This article starts from the practical production and summarizes the common problem points and improvement methods in the CNC machining process, as well as how to select the important factors of speed, feed rate, and cutting depth in different application fields for your reference.
1.Overcutting of workpiece
Reasons:
1) Dulling of the tool due to insufficient tool strength, length, or size.
2) Operator error.
3) Uniformity of cutting allowance (e.g. 0.5 on the contour side, 0.15 on the bottom side).
4) Inappropriate cutting parameters (e.g. too large tolerances, too fast SF setting).
Improvement:
1) Cutting tool principle: use larger tools instead of smaller ones, use shorter tools instead of longer ones.
2) Add chamfering program to keep the cutting allowance uniform (with the same allowance on the contour side and bottom side).
3) Reasonably adjust cutting parameters to round off sharp corners with a large cutting allowance.
4) Utilize the SF function of the machine to fine-tune the speed so that the machine can achieve optimal cutting results.
2.The Problem of Centering:
Reasons:
1) Operator's manual operation is not accurate.
2) There are burrs around the mold.
3) The centering bar has magnetism.
4) The four sides of the mold are not perpendicular.
Improvement:
1) Manual operation should be repeated and carefully checked, and the centering should be done as accurately as possible at the same point and height.
2) Use oilstone or file to remove burrs from the mold, then wipe it clean with a rag, and finally check it by hand.
3) Before centering the mold, demagnetize the centering bar (use ceramic centering bar or other options).
4) Check the verticality of the four sides of the mold with a caliper (if the verticality error is large, consult with the toolmaker for a solution).
3.The problem of tool setting
Reasons:
1) Inaccurate manual operation by operators.
2) Incorrect tool clamping.
3) Incorrect blade placement on the flying knife (the flying knife itself has certain errors).
4) Errors between the R knife, flat knife, and flying knife.
Improvement:
1) Manual operation should be repeated and carefully checked, and the tool setting should be done as accurately as possible at the same point.
2) The tool clamping should be cleaned with an air gun or wiped with a piece of torn cloth before use.
3) When testing the blade on the tool holder or the flat bottom surface, use one blade.
4) A separate tool setting program can be used to avoid errors between the R knife, flat knife, and flying knife.
4.Collision - Programming
Reasons:
1) Insufficient safety height or no setting (tool or chuck collides with the workpiece during rapid feed G00).
2) Wrong tool and actual program tool written on the program sheet.
3) Wrong tool length (blade length) and actual machining depth written on the program sheet.
4) Wrong Z-axis depth reading on the program sheet and actual Z-axis depth reading.
5) Incorrect 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 sheet and the actual program tool must match (use automatic program generation or program generation from pictures as much as possible).
3) Measure the actual machining depth on the workpiece and clearly write down the tool length and blade length on the program sheet (usually, the tool chuck length is 2-3MM higher than the workpiece, and the blade length avoidance is 0.5-1.0MM).
4) Take the actual Z-axis depth on the workpiece and clearly write it down on the program sheet (this operation is usually manual operation, and it should be checked repeatedly).
5.Collision - Operator
Reasons:
1) Depth Z-axis tool setting error.
2) Setting and operating errors (e.g. taking single-sided data without taking into account the cutting radius).
3) Using the wrong tool (e.g. using tool D10 to process with tool D4).
4) Program error (e.g. running program A7.NC instead of program A9.NC).
5) Manual operation error when the handwheel was turned in the wrong direction.
6) Manual rapid feed error when the wrong direction was selected (e.g. selecting +X when the correct direction was -X).
Improvements:
1) Depth Z-axis tool setting must be carefully checked to ensure that the tool is set correctly (e.g. on the bottom, top, or datum plane).
2) After setting and operating, the data must be checked and double-checked.
3) The tool must be checked against the program and the program sheet before being installed.
4) Programs must be run in sequence.
5) Operators must improve their familiarity with the machine when using manual operation.
6) When using manual rapid feed, the Z-axis can be raised above the workpiece before moving.
6.Surface Accuracy
Reasons:
1) Inappropriate cutting parameters, resulting in rough surface of the workpiece.
2) Dull cutting tool edge.
3) Tool holder too long, too long cutting edge clearance.
4) Poor chip evacuation, blowing, and lubrication.
5) Programming cutting path (can consider smooth milling as much as possible).
6) Workpiece burrs.
Improvement:
1) Reasonable cutting parameters, tolerance, allowance, speed and feed setting.
2) Operators are required to regularly check and replace cutting tools.
3) Operators are required to clamp the tool holder as short as possible and avoid too long cutting edge clearance.
4) Reasonable speed and feed setting for flat knives, R knives, and round-nose knives.
5) Workpiece burrs: It is directly related to the performance of the machine tool, cutting tool, and cutting path, so we need to understand the performance of the machine tool and make upcuts for the edges with burrs.
7. Chipping
* Too fast feed rate:
- Slow down to an appropriate feed rate.
* Too fast feed rate at the start of cutting:
- Slow down the feed rate at the start of cutting.
* Loose clamping (tool):
- Tighten the clamping.
* Loose clamping (workpiece):
- Tighten the clamping.
* Insufficient rigidity (tool):
- Use the shortest possible tool with a deep shank, and try counter-milling.
* Too sharp cutting edge of the tool:
- Change the fragile cutting edge angle, and sharpen the cutting edge.
* Insufficient rigidity of the machine and tool holder:
- Use a rigid machine and tool holder.
8.Wear
1) Machine speed is too high
-- Slow down, add enough coolant.
2) Hardened material
-- Use high-grade tools, tool materials, and 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) Feed rate is inappropriate (too low)
-- Increase the feed rate, try counter-milling.
5) Cutting angle is not suitable
-- Change to the appropriate cutting angle.
6) The tool's initial rake angle is too small
-- Change to a larger rake angle.
9.Destruction
1. Feed rate is too high.
-- Slow down the feed rate.
2. Cutting depth is too large.
-- Use a smaller cutting depth per tooth.
3. Tool length and overall length are too large.
-- Clamp the handle deeper, use a shorter tool, and try finishing cuts.
4. Wear is too much.
-- Re-sharpen at the beginning.
10.Vibration
1) Too fast feed and cutting speed
--Adjust the feed and cutting speed
2) Insufficient rigidity (machine tool and chuck)
--Use better machine tool and chuck or change the cutting conditions
3) Too large rake angle
--Change to a smaller rake angle and grind the cutting edge (use an oilstone)
4) Loose clamping
--Clamp the workpiece firmly
5) Consider speed, feed rate, and depth of cut
The relationships between speed, feed rate, and depth of cut are the most important factors in determining cutting effect. Inappropriate feed rates and speeds often result in reduced production, poor workpiece quality, and large tool damage.
Use the low speed range for:
High-hardness materials
Large-dimensional materials
Difficult-to-machine materials
Heavy cutting
Minimum tool wear
Longest tool life
Use the high speed range for:
Soft materials
Better surface quality
Smaller tool diameter
Light cutting
Fragile workpieces
Manual operation
Maximum machining efficiency
Non-metallic materials
Use high feed rates for:
Heavy, rough cutting
Steel structures
Easily machinable materials
Coarse cutting tools
Plane cutting
Low tensile strength materials
Coarse-toothed end mills;
Use low feed rates for:
Light machining, fine cutting
Brittle structures
Difficult-to-machine materials
Small cutting tools
Deep slot machining
High tensile strength materials
Finishing tools
