Example designation
0.5-20 BUTT-3A
- 0.5
- Nominal major diameter in inches.
- 20
- Threads per inch.
- BUTT
- Inch buttress profile.
- 3A
- Class 3 external fit; 3B identifies the internal fit.
Identify the loaded flank
The two flanks serve different roles. Carry the 7° load-flank orientation through the drawing, tool setup, and inspection plan; thread hand alone does not describe which flank carries the load.
Class and engagement
Choose 2A or 3A for the external thread and 2B or 3B for the internal thread. This calculation uses a single start and allows an explicit engagement length.