Coil and Pipe

Overview#

Coil generates helical solids: compression and extension springs, worm gears, screw threads, and flat spirals. Pipe generates a tube (solid or hollow) of constant section along a path: plumbing, wire, rails, hoses.

Both are listed by Autodesk as solid primitives. Neither needs a profile sketch: you pick a section shape and size.

Where to find it#

• Workspace: Design.
• Tab / panel: Solid tab > Create > Coil; Solid tab > Create > Pipe.
• Shortcut: none listed in Autodesk's shortcut reference.

Step by step#

Coil#

1. Optionally create a construction plane or sketch circle to define the coil's starting plane and diameter. Coil can also be started from any plane or planar face.
2. Choose Create > Coil.
3. Select the plane (or face) where the coil begins. Fusion then asks for a center; click the origin or a sketch point.
4. Enter the Diameter of the helix. Autodesk's creation steps call this value Diameter, while its reference page lists it as Radius, measured from the center of the coil to the center of the profile, so watch the on-canvas preview. The Section Position setting then places the wire inside, centered on, or outside that value.
5. Choose the Type: Revolution and Height, Revolution and Pitch, Height and Pitch, or Spiral.
6. Enter the values (for example Revolutions 8, Height 40 mm).
7. Set the Section shape (Circular, Square, Triangular External, Triangular Internal), Section Size, and Section Position (Inside, On Center, or Outside the coil diameter).
8. Optionally set Angle (taper angle, all types except Spiral) and click Rotation to switch between clockwise and counterclockwise (the reference describes this as the way to change the coil's handedness).

Note: not confirmed in Autodesk's documentation: Autodesk's step-by-step Coil topic describes the Rotation icon as rotating the coil 90 degrees around the Y axis, which conflicts with its reference page (clockwise/counterclockwise). Check the preview in your version.

9. OK.

Pipe#

1. Draw a path as a 2D or 3D sketch (lines and arcs, tangent arcs for bends).
2. Choose Create > Pipe.
3. Select the Path. Tangent Chain, when checked, includes tangentially connected geometry. Drag the Distance handle or type a percentage to cover only part of the path.
4. Choose the Section: Circular, Square, or Triangular (equilateral).
5. Enter Section Size.
6. Check Hollow and enter Section Thickness if you want a tube.
7. Pick the Operation and confirm.

Worked example: a compression spring#

1. Create > Coil, base plane XY, center at origin.
2. Diameter 20 mm, Type Revolution and Height, Revolutions 10, Height 50 mm.
3. Section Circular, Size 2 mm, Position On Center.
4. OK.

The spring has a 2 mm wire, 20 mm mean diameter, pitch 5 mm. For ground flat ends, Cut the ends with a Combine against a box.

Options reference#

Tips and pitfalls#

Tip: Create threads manually: make a cylinder and use Coil with Triangular section and Operation Cut for a custom thread profile, then use the Thread tool for the standard ones.

Tip: Use a 3D sketch path with the Pipe tool for wire harness routing, then use a hollow section for insulated cable.

Warning: Pipes with sharp bends fail when the bend radius is smaller than the pipe's outer radius. Add a sketch fillet to the path corners greater than the pipe radius.

Warning: Coils with high revolution counts and small section sizes generate heavy geometry. Use cosmetic or simplified springs in assemblies.

Warning: A coil that is Joined to a body but only touches it tangentially is a fragile contact. Overlap the wire into the body by a small amount.

• Pitch less than the wire diameter makes the coil self-intersect.
• Clockwise versus counterclockwise is switched with the Rotation control.
• For extension springs, add hook shapes with Sweep on a separate path and Combine Join.
• Use Pipe for rail guards with Hollow turned off and the path built from line and tangent-arc sketch segments.
• Pipe on a path with a start inside another body can be used with Cut to drill channels, such as cooling lines.
• Coil geometry is exact, so interference analysis between spring turns is meaningful.

Working out the numbers#

Springs and threads are numeric problems, so decide the numbers before opening the dialog.

Height versus overall length. Coil Height is measured from the center of the starting profile to the center of the ending profile. A spring modeled as Height 50 mm with a 2 mm wire is therefore 52 mm tall overall (1 mm of wire radius protrudes at each end). If a drawing calls for 50 mm free length, enter Height 48 mm.

Pitch and coil gap. Pitch is the axial advance per turn. The gap between adjacent turns is Pitch minus wire diameter. With a 2 mm wire and a 5 mm pitch the gap is 3 mm. When the gap reaches zero the spring is solid (fully compressed); a negative gap means the turns overlap and the solid fails. For a spring that must compress 12 mm across 10 active turns, the gap must be at least 1.2 mm per turn, so pitch is at least 3.2 mm for a 2 mm wire.

Mean diameter, inside diameter, outside diameter. Spring catalogs quote outside diameter (OD). With Section Position On Center and a 2 mm circular wire, a coil Diameter of 20 mm gives OD 22 mm and ID 18 mm. To hit OD 20 mm exactly, either use Diameter 18 mm with On Center, or keep Diameter 20 mm and pick Position Inside. Choose one convention and record it in a user parameter such as spring_OD.

Taper. Angle tilts the helix wall. A conical spring with a 5 degree angle over 40 mm of height grows by about 40 x tan(5 degrees) = 3.5 mm in radius at the far end. Large angles with many turns can make turns collide near the narrow end, so check pitch against wire diameter there.

Modeling a thread with Coil#

Fusion's Thread feature is the right tool for standard threads, but Coil gives you real, printable geometry with a profile you control:

1. Extrude a 10 mm diameter, 20 mm long cylinder.
2. Create > Coil on the top face, Diameter 10 mm, Type Height and Pitch, Height 18 mm, Pitch 1.5 mm.
3. Section Triangular (External), Size 1.2 mm (the circumscribed circle diameter), Position Inside so the profile lies within the shaft, Operation Cut.
4. Start the coil 1 mm below the top face so the thread runs out cleanly, and end it before the bottom.

The thread is a V-groove cut into the shaft. Because the sweep ends abruptly you will see a blunt end of the groove; a short chamfer on the shaft end hides the start. For a mating nut, repeat with Position Outside on a bore. As general guidance (not Autodesk data), FDM printing usually needs a few tenths of a millimetre of clearance.

Pipe details#

• The path may be an open or closed chain of lines, arcs and splines, in a 2D or 3D sketch. A closed loop gives a ring or frame.
• For Hollow, Section Thickness is the wall; if Section Size is the 12 mm outer dimension, a 1.5 mm wall leaves a 9 mm bore. Thickness must be less than half the size, otherwise the bore vanishes (general geometry).
• A solid round Pipe needs a bend centerline radius larger than the pipe radius, otherwise the inside of the bend folds over itself. A 20 mm OD pipe needs path corners above 10 mm radius; as general guidance (not Autodesk data), real tube bends are often 1.5 to 3 times the OD, so use 30 to 60 mm for realistic parts.
• Square and triangular sections may twist along 3D paths; check the result before you rely on it (general guidance).

Spiral type#

Autodesk describes Spiral as using the same settings as Revolution And Pitch, without an Angle value, so there is no Height or taper to set. Treat Pitch as the spacing between windings and check the preview. A clock mainspring with 6 revolutions and a 1.2 mm pitch, built from a 0.5 mm Square section (a circumscribed circle of 0.5 mm, so a side of about 0.35 mm), leaves a clear gap of roughly 0.8 mm if Pitch is the radial spacing.

Failure modes and fixes#

Practice exercise#

Build a small spring-loaded latch using only Coil and Pipe, and check each number.

1. Compression spring: Coil, Type Revolution and Pitch, Diameter 12 mm, Revolutions 8, Pitch 4 mm, Circular section 1.2 mm, Position On Center. Predict the helix height (8 x 4 = 32 mm) plus one wire diameter, about 33.2 mm overall, and the gap between turns (2.8 mm). Measure with Inspect > Measure to confirm. Predict the solid height (8 x 1.2 = 9.6 mm).
2. Create a second spring and click Rotation to switch clockwise/counterclockwise. Confirm in the preview that the helix reverses.
3. Bolt: extrude a 6 mm diameter, 30 mm cylinder. Cut a thread with Coil: Height and Pitch, Diameter 6 mm, Height 24 mm, Pitch 1 mm, Triangular (External), Size 0.7 mm, Position Inside, Operation Cut. Add a 0.5 mm chamfer on the tip.
4. Pipe: draw a path of a 40 mm line, a tangent arc of radius 30 mm turning 90 degrees, and a 40 mm line. Make a Hollow Pipe, Section Size 10 mm, Thickness 1 mm. Then edit the arc radius to 4 mm and observe the failure, then restore it.
5. Create user parameters wire_d and pitch in Modify > Change Parameters and drive the spring with them. Set pitch to 1.0 mm and note the error (turns collide), then set it to 2.5 mm.

Expected result: a 33 mm spring whose height you predicted before measuring, a threaded bolt, a bent tube that fails below the minimum radius, and a parametric spring whose pitch limit you found by breaking it.

Related#

• Sweep
• Hole and Thread
• Revolve
• Construction geometry (planes for the coil start)