Why do high-strength bolt holes require more than cutting accuracy?
𝗟𝗮𝘀𝗲𝗿 𝗰𝘂𝘁𝘁𝗶𝗻𝗴 𝗵𝗮𝘀 𝗰𝗵𝗮𝗻𝗴𝗲𝗱 𝘀𝘁𝗿𝘂𝗰𝘁𝘂𝗿𝗮𝗹 𝘀𝘁𝗲𝗲𝗹 𝗳𝗮𝗯𝗿𝗶𝗰𝗮𝘁𝗶𝗼𝗻
Laser cutting is becoming the preferred process for more structural steel applications. Higher power, faster cutting and improved automation have continued to expand what the process can do. For high-strength bolt holes, laser cutting can complete large quantities of holes quickly and accurately, without the separate drilling processes traditionally required.
CLICK THE LINK BELOW TO WATCH THE VIDEO
Laser-cut bolt holes.mp4
The cutting footage shown above was recorded on our 3rd-generation system in 2024.
But 𝗻𝗼𝘁 𝗲𝘃𝗲𝗿𝘆 𝘀𝘂𝗰𝗰𝗲𝘀𝘀𝗳𝘂𝗹 𝗰𝘂𝘁 𝗶𝘀 𝗮 𝘀𝘂𝗰𝗰𝗲𝘀𝘀𝗳𝘂𝗹 𝗲𝗻𝗴𝗶𝗻𝗲𝗲𝗿𝗶𝗻𝗴 𝗮𝗽𝗽𝗹𝗶𝗰𝗮𝘁𝗶𝗼𝗻.
A finished hole may have the correct diameter, position and appearance. However, dimensional accuracy alone cannot tell us whether the cutting process has affected the material around the hole.
Laser cutting is a 𝘁𝗵𝗲𝗿𝗺𝗮𝗹 𝗽𝗿𝗼𝗰𝗲𝘀𝘀.
The concentrated heat creates a narrow 𝗵𝗲𝗮𝘁-𝗮𝗳𝗳𝗲𝗰𝘁𝗲𝗱 𝘇𝗼𝗻𝗲 along the cut surface. If the process is not properly controlled, local hardness may increase, and ductility may decrease.
Under demanding service conditions, especially repeated or dynamic loading, these changes may become more significant.
That is why 𝗮 𝗴𝗼𝗼𝗱-𝗹𝗼𝗼𝗸𝗶𝗻𝗴 𝗵𝗼𝗹𝗲 𝗶𝘀 𝗻𝗼𝘁 𝗲𝗻𝗼𝘂𝗴𝗵.
𝗪𝗲 𝘁𝗲𝘀𝘁𝗲𝗱 𝘄𝗵𝗮𝘁 𝗰𝗮𝗻𝗻𝗼𝘁 𝗯𝗲 𝘀𝗲𝗲𝗻
To assess the effect of thermal cutting on the material, 𝗩𝗶𝗰𝗸𝗲𝗿𝘀 𝗵𝗮𝗿𝗱𝗻𝗲𝘀𝘀 𝘁𝗲𝘀𝘁𝗶𝗻𝗴 was carried out on a 𝗤𝟯𝟵𝟬𝗕 laser-cut test plate.
The testing method followed 𝗜𝗦𝗢 𝟲𝟱𝟬𝟳-𝟭:𝟮𝟬𝟭𝟴, with the results reported as 𝗛𝗩𝟭𝟬.
A total of 𝟭𝟱 𝗺𝗲𝗮𝘀𝘂𝗿𝗲𝗺𝗲𝗻𝘁𝘀 were taken along the thermally cut surface, covering three regions:
- Near the upper surface
- At the plate centreline
- Near the lower surface
The report used 𝗘𝗡 𝟭𝟬𝟵𝟬-𝟮:𝟮𝟬𝟭𝟴 as the evaluation basis and applied a specified maximum hardness value of:
𝟯𝟴𝟬 𝗛𝗩𝟭𝟬
All 15 measurements were below this limit.
The highest recorded value was 𝟯𝟲𝟴 𝗛𝗩𝟭𝟬, and the test result was assessed as compliant.
This confirms that excessive local hardening was not detected in the tested sample under the applied cutting conditions.
However, 𝗵𝗮𝗿𝗱𝗻𝗲𝘀𝘀 𝗶𝘀 𝗼𝗻𝗹𝘆 𝗼𝗻𝗲 𝗽𝗮𝗿𝘁 𝗼𝗳 𝘁𝗵𝗲 𝗲𝘃𝗮𝗹𝘂𝗮𝘁𝗶𝗼𝗻.
The result applies to the tested material, sample and cutting conditions. It does not, by itself, establish suitability for every steel grade, thickness or application.
𝗧𝗵𝗲 𝘀𝗲𝗿𝘃𝗶𝗰𝗲 𝗰𝗼𝗻𝗱𝗶𝘁𝗶𝗼𝗻 𝘀𝘁𝗶𝗹𝗹 𝗺𝗮𝘁𝘁𝗲𝗿𝘀
A bolt hole in a building and a bolt hole in a bridge may look exactly the same when they leave the factory.
But the loads they face can be very different.
Many building steel components are mainly subjected to relatively 𝘀𝘁𝗮𝘁𝗶𝗰 𝗹𝗼𝗮𝗱𝗶𝗻𝗴, although wind, seismic action and other variable loads may still need to be considered.
Bridge components and other dynamically loaded structures may be repeatedly subjected to vibration, traffic loads and stress cycles.
Under 𝗿𝗲𝗽𝗲𝗮𝘁𝗲𝗱 𝗹𝗼𝗮𝗱𝗶𝗻𝗴, a small defect or locally hardened area at the hole edge may become more significant over time.
One of our customers applies a stricter hardness control value of 𝟯𝟱𝟬 𝗛𝗩𝟭𝟬 and does not accept laser-cut bolt holes in dynamically loaded locations.
This is a 𝗰𝘂𝘀𝘁𝗼𝗺𝗲𝗿- 𝗮𝗻𝗱 𝗽𝗿𝗼𝗷𝗲𝗰𝘁-𝘀𝗽𝗲𝗰𝗶𝗳𝗶𝗰 𝗿𝗲𝗾𝘂𝗶𝗿𝗲𝗺𝗲𝗻𝘁, rather than a universal rule for every bridge or every international market.
More importantly, meeting a hardness value alone does not automatically prove that a laser-cut hole is suitable for a fatigue-critical component.
The steel grade, service condition, hole-edge quality, fatigue requirement and project approval process must all be considered.
The real distinction is not simply between 𝗯𝘂𝗶𝗹𝗱𝗶𝗻𝗴𝘀 𝗮𝗻𝗱 𝗯𝗿𝗶𝗱𝗴𝗲𝘀. It is between relatively static applications and applications exposed to significant 𝗱𝘆𝗻𝗮𝗺𝗶𝗰 𝗼𝗿 𝗳𝗮𝘁𝗶𝗴𝘂𝗲 𝗹𝗼𝗮𝗱𝗶𝗻𝗴.
Dynamic
Static
𝗖𝘂𝘁𝘁𝗶𝗻𝗴 𝘀𝘁𝗿𝗮𝘁𝗲𝗴𝘆 𝗮𝗹𝘀𝗼 𝗮𝗳𝗳𝗲𝗰𝘁𝘀 𝘁𝗵𝗲 𝗵𝗼𝗹𝗲
Even when laser cutting is suitable for the application, the cutting path still needs to be carefully designed. Piercing directly on the finished hole contour can leave a visible mark on the functional surface.
For this reason, the piercing point should be moved away from the finished hole edge by using a lead-in. However, 𝗻𝗼𝘁 𝗲𝘃𝗲𝗿𝘆 𝗹𝗲𝗮𝗱-𝗶𝗻 𝗽𝗿𝗼𝗱𝘂𝗰𝗲𝘀 𝘁𝗵𝗲 𝘀𝗮𝗺𝗲 𝗿𝗲𝘀𝘂𝗹𝘁. A 𝘀𝘁𝗿𝗮𝗶𝗴𝗵𝘁 𝗹𝗲𝗮𝗱-𝗶𝗻 is simple to program, but it enters the circular contour abruptly.
The transition between the straight path and the circular profile may leave a small corner or witness mark on the hole surface.
For high-strength bolt holes, we use an 𝗮𝗿𝗰 𝗹𝗲𝗮𝗱-𝗶𝗻.
Instead of meeting the circular contour abruptly, the cutting path gradually blends into the finished profile along an arc.
After completing the full circle, the cutting path continues slightly beyond the starting point, creating a 𝗰𝗼𝗻𝘁𝗿𝗼𝗹𝗹𝗲𝗱 𝗼𝘃𝗲𝗿𝗹𝗮𝗽.
This smooth transition between the starting and finishing points reduces local discontinuities and produces a 𝘀𝗺𝗼𝗼𝘁𝗵𝗲𝗿, 𝗺𝗼𝗿𝗲 𝗰𝗼𝗻𝘁𝗶𝗻𝘂𝗼𝘂𝘀 𝗵𝗼𝗹𝗲 𝘀𝘂𝗿𝗳𝗮𝗰𝗲.
It may appear to be a small programming detail, but it is an important part of producing a high-quality laser-cut bolt hole.
𝗞𝗻𝗼𝘄𝗶𝗻𝗴 𝘁𝗵𝗲 𝗯𝗼𝘂𝗻𝗱𝗮𝗿𝘆 𝗶𝘀 𝗽𝗮𝗿𝘁 𝗼𝗳 𝗲𝗻𝗴𝗶𝗻𝗲𝗲𝗿𝗶𝗻𝗴
Laser cutting can produce fast, accurate and consistent bolt holes in suitable structural steel applications. But the final result depends on more than cutting speed or machine power. It depends on the material, heat input, cutting strategy, hardness and the actual service condition.
For dynamically loaded or fatigue-critical components, requirements may be stricter, and further testing or project approval may still be necessary. A mature process is not one that claims to suit every application. It is one whose capabilities and boundaries are both understood.
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