Process and Programming in the Training of CNC Lathes

CNC Lathe

When beginners in numerical control technology start practicing on CNC machine tools, especially students from higher vocational colleges, they often face challenges despite having studied related technical courses. The lack of real-world production experience leads to incomplete understanding and difficulty applying theoretical knowledge effectively. This can result in parts that don't meet the desired specifications. To address this, practical training examples are used to analyze and discuss process planning and programming issues in depth.

1. Training Conditions

(1) Equipment: "GTC2E" CNC lathe from Shenzhen.

(2) Software: "SANYING" CNC lathe simulation system.

(3) Practical Example: Boring tool handle.

(4) Workpiece Blank: φ26mm round wood.

(5) Part Drawing: The boring tool handle is shown in the diagram below.

2. Process Planning

(1) First, carefully study the part drawing and break it down. The handle contour consists of a truncated cone, a cylinder, and three arc-connected surfaces. Determine the workpiece coordinate origin and calculate the coordinates of each vertex and the connection points of the curves. Refer to the note on the drawing for details.

(2) Tool selection should consider whether the tool’s structure and size will interfere with the already machined areas. During cutting, if the cutting point deviates from the tool tip, overcutting may occur. For smaller profile curves, shaped tools may be more suitable. Since the handle has a non-uniform curved profile connected by three arcs, the turning angle and auxiliary relief angle of the external turning tool must be considered. The main relief angle affects the shape of the workpiece, while the auxiliary relief angle ensures no interference with the machined surface. A calculated relief angle greater than 13.4° is required. Therefore, two types of tools were selected: a right-hand right-handed tool with a 90° main relief angle and a 15° auxiliary relief angle, and a cutting tool.

(3) Based on the selected tools, the right-hand right-handed tool is the primary machining tool. All possible features should be machined first, followed by changes in the taper and cut. Consider the width of the tool to reduce the number of tool changes and minimize idle time during the cutting stroke.

(4) Roughing and finishing processes must also be considered. Using a full contour cycle program simplifies programming but may include too many idle movements, which is inefficient. Instead, the R8mm ball end is roughed using a conical turning method, and the R30mm concave arc is roughed using a roughing cycle. After removing most of the material, the surface is finished using the same tool to ensure accuracy and surface finish requirements.

(5) Choosing appropriate cutting parameters is essential. Under the premise of ensuring processing quality and tool life, the machine tool and cutting tool performance should be fully utilized to maximize efficiency and minimize costs. During roughing, low cutting speeds, large depths of cut, and high feed rates are used. For finishing, high cutting speeds and small feed rates are recommended.

CNC Lathe

3. Programming and Training Considerations

(1) Programming must follow the process plan. Each pass should be clearly defined. First, determine the workpiece coordinate origin (the center of the right end of the handle) and calculate the coordinates of each corner point and the curve connection points. See the note on the drawing for reference. Accurately defining the starting point for each step is crucial, as it directly affects the final contour formation.

(2) According to the roughing, finishing, and tool division process, the roughing program (N00040–N00200) removes most of the excess material, while the finishing program (N00210–N00280) improves surface quality using T01 (external right-handed tool, 15° auxiliary angle). The cone and cutting procedures (N00300–N00340) use T02 (cutting tool, 4mm width). Be mindful of the actual tool nose and the influence of the knife width during programming. See N00300, N00320, etc., in the example program.

(3) Avoid using G00 directly to move the tool to the workpiece surface. Also, avoid using G00 when there is no contact between the tool and the workpiece. Instead, use G01 for safe movement. For example, in N00140, N00220, N00270, and N00310, G01 is used to prevent potential collisions and damage to the tool or workpiece.

(4) Accurate tool setting is essential. CNC programming is based on the tool tip as the reference point. Before training, ensure that the tool tip coordinates match the workpiece origin. Only then can the programmed tool movements produce the correct part contours.

(5) Use the simulation to check the tool path. While simulation can verify the correctness of the program, it may not detect all overcutting or interference issues during actual machining. Thus, even if the simulation looks good, the final part might not be complete due to unaccounted tool interactions.

4. Example Programming

The following is the programming for the boring tool handle on the "GTC2E" CNC lathe:

N00010 M03 S600

N00020 G00 X60 Z20

N00030 T1 (external right-handed tool, 15° relief angle)

N00040 G00 X16 Z2

N00050 G24 X36 W-10 F50

N00060 U-5

N00070 U-5

N00120 G00 X37 Z2

N00130 G22 L2

N00140 C00 U-31

N00150 G01 W-2 F60

N00160 G03 U15.2 W-5.5 R8

N00170 G03 U4.4 W-46.9 R85

N00180 G02 U2.4 W-17.6 R30

N00185 G01 U4

N00190 G00 W72

N00200 G80

N00210 G00 X0 S1000

N00220 G01 Z0 F40

N00230 G03 X15.2 Z-5.5 R8

N00240 G03 X19.6 Z-52.4 R85

N00250 G02 X22 Z-70 R30

N00260 G01 Z-95

N00270 G01 X26

N00280 G00 X60 Z20 S500

N00290 T2 (cutting blade, 4mm width)

N00300 G00 X24 Z-84

N00310 G01 X17 F30

N00320 G01 X16 Z-94

N00330 G01 X0

N00340 G00 X60 Z20

N00350 M05

N00360 M02

5. Conclusion

In summary, the numerical control training process includes several key steps: analyzing the part drawing, drafting the process plan, programming and inputting, tool setup, simulation, and finally machining the part. In the initial training, it's better not to aim for complex shapes or use every possible command at once, as this can lead to frustration. Instead, focus on a few representative surfaces and build your skills gradually. Through practical examples, trainees can develop their ability to create process plans, select tools, and program according to the drawings, ultimately achieving a deeper understanding and better results.

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