Well-run CNC shops in Jamnagar can usually hold a few hundredths of a millimetre on turned diameters and lengths, and tighter on a small number of critical features. Shops that run cam automatics typically work to looser tolerances.
These are rough guides, not promises. The real figure depends on the machine, its condition, the part shape and how the shop measures. The only reliable answer comes from measuring parts from that shop.
What drives the achievable tolerance
- The machine type and condition, since a worn spindle or slide limits accuracy.
- The part shape, because long, thin parts deflect and thin walls distort.
- The feature, as outer diameters are easier to hold than deep bores, threads or positions.
- Tool wear over a long run, which makes dimensions drift unless the shop corrects for it.
- Temperature, because brass expands as it warms during machining and in the shop.
- Measurement, since a shop cannot hold what it cannot measure.
How to put tolerances on your drawing
Specify tight tolerances only where the function needs them. Every extra decimal place adds cost, slows the machine and raises the scrap rate.
For other dimensions, refer to a general tolerance standard such as ISO 2768, stating the class, for example ISO 2768-m. Mark critical dimensions clearly and say how they will be checked.
State surface finish as an Ra value where it matters, such as on sealing faces. For threads, give the class, such as 6g or 6H for metric threads, so they can be gauged.
How to check a supplier's capability
Ask for a sample batch, not a single sample. Measure every critical dimension on a set of parts and look at the spread, not just whether each part is in tolerance.
For repeat orders, ask the supplier to record critical dimensions during the run. Process capability indices tell you whether the process is centred and stable, as explained in what Cpk is.
Gauges and measuring methods are covered in checking brass part dimensions. For complex parts, a coordinate measuring machine gives a full report.
Common gaps
Some shops quote tight tolerances to win the order and then deliver parts that pass only on the first sample. Others measure with hand calipers where a micrometer or gauge is needed. Ask what instrument they use for each critical dimension, and check its calibration date.
Another gap is geometric tolerances. Many smaller shops are used to plus-and-minus sizes but less familiar with concentricity, runout, perpendicularity or true position. If your drawing uses these, ask the supplier to explain how it will measure each one before it quotes. If it cannot, the part may need a shop with a coordinate measuring machine.
Next step
We audit manufacturers, measure samples and supervise production so tolerances hold beyond the first batch. To check a supplier against your drawing, book a free sourcing review.