Analyze the three major items that require measurement of precision when delivering a CNC machining center.

Analysis of the Key Elements in the Precision Acceptance of CNC Machining Centers

I. Introduction

II. Geometric Precision Inspection of CNC Machining Centers

(I) Inspection Items and Connotations

• Flatness of the Worktable Surface: As the clamping reference for workpieces, the flatness of the worktable surface directly affects the installation precision of workpieces and the planar quality after processing. If the flatness exceeds the tolerance, problems such as uneven thickness and deteriorated surface roughness will occur when processing planar workpieces.
• Mutual Perpendicularity of Movements in Each Coordinate Direction: The perpendicularity deviation among the X, Y, and Z coordinate axes will cause 扭曲变形 in the spatial geometric shape of the processed workpiece. For example, when milling a cuboid workpiece, the originally perpendicular edges will have angular deviations, seriously affecting the assembly performance of the workpiece.
• Parallelism of the Worktable Surface during Movements in the X and Y Coordinate Directions: This parallelism ensures that the relative position relationship between the cutting tool and the worktable surface remains constant when the tool moves in the X and Y plane. Otherwise, during planar milling, uneven machining allowances will occur, resulting in a decline in surface quality and even excessive wear of the cutting tool.
• Parallelism of the Side of the T-slot on the Worktable Surface during Movement in the X Coordinate Direction: For machining tasks that require fixture positioning using the T-slot, the accuracy of this parallelism is related to the accuracy of fixture installation, which in turn affects the positioning precision and machining precision of the workpiece.
• Axial Runout of the Spindle: The axial runout of the spindle will cause a tiny displacement of the cutting tool in the axial direction. During drilling, boring and other machining processes, it will result in errors in hole diameter size, deterioration of hole cylindricity, and an increase in surface roughness.
• Radial Runout of the Spindle Bore: It affects the clamping precision of the cutting tool, causing the radial position of the tool to be unstable during rotation. When milling the outer circle or boring holes, it will increase the contour shape error of the machined part, making it difficult to ensure roundness and cylindricity.
• Parallelism of the Spindle Axis when the Spindle Box Moves along the Z Coordinate Direction: This precision index is crucial for ensuring the consistency of the relative position between the cutting tool and the workpiece when machining at different Z-axis positions. If the parallelism is poor, uneven machining depths will occur during deep milling or boring.
• Perpendicularity of the Spindle Rotation Axis to the Worktable Surface: For vertical machining centers, this perpendicularity directly determines the precision of machining vertical surfaces and inclined surfaces. If there is a deviation, problems such as non-perpendicular vertical surfaces and inaccurate inclined surface angles will occur.
• Straightness of the Spindle Box Movement along the Z Coordinate Direction: The straightness error will cause the cutting tool to deviate from the ideal straight trajectory during movement along the Z-axis. When machining deep holes or multi-step surfaces, it will cause coaxiality errors between the steps and straightness errors of the holes.

(II) Commonly Used Inspection Tools

(III) Inspection Precautions

III. Positioning Precision Inspection of CNC Machining Centers

(I) Definition and Influencing Factors of Positioning Precision

(II) Inspection Contents

• Positioning Precision and Repetitive Positioning Precision of Each Linear Motion Axis: Positioning precision reflects the deviation range between the commanded position and the actual reached position of the coordinate axis, while repetitive positioning precision reflects the degree of position dispersion when the coordinate axis repeatedly moves to the same commanded position. For example, when performing contour milling, poor positioning precision will cause deviations between the machined contour shape and the designed contour, and poor repetitive positioning precision will lead to inconsistent machining trajectories when processing the same contour multiple times, affecting surface quality and dimensional precision.
• Return Precision of the Mechanical Origin of Each Linear Motion Axis: The mechanical origin is the reference point of the coordinate axis, and its return precision directly affects the accuracy of the initial position of the coordinate axis after the machine tool is powered on or the zero return operation is performed. If the return precision is not high, it may lead to deviations between the origin of the workpiece coordinate system in subsequent machining and the designed origin, resulting in systematic position errors in the entire machining process.
• Backlash of Each Linear Motion Axis: When the coordinate axis switches between forward and reverse movements, due to factors such as the clearance between mechanical transmission components and changes in friction, backlash will occur. In machining tasks with frequent forward and reverse movements, such as milling threads or performing reciprocating contour machining, backlash will cause “step” -like errors in the machining trajectory, affecting machining precision and surface quality.
• Positioning Precision and Repetitive Positioning Precision of Each Rotary Motion Axis (Rotary Worktable): For machining centers with rotary worktables, the positioning precision and repetitive positioning precision of the rotary motion axes are crucial for machining workpieces with circular indexing or multi-station processing. For example, when processing workpieces with complex circular distribution characteristics such as turbine blades, the precision of the rotary axis directly determines the angular precision and distribution uniformity among the blades.
• Return Precision of the Origin of Each Rotary Motion Axis: Similar to the linear motion axis, the return precision of the origin of the rotary motion axis affects the accuracy of its initial angular position after the zero return operation, and it is an important basis for ensuring the precision of multi-station processing or circular indexing processing.
• Backlash of Each Rotary Motion Axis: The backlash generated when the rotary axis switches between forward and reverse rotations will cause angular deviations when machining circular contours or performing angular indexing, affecting the shape precision and position precision of the workpiece.

(III) Inspection Methods and Equipment

IV. Cutting Precision Inspection of CNC Machining Centers

(I) Nature and Significance of Cutting Precision

(II) Inspection Classification and Contents

• Single Machining Precision Inspection
  • Boring Precision – Roundness, Cylindricity: Boring is a common machining process in machining centers. The roundness and cylindricity of the bored hole directly reflect the precision level of the machine tool when the rotary and linear motions work together. Roundness errors will lead to uneven hole diameter sizes, and cylindricity errors will cause the axis of the hole to bend, affecting the fitting precision with other parts.
  • Flatness and Step Difference of Planar Milling with End Mills: When milling a plane with an end mill, the flatness reflects the parallelism between the worktable surface and the tool movement plane and the uniform wear of the cutting edge of the tool, while the step difference reflects the consistency of the cutting depth of the tool at different positions during the planar milling process. If there is a step difference, it indicates that there are problems with the motion uniformity of the machine tool in the X and Y plane.
  • Perpendicularity and Parallelism of Side Milling with End Mills: When milling the side surface, the perpendicularity and parallelism respectively test the perpendicularity between the spindle rotation axis and the coordinate axis and the parallelism relationship between the tool and the reference surface when cutting on the side surface, which is of great significance for ensuring the shape precision and assembly precision of the side surface of the workpiece.
• Precision Inspection of Machining a Standard Comprehensive Test Piece
  • Contents of Cutting Precision Inspection for Horizontal Machining Centers
    • Precision of Bore Hole Spacing — in the X-axis Direction, Y-axis Direction, Diagonal Direction, and Hole Diameter Deviation: The precision of bore hole spacing comprehensively tests the positioning precision of the machine tool in the X and Y plane and the ability to control dimensional precision in different directions. The hole diameter deviation further reflects the precision stability of the boring process.
    • Straightness, Parallelism, Thickness Difference, and Perpendicularity of Milling the Surrounding Surfaces with End Mills: By milling the surrounding surfaces with end mills, the positional precision relationship of the tool relative to different surfaces of the workpiece can be detected during multi-axis linkage machining. Straightness, parallelism, and perpendicularity respectively test the geometric shape precision among the surfaces, and the thickness difference reflects the cutting depth control precision of the tool in the Z-axis direction.
    • Straightness, Parallelism, and Perpendicularity of Two-axis Linkage Milling of Straight Lines: Two-axis linkage milling of straight lines is a basic contour machining operation. This precision inspection can evaluate the trajectory precision of the machine tool when the X and Y axes move in coordination, which plays a key role in ensuring the precision of machining workpieces with various straight contour shapes.
    • Roundness of Arc Milling with End Mills: The precision of arc milling mainly tests the precision of the machine tool during arc interpolation motion. Roundness errors will affect the shape precision of workpieces with arc contours, such as bearing housings and gears.

(III) Conditions and Requirements for Cutting Precision Inspection

V. Conclusion