CNC machining often runs into repeat issues such as unstable dimensions, tool wear, vibration, poor chip control, and thread defects. Understanding the following 23 points can help improve machining efficiency, surface quality, and process stability.
1. What Has the Greatest Influence on Cutting Temperature, Cutting Force, and Tool Life?
Three factors strongly affect cutting temperature:
- cutting speed
- feed rate
- depth of cut
Cutting force is mainly influenced by:
- clearance angle
- feed rate
- cutting speed
Tool life is most sensitive to:
- cutting speed
- feed rate
- depth of cut
2. How Do Cutting Parameters Change Cutting Force?
When the depth of cut doubles, cutting force usually doubles.
When the feed rate doubles, cutting force increases by about 70%.
When cutting speed doubles, cutting force usually decreases gradually.
In other words, if you use G99 (feed per revolution), increasing cutting speed will not cause large changes in cutting force.
3. How Can You Judge Whether Cutting Force Is Normal?
In actual machining, cutting force can often be judged indirectly by:
- chip evacuation condition
- cutting temperature
- chip color and shape
If chips are removed smoothly and temperature stays within a normal range, cutting force is usually under control.
4. Why Does a Tool Sometimes Rub Instead of Cut at the Start of an Arc?
If the actual X value differs from the drawing diameter by more than 0.8 mm, a turning tool with a 52° secondary cutting edge angle may rub at the starting point, especially when turning a concave arc. This is common with tools using a 35° approach angle and 93° lead angle.
5. Chip Color and Approximate Temperature
Chip color can help estimate cutting temperature:
- white: below 200°C
- yellow: 220–240°C
- dark blue: around 290°C
- blue: 320–350°C
- purple-black: above 500°C
- red: above 800°C
6. Common G Codes in FANUC Oi Mate-TC / Oi-TC Systems
Some frequently used G codes include:
- G21: metric input
- G25: spindle speed fluctuation detection off
- G80: cancel fixed cycle
- G54: default work coordinate system
- G18: ZX plane selection
- G96 / G97: constant surface speed / cancel constant surface speed
- G99: feed per revolution
- G40: cancel tool nose radius compensation
- G22: stored stroke check on
- G67: cancel modal macro call
- G13.1: cancel polar coordinate interpolation
7. Key Points for Thread Machining
For thread cutting:
- external thread depth is usually about 1.3P
- internal thread depth is usually about 1.08P
Here, P means pitch.
A common spindle speed formula for threading is:
S = 1200 / pitch × safety factor
The safety factor is often around 0.8.
8. Manual Tool Nose Radius Compensation for Chamfering
When calculating manual compensation for tool nose radius during chamfering, the formula depends on cutting direction and chamfer angle.
For upward chamfering:
- Z = R × (1 - tan(α/2))
- X = R × (1 - tan(α/2)) × tan(α)
For downward chamfering, change the minus sign to a plus sign.
9. How Should You Adjust Parameters When Feed Increases?
A common adjustment rule is:
If feed rate increases by 0.05, spindle speed can be reduced by 50–80 rpm.
This helps reduce tool wear and slow down the rise of cutting force and temperature caused by higher feed.
10. What Is the Real Relationship Between Cutting Speed and Tool Damage?
At a constant feed rate, increasing cutting speed may slightly reduce cutting force.
However, if cutting speed becomes too high, tool wear increases quickly, which then causes:
- higher cutting force
- higher temperature
- greater internal stress
When cutting force and thermal stress exceed the insert limit, the tool may chip or break.
11. Important Practical Points in CNC Machining
Spindle Torque at Low Speed
Many economical CNC lathes use three-phase asynchronous motors with variable frequency control. Without mechanical gear reduction, spindle torque at low speed may be insufficient, causing overload and stalling.
Tool Life Management
Ideally, one tool should finish one part or one full shift whenever possible. For finishing large parts, avoid changing tools midway if possible.
Threading Efficiency
Use a higher but safe spindle speed in CNC threading to improve both productivity and thread quality.
Constant Surface Speed
Use G96 whenever suitable.
High-Speed Machining Principle
If feed exceeds the heat conduction speed into the workpiece, most heat leaves with the chips. This can reduce workpiece heating. It usually requires:
- high cutting speed
- high feed
- small depth of cut
Tool Nose Radius Compensation
Apply G41/G42 correctly to avoid contour errors.
12. Useful Technical References
Machining is easier when operators keep these references nearby:
- machinability table for different workpiece materials
- common threading pass and depth table
- geometric calculation formulas
- inch-to-millimeter conversion chart
13. Why Do Grooving Tools Vibrate or Break?
The main reasons are:
- excessive cutting force
- insufficient tool rigidity
Key factors include:
- tool overhang too long
- clearance angle too large or too small
- feed too low or too high
- machine rigidity insufficient
- grooving width too narrow for the cutting load
A shorter tool overhang and stronger tool body improve rigidity and reduce vibration.
14. Why Does Part Size Become Unstable During Machining?
At the start, a new tool cuts with lower force, so size stays normal.
As the tool wears, cutting force increases, which may cause the workpiece to shift in the chuck. This leads to dimensional variation.
15. G71 Usage Notes in FANUC Systems
In FANUC controls, the P and Q values in G71–G73 cycles must not exceed the actual sequence numbers in the full program. Otherwise, the control may alarm with a format error.
16. Two Common FANUC Subprogram Formats
Two common formats are:
- P0000000: first three digits = repeat count, last four digits = program number
- P0000 L000: first four digits = program number, last three digits after L = repeat count
17. Arc Machining Tip
If the arc starting point stays unchanged and the endpoint shifts by a mm in the Z direction, the diameter at the arc bottom shifts by a/2 mm.
18. Deep Hole Drilling Tips
For deep hole drilling:
- grind the flute properly to improve chip evacuation
- when drilling stainless steel, thin the chisel edge to avoid slipping
- if using cobalt drills, avoid grinding the flute excessively, or the drill may lose hardness from overheating
19. Bar Preparation Methods
Common material preparation methods include:
- one part from one blank
- two parts from one blank
- full bar machining
If thread shape becomes oval, loose material structure may be one possible cause. In some cases, repeating the thread pass can help.
20. Macro Programming
In controls that support macro programming, macros can replace repeated subprogram loops. This saves program numbers and makes the program easier to manage.
21. Reaming and Hole Enlargement Tips
If a drill produces excessive hole runout during enlargement:
- switch to a flat-bottom drill if suitable
- shorten the drill to improve rigidity
When drilling directly on a drill press, hole size may deviate, but fluctuation is often still limited.
22. Chip Breaking Control in Small Hole Turning
For turning small through-holes, the goal is to make chips curl continuously and exit smoothly.
Key points include:
- raise the tool position slightly if needed
- choose a suitable inclination angle
- set a proper cutting amount and feed rate
- avoid positioning the tool too low, or chip breaking may become unstable
A larger secondary cutting edge angle can help prevent broken chips from jamming against the tool bar.
23. Tips for Machining Copper Bores
When turning internal copper features, a slightly larger tool nose radius can help, such as R0.4 to R0.8.
This is especially useful for taper turning. Steel chips may not cause problems in the same geometry, but copper chips are more likely to clog and affect cutting stability.
Work With XY-GLOBAL for Reliable CNC Machining Support
At XY-GLOBAL, we understand that good machining is not only about running a program. It also depends on correct cutting parameters, stable process control, proper tooling, and practical production experience.
If you are looking for a reliable machining partner, XY-GLOBAL is ready to support your project.
You are welcome to contact us for technical discussion, quotation, or manufacturability review.
Send us your 2D or 3D drawings, and our team can help evaluate the machining feasibility and provide a suitable solution for your parts.
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