Drawing Tolerances & Dimensions
What tolerance do we hold in laser cutting, CNC bending and welding? The figures, and why material and thickness move them.
- Three words decide every tolerance: material, thickness, feature.
- Laser cutting: +/- 0.1 mm.
- Default: ISO 2768 applies unless a feature is marked tighter on the drawing.
- Why it moves: material grade and sheet thickness move springback and heat input, so the same tolerance does not cost the same on every part.
- Cost rule: mark only the interfaces that meet your own equipment. Tightening every joint raises price with no benefit.
The short answer
Every tolerance question is answered by the same three words: material, thickness, feature. Below is the honest version, process by process.
| Process | Typical working figure | What decides where inside that you land |
|---|---|---|
| Laser cutting | Around ±0.1 mm on the material ranges we normally run | Material grade and sheet thickness, primarily |
| CNC bending | General practice is ±0.5 mm on formed dimensions and about ±1° on bend angles | Thickness, bend radius, number of bends, tooling |
| Welding (distortion) | Expected movement, not a tolerance: controlled and clamped rather than eliminated | Heat input, joint length, sequence, fixture design |
| Untoleranced dimensions | Treated as general engineering tolerance rather than precision | Whatever the drawing states, otherwise general practice applies |
Why there is no single number
Buyers ask “what tolerance do you hold” as though there is one figure that applies to everything. There is not, and any supplier who gives you a single number without asking about the part is guessing in a way that costs you money later.
Three things change the answer every time:
- Material. Mild steel, stainless and aluminium do not behave the same under a laser or in a press brake. Springback alone differs enough to matter on a bend.
- Thickness. The same machine holds a tighter band on 1 mm sheet than it does on 10 mm plate. Our everyday work sits in a defined range — tube wall 1.2-3.0 mm, sheet and deck plate 0.8-3.0 mm — and the figures below are quoted for that range.
- The feature itself. A decorative hole in a side panel and a caster mounting hole that sets how four wheels track to each other are not the same problem. One needs a tolerance; the other needs nothing.
Laser cutting
On the sheet and tube material we normally run — typical shop frame tube of Ø20-50 mm with wall thickness 1.2-3.0 mm, and plate between 0.8 and 3.0 mm — a working figure of around ±0.1 mm is realistic for the cut profile.
That figure moves with material and thickness, and it moves for reasons that are physical rather than negotiable:
| Factor | Effect on what we can hold |
|---|---|
| Thicker material | Band widens. Thicker plate holds heat longer and the kerf behaves differently through the section. |
| Stainless versus mild steel | Different melt behaviour, different assist-gas response, so the achievable band is not identical. |
| Aluminium | Reflective and thermally conductive; needs different parameters again. |
| Feature size and spacing | Small holes close together move more, because the intervening material heats up. |
| Heat-affected zone | The edge changes character regardless of dimensional accuracy. Relevant if you are welding or coating afterwards. |
If you need to know what we can hold on your specific material and thickness, say so at quotation stage and we will answer against that combination, not against a brochure. The process itself is described in more detail on laser cutting for trolley parts.
CNC bending
Bending is where most tolerance arguments start, usually because the drawing asked for more precision than the function needed.
Working to general industry practice, formed dimensions typically sit at around ±0.5 mm and bend angles at around ±1°. On simple parts with good tooling and few bends we can do better; on long flanges, deep sections or multi-bend sequences where each bend adds to the next, those general figures are the realistic place to be.
| Factor | Effect on what we can hold |
|---|---|
| Number of bends | Tolerance accumulates. Ten bends do not have the same total deviation as one. |
| Bend radius relative to thickness | A tight radius in thick material is harder to hold than a generous one. |
| Springback | Varies by material and even by batch, and it is compensated rather than eliminated. |
| Grain direction of the sheet | Bending with or across the grain changes the result measurably. |
| Flange length | Short flanges are harder to locate consistently than long ones. |
More detail on how we set up a job is in CNC bending for trolley parts, including the minimum flange lengths and hole-to-bend clearances that make a part easy or difficult to hold.
Welding distortion
Welding does not have a tolerance in the same sense. It has movement, and the question is how much we control it and where.
On the thin-wall frames we build, a working expectation is distortion in the region of 0.5-1.5 mm per metre of weld length, before fixturing and sequence control. In practice that movement is managed, not simply measured after the fact:
- Fixtures designed for the specific frame, so repeat units are built the same way every time
- A weld sequence that balances heat input rather than dumping it all on one side
- Reinforcement placed structurally instead of relying on penetrating every joint
- Straightening after welding where the drawing allows it, and before it is treated as scrap
Where your assembly genuinely needs a flat or square datum, tell us which surfaces those are. We then control to those surfaces instead of spending equal effort everywhere — see quality and inspection for how those checks are recorded.
Where tightening pays for itself
Some dimensions deserve a tolerance callout and some do not. Marking every dimension on a drawing tight does not make a better product; it makes a more expensive one, because time goes into holding accuracy nobody benefits from.
Worth specifying explicitly:
| Feature | Why it matters |
|---|---|
| Caster mounting hole patterns | Decides whether four wheels track correctly or the cart pulls to one side |
| Hinge and latch positions | Doors that droop or fail to latch are usually a positional tolerance problem |
| Drawer slide mounting faces | Slides bind when the two sides are not parallel within tolerance |
| Anything mating with your existing equipment | The interface is where our tolerance becomes your problem |
| Nesting and stacking features | Accumulation across a large batch — see the arithmetic in stacking versus nesting |
Usually better left as general tolerance: overall outside dimensions, non-mating hole positions, panel edges that sit behind trim, cosmetic surfaces that get covered.
How we agree it
The sequence is simple and it protects both sides:
- You mark the features that genuinely need tolerance on the drawing, and leave the rest general
- We review drawing and material together, and tell you whether your callouts are achievable as specified, or suggest wording that gets the same function more cheaply
- The agreed drawing is signed off before anything is cut
- Unit one is measured against that drawing, with readings recorded — the mechanics are set out in first article and sample approval
- The batch runs to the approved standard
If you do not have a drawing yet, that is not a blocker. It is the situation most of our customers are in — see drawing and engineering service.
Questions buyers ask
Can you hold ±0.1 mm everywhere?
That figure describes laser cutting on the material and thickness ranges we run daily. It is not a blanket figure for bent or welded features. Tell us the feature and we will tell you what is realistic.
What if our drawing says nothing about tolerance?
Then general engineering practice applies, which is what the table above describes. We will flag anything that looks ambiguous rather than quietly picking the interpretation that suits us.
Do tighter tolerances cost more?
Usually yes, because holding them takes setup time, better tooling, more frequent inspection and a higher scrap allowance. The point of reviewing the drawing together is to spend that money only on the features that need it.
Will unit 500 match unit one?
That is what fixtures and first-article records are for. Between-batch variation is a separate question from within-batch variation, and repeatability over time is discussed in MOQ, lead time and repeat orders.
Can you inspect to our own drawing’s tolerance standard?
Yes, as long as we agree it before quoting and it is written into the order. Turning up with a tighter standard during goods-in is a conversation nobody enjoys.
What to send
The material, the thickness, and the two or three features where fit actually matters. That is enough for us to tell you what we can hold and what it will cost.
If you are still assembling the enquiry itself, start with the RFQ checklist. Otherwise tell us the material and thickness and we will answer against your part rather than against a general figure.
