Labs

Modeling Metric Threads

This is an attempt to create M6 metric threads for both external (male) threads and internal (female) threads. The basic procedure is by using rail revolve command, with precised measurement for profile curves, rail curves, and axis.

Step to create:

• Set the document. Put value to grid extents: 100 mm, snap spacing: 0.1 mm.
• Create the profile by using ISO rules for metric threads. Open thread on Wikipedia.
• We will model M6 threads for both external (male) and internal (female).
• Useful terms: Dmaj (Major Diameter) is set to 6 mm (defines the maximum size of external threads, and minimum size of internal threads), pitch is set to 1 mm, and height is set to 0.866 mm (equivalent for 0.866 x Pitch).

• Roll over to right viewport.
• Create the H curve (red) for 0.866 mm.

• Create the P curve for 1 mm, perpendicular to H curve. Use midpoint snap to positioned it.

• Create a polyline to connect three points (1 - 2 - 3) from H and P curve.

• Create a trim curve for 0.10825 mm (= H/8).
• Create an H/4 line, equivalent as 0.2165 mm.

• Create a P/16 line, equivalent as 0.0625 mm.

• Create a “rounded curve” as illustrated. Use curve command with degree set to 3. Shift the degree of the curve to 7 (see black curve) afterwards.
• Notice that two rounded curves needed, as “profile” for external and internal threads.

• Copy all curves and send them to separate layer. Trim the curves one another, then join them to get a profile curve.
• Offset the curve for 0.1 mm outward, to get a profile for external thread.

• Trick: notice that valleys should have rounded profiles for both external and internal threads. In other word, clipped peak of a thread should mate to rounded profile of the valley. This could be done by offsetting rounded profile to 0.1 outward, then trim offset line with profile curve for external threads. Thanks to Pascal, for pinpointing.
• Offset joined curve 0.1 mm outward. Change colors for both curves to differ them one another.

• Create major diameter (Dmaj) for 6 mm and axis curve by 1mm from base plane in z direction.

• Move created profile curve to quadrant of Dmaj. Using midpoint snap command to previous H/8 line. Midpoint of H/8 lines will be used as “base” to DMaj.

• Run helix command. Set mode into turns, set turns to 1, and pitch to 1, and reverse twist to no. Use previous axis curve as start and end, and use midpoint of external thread curve as radius point.
• Repeat helix command, and this time snap the radius point into midpoint of internal thread curve.
• Rails for internal and external threads are ready. Changed the color to avoid mistake when rail-revolving the profile curves.

• Run rail revolve command. Select profile curve for external (male) thread, and use the adjacent helix as rail. Use axis curve as start and end point.
• Re-run command for internal (female) thread.

• Voila! We now get basic surfaces for internal and external threads. Differ the colors to classify them. External (male) threads in blue, and internal  (female) threads in green.

• Copy the surface as many as needed, by using endpoint snap. Join them respectively.Name both polysurfaces, to keep geometries manageable.

• Create trim lines to cut away top and bottom parts.

• Run cap command to make external threads.
• Create a cylinder then run trim command with using created thread as trimmer to cut away the center of cylinder. Join the surfaces together to create an internal threads.

Done. :)

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