Starting from the actual production, this article summarizes the common problems and improvement methods of CNC machining process, as well as how to select the three important factors of speed, feed rate and cutting depth in different application scopes, for your reference.
01
Workpiece overcut
Reason:
1) Tool bounce, the tool strength is not enough, too long or too small, resulting in tool bounce.
2) Improper operation by the operator.
3) Uneven cutting allowance (such as: 0.5 on the side of the curved surface, 0.15 on the bottom surface)
4) Improper cutting parameters (such as: too large tolerance, SF setting too fast, etc.)
Improvement:
1) Tool use principle: large or small, short or long.
2) Add a corner clearing program, and keep the allowance as even as possible (keep the same allowance on the side and bottom).
3) Reasonably adjust the cutting parameters, and round the corners with large allowance.
4) Using the machine tool SF function, the operator fine-tunes the speed to achieve the best cutting effect of the machine tool.
02
Center problem
Reason:
1) The operator is inaccurate when operating manually.
2) There are burrs around the mold.
3) The centering rod is magnetic.
4) The four sides of the mold are not vertical.
Improvement:
1) Manual operation should be checked repeatedly and carefully, and the centering should be at the same point and height as much as possible.
2) Use an oil stone or file to remove burrs around the mold, then 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 other can be used).
4) Calibrate to check whether the four sides of the mold are vertical (large verticality error requires review of the plan with the fitter).
03
Tool setting problem
Reason:
1) The operator is not accurate during manual operation.
2) The tool is clamped incorrectly.
3) The blade on the flying tool is incorrect (the flying tool itself has a certain error).
4) There is an error between the R tool and the flat bottom tool and the flying tool.
Improvement:
1) Manual operation should be checked repeatedly and carefully, and the tool should be set at the same point as much as possible.
2) Blow it clean with an air gun or wipe it clean with a rag when the tool is clamped.
3) The blade on the flying cutter needs to measure the tool rod, and one blade can be used when the bottom surface is smooth.
4) A separate tool setting program can avoid the error between the R-cutting flat cutter and the flying cutter.
04
Collision-Programming
Reasons:
1) The safety height is not enough or not set (the tool or chuck hits 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) on the program sheet and the actual processing depth are written incorrectly.
4) The depth Z-axis reading on the program sheet and the actual Z-axis reading are written incorrectly.
5) The coordinate setting is wrong 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 should be consistent (try to use automatic program sheet or picture program sheet).
3) Measure the actual depth of machining on the workpiece, and write the length of the tool and the blade length clearly on the program sheet (generally, the tool holder length is 2-3MM higher than the workpiece, and the blade length is 0.5-1.0MM).
4) The actual Z-axis data on the workpiece is written clearly on the program sheet. (This operation is generally a manual operation and needs to be checked repeatedly).
05
Collision-Operator
Reason:
1) Depth Z-axis tool setting error.
2) The number of collisions and the number of operations are wrong (such as: single-side data acquisition has no feed radius, etc.).
3) Use the wrong tool (such as: D4 tool is processed with D10 tool).
4) The program is wrong (such as: A7.NC goes to A9.NC).
5) The handwheel is turned in the wrong direction during manual operation.
6) Press the wrong direction during manual rapid feed (such as: -X presses +X).
Improvement:
1) When setting the depth Z-axis tool, you must pay attention to where the tool is set. (bottom surface, top surface, analysis surface, etc.).
2) After the number of collisions and operations is completed, check repeatedly.
3) When clamping the tool, check it repeatedly with the program sheet and program before installing it.
4) The program should be run one by one in order.
5) When using manual operation, the operator should improve the proficiency of machine tool operation.
6) When moving manually quickly, the Z axis can be raised to the workpiece before moving.
06
Surface accuracy
Reasons:
1) The cutting parameters are unreasonable and the surface of the workpiece is rough.
2) The tool edge is not sharp.
3) The tool is clamped too long and the blade is too long to avoid air.
4) Chip removal, air blowing, and oil flushing are not good.
5) Programming tool feeding method (you can try to consider going down milling).
6) The workpiece has burrs.
Improvement:
1) The cutting parameters, tolerances, allowances, and speed feed settings should be reasonable.
2) The tool requires the operator to check 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 to avoid air.
4) For the down-cutting of 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, tools, and cutting methods, so we must understand the performance of the machine tools and make up the edges with burrs.
07
Broken edge
1) Feed too fast
--Slow down to a suitable feed speed.
2) Feed too fast at the beginning of cutting
--Slow down the feed speed at the beginning of cutting.
3) Loose clamping (tool)
--Clamp.
4) Loose clamping (workpiece)
--Clamp.
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 machine tool and tool holder with good rigidity.
08
Wear
1) Machine speed is too fast
--Slow down and add enough coolant.
2) Hardened material
--Use advanced tools and tool materials, and increase surface treatment methods.
3) Chip adhesion
--Change the feed speed, chip size or use cooling oil or air gun to clean the chips.
4) Inappropriate feed speed (too low)
--Increase the feed speed and try down milling.
5) Inappropriate cutting angle
--Change to an appropriate cutting angle.
6) The primary back angle of the tool is too small
--Change to a larger back angle.
09
Destruction
1) Feed too fast
--Slow down the feed speed.
2) Too much cutting amount
--Use a smaller cutting amount per blade.
3) Too much blade length and total length
-Clamp the handle a little deeper, use a shorter tool, and try down milling.
4) Too much wear
--Re-grind in the early stage.
10
Chatter marks
1) Feed and cutting speed are too fast
--Correct feed and cutting speed
2) Insufficient rigidity (machine tool and tool holder)
--Use better machine tool and tool holder or change cutting conditions
3) Too large back angle
--Change to a smaller back angle and process the edge band (grind the edge once with an oilstone)
4) Loose clamping
--Clamp the workpiece
5) Consider speed and feed rate
The relationship between speed, feed rate and cutting depth is the most important factor in determining the cutting effect. Inappropriate feed rate and speed often lead to reduced production, poor workpiece quality and severe tool damage.
Use the low speed range for:
High hardness materials
Highly resistant materials
Difficult to cut materials
Heavy cutting
Minimum tool wear
Longest tool life
Use the high speed range for
Soft materials
Better surface quality
Smaller tool outer diameter
Light cutting
Brittle workpieces
Manual operation
Maximum machining efficiency
Non-metallic materials
Use high feed rates for
Heavy and rough cutting
Steel structures
Easy-to-cut materials
Roughing tools
Flat cutting
Low tensile strength materials
Coarse tooth milling cutters
Use low feed rates for
Light and fine cutting
Brittle structures
Difficult-to-cut materials
Small tools
Deep grooving
High tensile strength materials
Fine cutting tools
