Thread-drawing modules for Openscad

Here are some modules to draw accurate, ISO-standard threads in Openscad. The specification comes from Wikipedia.

Openscad modules

The modules file is downloadable here. (Right-click and Save Link As...)

There are two front-end modules:

• metric_thread(diameter, pitch, length)
• english_thread(diameter, threads_per_inch, length)

metric_thread() input units are millimeters. english_thread() inputs are in inches, though the drawing dimensions are in millimeters.

There are two optional parameters that can be used with either of these modules:

• internal=true. Indicates that an internal thread (e.g., for a nut) will be drawn. Internal threads should be "cut out" from a solid using difference(). The default (internal=false) indicates that external threads (e.g., for a bolt) will be drawn. (The clearances for internal and external threads are different. See Wikipedia.)
• n_starts=1 (default). This provides the option to draw multi-start threads. These have two or more "parallel grooves" for the threads. For example, the DNA double-helix has two "starts". I used this option to create a sprocket for a Rohloff 14-speed bicycle hub, which has a proprietary six-start metric thread.

Older version

A previous version of the modules used a different approach, based on using linear_extrude() with "twist" and a thread profile in a plane normal to the threads' axis of rotation. The previous version is here. Providing enough segments to draw reasonably smooth surfaces created so many polyhedra, however, that it quickly appeared to overwhelm Openscad. ("Program error - application must close" or some such.)

The current approach, which uses a polyhedron for each segment of the thread, was inspired by Trevor Moseley's work, although the results there do not appear to adhere to the ISO standards (especially with regard to required clearances).

Spreadsheet. I created a spreadsheet to get data for the perpendicular-to-the-axis-cross-section of the ISO standard thread profiles. The spreadsheet is here.

Please contact me if you have any questions or suggestions.