Laser Cutting Focus Position Adjustment Guide

Master correct focus position adjustment methods to improve cutting quality and efficiency

Quick Start

Why adjust focus position?

Focus position directly affects energy density distribution, thereby impacting cutting speed, edge quality, and penetration capability. Different materials and thicknesses require different focus positions for optimal cutting results.

How often should focus be adjusted?

After new equipment installation, after replacing focusing lens, when cutting quality deteriorates, or perform quarterly calibration.

1. Focus Position Fundamentals

Focus Position Definitions

Negative Focus

Focus inside material

• Focus position: -1 ~ -5 mm

• Energy concentrated internally

• Suitable for thick plate cutting

• Strong penetration capability

Zero Focus

Focus at material surface

• Focus position: 0 mm

• Energy evenly distributed

• General cutting setting

• Balanced performance

Positive Focus

Focus above material

• Focus position: +0.5 ~ +2 mm

• Energy concentrated on surface

• Suitable for thin sheet cutting

• Good surface quality

Impact of Focus Position on Cutting Quality

• Penetration Capability: Negative focus enhances penetration, positive focus has weaker penetration
• Edge Quality: Zero focus and slight negative focus typically produce smoothest edges
• Dross Formation: Excessive negative focus can cause bottom dross
• Cutting Speed: Negative focus can increase speed, but too deep reduces quality

Optimal Focus Position by Material

Material Type	Thickness Range	Recommended Focus Position	Notes
Carbon Steel	1-6mm	-1 ~ 0 mm	Oxygen cutting
Carbon Steel	6-20mm	-2 ~ -1 mm	Oxygen cutting, thick plate
Stainless Steel	1-10mm	-1 ~ 0 mm	Nitrogen cutting
Aluminum Alloy	1-8mm	0 ~ +1 mm	Nitrogen cutting, positive or zero focus
Copper	1-5mm	+0.5 ~ +1 mm	Highly reflective material, positive focus
Acrylic	1-20mm	0 mm	CO₂ laser, zero focus

2. Focus Position Adjustment Methods

Combined with intelligent control systems, this significantly improves first-time success rate and consistency. Some manufacturers (such as OPMT Laser) offer adaptive focus/height tracking solutions that maintain stable focus under material warpage and long-duration processing conditions.

Method 1: Ramp Test (Most Common)

High accuracy, simple operation, suitable for most equipment

Recommended

Required Tools

45° ramp block or dedicated ramp tool
Test plate (typically thin stainless steel or carbon steel)
Tape measure or steel ruler

Procedure

1
Place Ramp Block
Place the 45° ramp block on the cutting table, ensuring the ramp surface faces the laser beam. Ramp height is typically 40-60mm.
2
Adjust Cutting Head Height
Move the cutting head above the ramp block, adjust Z-axis height so the nozzle is approximately 1-2mm from the ramp surface (normal cutting height).
3
Pulse Marking
Fire multiple pulse marks (5-10) along the ramp surface in the inclined direction, spaced 5-10mm apart. Use low power (100-300W) and short pulses (10-30ms).
4
Observe Results
Remove the ramp block and observe the pulse marks. The smallest, roundest, and deepest mark indicates the focus position. Use a tape measure to measure the vertical height from this point to the ramp base.
5
Calculate Focus Offset
Based on 45° ramp geometry, focus position offset = measured height - (ramp height / √2). Adjust Z-axis parameters to position focus at the desired location (typically at material surface or slightly inside).

Result Interpretation Tips

• Deepest Mark: Highest energy density, indicates focus position
• Mark Shape: Should be circular; elliptical shape indicates beam axis misalignment
• Mark Size: Smallest spot at focus, increases when defocused
• Repeatability: Recommend measuring 2-3 times and averaging for accuracy

Method 2: Dot Test

Simple and fast, suitable for quick checks and rough adjustments

Applicable Scenarios

Quick check if focus has shifted
Alternative method when dedicated ramp tool unavailable
Rapid verification during routine maintenance

Procedure

1.Fire multiple pulse marks at different Z heights on flat material (spaced 2-3mm apart)
2.Observe which mark is smallest, roundest, and penetrates deepest
3.This position indicates current focus; adjust Z-axis to match target position

Note: Dot test has lower accuracy, suitable only for quick checks. For precise adjustment, use the ramp test method.

Method 3: Auto-Focus System

High-end equipment feature with high automation level

Capacitive Focus Principle

Uses capacitance changes between nozzle and material surface to automatically detect and adjust focus position. Suitable for warped materials or scenarios requiring frequent thickness changes.

Operation Procedure

1.Enable auto-focus function in control system
2.Set target focus position offset (e.g., -1mm)
3.System automatically detects material surface and adjusts Z-axis
4.Begin cutting; system tracks focus position in real-time

Calibration Frequency Recommendations

• Daily: Check capacitive sensor cleanliness
• Weekly: Verify calibration accuracy (using standard block)
• Monthly: Recalibrate system (if deviation detected)

3. Focus Position Error Diagnosis

Cutting Characteristics: Focus Too High

×
Incomplete Penetration
Insufficient energy density to penetrate material bottom
×
Bottom Dross or Burrs
Insufficient energy prevents complete melt ejection
×
Significant Speed Reduction Required
Compensates for insufficient energy density
×
Excessive Surface Ablation
Energy concentrated on surface, insufficient at bottom

Solution: Lower Z-axis position to move focus downward

Cutting Characteristics: Focus Too Low

×
Excessively Wide Surface Kerf
Focus inside material, larger surface spot size
×
Poor Surface Quality, High Roughness
Insufficient surface energy density
×
Bottom Kerf Too Narrow or Burnt Edge
Energy concentrated at bottom
×
Non-Vertical Kerf, Wide at Top Narrow at Bottom
Uneven energy distribution

Solution: Raise Z-axis position to move focus upward

Quick Diagnostic Flowchart

Symptom → Possible Cause → Solution

No PenetrationFocus too high or insufficient powerLower Z-axis

Poor Surface QualityFocus too lowRaise Z-axis

Bottom DrossFocus too high or insufficient pressureLower Z-axis or increase pressure

Wide Top Narrow BottomFocus too lowRaise Z-axis

Excessively Wide KerfFocus misaligned or excessive powerRecalibrate

4. Frequently Asked Questions (FAQ)

Q1: How often should focus be recalibrated?

A: Depends on usage frequency and accuracy requirements:

High Precision Processing: Weekly check, calibrate immediately if deviation detected
General Cutting: Monthly periodic calibration
Required Calibration Situations: After lens replacement, after head collision, when cutting quality significantly deteriorates

Q2: How does lens contamination affect focus position?

A: Lens contamination does not change physical focus position, but:

Reduces transmittance, causing energy density decrease
Causes beam distortion, affecting spot quality
Results in cutting effects similar to "focus misalignment"
Recommend daily cleaning of protective lens, weekly inspection of focusing lens

Q3: How to select different lens focal lengths?

A: Focal length affects spot size and depth of focus:

• Short Focal Length (50-75mm): Small spot, high precision, suitable for thin sheet precision cutting

• Standard Focal Length (127mm/5 inches): Balanced performance, most commonly used

• Long Focal Length (190mm+): Large depth of focus, high tolerance, suitable for thick plate cutting

Q4: Does auto-focus system still require manual calibration?

A: Yes. Although auto-focus systems are convenient:

The system itself requires periodic calibration (typically monthly)
Different materials have different capacitive properties, which may affect detection accuracy
Recommend quarterly verification of auto-focus accuracy using ramp test method
When first using new materials, manually verify focus position

Related Guides

Material Thickness Cutting Parameters

View recommended focus positions for different materials

Laser Cutting Troubleshooting Guide

Diagnose and resolve cutting quality issues

Laser Focusing Lens Specifications

Understand characteristics of different focal length lenses

Related Tools

Laser Power Density Calculator

Calculate power density at different focus positions

Laser Cutting Kerf Width Calculator

Focus position affects kerf width

References:

TRUMPF Operating Manual - Laser Cutting Machine Focus Calibration Procedures
Bystronic Training Materials - Focus Position Adjustment and Optimization
ISO 9013 - Thermal Cutting Quality and Tolerance Standards
Industry Expert Experience and Best Practices

Last Updated: 2025-10-31 | This guide is based on operating manuals from mainstream laser equipment manufacturers and industry standards, provided for operator reference and learning.