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. Copyright Stuart B. Egli Page 1 of 16
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 Copyright Stuart B. Egli Page 2 of 16
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. Copyright Stuart B. Egli Page 3 of 16
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. Copyright Stuart B. Egli Page 4 of 16
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. Copyright Stuart B. Egli Page 5 of 16
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 = depth of thread, then: 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. Copyright Stuart B. Egli Page 6 of 16
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 Be aware of starting position of helical path Copyright Stuart B. Egli Page 7 of 16
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. Copyright Stuart B. Egli Page 8 of 16
STEP 5b: Constrain sides of triangle Use equal relation STEP 5c: Smart Dimension Included Angle All ANSI V-Threads use a 60 degree included angle Copyright Stuart B. Egli Page 9 of 16
STEP 5d: Dimension vertical side of triangle Dimension should equal.875 / TPI STEP 5e: Add Relation to position vertically Select corner vertex of triangle and origin. Hold control key while selecting the second point. Set Horizontal relation. Copyright Stuart B. Egli Page 10 of 16
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 Sketch just completed should already be selected. Click in path box and select helix. Copyright Stuart B. Egli Page 11 of 16
Finished You re done! Copyright Stuart B. Egli Page 12 of 16
Supplemental Tables and Diagrams Copyright Stuart B. Egli Page 13 of 16
Copyright Stuart B. Egli Page 14 of 16
Copyright Stuart B. Egli Page 15 of 16
UNIFIED SCREW THREADS Copyright Stuart B. Egli Page 16 of 16