Why Tolerances Matter Before You Design
Tolerances are not a quality problem -- they are a reality of the process, and every cutting process has them. Laser cutting, waterjet, punching, and plasma all produce parts with some degree of dimensional variation. What matters is knowing the tolerance band for your process before you design the part, not after the parts arrive.
If you design a tab-and-slot joint assuming zero clearance and plasma removes 0.080" of material per pass, the joint will not fit. If you add the right clearance upfront, it assembles cleanly. This guide gives you the numbers.
Kerf Width
Kerf is the material removed by the cut. The plasma arc does not trace an infinitely thin line -- it melts and blows away a channel of metal with a defined width.
On Can-Cut's fine-cut table, kerf width is typically 0.060" to 0.090" depending on material and thickness. Thicker material and aluminum tend toward the wider end; thin mild steel toward the narrower end.
The critical point: the kerf is centered on the programmed path. This means each side of the cut loses half the kerf width. If you draw a slot that is exactly 0.250" wide and the kerf is 0.080", the resulting slot will be approximately 0.330" wide -- not 0.250".
For most parts this does not matter. For tab-and-slot joinery, mating brackets, or precision fits, you need to account for it.
Kerf Compensation in Your CAD Tool
Many CAD and nesting applications offer automatic kerf compensation -- the toolpath is offset inward or outward by half the kerf width so the finished part matches the drawn dimension. Can-Cut's nesting system applies kerf compensation automatically. If you are designing for a tight fit, it is worth confirming with us whether compensation is being applied and in which direction.
Dimensional Accuracy
Can-Cut targets +/- 0.030" (approximately 1/32") on fine-cut plasma for overall part dimensions. Most parts land within this window reliably.
A few factors that affect accuracy in practice:
- Material flatness: Steel plate, especially in heavier gauges, is not always perfectly flat. Warped or bowed sheet forces the torch height controller to compensate, which can introduce minor deviation.
- Thickness: Thicker plate (1/2" and above) produces slightly more variance than thin gauge due to the wider kerf, longer heat exposure, and greater material mass influencing the melt zone.
- Aluminum: Aluminum's higher thermal conductivity means heat dissipates faster, which generally helps accuracy, but the softer material and different melt behavior mean dimensional variance can be comparable to steel on thicker gauges.
For parts where +/- 0.030" is insufficient, plasma cutting is not the right process. Laser cutting or machining will get you tighter.
Edge Bevel
Fine-cut plasma produces an edge bevel of approximately 1 to 3 degrees from vertical. This means the top face of the cut edge is very slightly wider than the bottom face (or vice versa, depending on which side the dross falls).
For the vast majority of applications -- welded brackets, painted panels, structural components -- this bevel is imperceptible and has no practical effect. It becomes relevant when:
- You are press-fitting a part into a precise bore
- You need a truly square mating edge for a gasket or seal
- You are anodizing aluminum and the slight chamfer affects visual uniformity
If a square edge is a hard requirement, note it when ordering. A light grind on the cut edge removes the bevel.
Dross
Dross is resolidified metal that adheres to the bottom edge of the cut. It forms when the molten metal is not completely ejected downward by the gas velocity -- some of it re-solidifies before it clears the bottom of the plate.
Fine-cut plasma minimizes dross significantly versus conventional plasma, but does not eliminate it entirely. What you can expect:
- Thin mild steel (18 ga - 1/4"): Minimal to no dross on a fresh consumable. Light dross on an aging nozzle.
- Thick mild steel (3/8" - 3/4"): Light to moderate dross on the bottom edge. Removes readily with a wire brush or light grind.
- Stainless steel: Generally less dross than mild steel at equivalent thickness.
- Aluminum: Typically very clean bottom edge with minimal dross.
Dross is a normal characteristic of plasma cut parts, not a defect. If your downstream process requires a perfectly clean bottom edge -- welding, assembly into a groove, or a tight clearance fit -- plan for a deburring or grinding step.
Heat-Affected Zone (HAZ)
Any thermal cutting process alters the base metal in a narrow band along the cut edge. This is the heat-affected zone. In the HAZ, the metal has been heated above its transformation temperature and then rapidly cooled by the surrounding mass, which can change its grain structure and hardness slightly.
- Thin gauge (18 ga - 3/16"): The HAZ is barely measurable -- a fraction of a millimeter. No practical effect on most applications.
- Mid-thickness (1/4" - 3/8"): HAZ extends approximately 0.5-1mm from the cut edge.
- Thick plate (1/2" - 3/4"): HAZ can extend 1-2mm. If you are welding directly on the cut edge of thick plate, a light pass with a grinder before welding restores full base metal properties at the fusion zone.
For decorative or structural parts that are not welded at the cut edge, the HAZ has no meaningful effect.
Design Recommendations
With the above in mind, here are practical rules for designing plasma cut parts:
Clearance Holes for Fasteners
Add 0.030" to 0.060" over nominal fastener diameter. For an M8 bolt (0.315"), draw the hole at 0.345"-0.375". This accounts for kerf, positional variance, and makes assembly possible without filing.
Tab-and-Slot Joints
Add 0.030" to the slot width over the tab thickness. A 1/4" (0.250") thick tab should go into a slot drawn at 0.280". This provides a snug slip fit after kerf compensation is applied.
Minimum Feature Size
Can-Cut's validator flags features below 1/16" (0.0625"). Features this small may not resolve cleanly -- the kerf width approaches the feature size, leaving a burned or missing detail. Redesign narrow features to be at least 1/16" at their narrowest point.
Minimum Hole Diameter
The minimum hole diameter for a clean cutout should be at least equal to the material thickness. On 1/4" steel, the smallest reliable hole is approximately 1/4" diameter. Smaller holes can be cut but will not be perfectly round and may have significant bevel.
Minimum Feature-to-Edge Spacing
Keep features at least 1x the material thickness away from the part edge. Narrower webs tend to warp from heat and may torch-through on thick material.
Minimum Web Between Holes
If multiple holes or islands are close together, keep at least 0.5" of material between them. Closer spacing increases heat buildup, which degrades cut quality on subsequent cuts in the same area.
What Can-Cut's Validator Flags
When you upload your DXF, the validator automatically checks:
- Features below the 1/16" minimum
- Parts that exceed the 48" x 96" table boundary
- Open paths that cannot be cut as a closed contour
- Duplicate or overlapping geometry
Tolerance-related feedback -- kerf adjustment, clearance recommendations -- is not automated but is available if you reach out before ordering.
Ready to Cut a Precision Part?
Upload your DXF and our system will flag any structural concerns before you place your order. Have a tight-tolerance application or a question about whether plasma is the right process for your job? Contact us at can-cut.ca and we will give you a straight answer.