When performing CNC programming, the programmer must determine the cutting parameters for each operation and encode them into the program. Cutting parameters include cutting speed, depth of cut, and feed rate. Different machining methods require the selection of different cutting parameters.
1. Principles for Selecting Cutting Parameters
For rough machining, the primary goal is generally to increase productivity, though economic efficiency and machining costs must also be considered. For semi-finish and finish machining, cutting efficiency, economic efficiency, and machining costs must be balanced while ensuring machining quality. Specific values should be determined based on machine tool manuals and cutting parameter handbooks, combined with practical experience.
To optimize tool life, the sequence for selecting cutting parameters is: first determine the depth of cut, then the feed rate, and finally the cutting speed.
2. Determining the Depth of Cut
The depth of cut is determined by the rigidity of the machine tool, the workpiece, and the cutting tool. Whenever rigidity permits, the depth of cut should ideally equal the workpiece's machining allowance; this minimizes the number of passes and improves productivity.
Principles for determining the depth of cut:
(1) When the required surface roughness is Ra 12.5 μm–25 μm and the machining allowance is less than 5–6 mm, a single roughing pass is sufficient to meet requirements. However, if the allowance is large, the rigidity of the machining system is poor, or the machine tool lacks sufficient power, the process may be split into multiple passes.
(2) When the required surface roughness is Ra 3.2 μm–12.5 μm, the process can be divided into rough machining and semi-finish machining stages. The depth of cut for rough machining is selected as described above. An allowance of 0.5–1.0 mm is left after rough machining to be removed during semi-finish machining.
(3) When the required surface roughness is Ra 0.8 μm–3.2 μm, the process can be divided into three stages: rough machining, semi-finish machining, and finish machining. The depth of cut for semi-finish machining is set at 1.5–2 mm. For finishing operations, the depth of cut is set between 0.3 mm and 0.5 mm.
3. Determination of feed rate
The feed rate is primarily selected based on the required machining accuracy and surface roughness, as well as the materials of the cutting tool and the workpiece. The maximum feed speed is limited by the rigidity of the machine tool and the performance of the feed system.
Principles for determining feed speed:
1) When workpiece quality requirements can be guaranteed, a higher feed speed may be selected to improve production efficiency; this is generally chosen within the range of 100–200 m/min.
2) When performing cut-off operations, machining deep holes, or using high-speed steel (HSS) tools, a lower feed speed is advisable; this is generally chosen within the range of 20–50 m/min.
3) When high machining accuracy and surface finish are required, a lower feed speed should be selected, generally within the range of 20–50 m/min.
4) During non-cutting tool movements (rapid traverse), especially when returning to the reference point ("homing") over a long distance, the maximum feed speed set by the machine's CNC system may be selected.
4. Determination of spindle speed
Spindle speed should be selected based on the permissible cutting speed and the diameter of the workpiece (or tool). The calculation formula is:
n = 1000v / πD
v - Cutting speed (m/min), determined by tool durability;
n - Spindle speed (r/min);
D - Workpiece diameter or tool diameter (mm).
The calculated spindle speed (n) should ultimately be adjusted to match an available speed setting on the machine tool (or the closest available value) as specified in the machine manual. In summary, specific cutting parameter values should be determined using an analogical approach based on machine tool performance, relevant manuals, and practical experience. At the same time, the spindle speed, depth of cut, and feed speed should be balanced against one another to achieve optimal cutting parameters.
Reference formula:
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(I) Depth of cut (ap)
The perpendicular distance between the machined surface and the surface yet to be machined is defined as the depth of cut. Depth of cut ($a_p$) is the depth measured perpendicular to the working plane, passing through the base point of the cutting edge; it represents the depth to which the turning tool penetrates the workpiece during each feed movement, and is therefore also known as the "cutting depth." Based on this definition, when turning an external cylindrical surface longitudinally, the depth of cut can be calculated using the following formula:
$a_p = (d_w - d_m) / 2$
Where $a_p$ = depth of cut (mm);
$d_w$ = diameter of the workpiece surface to be machined (mm);
$d_m$ = diameter of the machined workpiece surface (mm).
Example 1: Given a workpiece surface diameter of $\Phi$95 mm to be machined; if it is turned to a diameter of $\Phi$90 mm in a single pass, calculate the depth of cut.
Solution: $a_p = (d_w - d_m) / 2 = (95 - 90) / 2 = 2.5$ mm.
(II) Feed ($f$)
The relative displacement between the tool and the workpiece in the direction of the feed motion for each revolution of the workpiece or tool.
Depending on the feed direction, it is classified into longitudinal feed and transverse feed; longitudinal feed refers to the feed along the direction of the lathe bed guideways, while transverse feed refers to the feed perpendicular to the direction of the lathe bed guideways.
(Note) Feed speed ($v_f$) refers to the instantaneous speed of a selected point on the cutting edge relative to the workpiece during the feed motion.
$v_f = fn$
Where $v_f$ = feed speed (mm/s);
$n$ = spindle speed (r/s);
$f$ = feed (mm/s).
(III) Cutting speed ($v_c$)
The instantaneous speed of a selected point on the cutting edge relative to the workpiece during the primary cutting motion. The calculation formula is as follows:
$v_c = (\pi d_w n) / 1000$
Where $v_c$ = cutting speed (m/min);
$d_w$ = diameter of the workpiece surface to be machined (mm);
$n$ = workpiece rotational speed (r/min). Calculations should be based on the maximum cutting speed; for instance, when turning, the calculation uses the diameter of the surface to be machined, as the speed is highest and tool wear is most rapid at that point.
Example 2: Turning the outer diameter of a workpiece with a diameter of Φ60 mm; the selected lathe spindle speed is 600 r/min. Calculate vc.
Solution: vc = (π × dw × n) / 1000 = 3.14 × 60 × 600 / 1000 = 113 m/min
In actual production, the workpiece diameter is usually known. The cutting speed is selected based on factors such as workpiece material, tool material, and machining requirements; this speed is then converted into the lathe spindle speed to adjust the machine, resulting in the following formula:
n = (1000 × vc) / (π × dw)
Example 3: Turning the outer diameter of a pulley (Φ260 mm) on a CA6140 horizontal lathe; select vc = 90 m/min and calculate n.
Solution: n = (1000 × vc) / (π × dw) = (1000 × 90) / (3.14 × 260) = 110 r/min
After calculating the spindle speed, select a value close to those listed on the machine's nameplate; in this case, select n = 100 r/min as the actual lathe speed.
III. Summary:
Cutting parameters
1. Depth of cut ap (mm): ap = (dw - dm) / 2 (mm)
2. Feed rate f (mm/r)
3. Cutting speed vc (m/min): vc = π × d × n / 1000 (m/min)
n = 1000 × vc / (π × d) (r/min)
