Additive Helix¶

In one sentence: Sweep a closed 2-D profile sketch along a helical path to create a coil, thread, or spiral solid and fuse it into the active Body.

Overview¶

Workbench: Part Design · Menu: Part Design → Additive Features → Additive Helix
Toolbar: Part Design Modeling Features · Shortcut: none

Additive Helix takes a closed sketch and sweeps it along an automatically generated helix, producing a solid coil or thread form that is fused into the active Body. Unlike Additive Pipe, you do not need to draw the helical path yourself — the tool computes it from four geometric parameters (pitch, height, turns, and cone angle) that you supply in four convenient input modes. Use it for coil springs, screw threads, worm gears, or any other solid with a helical geometry.

Intuition¶

Think of wrapping a ribbon around a cylinder: the ribbon is the profile and the cylinder's surface traces the helix. The tightness of the wrapping is the pitch (axial distance per turn), the height of the cylinder is the total height of the helix, and the number of times the ribbon goes around is the turns count.

What makes Additive Helix special compared to Additive Pipe with a manually drawn spine is that the helix geometry is fully parametric and self-consistent: change the pitch and the turns count updates, change the height and the pitch recalculates — depending on which mode you choose to drive the geometry.

For conical spirals (like a watch spring or a decreasing-pitch compression spring), a non-zero Cone angle makes the radius grow or shrink as the helix climbs.

When to use it¶

You need a coil spring — a circular profile swept along a cylindrical helix.
You want to add a helical thread form to a shaft (Additive Helix adds the thread material; use Subtractive Helix to cut a thread groove).
You are modelling a worm gear tooth, an Archimedes screw, or any helically extruded feature.
You need a conical spiral where the radius changes with each turn.
You need a flat Archimedean spiral (set height to zero and use Height-Turns-Growth mode with a non-zero growth value).

When NOT to use it¶

The path is an arbitrary curve, not a helix — use Additive Pipe with a manually drawn spine.
The cross-section is constant and the path is straight — use Pad.
The shape is axisymmetric — use Revolution.
You want to cut a helical groove — use Subtractive Helix, which has identical parameters but removes material.

Step-by-step walkthrough¶

Prerequisites: An active Body with at least one closed, fully-constrained sketch. The sketch should be placed on a plane that is perpendicular to (or near) the desired helix axis, and it should be offset from that axis (centred sketches produce self-intersecting geometry).

Select the profile sketch in the model tree or viewport.
Activate Part Design → Additive Features → Additive Helix. The task panel opens. FreeCAD automatically proposes a pitch and height based on the bounding box of your profile sketch to avoid self-intersection.
Choose the Axis. The dropdown lists the Body origin axes (X, Y, Z), the sketch's horizontal, vertical, and normal axes, and any custom reference. The sketch's normal axis (perpendicular to its plane) is often the right choice.
Choose the Mode. Pick the combination of inputs that matches what you know about your desired helix. See the Parameters table for a description of each mode.
Fill in the primary inputs for the chosen mode (e.g., Pitch and Height for Pitch-Height-Angle mode). The derived parameters are computed and displayed as read-only values.
Set the Cone angle if you want a conical spiral (positive = radius grows upward, negative = radius shrinks). Leave at 0° for a cylindrical helix.
Check "Left handed" to reverse the chirality from right-handed (clockwise when looking along the axis) to left-handed (counter-clockwise).
Check "Reversed" to make the helix travel in the opposite direction along the axis.
Click OK. FreeCAD sweeps the profile along the helix and fuses the result into the Body.

Tip

If the task panel shows a "self-intersecting" status warning, increase the pitch relative to the profile size. The profile's cross-section must fit inside the axial distance between turns to avoid overlapping geometry.

Parameters¶

Parameter	Type	Default	Description
Axis	Dropdown	(Body origin Y-axis)	The central axis of the helix. Choices: Body X/Y/Z axes, sketch horizontal/vertical/normal axes, or an arbitrary reference.
Mode	Dropdown	`Pitch-Height-Angle`	Input mode. Determines which three parameters are primary inputs and which are derived. See mode table below.
Pitch	Length (mm)	10.0 mm	Axial distance between two adjacent turns. Primary input in `Pitch-Height-Angle` and `Pitch-Turns-Angle` modes; derived in others. Must be > 0 when it is a primary input.
Height	Length (mm)	30.0 mm	Total axial length of the helix path (does not include the extent of the profile itself). Primary input in `Pitch-Height-Angle`, `Height-Turns-Angle`, and `Height-Turns-Growth` modes; derived in `Pitch-Turns-Angle` mode.
Turns	Float	3.0	Number of full turns. Primary input in `Pitch-Turns-Angle`, `Height-Turns-Angle`, and `Height-Turns-Growth` modes; derived in `Pitch-Height-Angle` mode. Minimum value: `Precision::Confusion()` (effectively > 0).
Cone angle	Angle (°)	0°	Half-angle of the cone that envelopes the helix. `0°` = cylindrical helix. Positive = radius grows in the axis direction; negative = radius shrinks. Valid range: −89° to +89°.
Radial growth	Length (mm)	0.0 mm	Radius increase per turn. Primary input only in `Height-Turns-Growth` mode; derived from `Cone angle` in all other modes.
Left handed	Checkbox	Off	When on, the helix winds counter-clockwise when viewed along its axis (left-hand thread convention).
Reversed	Checkbox	Off	When on, the helix travels in the opposite direction along the axis.
Tolerance	Float	0.1	Fusion tolerance multiplier. Increase if the helical solid does not merge cleanly with the existing Body. Minimum 0.1.

Input modes:

Mode	Primary inputs	Derived
Pitch-Height-Angle	Pitch, Height, Cone angle	Turns, Radial growth
Pitch-Turns-Angle	Pitch, Turns, Cone angle	Height, Radial growth
Height-Turns-Angle	Height, Turns, Cone angle	Pitch, Radial growth
Height-Turns-Growth	Height, Turns, Radial growth	Pitch, Cone angle

Mode in depth¶

The Mode dropdown is the first parameter to set. It determines which three of the four numeric helix parameters — Pitch, Height, Turns, and Cone angle (or Radial growth) — are the primary inputs you type in, and which one is derived automatically. The greyed-out field in the task panel is the derived value; changing any primary input recalculates it instantly.

Think of it as choosing which constraints to drive: you always have three degrees of freedom, the mode picks which three.

Pitch-Height-Angle (default)¶

You enter Pitch and Height; FreeCAD computes Turns. Cone angle is always a separate, independent input for conical helices.

Intuition: "I know the thread pitch (distance between turns) and the total length of the feature. Tell me how many turns that gives."

Use this mode when: - Designing standard screws or threads where pitch and length are specified (e.g. M6×1.0 thread, 20 mm long — pitch and height are the design inputs) - Modelling coil springs where the wire pitch and overall spring length are the known dimensions - The turn count is a consequence, not a requirement

Watch out for: If Height ÷ Pitch is not an integer, the helix terminates mid-turn. The solid ends at a partial spiral face, which may be unexpected. If you need a whole number of turns, switch to Pitch-Turns-Angle instead.

Pitch-Turns-Angle¶

You enter Pitch and Turns; FreeCAD computes Height.

Intuition: "I need exactly 5 full turns with a 1.5 mm pitch. How long is that?"

Use this mode when: - The number of engaging thread turns is the critical design variable (minimum thread engagement is typically 1.5 × nominal diameter for steel-into-steel) - Designing springs where the coil count is specified (number of active coils) - Fitting a helix into a feature where the turn count, not the length, is fixed

Watch out for: The derived Height is Pitch × Turns. If you later change either primary input and the new Height no longer fits in the available space, the helix will protrude through other geometry without warning.

Height-Turns-Angle¶

You enter Height and Turns; FreeCAD computes Pitch.

Intuition: "It must fit in a 25 mm pocket and I need at least 8 turns. What pitch does that require?"

Use this mode when: - The axial space is constrained by the design (e.g. a recess of known depth) and a minimum number of turns is required (e.g. minimum thread engagement) - Both height and turn count are specified by a drawing or standard, and pitch is the outcome

Watch out for: The derived Pitch = Height ÷ Turns. This often yields a non-standard pitch value — verify it is achievable in manufacturing before committing. For standard thread pitches, use Pitch-Height-Angle or Pitch-Turns-Angle instead.

Height-Turns-Growth¶

You enter Height and Turns; FreeCAD computes Pitch. Instead of Cone angle, the radial shape is controlled by Radial growth (the increase in radius per full turn). This mode is designed for Archimedean spirals, not cylindrical or conical helices.

Intuition: "I want a flat scroll spring that grows 3 mm outward each turn and completes 4 turns in a 10 mm axial height." Height ≈ 0 gives a purely flat spiral.

Use this mode when: - Creating scroll springs, decorative spirals, or cam profiles where the radius expands (or contracts) linearly with angle - Setting Height to 0 to produce a flat planar spiral (zero pitch, expanding radius only — requires a small non-zero Tolerance to fuse cleanly) - The Cone angle concept does not apply and radial growth per turn is the natural design input

Watch out for: Cone angle is unavailable in this mode (it is replaced by Radial growth — they are mutually exclusive ways to describe the radial shape). A negative Radial growth produces an inward-contracting spiral; the radius must remain positive at all turns or the helix collapses and fails.

Note

The Outside (Remove outside of profile) property is present in the source code but is hidden for Additive Helix — it only applies to Subtractive Helix, where it controls whether the material inside or outside the swept volume is removed.

Advanced usage¶

Flat Archimedean spirals¶

To create a flat spiral (zero height), use Height-Turns-Growth mode and set Height to 0. The result is a flat disc-like spiral whose radius increases by the Radial growth value each turn. This is useful for scroll springs or decorative spiral features. Note that with zero height and more than one turn, the helix path collapses onto a plane, which may require adjusting the Tolerance parameter for the fusion to succeed.

Self-intersection avoidance¶

FreeCAD proposes an initial pitch based on the profile bounding box (roughly 1.1 × the bounding-box diagonal). This is a conservative estimate — you can usually reduce the pitch significantly for compact profiles. The status label at the top of the task panel shows "Valid" when the proposed geometry is self-intersection-free and an error message when it is not.

Thread profiles¶

For external threads, draw the thread-tooth profile as a closed triangular or trapezoidal sketch on a plane perpendicular to the shaft axis, positioned so that the sketch is at the thread's outer diameter. Use the shaft's Z-axis as the helix axis. Set the pitch to match the thread standard (e.g. 1.0 mm for M6×1.0). After the Additive Helix, use a Revolution or Pad to add the core shaft body.

Conical coil springs¶

A non-zero Cone angle produces a conical coil spring (like a tapered compression spring). Positive angles make the coil wider towards the top of the axis direction; negative angles narrow it. The cone angle is the half-angle of the imaginary cone that the helix wraps around — a 10° cone angle on a 5 mm radius spring increases the radius by about 0.88 mm per 5 mm of height.

Common mistakes and pitfalls¶

Error: Pitch too small

Cause: In Pitch-Height-Angle or Pitch-Turns-Angle mode the pitch value is zero or below the geometric precision threshold.
Fix: Set Pitch to a positive value larger than the axial extent of your profile cross-section to avoid self-intersection.

Result is self-intersecting

Cause: The profile is too large relative to the pitch, so adjacent turns of the sweep overlap. The status label in the task panel will show a warning.
Fix: Reduce the profile cross-section size, increase the pitch, or use a larger axis offset (move the sketch further from the axis).

Error: Face must be planar

Cause: The profile sketch is attached to a non-planar face. The helix sweep algorithm requires a planar starting face.
Fix: Attach the profile sketch to a flat datum plane or a planar model face.

Adding the helix failed (fusion error)

Cause: The swept solid does not cleanly merge with the existing Body, often because of very tight geometry or a large model with small helix features.
Fix: Increase the Tolerance parameter in the task panel. Values above 1.0 are sometimes necessary for very large parts.

Helix direction looks wrong

Cause: The axis direction or the Reversed flag may be producing a helix that climbs in the opposite direction from what was intended.
Fix: Toggle the Reversed checkbox, or choose a different axis (e.g., swap from +Z to −Z direction by selecting the opposite origin axis).

Python API¶

Minimal example¶

import FreeCAD as App
import Part
import Sketcher

doc = App.newDocument()

# Create the body
body = doc.addObject("PartDesign::Body", "Body")

# Profile sketch: a small circle of radius 2 mm on the XY plane
# The circle is offset from the origin so it does not intersect the Z-axis
profile = doc.addObject("Sketcher::SketchObject", "Profile")
body.addObject(profile)
profile.addGeometry(Part.Circle(App.Vector(10, 0, 0), App.Vector(0, 0, 1), 2), False)
profile.addConstraint(Sketcher.Constraint("DistanceX", -1, 1, 0, 3, 10))  # TODO: verify
doc.recompute()

# Create the Additive Helix
helix = doc.addObject("PartDesign::AdditiveHelix", "AdditiveHelix")
body.addObject(helix)
helix.Profile = profile
helix.ReferenceAxis = (doc.getObject("Body").Origin.Z_Axis, [""])  # TODO: verify axis reference
helix.Mode = "pitch-height-angle"
helix.Pitch = 5.0      # mm per turn
helix.Height = 30.0    # total axial length
helix.Angle = 0.0      # cylindrical (no cone)
helix.LeftHanded = False
helix.Reversed = False
doc.recompute()

print(f"Volume: {helix.Shape.Volume:.4f} mm³")

Key properties¶

Property	Type	Default	Description
`Profile`	`App::PropertyLinkSub`	—	The cross-section sketch to sweep. Inherited from `ProfileBased`.
`ReferenceAxis`	`App::PropertyLinkSub`	—	The axis object and optional sub-element. When set, `Base` and `Axis` are derived automatically.
`Mode`	`App::PropertyEnumeration`	`"pitch-height-angle"`	Input mode. Values: `"pitch-height-angle"`, `"pitch-turns-angle"`, `"height-turns-angle"`, `"height-turns-growth"`.
`Pitch`	`App::PropertyLength`	`10.0`	Axial distance per turn in mm.
`Height`	`App::PropertyLength`	`30.0`	Total axial length of the helix path in mm.
`Turns`	`App::PropertyFloatConstraint`	`3.0`	Number of full turns. Must be > 0.
`Angle`	`App::PropertyAngle`	`0.0`	Cone half-angle in degrees. Range: −89° to +89°.
`Growth`	`App::PropertyDistance`	`0.0`	Radial growth per turn in mm. Used in `"height-turns-growth"` mode; otherwise derived.
`LeftHanded`	`App::PropertyBool`	`False`	When `True`, helix winds counter-clockwise (left-hand thread direction).
`Reversed`	`App::PropertyBool`	`False`	Reverses the axial direction of the helix.
`Tolerance`	`App::PropertyFloatConstraint`	`0.1`	Fusion tolerance multiplier. Minimum 0.1.
`Base`	`App::PropertyVector`	`(0,0,0)`	Start point of the helix axis. Read-only; derived from `ReferenceAxis`.
`Axis`	`App::PropertyVector`	`(0,1,0)`	Direction of the helix axis. Read-only; derived from `ReferenceAxis`.