Do identical parts in Onshape always have identical face-local coordinate systems?

he_qiu143

Hi everyone,

I'm investigating how consistent the face-local coordinate system (X, Y, Z) is in Onshape when working with geometrically similar but separately created parts.

Here's what I tested:

• I created two cylinders in two separate Part Studios using the GUI.
• Both were drawn manually using a sketch and then extruded to form a solid body.
• The cylinders are nearly identical — the only difference is a small variation in the sketch center and radius due to manual input.
• I then selected the top face of each cylinder (the end cap), and used evSurfaceDefinition to extract the local frame.

What I found:

• The origin, normal (Z), and X direction of the face-local coordinate systems were essentially identical between the two cylinders.
• This was surprising to me, as I assumed that even small differences in sketch parameters might cause differences in surface parameterization and thus affect the local X/Y directions.

I think if local frames are stable and predictable, we can represent geometry more compactly and deterministically — which is highly valuable in design automation and procedural modeling.

So my questions are:

1. Does Onshape attempt to preserve consistent local (X, Y, Z) directions for similar faces across similar parts, even if they are created independently in different studios?
2. Is this consistency guaranteed(depend on Parasolid underneath?), or just coincidental in this case?
3. What factors (e.g., sketch orientation, face loop order, extrusion direction) can influence the local X/Y axes, assuming the face normal (Z) is stable?
4. Is there a recommended way to define or extract a stable local frame for a face, especially when building directional or dependent features?

I’d love to better understand how Onshape (and Parasolid underneath) handles this, especially for reproducible modeling workflows where face orientation matters.

Thanks in advance for any insights!