Making 3D Threads in Feature Based Solid Modelers

Making 3D Threads in Feature Based Solid Modelers

THREAD BASICS

Making true geometric threads in feature-based solid modelers is a fairly straightforward process and can be handled using several different approaches. Before this is done the question must be asked “Why do the threads need to be modeled in 3D?”

Doing so requires significant effort, serves little purpose for drafting purposes, and adds significantly greater demands on computer resources. This is especially true in parts/assemblies with numerous threaded

features/components. In fact, completely representing threads showing crests and roots (see figure 1), is prohibitive for that among other reasons.

When it is considered that most threads are produced using dies, taps, or canned cycles on CNC machines, the geometry created in 3D serves little purpose except for cosmetics, and in some cases, analysis.

Screw Thread Terms

Fig. 1: Definition of parameters defining standard ‘V’ threads

When Do You Need to Model Threads

The questions thus comes to mind “Why model threads and when should we do so?”

There is no simple answer. In fact additional questions can be asked such as “If we do not model threads how should we model/represent threaded features?” and “What are the downstream ramifications of whichever approach we choose?”

The remainder of this tutorial assumes you have resolved those questions and have chosen to model a ‘V’ thread geometrically. The following figures/tables provide additional definitions/information about thread

specifications.

The following figure defines the parameters used to define ANSI UNC threads. The first example shows a ‘Sharp V’ thread that in reality cannot exist. Interestingly enough, it is also impossible to model using SolidWorks. The ‘American national’ form shown is more realistic and is more commonly expected.

Screw Thread Forms

ANSI UNC thread form details

UNC Unified Coarse Thread ANSI B1.1

The next figure illustrates the definition of ANSI B1.1 UNC threads and the table lists key parameters used to define such threads. Please not that this is a partial table for illustration purposes only.

Nominal Diameter Major Dia (inch) Major Dia (mm) Tap Drill Size (mm) TPI Pitch (mm) N 5 - 40 UNC 0.125 3.175 2.65 40 0.635 N 6 - 32 UNC 0.138 3.505 2.85 32 0.794 N 8 - 32 UNC 0.164 4.166 3.50 32 0.794 N 10 – 24 UNC 0.190 4.826 4.00 24 1.058 N 12 – 24 UNC 0.216 5.486 4.65 24 1.058 1/4" – 20 UNC 0.250 6.350 5.35 20 1.270 5/16" – 18 UNC 0.313 7.938 6.80 18 1.411 3/8" – 16 UNC 0.375 9.525 8.25 16 1.587 7/16" – 14 UNC 0.438 11.112 9.65 14 1.814 1/2" – 13 UNC 0.500 12.700 11.15 13 1.954 9/16" – 12 UNC 0.563 14.288 12.60 12 2.117 5/8" – 11 UNC 0.625 15.875 14.05 11 2.309 3/4" – 10 UNC 0.750 19.050 17.00 10 2.540 7/8" - 9 UNC 0.875 22.225 20.00 9 2.822 1" - 8 UNC 1.000 25.400 22.25 8 3.175

EXAMPLE – CREATING EXTERNAL THREADS USING SOLIDWORKS

The following example uses SolidWorks to create an external thread on a cylindrical feature. This feature could be created using either the Extruded feature or Revolved feature commands. This is a basic example and has certain flaws that will be discussed in follow-up documents.

In this example we are going to make a ½”-13 UNC thread by cutting it (using material removal) from a .5” diameter cylindrical feature.

A .5” diameter cylindrical feature is the starting point for this process. The general approach will be to use a SWEPT CUT feature based on a helical path and a triangular profile to cut the thread from the cylinder. (An alternate approach would be to use material addition and use the SWEPT BOSS feature to add the threads to a cylindrical feature.)

The following is a summary of the steps to be followed: a .5” diameter cylindrical feature in your part 1. Create a new sketch and create a .5” diameter circle in the plan defining the starting plane for the

threaded feature (create a center line

2. Use that sketched feature to create a helical curve is created to use as the path for the SWEPT feature 3. Create a chamfer on the end to be threaded. Use thread depth and 45 deg chamfer

4. Create a new sketch on a plane containing the cylinder/helix axis 5. Create a profile on that plane to ‘cut’ the thread

6. Create a SWEPT CUT feature to cut the thread into the base feature

STEP 1: Create cylindrical feature using REVOLVE or EXTRUDE

.5” DIA cylindrical feature

The starting point for this tutorial is a .5” diameter cylindrical feature. This can be created by either using a circle as the base sketch for an EXTRUDED BOSS/BASE or by creating a REVOLVED BASE/BOSS. This tutorial assumes the reader knows the procedure to produce these features.

STEP 2: Create sketch on end and convert edge to sketch curve

Create a sketch on the end face and a circle in that sketch

The next step is to create a sketch on the end of the cylinder and a circle in that sketch. This will be used to produce the helical path used for the thread.

In this example the CONVERT ENTITIES command was used to project the edge of the cylinder onto the sketch plane. This end result could also have been produced by drawing a circle but, depending on how it is created, additional relations may be required to maintain the integrity of the design if dimensions are modified. This is also a viable method if the end has already been chamfered or filleted.

STEP 3: Create Helical Curve using sketch created in previous step

Set appropriate direction using the ‘Reverse direction’ check box

Several options are available when creating a HELICAL curve. In this case we will use the ‘HEIGHT and REVOLUTIONS.’ Since the desired thread is 13 threads per inch (TPI) then for each inch of thread extending along the length of the cylinder we need 13 revolutions, or coils, in the helix. Note that we also must select the ‘REVERSE DIRECTION’ option in the dialog as well.

It should also be noted that the starting angle selected is 0 degrees. Although which angle is used is not it does impact which plane should be used in the next step. By using the previously stated options the result shown above should be produced. At this point exit the sketch and move on to the next step.

STEP 3: Use ‘Height and Revolutions’ Option to complete helical path

Revolutions = TPI for each Inch of Thread, in this case 13 thread per each inch of length

Sharp V-thread

The sides of the thread form an angle of 60 degrees with each other. The top and bottom of the thread are, theoretically, sharp. however in practice it is necessary to make the thread with a slight flat. There is no standard adopted for this flat,

but it is usually made about one-twenty-fifth of the pitch. If p = pitch of thread, and

d

The chamfer feature made in the next step distance should be approximately the same as the thread depth which is calculated using the information in the above figure and equation.

STEP 4: Chamfer end of shaft to be threaded

Chamfer should be approximately thread depth. Use .866/TPI for depth

STEP 5: Create profile to sweep along helical path to ‘Cut’ thread

The next step is to create a sketch to ‘cut’ the thread. For clarity this is created in a plane that contains the starting point of the helix. To do this we select the ‘TOP PLANE’ from the tree to define the sketch plane.

STEP 5a: Draw triangle

Draw slightly off to the side to avoid creating relations that may not be desired.

The profile used to cut the thread is simply a triangle. The critical dimensions will be the width and the included angle.

For a UNC thread the included angle will always be 60 degrees. For a pure ‘V’ thread the width will be the inverse of the pitch. In this case that will be 1/13”.

The only problem with this is that if that number is used SolidWorks will fail to create the SWEPT CUT feature desired. The width used must be less than the actual pitch. To more accurately represent a UNIFIED thread we will reduce the width of the cutter to .875 times the pitch. This will also allow the SWEPT CUT function to work.

STEP 5b: Constrain sides of triangle

Use ‘equal’ relation

STEP 5c: Smart Dimension Included Angle

STEP 5d: Dimension vertical side of triangle

Dimension should equal .875 / TPI

STEP 5f: Add relation to vertical side of triangle

Select vertical side of triangle and corner vertex where chamfer meets outer major diameter of shaft. Add ‘coincident’ relation. Exit sketch.

STEP 6: Create ‘SWEPT CUT’ Feature

Finished