrate to check whether the four sides of the mold are vertical (large vertical error requires reviewing the plan with fitter).

Tool setting problem

The Reasons:

1. Inaccurate during manual operation by the operator.
2. The tool clamping is wrong.
3. The blade on the flying knife is wrong (the flying knife itself has a certain error).
4. There is an error between the R knife, the flat-bottom knife and the flying knife.

Improvement:

1. Manual operation should be carefully checked repeatedly, and the tool setting should be at the same point as much as possible.
2. When the tool is clamped, blow it with an air gun or wipe it with a rag.
3. One blade can be used when the upper blade of the flying knife needs to be measured on the shank and the smooth bottom surface.
4. A separate tool setting program can avoid the error between R knife, flat knife and flying knife.

Collision-Programming

The Reasons:

1. The safety height is insufficient or not set (the tool or the chuck hits the workpiece during rapid traverse G00).
2. The tool on the program sheet and the actual program tool are written incorrectly.
3. The tool length (blade length) and the actual machining depth on the program sheet are incorrectly written.
4. The depth Z-axis access and actual Z-axis access on the program sheet are incorrectly written.
5. Wrong coordinate setting during programming.

Improvement:

1. Accurately measure the height of the workpiece and ensure that the safety height is above the workpiece.
2. The tool on the program list must be consistent with the actual program tool (try to use the automatic program list or the picture to display the program list).
3. Measure the actual machining depth on the workpiece, and write clearly the length and blade length of the tool on the program sheet (generally, the length of the tool holder is 2-3MM higher than the workpiece, and the blade length is 0.5-1.0MM).
4. Take the actual number of Z axis on the workpiece and write it clearly on the program sheet. (This operation is generally written manually and needs to be checked repeatedly)

Collision-operator

The Reasons:

1. The depth Z axis tool setting error.
2. The number of hits and operations in the division are wrong (such as: there is no infeed radius in the unilateral reading, etc.).
3. Use the wrong tool (for example: D4 tool is processed with D10 tool).
4. The program goes wrong (for example: A7.NC goes A9.NC).
5. The handwheel is turned in the wrong direction during manual operation.
6. Press the wrong direction during manual rapid feed (for example: -X press +X).

Improvement:

1. For depth Z axis tool setting, you must pay attention to the position of the tool. (Bottom surface, top surface, analysis surface, etc.).
2. Check repeatedly after the count and the operation are completed.
3. When clamping the tool, check it repeatedly with the program sheet and the program before loading it.
4. The program should go one by one in order.
5. When using manual operation, the operator himself should strengthen the operation proficiency of the machine tool.
6. In manual rapid traverse, you can first raise the Z axis to the workpiece and move.

Curved Surface Accuracy

The Reasons:

1. Unreasonable cutting parameters, rough surface of the workpiece.
2. The cutting edge of the tool is not sharp.
3. The tool clamp is too long, and the blade is too long to avoid the air.
4. Chip removal, air blowing and oil flushing are not good.
5. Program the way of tool movement (you can consider down milling as much as possible).
6. The workpiece has burrs.

Improvement

1. The cutting parameters, tolerances, margins, and speed feed settings should be reasonable.
2. The tool requires the operator to inspect and replace it irregularly.
3. When clamping the tool, the operator is required to clamp as short as possible, and the cutting edge should not be too long.
4. For flat knives, R knives, and round nose knives, the speed feed setting should be reasonable.
5. The workpiece has burrs: it is directly related to our machine tools, cutting tools and cutting methods. So we have to understand the performance of the machine tool and make up the edges with burrs.

Crumble

Reasons And Improvements:

1. Feed too fast

–Slow down to a suitable feed rate

2. Feed too fast at the beginning of cutting

–Slow down the feed rate at the beginning of cutting

3. Loose clamping (tool)

–Clamping

4. Loose clamping (workpiece)

–Clamping

5. Insufficient rigidity (tool)

–Use the shortest tool allowed, clamp the shank deeper, and try down milling

6. The cutting edge of the tool is too sharp

–Change the fragile cutting edge angle, once the edge

7. Insufficient rigidity of machine tool and tool holder

–Use a rigid machine tool and tool holder

Wear

Reasons And Improvements:

1. The machine speed is 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 rate, chip size or use cooling oil or air gun to clean the chips

4. Improper feed rate (too low)

–Increase the feed rate, try down milling

5. Unsuitable cutting angle

–Change to proper cutting angle

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

–Change to a larger rear angle

Damage

Reasons And Improvements:

1. Feed too fast

–Slow down the feed rate

2. Too much cutting

–Use a smaller cutting amount per edge

3. The blade length and overall length are too large

–Clip the shank deeper, use a short tool, try down milling

4. Too much wear

–Re-grind at the beginning

Chatter Marks

Reasons And Improvements:

1. Feed and cutting speed are too fast

-Correct the 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 back angle is too large

–Change to a smaller relief angle and process the cutting edge (grind the edge once with oil stone)

4. Loose clamping

–Clamp the workpiece

Consider Speed And Feed

The interrelationship of the three factors of speed, feed and depth of cut is the most important factor in determining the cutting effect. Improper feed and speed often result in reduced production, poor workpiece quality, and large tool damage.

Use low speed range for:

High hardness materials

Wayward material

Materials that are difficult to cut

Heavy cutting

Minimal tool wear

Longest tool life

Use high speed range for

Soft material

Better surface quality

Smaller tool outer diameter

Light cutting

Brittle parts

Manual operation

Maximum processing efficiency

Non-metallic materials

Use high feed for

Heavy and rough cutting

Steel structure

Easy to process materials

Roughing tool

Plane cutting

Low tensile strength material

Coarse cutter

Use low feed for

Light machining, fine cutting

Brittle structure

Difficult to process materials

Small tool

Deep vertical groove processing

High tensile strength material

Finishing tool