GD&T position workspace
Enter measured deviations and feature sizes, and see the true position, the bonus tolerance the modifier actually earns, the virtual and resultant condition where they exist, and the tolerance zone drawn to the same scale as the measured point. Millimetres or inches. Everything stays in your browser.
Control and inputs
Locate a feature axis inside a cylindrical tolerance zone at its basic location.
Units
Feature of size
At MMC. RFS grants no bonus tolerance.
Datum references
Feature control frame and zone
Model: ASME Y14.5-2018. Values in mm.
Active model: TP = 2 x sqrt(X^2 + Y^2)
Result
DatumLab case sheet
Bracket hole pattern · ASME Y14.5-2018 · Position
- Stated tolerance
- 0.250 mm
- Bonus tolerance
- 0.180 mm
- Total allowed
- 0.430 mm
- Virtual condition
- 9.750 mm
- Resultant condition
- 10.650 mm
- Datum shift available
- None
- Size status
- Within 10.000 to 10.200 mm
The feature is inside its size limits and inside the total allowed tolerance.
Saved cases on this device
No saved cases yet. Enter a measurement above, name it, and press Save case. Everything stays in this browser.
Practice questions
156 authored questions across 14 topics: Position at RFS, Position at MMC and bonus tolerance, Least material condition, Virtual and resultant condition, Datum shift, Reading a feature control frame, Datums and precedence, Size limits and Rule #1, Profile, Runout, Orientation, Form, Composite position, Inspection and gaging. Each one shows the reasoning and names the section of ASME Y14.5-2018 the rule comes from. Every question here is written from scratch; no standard or course material is reproduced.
What does the leftmost compartment of any feature control frame always contain?
Symbol, then tolerance, then datums.
That is three of 156. The practice surface filters by topic and difficulty, keeps a mistakes notebook and a mastery table, and runs a timed exam simulation.
Geometric characteristic symbols
The two profile symbols are the pair most often drawn wrong: profile of a surface and profile of a line are different characters and different controls. The full symbol referencecarries all fourteen characteristics with their datum requirements.
| Symbol | Characteristic | What it controls |
|---|---|---|
| ⌖ | Position | Locate a feature axis or centre plane inside a tolerance zone at its basic location. |
| ⌓ | Profile of a surface | Hold a surface inside a zone disposed about the true profile. |
| ⌒ | Profile of a line | Hold each cross section of a surface inside a two dimensional zone about the true profile. |
| ↗ | Circular runout | Limit the indicator movement at each single circular element as the part rotates. |
| ⏥ | Flatness | Hold a surface between two parallel planes. |
| ⏤ | Straightness | Hold a line element or an axis between two parallel lines or inside a cylinder. |
| ○ | Circularity | Hold each circular cross section between two concentric circles. |
| ⌭ | Cylindricity | Hold a cylindrical surface between two concentric cylinders. |
| ⊥ | Perpendicularity | Hold a feature at 90 degrees to a datum. |
| ∥ | Parallelism | Hold a feature parallel to a datum. |
| ∠ | Angularity | Hold a feature at a basic angle to a datum. |
Datum precedence in the planar arrangement
| Datum | Role | Degrees of freedom constrained | Which ones |
|---|---|---|---|
| A | Primary plane | 3 | One translation and two rotations. |
| B | Secondary plane or width | 2 | One translation and one rotation. |
| C | Tertiary stop | 1 | The last translation. |
A primary cylindrical datum feature constrains four degrees of freedom, two translations and two rotations, so the three, two, one split above applies to the planar arrangement only.
Frequently asked questions
How is true position calculated from X and Y deviations?
True position is 2 x sqrt(X^2 + Y^2). The position zone for a feature axis is a cylinder, so the drawing states a diameter, and the measured radial offset of the actual axis is doubled to compare against it. A deviation of 0.06 mm in X and 0.08 mm in Y is a radial offset of 0.10 mm and a true position of 0.20 mm.
How much bonus tolerance does the MMC modifier give?
Bonus equals the departure of the produced feature from its maximum material condition size: for a hole the actual size minus MMC, for a pin MMC minus the actual size. It is bounded by the size tolerance, so it can never exceed the distance from MMC to LMC. A feature produced outside its size limits is a size rejection and earns no bonus at all.
Why does the tool show no virtual condition when the tolerance is RFS?
Virtual condition is a constant worst case boundary, and it only exists because the geometric tolerance is tied to a fixed material condition. Regardless of feature size means the stated tolerance applies at every produced size, so no constant boundary is created and quoting one would describe a boundary the drawing never made.
Is datum shift just extra tolerance added to the zone?
No. Bonus tolerance widens the tolerance zone of the feature being controlled. Datum shift is freedom for the datum reference frame to move relative to the part, granted when a datum feature is referenced at MMB or LMB, and it only helps when the shift is available in the direction the feature actually moved. DatumLab reports it separately for that reason and never folds it into the acceptance limit.
Can I work in inches?
Yes. The unit switch converts every value in the workspace at once, using the exact international inch of 25.4 mm, so a calculation never mixes millimetres and inches. Millimetre results are shown to three decimals and inch results to four.
Does anything I enter leave my browser?
No. Every calculation runs in the page, saved cases are written to this browser's local storage under a versioned key, and the export button writes a JSON file to your own device. There is no account and no upload.