How to Model Screw with AutoCAD (cont'd)

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.

See Also:

• Creating a 3D Screw

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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