Common Issues in CNC Machining and Improvement Methods

This guide addresses common issues encountered during CNC machining processes, along with improvement methods. It also covers how to select speed, feed rate, and cutting depth for different applications.

01 Overcutting Workpiece

Causes:

1. Tool deflection due to insufficient tool strength or excessively long/small tools.

2. Operator error.

3. Uneven stock allowance (e.g., leaving 0.5mm on side surfaces and 0.15mm on bottom surfaces).

4. Improper cutting parameters (e.g., too large tolerances, SF settings too fast).

Improvements:

1. Use the principle of "larger if possible, shorter if possible" when selecting tools.

2. Add corner cleaning programs and try to maintain uniform stock allowances (consistent allowances on sides and bottoms).

3. Adjust cutting parameters reasonably, and round corners where stock allowances are large.

4. Utilize the machine's SF function, allowing operators to fine-tune speeds for optimal cutting performance.

02 Centering Issues

Causes:

1. Inaccurate manual operation by the operator.

2. Burr around the mold.

3. Magnetic centering pins.

4. Non-perpendicular edges of the mold.

Improvements:

1. Carefully check manual operations repeatedly; center at the same point and height whenever possible.

2. Remove burrs around the mold using oil stones or files, clean with a cloth, and confirm manually.

3. Demagnetize the centering pin before use (use ceramic pins or alternatives).

4. Check the perpendicularity of the mold edges with a dial indicator (address significant errors with fitters).

03 Tool Setting Issues

Causes:

1. Inaccurate manual operation by the operator.

2. Incorrect tool installation.

3. Errors in fly cutter blades.

4. Differences between R tools, flat-bottomed tools, and fly cutters.

Improvements:

1. Carefully check manual operations repeatedly; set tools at the same point.

2. Clean tool holders with an air gun or cloth before installation.

3. Measure fly cutter blades and ensure bottom surfaces are smooth.

4. Develop a separate tool-setting program to avoid discrepancies between R tools, flat-bottomed tools, and fly cutters.

04 Machine Collisions – Programming

Causes:

1. Insufficient safety height or not set (tool or chuck collides with the workpiece during rapid feed G00).

2. Mismatch between tool lists and actual program tools.

3. Incorrect tool length (flute length) or processing depth.

4. Incorrect Z-axis values in the program list vs. actual values.

5. Coordinate setting errors during programming.

Improvements:

1. Accurately measure workpiece height and ensure safety height is above the workpiece.

2. Ensure consistency between tool lists and actual program tools (preferably auto-generated tool lists or image-based lists).

3. Measure actual processing depths and clearly specify tool lengths and flute lengths in the tool list (typically, holder extends 2-3mm above the workpiece, with a clearance of 0.5-1.0mm for the cutting edge).

4. Verify actual Z-axis values on the workpiece and clearly document them in the tool list (manually entered data should be double-checked).

05 Machine Collisions – Operators

Causes:

1. Incorrect Z-axis tool setting.

2. Errors in centering and measurement (e.g., neglecting tool radius).

3. Using the wrong tool (e.g., using a D10 tool instead of a D4 tool).

4. Running the wrong program (e.g., running A9. NC instead of A7.NC).

5. Incorrect direction during manual operation.

6. Pressing the wrong direction during rapid feed (e.g., pressing +X instead of -X).

Improvements:

1. Pay attention to the position when setting the Z-axis tool (bottom, top, analysis surface).

2. Double-check centering and measurements after completion.

3. Verify tool lists and programs before installing tools.

4. Follow program steps sequentially.

5. Enhance operator proficiency in machine operation.

6. Elevate the Z-axis above the workpiece before moving it manually.

06 Surface Precision Issues

Causes:

1. Unreasonable cutting parameters leading to rough surfaces.

2. Dull cutting edges.

3. Excessively long tool setup with excessive overhang.

4. Poor chip evacuation, lack of blowing air, or coolant.

5. Inefficient toolpaths (consider climb milling).

6. Burr formation on the workpiece.

Improvements:

1. Set reasonable cutting parameters, tolerances, stock allowances, and speeds/feeds.

2. Regularly inspect and replace cutting tools.

3. Minimize tool overhang as much as possible.

4. Set appropriate speeds/feeds for flat, R, and ball nose tools.

5. Address burr formation by understanding machine capabilities and adding finishing cuts.

07 Chipped Cutting Edges

Causes:

1. Too fast feed rate.

•Slow down to an appropriate feed rate.

2. Fast initial feed during cutting.

•Slow down the feed rate at the start of cutting.

3. Loose tool clamping.

•Tighten the tool.

4. Loose workpiece clamping.

•Tighten the workpiece.

5. Insufficient rigidity (tool).

•Use the shortest allowable tool with deeper clamping, consider climb milling.

6. Too sharp cutting edges.

•Modify the fragile cutting edge angle.

7. Insufficient rigidity (machine and toolholder).

•Use more rigid machines and toolholders.

08 Tool Wear

Causes:

1. Too high spindle speed.

•Reduce speed and apply sufficient coolant.

2. Hardened materials.

•Use advanced tool materials and coatings.

3. Chip adhesion.

•Adjust feed rate or chip size, use coolant or compressed air to clear chips.

4. Too low feed rate.

•Increase feed rate, consider climb milling.

5. Incorrect cutting angles.

•Adjust to appropriate cutting angles.

6. Too small clearance angle.

•Increase the clearance angle.

09 Tool Breakage

Causes:

1. Too fast feed rate.

•Slow down the feed rate.

2. Too large cutting volume.

•Use smaller per-edge cutting volumes.

3. Too long overall tool length.

•Clamp the tool deeper, use shorter tools, consider climb milling.

4. Excessive wear.

•Resharpen tools early in their life cycle.

10 Vibration Marks

Causes:

1. Too fast feed and cutting speeds.

•Adjust feed and cutting speeds.

2. Insufficient rigidity (machine and toolholder).

•Use better machines and toolholders or change cutting conditions.

3. Too large clearance angle.

•Reduce the clearance angle, finish the cutting edge (use an oil stone to hone the edge).

4. Loose clamping.

•Tighten the workpiece.

Considerations for Speed, Feed Rate, and Cutting Depth

The relationship between speed, feed rate, and cutting depth is crucial for optimal machining results. Inappropriate settings can reduce productivity, degrade part quality, and increase tool wear.

Low Speed Range Used For:

•High hardness materials

•Tough materials

•Difficult-to-machine materials

•Heavy cuts

•Minimal tool wear

•Longest tool life

High Speed Range Used For:

•Soft materials

•Better surface finish

•Smaller tool diameters

•Light cuts

•Brittle materials

•Manual operations

•Maximum efficiency

•Non-metallic materials

High Feed Rate Used For:

•Heavy, rough cuts

•Rigid structures

•Easy-to-machine materials

•Roughing tools

•Planar cutting

•Low tensile strength materials

•Coarse pitch milling cutters

Low Feed Rate Used For:

•Light, finishing cuts

•Brittle structures

•Difficult-to-machine materials

•Small tools

•Deep slotting

•High tensile strength materials

•Finishing tools