During the machining process of CNC walking-head lathes, there is a high requirement for machining accuracy. How to solve the problem of machining accuracy errors? Operators need to carefully analyze the causes affecting the machining accuracy of CNC walking-head lathes in order to get to the root of the problem and find the correct solution.

1. Machining Principle Error

Machining principle error refers to the error generated by using approximate tool contour or approximate transmission relationship during machining. This type of error commonly occurs in thread machining, gear machining, and complex surface machining.

For example, when a gear hob is used on a lathe to machine involute gears, an Archimedean basic worm or a normal straight-sided basic worm is often adopted instead of the involute basic worm to facilitate hob manufacturing, resulting in errors in the gear's involute tooth profile. Another example is when machining module worms; since the worm's pitch equals the worm wheel's circular pitch (i.e., mπ), where m is the module, and π is an irrational number, but the number of teeth on the exchange gears of the lathe is limited. When selecting exchange gears, π must be approximated as a fractional value (π = 3.1415) for calculation, which causes inaccuracies in the tool's forming motion (helical motion) relative to the workpiece, leading to pitch errors.

In machining, approximate machining is generally employed. Under the premise that the theoretical error can meet the machining accuracy requirements (≤10%-15% of dimensional tolerance), this approach is used to improve productivity and economy.

2. Adjustment Error

The adjustment error of a through-the-heart CNC machine tool refers to the error caused by inaccurate adjustment.

3. Machine Tool Errors

Machine tool errors refer to the manufacturing errors, installation errors, and wear of walking-head automatic lathes. They mainly include guide rail orientation errors, spindle rotational errors, and transmission errors in the machine tool's transmission chain.

1) Guide Rail Orientation Accuracy — The degree to which the actual motion direction of the moving parts of the guide rail pair conforms to the ideal motion direction. It mainly includes: ① Straightness of the guide rail in the horizontal plane (Δy) and vertical plane (bending, Δz); ② Parallelism of the front and rear guide rails (twist); ③ Parallelism or perpendicularity errors of the guide rail relative to the spindle rotational axis in the horizontal and vertical planes.

2) The impact of guide rail orientation accuracy on cutting machining mainly considers the relative displacement between the tool and workpiece caused by guide rail errors in the error-sensitive direction. During CNC walking-head lathe machining, the error-sensitive direction is horizontal; processing errors caused by orientation errors in the vertical direction can be ignored. During boring, the error-sensitive direction changes with the rotation of the tool. During planing, the error-sensitive direction is vertical, and the straightness of the bed guideways in the vertical plane causes linear and flatness errors in the machined surface.

Spindle Rotation Error of Machine Tools

The spindle rotation error of CNC walking-head machine tools refers to the deviation of the actual rotating axis from the ideal rotating axis. It mainly includes spindle end face circular runout, spindle radial circular runout, and spindle geometric axis angular oscillation.

1) Impact of Spindle End Face Circular Runout on CNC Walking-Head Machining Accuracy: ① No effect when machining cylindrical surfaces; ② When turning or boring end faces, it will cause perpendicularity error between the end face and the cylindrical axis or flatness error of the end face; ③ When machining threads, it will result in pitch periodic error.

2) Impact of Spindle Radial Circular Runout on Machining Accuracy: ① If the radial rotational error is manifested as the actual axis undergoing simple harmonic linear motion in the y-axis coordinate direction, the hole bored by a boring machine will be an elliptical hole, with the roundness error equal to the amplitude of the radial circular runout; while the hole turned by a lathe is unaffected; ② If the spindle geometric axis undergoes eccentric motion, both turning and boring can produce a circle with a radius equal to the distance from the tool tip to the average axis.

3) Impact of Spindle Geometric Axis Angular Oscillation on Machining Accuracy: ① The geometric axis forms a conical trajectory with a certain cone angle relative to the average axis in space. From each cross-section, it is equivalent to the geometric axis center oscillating eccentrically around the average axis center, but the eccentricity varies along the axial direction; ② When the geometric axis swings within a certain plane, from each cross-section, it is equivalent to the actual axis undergoing simple harmonic linear motion within a plane, but the runout amplitude varies along the axial direction; ③ In reality, the angular oscillation of the spindle geometric axis is a superposition of the above two types.

Transmission Error of Machine Tool Transmission Chain

The transmission error of the CNC walking-head machine tool transmission chain refers to the relative motion error between the first and last transmission elements in the transmission chain.

4. Manufacturing Errors and Wear of Jigs

The main errors of jigs for CNC walking-head lathes refer to: 1) Manufacturing errors of positioning elements, tool guiding elements, indexing mechanisms, and jig bodies; 2) Relative dimensional errors between the working surfaces of the above elements after jig assembly; 3) Wear of the working surfaces during the use of the jig.

5. Manufacturing Errors and Wear of Tools

During the machining process of CNC walking-head lathes, the impact of tool errors on machining accuracy varies depending on the type of tool.

1) The dimensional accuracy of fixed-size tools (such as drills, reamers, keyway milling cutters, and round broaches) directly affects the dimensional accuracy of the workpiece.

2) The shape accuracy of forming tools (such as forming turning tools, forming milling cutters, and forming grinding wheels) directly affects the shape accuracy of the workpiece.

3) The tool edge shape errors of generating tools (such as gear hobs, spline hobs, and shaping cutters) affect the shape accuracy of the machined surface.

4) For general tools (such as turning tools, boring tools, and milling cutters), their manufacturing accuracy does not directly affect machining accuracy, but these tools are prone to wear.

6. Deformation of the Process System Under Force

During machining on a walking-head lathe, the process system deforms under the action of cutting forces, clamping forces, gravity, and inertial forces. This deformation disrupts the relative positional relationships of the adjusted components of the process system, leading to machining errors and affecting the stability of the machining process. The main considerations include machine tool deformation, workpiece deformation, and total deformation of the process system.

7. Thermal Deformation of the Process System

During machining, the process system heats up and deforms due to heat generated by internal sources (cutting heat, friction heat) or external sources (ambient temperature, thermal radiation). This thermal deformation affects machining accuracy. In large workpiece machining and precision machining, machining errors caused by process system thermal deformation account for 40%-70% of the total machining errors.

The impact of workpiece thermal deformation on machining includes uniform heating of the workpiece and non-uniform heating of the workpiece.

8. Residual Stresses Inside the Workpiece

Sources of residual stresses: 1) Residual stresses generated during blank manufacturing and heat treatment; 2) Residual stresses from cold straightening; 3) Residual stresses from cutting machining.

9. Impact of Clamping Force on Machining Accuracy

When the workpiece is clamped, due to low workpiece rigidity or improper clamping force application points, the workpiece deforms accordingly, causing machining errors.

10. Impact of Cutting Force on Machining Accuracy

Considering only machine tool deformation, for machining shaft parts, the machine tool's deformation under cutting force causes the machined workpiece to have a saddle shape (thicker at both ends and thinner in the middle), i.e., cylindrical error. Considering only workpiece deformation, for machining shaft parts, the workpiece's deformation under cutting force causes the machined workpiece to have a barrel shape (thinner at both ends and thicker in the middle). For machining hole parts, considering only machine tool or workpiece deformation separately, the shape of the machined workpiece is the reverse of that of the machined shaft parts.