How to Model Screw with AutoCAD If You Must

December 2, 2011 | Comments

A recurring topic in CAD newsgroups is making ‘real’ screw threads. CAD users insist on modeling parts that are accurate to every detail down to thousandths of an inch. Hours may be spent on this endeavor. This can lead to much frustration.

Don’t Do It

My usual answer when asked how make real screws is “Don’t do it!” Draw them in 2D. AutoCAD strains itself on solids. A realistically modeled length of thread can bogs down most computers. And when it gets right down to it, a true screw thread is impossible in AutoCAD. Some CAD programs, like SolidWorks can make true threads, but still file size balloons and performance suffers.

Do it In the Flat

A 2D representation of a screw is usually sufficient to convey its use in an assembly. A 2D depiction of a screw thread can look quite real. The time spent on creating a realistic 3D model as well as the time needed to then print it with hidden lines removed  is better spent on something productive.

Table 1: Representing screw threads in 2D.

Method 1: Not very realistic.

  • Fast. Just arrayed lines.

Method 2: Semi-realistic

  • Arrayed lines and zig-zag.

Method 3: Realistic

  • Teeth offset, then arrayed lines

Method 4: Real

  • Splines and overlaps. Lots of time.

Really real

Actual model for comparison

My personal preference for representing threads in 2D is method 2 or 3. The first example just doesn’t look like threads. It’ll be a cold day when I make threads with splines!

Showing Threads in Isometric Views

The standard flat drawings aren’t appropriate all the time. Perspectives, assembly manuals, and presentations are a some of those times. One firm covered most of their bases by including an isometric view in each of their block drawings. Just the two isometric views — when rotated or flipped — can account for many different perspectives.

But there are other views than just isometric, right? Then make a solid model. There are thousands of times when a 2D drawing isn’t appropriate. By time you draw two isometric views, or just one real projection, you can make a solid model that can be viewed from an angle.

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

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