## How to create a threaded cap By: Omar Metwally, M.D.

Objective: Design and 3D print a threaded cap that creates a water-tight seal with a threaded glass jar.

Environment: Autodesk Fusion 360

Glass jar dimensions:

Jar Outer Diameter (measured over threads): 48.06 mm

Approach: Design a custom “coil” element.

Coil diameter (CD): Diameter of a zero-thickness coil as viewed from above (i.e. in the XY plane).

Jar Outer Diameter (JOD): Diameter of the jar opening, measured with Vernier calipers. Includes threads.

Coils: Protruding rings comprising the thread.

Section size (S): Coil thickness

Height (H): vertical height of the thread

Metric Pitch (P): distance between two adjacent coils

Revolutions (R): The vertical distance traveled by the coil (i.e. along the Z-axis of a cylinder) as it completes a 360 degree turn, beginning and ending at the same X and Y coordinates (i.e. looking down at the cylinder from above).

Tolerance (T): Arbitrary distance added to the coil diameter (CD) to allow the cap coils to interface with the glass jar threads.

Method

1. Create a coil with coil diameter (CD) equal to the jar outer diameter (JOD).

CD = JOD

In this case,

Jar outer diameter (JOD) = coil diameter (CD) = 48.06 mm

S = 2 mm

H = 11.88 mm

T = 0.2 mm

Angle = 0.0 degrees

P = 3.5 mm

2. Calculate coil revolutions (R), remembering that one revolution is a 360 degree turn.

R = H / P

Revolutions = Height / Pitch

In this case,

R = 11.88 / 3.5 mm = 3.39 mm

3. Create the rest of the cap

The cap comprises the cap body combined with the coil. The body is a hollow (shelled) cylinder, and the coils can be either inside or outside the body. In this case, the coils are inside the body to create the shape of a female cap. The body inner diameter (BID) equals the coil diameter (CD) plus tolerance (T). That is,

BID = CD + T

In this case, we’ll use a tolerance (T) of 0.2 mm for a PLA print, and the body will be 2 mm thick, such that:

BID = 48.06 + 0.2 mm = 48.26 mm

BOD = BID + (2*2 mm) = 52.26 mm

RESULTS