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Error processing SSI file
How to Model Screw with AutoCAD (cont'd)

Solids: Stretching the CPU Budget

Rotating Flat Drawing

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 realistic

  • Fast, just two cylinders.
  • This is passable in many drawings. 
  • Makes for small file size that renders and shades very fast.
  • Detail rendering looks poor.

Method 2: Semi-realistic

  • Chamfer and socketed.
  • The head detail adds more to clarity and recognition than the threads.

Method 3: A little more realistic (preferrered)

  • Same as before, but with 3D thread line drawn. My favorite. 
  • 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.

'True' screw 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.

 Rendered 1/4-28 SHCS

The final product, real within 0.01%

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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 Virginia.

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