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NX – Modeling a tapered thread

Charles-Etienne Lavoie - Friday, May 04, 2012

Currently, the NX Thread command can be used to create a fully modeled straight thread. When
this command is run and the Detailed Thread type is selected a fully modeled thread will be
created. NX provides Modeling tools which allow users to create fully modeled tapered threads.
The Variational Sweep is one of these tools.


1. Create a Datum CSYS on the centerline of the thread at the start location of the tapered


2. Create the following expressions in the Expression editor.



ANGLE will be the included angle of the thread profile. This is typically 60 degrees.
L will be the length of the thread.
P is the thread Pitch which is the distance from thread to thread.
START_DIA is the diameter at the start end of the thread.
TAPER is the taper of the thread.
END_R will be the calculated value L*TAN(TAPER)+STRT_R.
STRT_R will be calculated as START_DIA/2.


All expressions should be created as Length type expressions except for the ANGLE
and TAPER variables. These two need to be set to the Angle expression type. If these
variables are not created as Angle type expressions they will not be selectable when
creating the feature.

3. Start the process by creating a Helix curve.


The Number of Turns will be calculated by dividing the Length by the Pitch or L/P using
the defined expressions. The Pitch variable will be specified using the expression P.

4. To create the tapered helix the Radius Method Use Law will be used. When selected
the Law Function window will be displayed. At this point select the Linear type.


5. Specify the Start and End radius values by supplying these expression variables.


Note that the tolerance of the helix can greatly influence the accuracy of the thread.
Initially the helix will be created to the model tolerance in effect when created. This can
be found at Preferences => Modeling => Distance Tolerance.

If the accuracy needs to be improved after the helix is created a higher tolerance can be
specified by editing the helix and changing the tolerance value.

6. After the helix is created select Insert => Sweep => Variational Sweep. Select the helix
curve as the path. For Plane Orientation pick the Through Axis option and select the
centerline of the helix for the vector. For the Sketch Orientation select the same axis.


7. When OK is pressed a Sketch will be created. At this point create the profile of the
thread. Constrain all geometry to the point that was created on the helix curve when the
Variational Sweep operation was started. This is an important step.


It is significant that the width of the thread be smaller than the Pitch (P-.01). If this width
value is too large then the model will intersect itself as it sweeps along the helix guide
curve. This would cause an invalid solid to be created.

8. When the sweep is complete a hollow thread profile will be created as seen below.


9. The thread would be completed by Uniting it to the model of the base of the thread.


This same procedure can be used to create a multi-lead thread. When creating the
Variable Sweep Sketch of the thread profile create two threads at half the Pitch in width.
See the sketch below along with the picture of the resultant multi-lead thread. The colors
of the different leads have been altered for emphasis.


Using tools provided in NX, users can quickly and easily model complex features.
Randall Waser

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