Reference planes
Build sketches on surfaces that do not exist yet — angled brackets, offset faces, and mid-planes.
By the end you can
- Create offset, angled, and mid-planes to sketch on
- Choose a plane that keeps a part symmetric about the origin
- Explain why sketching on a face can break a model later
On this page · 4 sections
The three default planes get you through a flat plate. The moment you need a gusset at 30 degrees, or a boss standing off a surface, you need a plane of your own.
The three you will actually use
- Offset
- A plane parallel to an existing one, a set distance away. Use it for a sketch that sits above a surface — a standoff boss, or a second plate in a sandwich.
- Angled
- A plane rotated about an edge or axis by a given angle. This is how an angled intake bracket gets modelled without any trigonometry on your part.
- Mid-plane
- A plane exactly halfway between two references. The most useful of the three, because it keeps a part symmetric for free.
Why not just sketch on a face?
You can. Onshape will let you click any flat face and start sketching. It is faster in the moment and it is the single most common cause of a model that explodes two weeks later.
The topological naming problem
When you sketch on a face, the sketch is attached to that specific face by an internal name. Change an earlier feature so the face is split, merged, or regenerated, and the sketch loses its reference — taking every feature built on it down as well.
A reference plane does not have this problem. It is defined against the origin or against other planes, which do not disappear when you edit a fillet.
The practical rule
Sketch on a face only when the face is created by something that will never change — and be honest about how often that is actually true on a competition robot in November.
Symmetry is the real prize
Build a drivetrain from a mid-plane and the whole thing stays centred on the origin. That matters more than it sounds:
- Mirroring a mechanism to the other side of the robot becomes one feature, not a rebuild.
- Assembly mates snap to the origin cleanly instead of needing offsets.
- Your centre of mass readout means something, because the model is actually centred.
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Create a mid-plane between the two drivetrain side plates. Select both inner faces, choose mid-plane, done.
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Sketch the cross-member profile on it. The part is now symmetric by construction rather than by careful arithmetic.
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Change the drivetrain width. The cross-member follows and stays centred, because the plane it lives on is defined by the two plates rather than by a number you typed.
Check yourself
You sketch a motor mount directly on the outer face of a drivetrain plate. Later you add a fillet that rounds that face's edge. What is the risk?
Next
Sketches are the foundation. The next unit turns them into solid geometry.
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