Common problems and improvement methods in CNC machining process

Common problems and improvement methods in CNC Machining process, as well as how to choose the three important factors of speed, feed rate, and cutting depth in different application categories.

1. Workpiece over cutting
Reason:
1. When the tool strength is not long enough or too small, it may cause the tool to bounce.
2. Improper operator operation.
3. Uneven cutting allowance (such as leaving 0.5 on the side of the curved surface and 0.15 on the bottom)
4. Improper cutting parameters (such as too large tolerance, SF setting too fast, etc.)

Improvement:
1. Principle of using a knife: it can be large but not small, and it can be short but not long.
2. Add a corner cleaning program, and try to leave the margin as evenly as possible (the margin on the side and bottom should be consistent).
3. Reasonably adjust cutting parameters and round corners with large margin.
4. By utilizing the SF function of the machine tool, the operator can adjust the speed to achieve the best cutting effect.

2.Midpoint problem
Reason:
1. Inaccurate manual operation by the operator.
2. There are burrs around the mold.
3. The dividing rod has magnetism.
4. The four sides of the mold are not perpendicular.

Improvement:
1. Manual operation should be carefully checked repeatedly, and the center should be at the same point and height as much as possible.
2. Use an oilstone or file to remove burrs around the mold, wipe it clean with a rag, and finally confirm by hand.
3.Before dividing the mold, demagnetize the dividing rod (using ceramic dividing rods or other materials).
4. Check if the four sides of the mold are vertical with the calibration table (if there is a large verticality error, it is necessary to review the plan with the fitter)

3. Knife alignment issue
Reason:
1. Inaccurate manual operation by the operator.
2. Incorrect tool clamping.
3. The blade on the flying knife is incorrect (the flying knife itself has a certain degree of error).
4. There is an error between the R knife and the flat-bottomed knife and the flying knife.

Improvement:
1. Manual operation should be carefully checked repeatedly, and the knife should be aligned at the same point as much as possible.
2. When clamping the cutting tools, use an air gun to blow them clean or wipe them clean with a cloth.
3. When measuring the blade on the flying knife, one blade can be used to measure the tool holder and the bottom surface.
4. Create a separate tool alignment program to avoid errors between R flat and flying knives.

4. Collision - Programming
Reason:
1. Insufficient or no safety height (when the tool or chuck collides with the workpiece during rapid feed G00).
2. The tool on the program sheet and the actual program tool are written incorrectly.
3. The tool length (blade length) and actual machining depth on the program sheet are written incorrectly.
4. The depth Z-axis retrieval and actual Z-axis retrieval on the program sheet are written incorrectly.
5. Coordinate setting error during programming.

Improvement:
1. Accurately measuring the height of the workpiece also ensures that the safe height is above the workpiece.
2. The tools on the program sheet should be consistent with the actual program tools (try to use automatic or image generated program sheets).
3. Measure the actual depth of machining 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 should be avoided by 0.5-1.0mm).
4. Take the actual Z-axis data on the workpiece and write it clearly on the program sheet. (This operation is usually manual and needs to be checked repeatedly.).

5. Collision - Operator
Reason:

1. Depth Z-axis tool alignment error.

2. There are errors in the number of hits and operations during the division (such as unilateral data retrieval without feed radius, etc.).

3. Use the wrong knife (such as using D10 knife for machining with D4 knife).

4. Program error (e.g. A7. NC went to A9. NC).

5. The handwheel is turned in the wrong direction during manual operation.

6. When manually rapid feeding, press the wrong direction (such as - X pressing+X).

Improvement:

1. It is important to pay attention to the position of the tool when aligning the depth Z-axis. (Bottom, top, analysis surface, etc.).
2. Repeated checks should be carried out after the completion of the middle collision and operation numbers.
3. When clamping the tool, it is necessary to repeatedly check and compare it with the program sheet and program before installing it.
4. The program should be executed in sequence one by one.
5.When using manual operation, the operator should enhance the proficiency of the machine tool operation.
6.When manually moving quickly, the Z-axis can be raised above the workpiece before moving.

6. Surface accuracy
Reason:
1. The cutting parameters are unreasonable, and the surface of 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 avoidance distance is too long.
4. Chip removal, blowing air, and oil flushing are not good.
5. Programming the cutting method (consider smooth milling as much as possible).
6. The workpiece has burrs.

Improvement:

1. Cutting parameters, tolerances, allowances, 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 it as short as possible and the blade should not be too long in the air.
4. For the downward cutting of flat knives, R knives, and round nose knives, the speed feed setting should be reasonable.
5. Burrs on the workpiece: This is directly related to our machine tool, cutting tool, and cutting method. So we need to understand the performance of the machine tool and make up for burrs on the edges.

7. Broken blade
Reason and improvement:

1. Feed too fast

--Slow down to the appropriate feed rate

2. The feed rate is 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 allowable knife, clamp the handle a little deeper, and also try milling clockwise

6. The cutting edge of the tool is too sharp

--Changing the fragile cutting edge angle, one cutting edge

7. Insufficient rigidity of machine tools and tool holders

--Use rigid machine tools and tool holders

8. Wear and tear

Reason and improvement:

1. The machine speed is too fast

--Slow down and add enough coolant

2. Hardening materials

--Using advanced cutting tools and tool materials to increase surface treatment methods

3. Chip adhesion

--Change feed rate, chip size, or clean chips with cooling oil or air gun

4. Improper feed rate (too low)

--Increase the feed rate and try the forward milling process

5. Inappropriate cutting angle

--Change to an appropriate cutting angle

6. The initial back angle of the tool is too small

--Change to a larger rear corner

9. Destruction

Reason and improvement:

1. Feed too fast

--Slow down the feed rate

2. The cutting amount is too large

--Using a smaller cutting amount per edge

3. The blade length and total length are too large

--Clamp the handle deeper and use a shorter knife to try milling it clockwise

4. Excessive wear and tear

--Grind again in the initial stage

10. Vibration pattern

Reason and improvement:

1. The feed and cutting speeds are too fast

--Correction of 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 back angle and machine the cutting edge (grind the edge once with an oilstone)

4. Loose clamping

--Clamping the workpiece

◆ Consider speed and feed rate

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The interrelationship between the three factors of speed, feed rate, and cutting depth is the most important factor determining the cutting effect. Inappropriate feed rate and speed often lead to reduced production, poor workpiece quality, and significant tool damage.

Use low speed range for:



High hardness materials

Materials with high capriciousness

Difficult to cut materials.

Heavy cutting

Minimum tool wear

Longest tool life

Use high speed range for



Soft materials

Better surface quality

Smaller tool outer diameter

Light cutting

Workpieces with high brittleness

Manual operation

Maximum processing efficiency

Non metallic materials

Use high speed range for



Soft materials

Better surface quality

Smaller tool outer diameter

Light cutting

Workpieces with high brittleness

Manual operation

Maximum processing efficiency

Non metallic materials

Use low feed rate for

Light machining, precision cutting

Brittle structure

Difficult to process materials.

Small cutting tools

Deep groove processing

High tensile strength materials

Precision machining tools