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How to Model Screw with AutoCAD (cont'd)
Solids: Stretching the CPU Budget
So if you absolutely insist on creating a solid
model to represent threads, I'll show you how.
However, please do as little detail as you can
get away with.
Table 2: Several methods of making 3D
models of screws.
Method 1: Not very
- Fast, just two
- This is passable in
- Makes for small file
size that renders and shades very fast.
- Detail rendering looks
Method 2: Semi-realistic
- Chamfer and socketed.
- The head detail adds more to clarity and recognition
than the threads.
Method 3: A little more
- Same as before, but with 3D thread line drawn. My
- Always recognizable, but small file size. The shank
is the tooth root diameter, and 3dspiral.lsp
is used to draw a helical line around it. Remember to
block both or you'll "lose" the threads.
Method 4: Almost there
- Revolved half-sections with teeth.
- Looks more realistic than #3 in details, close-ups,
and side views.
- File size is doubles compared to #3
Method 5: Really real
- CPU killer.
- Solid threads made with multiples of half the
intersection of two offset solid triangles.
- See next note.
threads in AutoCAD.
The technically correct answer is that a perfect
screw thread cannot be modeled in AutoCAD. The figure above left
picture shows that shapes extruded along polylines 'rotate
around themselves' and don't keep a constant relationship to the
extrude path. So you can't extrude screw threads. But all is not
lost, you can make an approximation. If accuracy of ±0.01% is
close enough for you, then you can make half of one screw-thread
with the offset intersection of two revolved triangles. Two
halves unioned make a whole (see picture below) and so on and so
forth until you have the amount of thread you want. The AutoCAD
users at Burris Optical wrote the procedure into a lisp
that does almost the entire operation, but you'll still have to
construct the heads.
The final product, real within 0.01%
back to beginning>>
About the Author
James is a veteran designer well versed in 3D. He
has been working in production design and manufacturing for ten
years. He models with AutoCAD and SolidWorks. James lives in