How to judge whether the cutting head of servo pump water jet cutting machine needs to be replaced?

Guide for judging the replacement of cutting head of servo pump water jet cutting machine

Cutting head is the core executive part of servo pump water jet cutting machine, which directly determines cutting accuracy, efficiency and incision quality. Its core vulnerable parts (nozzle, jewel seat, guide sleeve, etc.) will gradually wear with the use time or working conditions, and it is necessary to comprehensively judge whether to replace them through three dimensions: performance, component state and abnormal parameters. The specific judgment criteria and methods are as follows:

First, judging from the abnormal cutting effect: intuitively reflect the core issues.

The cutting effect is the most direct embodiment of the state of the cutting head. If the following abnormalities occur, it is highly probable that the relevant parts of the cutting head need to be inspected or replaced:

1. The quality of incision is obviously decreased.

Serious edge chipping/burr: when cutting hard and brittle materials such as metal and glass, the edge of the cut is irregularly chipped (for example, the burr of the metal plate cut exceeds 0.1mm), or the corner of the stone and ceramic cut falls off; Especially when cutting materials with a thickness of > ＞20mm, the edge collapse range extends from the edge to the inside (normally ≤0.5mm), so it is necessary to check whether the nozzle is worn first.

Deviation of cut verticality exceeds the standard: when using a square ruler or a laser thickness gauge to detect the cut, the verticality error is more than 0.5 (for example, when cutting 10mm stainless steel, the upper and lower ends of the cut deviate more than 0.087mm), and after excluding the factors such as "the workbench is not horizontal and the high pressure is unstable", it may be that the guide sleeve of the cutting head is deformed or the nozzle center deviates, so the cutting head assembly needs to be replaced.

Rough surface/abnormal grain: after normal cutting, the surface of the cut should be smooth (Ra≤3.2μm). If there are "spiral grain" and "uneven steps", or the cutting speed has not changed but the surface roughness has suddenly increased (for example, Ra has increased from 2.0μm to 6.3μm), the nozzle aperture is mostly worn and enlarged (for example, the original aperture is ground to 0.25mm), which leads to the diffusion of high-pressure water jet.

2. Continuous deviation of cutting accuracy

Size error is out of range: when cutting a standard sample (e.g. 100mm×100mm square), the deviation between the actual size and the set size is > 0.05mm (normally it should be ≤±0.03mm), and there is still no improvement after calibrating the numerical control system and guide rail for many times. It may be that the coaxiality between the nozzle of the cutting head and the gemstone holder is invalid (e.g. the gemstone holder is loose), resulting in water beam deviation.

Poor positioning deviation/repetitive accuracy: after multiple cuts in the same position, the center of the cut deviates more than 0.02mm (repetitive positioning accuracy is required to be ≤0.01mm), or the cutting path appears "slight deviation" (for example, straight cutting turns into a slight curve), so it is necessary to check whether the connecting seat of the cutting head is worn (for example, the thread is loose and the positioning pin is deformed), and replace the whole cutting head if necessary.

3. The cutting efficiency is obviously reduced.

Cutting speed is forced to be reduced: in order to ensure the quality of incision, the original cutting speed needs to be reduced by more than 10% (for example, the original cutting speed of 10mm stainless steel is 100mm/min, and now it needs to be reduced to below 85mm/min), and the pressure of high-pressure pump has been adjusted to the rated value (for example, 380MPa), indicating that the energy of water beam in the cutting head is attenuated (for example, the flow rate is reduced due to nozzle blockage or wear).

Abnormal increase in abrasive consumption: Under the same cutting amount, the abrasive consumption is more than 20% than normal (for example, 1.2kg of abrasive was originally consumed for cutting 1㎡ stainless steel, but now it needs more than 1.5kg), which may be due to the abrasion of the abrasive mixing cavity of the cutting head (for example, the gap between the cavity walls is widened and the abrasive is lost without being fully mixed with the water beam), so it is necessary to replace the mixing cavity or the cutting head.

Second, judging from the state of the cutting head components: directly check the wearing parts.

By disassembling or visually inspecting the core components of the cutting head, if the following signs of wear and damage are found, the corresponding components or the whole cutting head should be replaced immediately:

1. Nozzle (core wearing parts)

Enlarged aperture wear: measure nozzle aperture with micrometer (normal new nozzle aperture error ≤±0.005mm). If the initial value of aperture ratio is more than 0.02mm after wear (for example, the original aperture of 0.2mm is ground to 0.22mm), the water beam will change from "concentrated column" to "diffusion cone", which directly affects the accuracy;

Nozzle mouth deformation/crack: visually observe the nozzle outlet end or use a magnifying glass. If there is "edge bump deformation" or "inner wall crack" (mostly due to high-pressure water impact or abrasive scratch), even if the aperture is not out of tolerance, it will lead to water jet disorder and need to be replaced immediately.

2. Gemstone holder and gem (key parts of high-pressure seal)

Damaged/scratched gemstone: the gemstone is the "guiding core" of the high-pressure water beam. If there are "surface collapse" and "internal cracks" (possibly due to foreign objects entering or overpressure impact), the high-pressure water will leak (water drops seep from the joint between the cutting head and the gemstone holder), and the gemstone holder assembly needs to be replaced;

Failure of the seal of the jewel seat: after the cutting head is removed, check the sealing surface between the jewel seat and the main body of the cutting head. If it is found that the gasket is aging (such as hardening and cracking) or the sealing surface is scratched (with obvious scratches), high-pressure water will leak from the seal (water mist can be seen during operation), so it is necessary to replace the gasket or jewel seat.

3. Guide sleeve and mixing cavity

Abrasion of the inner wall of the diversion sleeve: the diversion sleeve is used to guide the abrasive into the water beam. If there are "obvious scratches" and "local depressions" on the inner wall (caused by long-term abrasion of the abrasive), the abrasive will be unevenly mixed and the abrasive particles in the water belt will splash; Measure the inner diameter of the diversion sleeve with calipers, and replace it if it is more than 0.1mm larger than the initial value;

Blockage/deformation of mixing cavity: The mixing cavity is a "mixing chamber" of water jet and abrasive. If the water jet is blocked due to abrasive caking (caused by moisture) or foreign matter entering (caused by sudden thinning or cut-off of water jet during cutting), or the cavity wall is deformed (caused by high temperature or overpressure), it needs to be disassembled and cleaned. If it cannot be recovered after cleaning, the mixing cavity or cutting head needs to be replaced.

Third, judging from the abnormal operation parameters of the equipment: indirectly reflecting the state of the components.

The failure of the cutting head can be indirectly judged by the change of the operating parameters of the servo pump water jet cutting machine. The common anomalies are as follows:

1. Abnormal high pressure fluctuation

During normal operation, the high pressure should be stable within the set value of 5 MPa (if 380MPa is set, the fluctuation should be ≤ 375-385 MPa); If the pressure fluctuates frequently (for example, it drops to 350MPa and then rises again), and after the "servo pump failure and pipeline leakage" are eliminated, the nozzle of the cutting head is mostly blocked or worn (the pressure fluctuates due to the change of water flow), it is necessary to check the cutting head.

2. The servo pump current rises abnormally.

The servo pump current should be ≤ 60% of the rated current at no load (for example, the rated current of 75kW servo pump is 150A, and the no-load current should be ≤ 90a); If the current is more than 15% higher than normal when the load is running (for example, the current is 120A when cutting, and now it is 140A), and the cutting speed is not improved, it may be that the resistance of the cutting head is increased (for example, the nozzle is blocked and the mixing chamber is worn, resulting in poor water flow), so it is necessary to check the cutting head.

3. Abnormal water flow/abrasive flow

If the actual flow rate is lower than the rated value by more than 10% (for example, the rated flow rate is 2.5L/min, but the actual flow rate is only 2.2L/min), and the pipeline is not blocked, it indicates that the nozzle of the cutting head is worn (the aperture is enlarged but the flow rate is decreased instead, and the effective flow rate is reduced due to the diffusion of water beam);

The abrasive flow meter shows "unstable flow" (if it is set at 1.2kg/h, it actually fluctuates at 1.0-1.4kg/h), and "the abrasive tank is blocked and the gas pipe pressure is unstable" is ruled out. It is necessary to check whether the abrasive inlet of the cutting head is worn (leading to unstable air intake).

4. Verification after replacement: Ensure that the new cutting head works normally.

After the cutting head (or core component) is replaced, it needs to be verified by the following steps to avoid secondary failure due to installation or component problems:

No-load test: after installing a new cutting head, adjust the high pressure to 100MPa (low pressure) and run for 10 minutes, and check that there is no leakage at the joint of the cutting head (wipe it with paper towel without watermark), and the water jet is "straight column" (no diffusion and deviation);

Trial cutting verification: cut the standard sample according to the original cutting parameters (such as material, thickness and speed), check the notch quality (no edge collapse, up to standard roughness) and dimensional accuracy (error ≤±0.03mm), and confirm that it is consistent with the initial state of the new equipment;

Parameter recording: record the initial parameters of the new cutting head (such as aperture, water flow rate and cutting speed) as the basis for subsequent wear judgment (replacement can be predicted when the next aperture is ground to the initial value of+0.02mm).

To sum up, to judge whether the cutting head of the servo pump water jet cutting machine should be replaced, it is necessary to combine the comprehensive evaluation of "cutting effect, component state and operating parameters" to avoid "equipment failure caused by excessive use" or "waste caused by premature replacement". It is suggested to set up a cutting head usage account to record the replacement time and cutting amount (such as cumulative cutting area), and form a regular inspection and replacement mechanism based on the above criteria (usually the service life of nozzle is 50-100 hours, and the gem holder is 200-300 hours, depending on the cutting materials and working conditions).