A tab that will not enter its slot, a hole that finishes undersize, or a screen panel that sits out of alignment can often be traced back to one small detail: laser kerf allowances. The laser beam removes material as it cuts, so the finished edge cannot sit exactly on the same line as the original CAD geometry without the cutting path being compensated.
For fabricators, builders, designers and manufacturers, this is not a theoretical issue. Kerf affects whether parts assemble cleanly, whether a tolerance is achievable, and whether a job reaches site ready for the next operation. Getting it right starts with understanding what should be handled in the drawing and what should be handled in the cutting program.
What are laser kerf allowances?
Kerf is the width of material removed by the laser. It is the narrow cut line left behind as the beam passes through sheet, plate or other suitable material. A laser cutting machine follows a programmed toolpath, and that path is offset from the finished profile by an amount that accounts for the kerf.
The term laser kerf allowances is commonly used in two ways. It can describe the automatic compensation applied by the programmer so external profiles are not cut undersize and internal features are not cut oversize. It can also describe intentional clearance added to a design where parts must fit together, bend, accept hardware or allow for coating.
Those are related but different decisions. Standard kerf compensation puts the cut edge in the correct position. Assembly clearance is a design choice based on how the component will be used. Confusing the two can produce parts that are technically cut accurately but do not perform as intended.
Why kerf changes the finished part
If a laser cut were programmed directly down the centre of every CAD line without compensation, an external part would lose material all the way around its perimeter. A circular blank would finish smaller than drawn. An internal hole would grow larger than drawn. Correct toolpath offset reverses that effect, placing the edge close to the nominal dimension specified in the file.
The exact kerf is not fixed. It depends on material type and thickness, laser settings, focal position, assist gas, cut speed and the quality required at the edge. A fine feature in thin stainless steel behaves differently from a profile cut through heavier mild steel or aluminium.
This is why nominal kerf figures should be treated as a starting point, not a blanket rule for every job. A capable laser operator selects and verifies cutting parameters for the material and required result, then applies the suitable compensation in the programming stage.
Internal features need particular attention
Small holes, narrow slots and sharp internal corners are where design assumptions become visible fastest. Even with proper kerf compensation, very small features can be affected by heat input, beam geometry and material movement. The entry point of a cut can also matter on highly visible work.
For example, a slot designed to accept a matching tab may need intentional extra clearance. The amount depends on sheet thickness, tab length, whether the assembly is welded, and whether it needs to slide together by hand before tack-up. A decorative panel may tolerate a minor visual variation inside a small feature, while a precision mounting plate may not.
Kerf compensation is not assembly clearance
A common request is to make every tab and slot exactly the same nominal width. That can work in selected applications, but it does not automatically guarantee an easy fit. Manufacturing tolerance, material variation, burr condition, coating thickness and weld distortion can all affect the final assembly.
For press-fit or self-jigging components, the required clearance should be nominated as part of the design brief. A loose location fit, a hand-assembly fit and an interference fit each need different geometry. The right approach depends on the project, not a default number copied from a previous job.
Powder coating, paint and galvanising deserve their own allowance. These finishes add thickness to surfaces and can make close-fitting tabs, holes and folded components difficult to assemble after coating. If a part will receive a finish after cutting, advise this before programming begins.
How to prepare a drawing for laser cutting
The best laser-cut parts begin with clean, unambiguous files. Supply geometry at the finished, nominal size unless you have specifically agreed that the drawing should include a design allowance. In most cases, the laser programmer applies normal kerf compensation in the CAM software. Manually offsetting all profiles in CAD as well can create a double allowance and inaccurate parts.
Use 1:1 scale and clearly identify the material grade, thickness, quantity and any critical dimensions. If certain holes, slots or outside dimensions are functional, mark them as critical and include the tolerance required. This gives the cutting team the information needed to select the right process and inspect the finished work appropriately.
Avoid duplicate lines, overlapping entities and tiny unintended segments in DXF files. These can lead to repeated cutting, pauses or unwanted marks. Closed profiles should be genuinely closed, and internal cut-outs should be kept on the correct layer where a layered file is used.
Where several parts interlock, send the complete assembly drawing as well as the individual profiles. It is much easier to review fit before cutting than to alter a production run after components have been delivered.
Holes, slots and corners
A hole can be laser cut accurately, but a punched, drilled or reamed finish may be the better choice where the feature is very small or the tolerance is especially tight. The same applies to holes that will be tapped. Consider whether thread engagement, heat effects and hole roundness suit the intended fastener.
Internal corners naturally retain a radius because the laser beam has width. If a square tab must seat fully into an inside corner, add a corner relief or dog-bone detail to the receiving slot. This small design change prevents the radius at the bottom of the internal corner from stopping the tab before it reaches its intended position.
For narrow slots, specify the required finished width and function rather than assuming a generic kerf adjustment will solve the fit. The cut team can then assess whether laser cutting is suitable and whether a test part is worthwhile.
Tolerance, edge quality and material movement
Laser cutting is a highly accurate production process, but every job has practical tolerances. Long, thin strips can move as internal stress is released. Large sheets can contain variation. Heat from the laser can influence fine features or closely spaced cuts, particularly when the material is thin or the design is densely nested.
Cut order and nesting strategy help manage these effects. Parts may be held in the sheet with small tabs until the final stage, or the program may sequence cuts to reduce heat concentration. These choices protect accuracy and reduce the chance of small components tipping or moving during processing.
Edge quality also involves trade-offs. A setting selected for maximum speed may not provide the same edge finish as one selected for a premium visible edge. For architectural screens, exposed brackets and customer-facing components, it is worth discussing the required appearance. For concealed production parts heading to welding or machining, turnaround and cost may carry more weight.
When waterjet is the better option
Laser is an efficient choice for many steel, stainless steel and aluminium profiles, especially where speed and repeatability are priorities. However, laser kerf allowances are only one part of process selection.
Waterjet cutting uses a high-pressure stream rather than heat. It is well suited to heat-sensitive materials, thicker sections, laminates, rubber, foam, tiles and applications where avoiding heat-affected zones matters. It also provides a practical alternative where material characteristics or design requirements make laser less suitable.
At Waterjet & Laser SA, having both CNC laser and waterjet cutting onsite means the conversation can focus on the finished part rather than forcing every job through one process. For some work, the fastest laser cut is the right answer. For others, cold-cut waterjet processing protects the material and delivers the better outcome.
Ask the fitting question before cutting
The most useful question is not simply, “What is the kerf?” It is, “What does this part need to do after it is cut?” Will it locate a fabricated frame, accept a bearing, form part of a decorative screen, be welded, coated, folded or installed on site?
Provide that context with the drawing. A short note about fit, finish and downstream work can prevent rework, delays and unnecessary cost. Accurate cutting starts with accurate geometry, but the best result comes from matching the allowance, process and tolerance to the job at hand.