Knowledge of CNC Lathe Machining
1. Factors affecting cutting temperature: cutting speed, feed rate, depth of cut;
Factors affecting cutting force: depth of cut, feed rate, cutting speed;
Factors affecting tool life: cutting speed, feed rate, depth of cut.
2. When the depth of cut is doubled, the cutting force doubles;
When the feed rate is doubled, the cutting force increases by approximately 70%;
When the cutting speed is doubled, the cutting force gradually decreases;
In other words, if using G99, the cutting force does not change significantly when the cutting speed increases.
3. One can judge whether the cutting force and cutting temperature are within normal ranges based on the chip evacuation.
4. When machining a concave arc where the difference between the measured actual value (X) and the blueprint diameter (Y) exceeds 0.8, using a turning tool with a secondary cutting edge angle of 52° ( the commonly used tool with a 35° insert angle and a 93° principal cutting edge angle) may result in tool rubbing at the starting point of the arc.
5. Temperatures indicated by chip color:
White: < 200°C
Yellow: 220–240°C
Dark blue: 290°C
Blue: 320–350°C
Purple-black: > 500°C
Red: > 800°C
6. Typical default G-codes for FANUC 0i-MTC:
G21: Metric input
G25: Spindle speed fluctuation detection OFF
G80: Canned cycle cancel
G54: Default coordinate system
G18: ZX plane selection
G96 (G97): Constant surface speed control
G99: Feed per revolution
G40: Tool nose radius compensation cancel (G41/G42)
G22: Stored stroke check ON
G67: Macro modal call cancel
G64: (Not specified/Unclear)
G13.1: Polar coordinate interpolation cancel
7. External thread depth is generally 1.3P; internal thread depth is 1.08P.
8. Threading spindle speed = 1200 / Thread pitch × Safety factor (typically 0.8).
9. Manual tool nose radius (R) compensation formulas: For chamfering from bottom to top: Z = R × (1 - tan(a/2)), X = R × (1 - tan(a/2)) × tan(a). For chamfering from top to bottom, simply change the minus sign to a plus sign.
10. For every 0.05 increase in feed rate, reduce spindle speed by 50–80 RPM. Lowering the speed reduces tool wear and slows the rate of increase in cutting force; this compensates for the rise in cutting force and temperature caused by the increased feed rate.
11. Cutting speed and cutting force significantly impact the tool; excessive cutting force is a primary cause of tool chipping or breakage. The relationship between cutting speed and cutting force is as follows: at a constant feed rate, increasing cutting speed causes a gradual decrease in cutting force; however, higher speeds also accelerate tool wear, which eventually leads to increased cutting force and higher temperatures. When the combined cutting force and internal stress exceed the insert's structural limits, tool failure (chipping or breakage) occurs (this is also influenced by factors such as stress induced by temperature fluctuations and the reduction of material hardness).
12. When performing CNC turning, pay special attention to the following points:
(1) Most economical CNC lathes in my country typically use standard three-phase asynchronous motors with variable frequency drives (VFDs) for stepless speed control. Without mechanical gear reduction, spindle torque is often insufficient at low speeds; excessive cutting loads can easily cause the spindle to stall (though some machines effectively resolve this issue by incorporating gear ranges);
(2) Aim to complete the machining of a single part or a full work shift with the same tool; for the finish machining of large components, it is particularly important to avoid mid-process tool changes to ensure the operation can be completed in a single pass;
(3) When thread turning on a CNC lathe, use the highest feasible speed to ensure high quality and production efficiency;
(4) Use G96 (constant surface speed control) whenever possible;
(5) The fundamental concept of high-speed machining is to set the feed rate higher than the rate of heat conduction into the workpiece; this allows cutting heat to be carried away with the chips, isolating the heat from the workpiece and preventing or minimizing temperature rise. Therefore, high-speed machining involves combining high cutting speeds and high feed rates with a reduced depth of cut;
(6) Pay attention to tool nose radius (TNR) compensation.
13. Vibration and tool breakage frequently occur during grooving operations; the root cause is typically excessive cutting force combined with insufficient tool rigidity. Rigidity improves-and the tool can withstand greater cutting forces-when the tool overhang is shorter, the clearance angle is smaller, and the insert surface area is larger. While a wider grooving tool can withstand greater forces, it also generates higher cutting forces; conversely, a narrower tool has a lower load capacity but generates less cutting force.
14. Causes of vibration during grooving operations:
(1) Excessive tool overhang, resulting in reduced rigidity;
(2) Feed rate is too slow, causing the unit cutting force to increase and triggering significant vibration (Formula: P = F / (depth of cut × f), where P is unit cutting force and F is cutting force); additionally, excessively high spindle speed can also cause chatter;
(3) Insufficient machine tool rigidity-meaning the tool can withstand the cutting force, but the machine cannot; simply put, the machine lacks the capability to handle the cut. Generally, new machines do not exhibit this issue; machines that do are usually either very old or have been subjected to "machine killers" (operators who abuse the equipment).
15. When machining a part, dimensions appear correct initially, but after several hours of operation, dimensions shift and become unstable.
The reason may be that the tool was new at the start, resulting in relatively low cutting forces. However, after machining for a period, tool wear increases the cutting force, causing the workpiece to shift within the chuck, leading to dimensional changes and instability.
16. When using G71, the P and Q values must not exceed the sequence numbers defined in the program; otherwise, an alarm indicating an incorrect G71-G73 command format will occur (at least on FANUC systems).
17. There are two formats for subroutines in FANUC systems:
(1) P000 0000: The first three digits represent the number of repetitions (loops), and the last four digits represent the program number;
(2) P0000L000: The first four digits represent the program number, and the three digits following "L" represent the number of repetitions.
18. If the starting point of an arc remains fixed while the endpoint shifts by *a* mm in the Z-direction, the position of the arc's base diameter shifts by a/2.
19. When drilling deep holes, the drill bit's cutting flutes are not ground down, in order to facilitate chip evacuation.
20. If a custom fixture mounted on the tool turret is used for drilling, rotating the drill bit can alter the diameter of the drilled hole.
21. When spotting or drilling stainless steel, the center drill or drill bit must be small; otherwise, it will not penetrate. When using cobalt drills, do not regrind the flutes to avoid annealing the bit during the drilling process.
22. Material cutting (blanking) generally falls into three categories based on the process: cutting one blank at a time, cutting two parts at a time, or cutting the entire bar stock at once.
23. If an oval shape occurs when threading, the workpiece may have shifted or loosened; simply make a few additional passes with the threading tool to correct it.
24. In systems that support macro programming, macros can replace subroutine loops; this saves program numbers and avoids various complications.
25. If a standard drill bit causes excessive runout when enlarging a hole, switch to a flat-bottom drill; ensure the twist drill used is short to maximize rigidity.
26. When boring small through-holes, aim for continuous chip curling so chips are discharged from the rear. Key points for chip curling: 1. Position the tool tip slightly higher; 2. Use appropriate edge inclination, depth of cut, and feed rate. Avoid setting the tool too low, as this causes chip breakage. A large secondary cutting edge angle prevents broken chips from jamming the tool shank, whereas a small angle allows broken chips to wedge against the shank, creating a hazard.
27. A larger tool shank cross-section reduces the risk of chatter. Additionally, wrapping a strong rubber band around the tool shank can help dampen vibrations.
28. When boring copper, use a slightly larger tool nose radius (R0.1–R0.8), especially when machining tapered bores; while steel parts might tolerate smaller radii, copper parts are prone to chip jamming.
