Need a thread, but the Thread/Tap command just won’t do the trick?
Needles Not Necessary
Perhaps you’re building a hole, or maybe drawing a bolt or a nut. The 3D geometry is finished and the shape looks great, except it’s missing a key detail: No threads! In this post, we’ll look at a couple of ways to add threads to our 3D model, but we’ll primarily focus on manually building them.
It’s in the Name
Option one (and likely the easiest) is to use the Thread/Tap command (
). This is usually the place to start: Afterall, it’s got thread right there in the name!
You can also add a Thread when building a hole by heading to the Thread Definition tab and enabling the “Threaded” option near the top. The settings are identical.
A Misleading Name
The major issue with the thread command is that it does not create or modify the 3D shape geometry. So, there’ll be no actual 3D thread shown in the model. Fortunately, these threads will show on the 2D drawing. If you simply need your threads/taps on the drawing so that an experienced machinist will take care of the rest of it, this is generally a fine way to go.
Is this hole threaded?
If you need to see the Threads and Taps built using this method, run the Thread/Tap Analysis (
) command.
Yes!
Unfortunately, if the 3D model needs to show the physical cuts from the threads and taps, which leads us to option number two.
Solution: Ribs & Slots
To get a properly cut thread, geometry will need to be manually created. In general, this will be a combination of the Helix and Rib/Slot commands.
Setting it up
In this example, the goal is to thread the outside of the shaft shown below. To get the correct geometry for a Rib or Slot, we’ll need to create a helix and a triangular sketch.
The Helix needs two reference items: an axis line and a starting point. The distance between these two controls the radius of a helix. It’s not mandatory to set these up ahead of time (they can be created on the fly by right-clicking), but it’s generally advisable.
With the new geometry built, head over to Generative Shape Design to find the Helix command (
). Select the references, then key in the correct size values. It may be necessary to oversize the helix to accommodate a proper cut. Modifying the ”Starting Angle” is an easy way to extend the bottom side. Here, an extra 360 degrees in the negative direction adds another turn to the bottom of the helix.
The height and starting angles are tweaked...
...to get a slightly oversized helix.
With the helix out of the way, the next step is to create a Rib or Slot, depending on whether material is added or removed for the desired shape. In this case, we’ll be cutting the threads from the outside of the shaft, so we’ll use Slot to remove material.
Find a plane that intersects the helix. In this case, I’ll use the YZ plane. Create a Positioned Sketch (
) on the plane, using the helix starting point as the sketch origin. Adjust orientation as needed.
Draw the sketch for a single thread. I’m using approximate dimensions for an M12 hole.
For reference, this image (courtesy of Wikipedia) illustrates the relationship between thread diameter and pitch.
Create a Slot (
) from the existing helix and positioned sketch.
Tuck here, nip there
The results are... not right. There’s a twist that’s developed as the slot follows the helix, throwing off the shape of the thread.
The width between threads is varying and incorrect.
To counteract this, change the “Profile control” to “Pulling Direction” and use the helix axis (Line.1 in this model) from earlier. With this change, the triangle sketch will now stay aligned in a vertical orientation; it can no longer rotate clockwise or counterclockwise in the YZ plane.
Now, the sketch follows as it should, creating the correct shape.
Nice, even, properly-spaced threads.
Threaded
Depending on what you need out your 3D models, threads can be an easy addition or a small endeavor. Hopefully, these details have clarified the steps in building successful threads. Still looking for finer details and more info on helixes, ribs, slots and positioning sketches? Check them all out in the CATIA V5: Advanced Part Design class where we cover these items and a whole lot more.
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