High-tech modeling for garden railroaders

Part 25: How to make a nut-bolt-washer (NBW) casting for a 1920s narrow-gauge log car
I have been wanting to build a 1:20.32 narrow gauge log car like the one built by Howard Maculasy. As usual, I am going to make most of the parts because that is what I do. To start with, I am going to make a Nut-Bolt-Washer (NBW) as described in the article. He uses a 3/4” bolt, 2” hex nut with a washer. Since I model 1920's narrow gauge I am going to substitute the hex head nut with a square nut common in that era.

I will be 3D printing these parts; however I must warn you that a Filament Deposition Modeling (FDM) type printer does not have the accuracy you need to make these parts. I will be using a sterolithographic (SLA) printer (Product: Zortrax Inspire) for its accuracy. In the x and y plane it is accurate to 50 microns (0.00196”) and in the z plane it is accurate to 25 microns (0.00098”). It takes longer to print, the material is more expensive, but in this case the accuracy is what is needed.

As an Electronics Engineer, I had a cursory knowledge of nuts, bolts, threads and the like but not a detailed knowledge that is required to design some of these things. My reference is Machinery's Handbook, 24th Ed, by Industrial Press. (They are now in their 30th Ed. but things have not changed that much.) You can also find a lot of this information on the web, but I like my reference books and this one was getting dusty anyway.

Note: I now have a website. Learn more at https://rallecrailroad.com/
Threads for nut-bolt-washer casting
Photo 1
I am going to use what is called a 3/4-10 UNC Class 2b thread. That is a major diameter of 3/4”, 10 threads per inch (TPI), Unified Coarse thread. Class 2b in the tolerance class for the threads which I am told is the standard class of thread tolerance.

From my reference book I find the following dimensions that I need for an internal thread:
Dmajor: 0.750”
Dminor: 0.6309”
Pitch: 1/TPI = 0.1”
The above definitions are defined to the left:
For the scale that I am working in:
Dmajor: 0.0369”
Dminor: 0.0310”
Pitch: 1/TPI = 0.0049”

See photo 1.
Onshape software to create a new sketch
Photo 2
Building the bolt
I need to know some additional information before I proceed with Onshape. The prototypical height of the nut that I am going to use is 1 ¼” (0.0615” scale)(heavy nut) and a 2” (0.0130)(heavy washer)(0.0745” total). I am going to leave the same amount, 0.0745”, above the nut. The total length of the threaded portion is: 0.1491”. I will be leaving a shaft of diameter 0.060” down 0.100” so that can be glued in. Now on to Onshape.com. (Previous articles will show you how to get logged on and setup.)

Once you are in the Onshape drawing, you will have to first click on “Sketch”. See photo 2.
Choose sketch plane in Onshape
Photo 3
Once you have the sketch pull down you can do some things that I just learned. If you highlight the sketch plane dialog as shown in photo 3, you can enter a name for the sketch plane. This will give you some idea as to what planes mean what later on. Just click on the pencil as shown by the red arrow and type in the name. I chose the name “Bottom of screw” for my sketch plane.
Onshape minimum diameter of nut
Photo 4
Now I am going to draw a circle that is going to start to represent the minor diameter of the screw. Click on the circle pointed to by the red arrow and draw the circle on the sketch plane “Bottom of screw." See photo 4.
OnShape define diameter of screw
Photo 5
Once the circle is drawn we will have to define the diameter of the circle; that would be the minor diameter of the screw, using the dimension icon as shown by the red arrow. See photo 5.
Onshape enter minor diameter of screw
Photo 6
I clicked on the circle and hit enter. A dialog box now appears. I entered the minor diameter 0.0310. Remember to hit return to enter the number. See photo 6.
Onshape 3D photo of screw
Photo 7
You probably cannot see the circle when it is dimensioned so just use your scroll wheel to zoom in. I use the “cube” in the upper right corner to get into a 3D picture. See photo 7.
Onshape extrude the circle of screw
Photo 8
Now we need to extrude the circle to make a round bar of the minor diameter. First select the circle by clicking on it (should turn a dirty yellow), then use the extrude icon in the upper left hand corner shown by the red arrow. See photo 8.
Onshape extrusion depth of screw
Photo 9
Once the extrude icon is clicked, you can enter the depth of the extrusion: 0.1491. This is the depth of the screw portion only, not the post. See photo 9. We will be adding the post later on.
Onshape creating screw thread
Photo 10
For some of you this is a review of things that we have covered before. Please see previous articles for additional tips and help. Now we are going to cover something new: creating a thread.

We want the thread to start at the top and go down the whole length of the rod: 0.1491”. First select the Helix icon shown on the right. See photo 10.
Onshape screw thread details
Photo 11
There are four things that I filled out. First I identified the helix as “Threads”, next I selected from the drop down “Pitch”, next I selected the “Face of Minor diameter rod” by clicking in the box then click the outer face of the rod, next the pitch I previously calculated as being 0.0049 in. See photo 11.

Things don't get easier from here. Now we have to make the threads. First let us go to the Front sketch plane. The first thing we need to do is select the top edge of the rod.
Onshape show screw threat constraints
Photo 12
The Project/Convert will take the edge that we have selected and project it onto the selected sketch plane. If you check the “Show constraints” box in the Sketch plane drop down you will see little boxes that show you successfully executed the command. See photo 12.

Onshape make top of screw
Photo 13
Next we draw a trapezoid on the end of the helix. Use the line tool to do this. Make sure it is fairly large compared to the threads, we have some dimensions to add. You can see the constraints and the shadowed, enclosed area. That means that you have done it right. See photo 13.
Onshape add dimensions to screw top
Photo 14
At this point we need to add three dimensions. First is the base of the trapezoid. Using the dimensioning icon, click on the line at the very bottom of the trapezoid. You will need to enter the pitch: 0.0049. At each of the bottom corners of the trapezoid dimension an angle of 60 degrees. It should look something like photo 14.
Onshape top of screw menu options
Photo 15
Now we are ready to dimension the top of the trapezoid. First we are going to create a construction line along the axis of the rod. We do this using the line tool and run a line from the center top of the rod up a little. You will note that this creates a line; we want a center line. The construction icon is in photo 15.
Onshape add dimensions to screw top
Photo 16
We want a construction icon because when all is said and done the line will disappear without our needing to delete it.

Now using the dimensioning icon create a dimension between the top of the construction line to the top of the trapezoid in photo 16.
Onshape major radius of screw top
Photo 17
Then enter the major radius: 0.0369/2. You should get the following as shown in photo 17.
Onshape sweep icon
Photo 18
Now we need to create a thread the entire length. To do this we will use the sweep icon. See photo 18.
Onshape Sweep path
Photo 19
Right clicking the sweep icon gives the following box requesting information. First “Faces and sketch regions to sweep” is the trapezoid. Next the “Sweep path” is the helix. Right click on each one after selecting the box and right click on the check mark when completed. See photo 19.
Onshape riser does not line up
Photo 20
If you now zoom out using the thumb wheel you will see the entire thread. But there are some abstracts left from our sweep. Most noticeable is the rising thread that does not match the rod. If you look, they are on the top and the bottom. See photo 20 of closeup view.
Onshape fixing top of screw
Photo 21
Here is how we are going to take care of that. Click the top rod surface and it should turn a dirty yellow. Hover your mouse over the top of the selected surface and RIGHT CLICK. A drop down should appear and left click on “View normal to”. We will need to create another sketch with the top of the rod as the sketch plane. I called this sketch “Top abstract removal”. Now draw a circle using the circle icon whose diameter should be outside the threads, but centered on the center of the rod. Now click on the extrude icon and first click on “remove”. You might notice that it looks like it is going to remove the whole screw but next to “blind” there is another icon, arrows. Click on that and the removal should reverse. For depth I am going to take about 0.005”. There is another check box called “Second end position”. This one will go in the opposite direction. I entered 0.001”. Now click the “Merge with all” check box. See photo 21.

Flip the part over and do the exact same thing with the bottom.
Onshape add non-threaded rod
Photo 22
Now for something simple, we have to add the non-threaded rod so I can glue this into a hole in the wood. I am going to use a #0-80 major diameter which is 0.06”. This should be covered up by the two inch washer which would be 0.098”. It will be 0.100” long. As they say in college, “I am going to leave this as an exercise for the student”. This is what it should look like in photo 22.
Onshape extrude screw
Photo 23
The washer should also be easy for you now. Just a circle and extrude. See photo 23.
I got most of the information in making this very difficult (for me) part from a YouTube episode named “How to make a bolt in Onshape” by Matthew Barbercheck. I would suggest that you look at it before starting this project. I reviewed it over a dozen times. Next comes the nut. Another difficult part but we will be using much of what we learned. If you have any questions you can email me at service@rallecrailroad.com and remember this will only come out looking like what we tried to build using a SLA printer or another type of printer with high resolution.


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