Comparison of Structural Types of Five-Axis CNC Machine Tools
The five axes of a five-axis machine tool typically consist of three linear axes plus two rotary axes, but their structural configurations can vary significantly. These different structural forms lead to differences in rigidity, dynamic performance, and precision stability. This article focuses on analyzing and comparing two common structures of five-axis machines: one with a swiveling vertical spindle (head-tilting type), and the other with a stationary vertical spindle and swiveling table (cradle-type), to help users better understand their designs and select the most suitable machine according to their product characteristics and economic conditions.
1. Comparison of Spindle Rigidity
In head-tilting machines, the spindle swings during machining, resulting in relatively lower spindle rigidity. In contrast, with table-tilting machines, the table swings while the spindle remains stationary, so the spindle’s rigidity is unaffected.
2. Comparison of Machining Efficiency
Due to different pivot points, the head-tilting spindle must rotate at a larger angle to achieve the same tool tip displacement (similar to a lever principle). As a result, when machining the same part, head-tilting machines are generally less efficient.
3. Effect of Tool Length on Machining Accuracy
Swing error = Arm length × Swing angle
In head-tilting machines, the tool length contributes to the swing arm length. This means that longer tools increase the swing arm length, thereby amplifying the error. In contrast, in cradle-type machines, the tool length has no effect on the swing arm length, and therefore, does not affect the swing error.
4. Position and Shape Errors from Swing Motion
Both types of machines generate positioning errors during swing motion. However, head-tilting machines also introduce shape errors, whereas table-tilting machines do not.
When the head tilts, the tool’s rotation causes the feed direction to deviate from the tool’s rotation center. This leads not only to positioning errors in holes but also to shape errors — such as wedge-shaped holes.
In table-tilting machines, the feed direction of the spindle and the rotation center of the tool remain aligned, so while positioning errors may occur, no additional shape errors are introduced. This is a clear advantage of the cradle-type structure.
5. Comparison of Machining Size Range
As the spindle tilts in head-tilting machines, the machining range in the workpiece diameter direction decreases — meaning the tilting motion consumes part of the machine’s travel. As a result, the maximum workpiece diameter that can be machined in five-axis mode is smaller than in three-axis mode.
In table-tilting machines, the tilting of the table does not affect the horizontal size of the workpiece. In this sense, the maximum part size remains the same in both three-axis and five-axis machining. However, the actual range is still limited by possible interference from the machine’s structure. Therefore, users should request an interference diagram from the machine tool supplier to evaluate this. In general, for the same travel, a cradle-type table-tilting machine offers a larger five-axis machining range than a head-tilting machine.
Summary
Cradle-Type Table-Tilting Machines:
• Better spindle rigidity
• Higher machining efficiency
• Tool length does not affect accuracy
• No shape errors during machining
• Larger five-axis machining range for the same travel
Head-Tilting Machines:
• Lower spindle rigidity
• Lower machining efficiency
• Accuracy decreases with longer tool length
• Shape errors may occur during machining
• Tilting consumes travel, reducing max workpiece size
Additional Note:
In table-tilting machines, due to the need to overcome the weight of the workpiece, heavier parts place higher demands on the fixtures. Also, when machining large or heavy parts, the machine may not be able to tilt the table at all — in which case, a head-tilting configuration becomes the only viable option. |
