How to ensure the precision in the machining process of the die carving machine?

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It is a systematic project to ensure the machining accuracy of the die carving machine, which involves many links such as equipment performance, material characteristics, tool state, parameter setting, environmental control and so on. The following details the specific safeguard measures from the key influencing factors:

I. Accuracy of the equipment itself: Basic guarantee

The mechanical accuracy of the die carving machine is the premise of machining accuracy, and it is necessary to ensure that the equipment is in the best state through calibration and maintenance.

Mechanical structure calibration

Accuracy of guide rail and screw rod: the parallelism and verticality errors of guide rail (linear guide rail or dovetail guide rail) will directly lead to straightness and flatness deviation of machined parts. It is necessary to detect the positioning accuracy and repeated positioning accuracy of X/Y/Z axis regularly (for example, every month), and correct the errors through the functions of "pitch error compensation" and "reverse clearance compensation" of the numerical control system (for example, if there is a 0.01mm reverse clearance of the screw rod, a compensation value can be set in the system to offset the return error).

Accuracy of spindle: the radial runout (≤0.005mm) and axial movement of spindle will lead to the deviation of tool rotation, and then affect the machining size. It is necessary to regularly detect the spindle runout with a dial indicator. If it is out of tolerance, it is necessary to replace the spindle bearing or adjust the pre-tightening force (especially the high-speed spindle should be strictly controlled).

Worktable flatness: The flatness error of the worktable (≤ 0.02 mm/m) will lead to uneven material placement and depth deviation during processing. It can be adjusted by grinding the workbench or leveling the pad to ensure that the material is completely attached to the workbench.

Parameter optimization of CNC system

The "acceleration parameters" and "acceleration/deceleration curves" of numerical control systems (such as Wei Hong, New Generation and Siemens) need to match the rigidity of the equipment: excessive acceleration will easily lead to inertial vibration, while too small will affect the efficiency, so it needs to be adjusted according to the hardness of the processed materials (metal needs low acceleration, and wood can be slightly higher).

Enable the "smooth transition" function: automatically optimize the speed at the corner of the path to avoid the tool deflection caused by sudden stop and sharp turn (especially when machining the sharp corner edge of the tool die, the corner accuracy should be ensured).

Second, material control: reduce deformation and error.

The stability of the substrate (wood, plywood, metal plate) of the die directly affects the precision after machining, and it needs to be controlled from pretreatment to fixation.

Material pretreatment

Stress release: wood or plywood should be dried (moisture content ≤12%) and aged to avoid deformation caused by internal stress release after processing (for example, the undried wood may shrink after processing, resulting in narrower width of the cutter slot).

Smoothness screening: check the surface of the material with a leveling instrument or a laser leveling instrument. Materials with curvature > ＞0.5mm/m/m should be eliminated or flattened before use (otherwise, the deformation of the fixed material will be transferred to the processing size).

Material fixing mode

Uniform clamping force: when using multi-point fixture or vacuum adsorption, it is necessary to ensure uniform stress on the material (to avoid local deformation of the material caused by single-point clamping). For example, metal plate processing needs to be supported by equal height pads to prevent deformation of suspended parts due to cutting force during processing.

Uniform positioning datum: take the same side or hole position of the material as the positioning datum (such as using positioning pins to cooperate with fixtures) to avoid datum deviation caused by multiple clamping (especially when processing in batches, the positioning consistency of each material should be ensured).

Third, tool management: reduce cutting errors

The accuracy, installation state and wear degree of the tool are the key factors affecting the machining accuracy, which need to be controlled from the whole process of "selection, assembly, use and replacement".

Tool selection and accuracy

The accuracy of the tool itself (such as the concentricity of the handle and the blade runout) should meet the standards: the blade runout of the high-speed steel tool is ≤0.01mm, and that of the cemented carbide tool is ≤0.005mm (which can be detected by the tool presetter).

Select cutting tools according to machining requirements:

Machining the cutting edge of the knife die (high sharpness is required): choose a single-edged spiral knife or a straight-edged knife to avoid the dimensional deviation caused by the overlapping cutting edges of the double-edged knife;

Machining deep groove (such as the groove of the tool seat of the tool die): choose a tool with the length-diameter ratio ≤5:1 (too long and easy to chatter), and use a rigid tool holder if necessary.

Tool installation and calibration

Concentricity calibration: when installing the tool, use the tool holder calibrator to ensure that the tool is concentric with the spindle (deflection ≤0.003mm), otherwise radial force will be generated when rotating at high speed, which will lead to larger machining size (such as wider cutter groove width).

Accuracy of tool alignment: use an automatic tool alignment instrument (with an accuracy of ±0.001mm) to set the zero point of Z axis, so as to avoid the error of manual tool alignment (such as probing with paper) (the error of manual tool alignment may reach 0.01-0.03mm).

Tool wear monitoring

In the process of machining, tool wear will lead to the increase of cutting force, rough machining surface and even dimensional deviation (such as shallow depth of tool groove). It needs to be monitored in the following ways:

Observe the machined surface: when there are obvious burrs and coarse lines, stop the machine in time to check the tool;

Set tool life: according to the hardness and machining capacity of the material, preset the service life of the tool (for example, when machining metal, it is necessary to check the wear of the cemented carbide tool every 50m), and replace it when it expires.

4. Optimization of machining parameters: reducing cutting deformation.

Unreasonable parameter setting leads to problems such as "giving way to the tool" (the tool is bent by cutting force) and thermal deformation of the material, which needs to be accurately matched according to the characteristics of the material and the tool.

Core parameter matching principle

Parameter setting is based on an example (metal die processing)

Spindle speed The smaller the tool diameter, the higher the speed (to avoid excessive cutting force); The harder the material, the higher the rotating speed (reducing friction heat). φ 3mm cemented carbide cutter: 15,000-20,000 r/min.

Feed speed is inversely proportional to cutting depth and material hardness (hard material/deep cutting requires low speed). 45 # steel processing: feed speed is 500-800mm/min.

Cutting depth Single depth ≤ 1/3 of tool diameter (hard material). When the total depth of cutting tool load is reduced by 5mm by layered cutting, it will be cut in three times (2mm+2mm+1mm).

Special measures to avoid "giving way to the knife"

When machining long and narrow grooves or thin-walled structures, "reciprocating cutting" is adopted instead of "unidirectional cutting" to reduce the deflection caused by unilateral force of the cutter;

For high hardness materials (such as stainless steel die), pre-drill the process hole and start cutting from the hole to avoid the impact deformation when the tool cuts from the edge.

V. Program and path optimization: reducing path errors

Whether the machining path generated by CAD/CAM program is reasonable or not directly affects the final accuracy, and the following points should be emphatically checked:

Path accuracy verification

Simulate the path with software (such as Mastercam's "solid simulation"), and check whether there is over-cutting (the tool cuts into the non-machining area), under-cutting (the set depth is not reached) or path overlapping (resulting in large local size).

Ensure that the starting point/ending point of the path is consistent with the zero point of the coordinate system: for example, the cutting edge positioning of the die should take the reference point of the drawing as the origin to avoid the overall dimensional deviation caused by the coordinate system deviation.

Path smoothness optimization

An "arc transition" is set at the corner (instead of a right-angle corner) to reduce the sudden stop and sharp turn of the tool at the corner and avoid the dimensional deviation caused by inertia (especially for the sharp corner cutting edge of the tool die, the R angle accuracy should be ensured).

Avoid "tool jumping" (rapid movement of non-machining area) when the tool collides with the fixture or material, and the path should be raised to a safe height (≥ material thickness+5mm).

VI. Environment and Real-time Monitoring: Reducing External Interference