How to read the number
A flatness figure of 0.1 mm/m² means that over a one metre square reference area, the machined surface does not deviate from a true plane by more than 0.1 mm. It is a local flatness specification, not a statement about the whole table at any size.
This distinction matters when a table is long. A 4000 mm platform is not judged as a single 4 metre by 2 metre plane with one overall figure; flatness is assessed over reference areas across the surface. When you compare quotations, check that both suppliers are quoting the same reference area, or the numbers are not comparable.
Which grade fits which job
| Grade | Deviation | Typically specified for |
|---|---|---|
| Standard | 0.1 mm/m² | General fabrication, frames, bases and routine welded assemblies |
| Precision | 0.05 mm/m² | Assemblies with a defined position tolerance, jigs and repeat fixtures |
| High precision | 0.02 mm/m² | Tight-tolerance work where the table surface is part of the measurement reference |
Why over-specifying costs you
Flatness is produced by machining, and holding a tighter tolerance takes more passes and more inspection time. If the assembly being welded has a tolerance of plus or minus 0.5 mm, paying for a 0.02 mm/m² surface does not improve the finished part — the welding distortion is an order of magnitude larger than the surface deviation.
The grade is worth paying for when the table is used as a reference: when you are checking a part against the table, setting up a jig from the table datum, or building an assembly where stack-up tolerance is already tight. In those cases the surface deviation contributes directly to the result.
What else affects the result
- Platform height and reinforcement. A 100 mm body and a 200 mm body use different structures, and the internal reinforcement affects local deflection at clamping points. Custom heights are available for project-specific requirements.
- Support arrangement. How the table is supported, and how level the floor is, affects how the surface sits in use. Levelling feet allow the table to be brought into level after positioning.
- How you use the table. Heavy parts concentrated on one area, or clamping that pulls the surface, influence the working accuracy more than the delivered grade on its own.
How to check flatness on a table
You do not need a coordinate measuring machine to check a welded table. The three methods used in most shops are a precision straight edge with feeler gauges, a dial indicator on a stand, and a precision or electronic level for longer spans.
With a straight edge, place it across the surface and try to pass a feeler gauge under it at several positions. That finds high spots quickly but only tells you a number where you can feel the gap. A dial indicator on a magnetic or tripod stand gives a readout you can record, provided you sweep from a known reference rather than judging the table height itself. For long platforms, work in overlapping reference areas instead of trying to judge the whole surface in one pass.
- Precision straight edge — fast check, good for finding high spots.
- Dial indicator on a stand — gives a recordable reading against the specified grade.
- Precision or electronic level — practical on long tables and for checking how the table sits once installed.
- A straight edge on its own does not produce the number, so pair it with a gauge when the grade is contractual.
What to record as acceptance data
When flatness is a specification item, agree before delivery what will be measured, with which instrument, over which reference area, and what counts as a pass. That turns a figure on a quotation into something both sides can verify.
- The grade and the reference area, for example 0.1 mm over a 1 m² area.
- Which faces are covered: the top face only, or the side working faces as well.
- The instrument used and its own accuracy, since it must be finer than the grade claimed.
- Where readings are taken and how many points or areas are measured.
- Whether the check happens at the works before packing, on arrival, or both.
Common errors when checking flatness
Most flatness disagreements trace back to the measurement setup rather than the table. These are the ones that cause the most argument.
- Judging the whole table against one long reference instead of measuring reference areas.
- Checking on an uneven floor, or before the levelling feet have been set.
- Using an instrument whose own deviation is larger than the grade being claimed.
- Measuring a surface that has spatter, debris or a part resting on it.
- Comparing two quotations that used different reference areas, so the numbers were never equivalent.
Specifying it on an RFQ
State the grade together with the reference area, the table size, the platform height and the hole system. That combination is what a supplier needs to quote a real price. A request that only says "flat table" usually comes back with the standard grade, which may or may not be what the job needs.