Flatness Tolerance on Large Machined Weldments: What to Specify
Flatness is the most frequently over-specified characteristic on a heavy frame drawing. It is also one of the most expensive, because tightening it does not add a little cost, it adds a finishing pass, a second stress relief, a different machine, or all three.
Here is what the tolerance means, what is realistically achievable, and how to write one that buys you what you need.
What flatness actually is
Flatness is a form tolerance. It defines a zone between two parallel planes, and the entire surface must lie inside it. It says nothing about orientation or location relative to anything else, a surface can be perfectly flat and badly tilted, and still pass.
That distinction matters, because what people often want is not flatness but parallelism between two mounting planes, or coplanarity across several separate pads. Specifying flatness when you mean coplanarity is a common and expensive mix-up: four pads can each be beautifully flat and still sit on four different planes.
If what matters is that four feet land on one plane, say coplanarity or use a profile-of-a-surface call-out against a datum. Your fabricator will thank you and your quote will be lower.
Realistic figures
Rounded numbers from working practice on carbon steel weldments:
| Condition | Typical flatness | Notes |
|---|---|---|
| As-welded, no relief | 0.3–0.8 mm per 300 mm | Highly variable with joint layout |
| As-welded, stress relieved | 0.3–0.5 mm per 300 mm | More consistent, not tighter |
| Machined, standard | 0.05 mm/m | The common default |
| Machined, tight | 0.02 mm/m | Needs a good machine and a second relief |
| Machined, precision | Below 0.02 mm/m | Specialist work, temperature-controlled, expensive |
Two things to notice. First, stress relief does not much improve as-welded flatness, it makes the subsequent machining hold. Second, the step from 0.05 to 0.02 mm/m is not a 2.5× cost increase; it is a change in process.
Tolerance should scale with size
A practical rule: achievable flatness scales roughly with the square root of the characteristic dimension. Double the length of a surface and you should expect to allow roughly 1.4× the tolerance for the same manufacturing difficulty.
A 0.05 mm flatness on a 300 mm pad is routine. The same 0.05 mm across 3 m is a different order of problem, the machine’s own way error, thermal growth in the part during the cut, and how the piece is supported all become dominant.
This is why flatness is usually specified per unit length (0.05 mm/m) rather than as one total number. If you write a single total value across a long surface, check what it implies per metre before you release the drawing.
The sequence that makes tolerance achievable
Flatness is bought in the process, not at the inspection bench:
- Design for symmetry. A weld layout balanced about the neutral axis moves far less than one welded from one side. See welding distortion control on heavy plate.
- Stress relieve before machining. Non-negotiable if you want the surface to stay put, stress relief for welded machine bases.
- Rough machine, relieve again if thick, then finish. Removing a lot of metal releases stress. On heavy sections, a second relief between passes is what holds the tighter numbers.
- Support the part the way it will be supported in service. A 4 m frame clamped rigidly to a machine table can be machined dead flat and spring on release. Good shops machine on three-point support that mimics the foundation.
- Let it reach temperature. A part brought in from a cold yard and cut immediately will grow during the operation.
Measuring it, and agreeing how
A tolerance you cannot agree on measuring is not a tolerance. On large surfaces the method changes the number:
- Straightedge and feeler gauge: quick, crude, fine for as-welded checks
- Precision level and sweep: traditional, good on machine ways, slow
- Laser tracker: the practical default for large frames; accuracy typically in the tens of microns over metres
- Large-envelope CMM: most accurate, limited by machine size, usually a subcontract
Specify the method, the number of points and the support condition. “Flatness 0.05 mm/m, measured by laser tracker on three-point support, minimum 25 points” is a tolerance. “Must be flat” is a conversation. More on the instruments in dimensional inspection of large frames.
Temperature is part of the specification
Steel expands roughly 12 µm per metre per °C. On a 4 m surface, a 10 °C difference between the shop floor and your plant is about 0.5 mm of length change, and if the part is heated unevenly, some of that shows up as form error.
For anything tighter than 0.05 mm/m, state the inspection temperature (20 °C is the usual reference) and allow soak time. Without it, you and the shop can measure the same part on the same day and disagree.
Four questions before you release the drawing
- Do I mean flatness, or coplanarity, or parallelism? They have different costs.
- Does this surface need machining at all? A blasted and ground surface is far cheaper and often adequate for a non-critical mount.
- Can I shim instead? Designing in a 3 mm shim pack is free. Machining a second plane is not.
- What number did the calculation give? Not the number on the last drawing. Inherited tolerances are the single most common source of avoidable machining cost, see what drives the cost of heavy frames and skids.
Have a frame with a flatness call-out you are not sure is realistic, or affordable? Send the drawing and we will tell you what it implies in process terms, within 24 business hours.