| Feature | Held in normal production | What it costs |
|---|---|---|
| Milled linear dimension, no call out | ±0.05 mm | The general tolerance. Free. |
| Milled linear dimension, called out | ±0.01 mm | A finishing pass and an inspection step. Small cost. |
| Turned diameter | ±0.01 mm | Normal on a turned part. |
| Ground or honed bore | ±0.005 mm | A separate operation. Real cost, use it only on bearing and seal bores. |
| Bore position, single setup | ±0.02 mm | Free if the part is already machined in that setup. |
| Bolt circle true position, single setup | 0.04 mm | The number that decides whether motors interchange. |
| Flatness, machined face | 0.02 mm over 100 mm | Needs the face machined last on a pocketed part. |
| Perpendicularity, one setup | 0.02 mm | Cheap in one setup, expensive across two. |
| Surface finish, as milled | Ra 1.6 | Standard. Ra 0.8 is a light pass. Below that is another process. |
| Minimum wall, 6061 | 0.6 mm | Short walls with support. Tall thin walls are quoted case by case. |
| Minimum wall, 7075 | 0.8 mm | Springier than 6061, so lighter cuts. |
| Minimum internal corner radius | 1 mm | Possible and slow. 3 mm is sensible, 5 mm is cheap. |
These are the figures this shop works to, not a theoretical machine specification. Thin wall, deep pocket and long reach work is quoted case by case, because the limit there is deflection and chatter rather than the machine.
Which features on a UAV part deserve a tolerance
Almost every part on an aircraft has two or three features doing the real work. Tolerance those and let the rest run to the general tolerance.
| Part | Features that matter | Everything else |
|---|---|---|
| Arm or boom clamp | Bore, slot closure, clamp width | General tolerance |
| Motor mount | Bolt circle true position, mating face flatness | General tolerance |
| Gimbal bracket | Bearing seats, coaxiality, motor face squareness | General tolerance |
| Sealed enclosure | Groove depth and width, face flatness | General tolerance |
| Heat sink or cold plate | Contact face flatness and finish | Fin geometry, general tolerance |
| Plate or chassis | Hole pattern position, flatness | Profile, pockets, lightening holes |
Surface finish, in one table
| Ra | How | Cost |
|---|---|---|
| 3.2 | Roughing pass left on | Cheapest |
| 1.6 | Normal finishing pass | Standard |
| 0.8 | Light pass or finishing tool | Small premium |
| 0.4 | Grinding or honing | Separate operation |
| 0.2 | Lapping or polishing | Specialist |
The real step is between 0.8 and 0.4. Above that line you are asking for a better cut, below it you are asking for another process and another handling step.
General notes worth more than tight tolerances
- Break all sharp edges 0.2 to 0.4 mm unless otherwise stated.
- Drilled depth and tapped depth given separately on blind holes.
- Dimensions apply after coating, or before, stated once.
- Masked features listed by number.
- Datums chosen to match how the part assembles, not the outside faces.
- The two or three critical features identified as critical.
That last line is not a standard drawing note and it is the most useful one you can add. It tells the shop where to spend the effort.
How we prove it
Every first order and every drawing revision gets a first article report against your drawing, measured on a CMM and an optical comparator here, with actual readings rather than pass and fail ticks. Lot inspection follows a sample plan agreed at quote stage.
We are not ISO 9001 or AS9100 registered and no page on this site says otherwise. Measured evidence is what we offer instead.
Send the drawing with the critical features marked
We will tell you which tolerances are doing nothing except adding cost, before you order. Reply within 24 business hours.