Common CNC Machining Problems and Improvement Methods
Workpiece overcutting
Causes:
1) Tool bounce: Insufficient tool strength, excessive length, or insufficient size can cause tool bounce.
2) Improper operator operation.
3) Uneven cutting allowance (e.g., 0.5mm for curved surfaces, 0.15mm for the bottom).
4) Inappropriate cutting parameters (e.g., excessive tolerances, SF settings).
Improvements:
1) Tool usage principle: use larger tools whenever possible, shorter tools whenever possible.
2) Add a corner clearing program, leaving as even a margin as possible (the margin on the side and bottom should be the same).
3) Adjust cutting parameters appropriately, rounding off corners with large margins.
4) Utilize the machine tool's SF function for operation.The operator fine-tunes the speed to achieve the best cutting effect from the machine tool.
Problem of dividing the middle
Causes:
1) Inaccurate manual operation by the operator.
2) Burrs around the mold.
3) Magnetism on the centering rod.
4) The four sides of the mold are not perpendicular.
Improvements:
1) Manual operation requires repeated and careful checks, ensuring the centering is done at the same point and height as much as possible.
2) Deburr the mold perimeter with an oilstone or file, wipe it clean with a rag and finally confirm by hand.
3) Demagnetize the centering rod before centering the mold (a ceramic centering rod or other suitable tool can be used).
4) Use a dial indicator to check if the four sides of the mold are perpendicular (if the perpendicularity error is large, a solution needs to be discussed with the fitter).
Knife-off problem
Causes:
1) Inaccurate manual operation by the operator.
2) Incorrect tool clamping.
3) Incorrect insert on the fly knife (the fly knife itself has a certain degree of error).
4) Errors exist between the R-blade, the flat-bottomed knife, and the fly knife.
Improvements:
1) Manual operation requires repeated and careful checks before machining process, and tool setting should be performed at the same point as much as possible.
2) When clamping tools, clean them with an air gun or wipe them clean with a rag.
3) When using a fly knife, measure the tool shank; a single tool can be used when finishing the bottom surface.
4) Create a separate tool setting program to avoid errors between R-cut tools, flat tools, and fly knives.
Crash - Programming
Causes:
1) Insufficient or missing safety height (tool or chuck hits workpiece during rapid feed G00).
2) Incorrect tool selection on the program sheet and incorrect actual tool selection in the program.
3) Incorrect tool length (cutting edge length) on the program sheet and incorrect actual machining depth.
4) Incorrect Z-axis depth selection on the program sheet and incorrect actual Z-axis depth selection.
5) Incorrect coordinate settings during CNC programming.
Improvements:
1) Accurately measure the height of the workpiece and ensure that the safe height is above the workpiece.
2) The cutting tools on the program sheet must match the actual cutting tools in the program (use automatic program sheet generation or image-based program sheet generation whenever possible).
3) Measure the actual machining depth on the workpiece and clearly write the tool length and cutting edge length on the program sheet (generally, the tool clamping length should be 2-3mm higher than the workpiece, and the cutting edge length clearance should be 0.5-1.0mm).
4) Take the actual Z-axis measurements on the workpiece and clearly write them on the program sheet. (This operation is generally manual and should be repeatedly checked after writing).
Collision - Operator
Causes:
1) Incorrect Z-axis depth setting.
2) Errors in centering and operation (e.g., no feed radius for single-sided data entry).
3) Using the wrong tool (e.g., using D10 tool instead of D4 tool).
4) Incorrect program execution (e.g., A7.NC being executed as A9.NC).
5) Incorrect handwheel rotation during manual operation.
6) Incorrect direction pressed during manual rapid feed (e.g., pressing +X instead of -X).
Improvements:
1) When setting the Z-axis depth, pay close attention to the tool setting position (bottom surface, top surface, analysis surface, etc.).
2) After centering and counting, repeatedly check the results.
3) When clamping the tool, repeatedly check against the program sheet and program before installing it.
4) Run the program line by line in sequence.
5) When using manual operation, the operator should improve their proficiency in operating the machine tool.
6) When performing rapid manual traverse, raise the Z-axis above the workpiece before moving.
Surface accuracy
Causes:
1) Inappropriate cutting parameters, resulting in a rough workpiece surface.
2) Dull cutting edge.
3) Overly long tool clamping, leading to excessive clearance between the cutting edge.
4) Poor chip removal, air blowing, and oil flushing.
5) Incorrect tool path programming (preferably consider climb milling).
6) Burrs on the workpiece.
Improvements:
1) Cutting parameters, tolerances, allowances, and speed/feed settings should be reasonable.
2) Operators should regularly inspect and replace cutting tools.
3) When clamping tools, operators should clamp them as short as possible, and the cutting edge clearance shouldn't be too long.
4) For downward cutting with flat cutters, R-cutters, and ball nose cutters, speed/feed settings should be reasonable.
5) Burrs on the workpiece: This is directly related to our machine tool, cutting tools, and tool path, so we need to understand the machine tool's performance and perform additional cutting on burr-prone edges.
Chipping edge
1) The feed rate is too fast - slow it down to a suitable feed rate.
2) The feed rate is too fast at the start of cutting -- slow down the feed rate at the start of cutting.
3) Clamping loose (tool) -- clamping.
4) Clamping loose (workpiece) -- clamping.
5) Insufficient rigidity (tool) -- Use the shortest tool allowed, clamp the shank deeper, and also try climb milling.
6) The cutting edge of the tool is too sharp -- change the fragile cutting edge angle, one-time edge.
7) Insufficient rigidity of machine tools and tool holders -- Use machine tools and tool holders with good rigidity.
Wear
1) The machine speed is too fast - slow it down and add enough coolant.
2) Hardening materials - Use high-grade cutting tools and materials to increase surface treatment methods.
3) Chip adhesion - change the feed rate, chip size, or clean the chips with cooling oil or an air gun.
4) Inappropriate feed rate (too low) -- Increase the feed rate and try climb milling.
5) The cutting angle is not suitable -- change it to an appropriate cutting angle.
6) The clearance angle of the tool is too small -- change it to a larger clearance angle.
Destroy
1) Feeding too fast -- slow down the feed rate.
2) The cutting amount is too large -- use a smaller cutting amount per blade.
3) The cutting edge length and overall length are too large -- clamp the shank deeper, use a short cutter, and try climb milling.
4) Excessive wear - re-grind in the initial stage.
Vibration wave
1) Feed and cutting speeds are too high -- Correct feed and cutting speeds
2) Insufficient rigidity (machine tool and tool holder) -- Use a better machine tool and tool holder or change the cutting conditions.
3) The clearance angle is too large – change it to a smaller clearance angle and machine the cutting edge (sharpen the edge once with an oilstone).
4) Clamping loosening -- clamping the workpiece
5) Consider speed and feed rate