3D Print Captured Nuts Without Pausing Your Print.

I am often responsible for rapid prototyping parts on the job as a mechanical engineer. Regularly my designs require the use of captured nuts to joint parts together. It is possible to add a pause in your 3D print and embed nuts, but often my prints last days and run when I am not around to place nuts in the print.

In this Instuctable I will show you my technique for creating captured nuts of any size that can be placed and strongly retained after the print is complete. I have used many sizes of these in fixtures that have been through 10,000 cycles of fatigue testing at 800g's without failure or loosening. While this isn't a revolutionary idea, I have scientific testing behind the sizing of these captured nuts. I hope I can save you time, teach you a CAD trick, or inspire you to come up with a better way to hold onto your nuts!

Required Materials:

Vernier calipers.
Nuts and bolts (I am using M6 in this example).
3D printer or 3D printing service such as 3DHubs and through Instructables.
CAD (Computer Aided Design) software capable of saving .stl files. I am using Solidworks 2015.
Hammer.
Punch or screwdriver for driving nuts into holes.

Special Note:

Your 3D printer must be well calibrated dimensionally or you will need to adjust my numbers to suit your printer.

Two styles of captures

I will show you my two different styles for capturing the nuts. I call the first style "pocket style" as a hexagonal pocket is used to hold the nut. The pocket style is shown as the left part in the intro picture. As you will see this style captures the nut using detents (little bumps) in the corners of the pocket. This style works great in tension (pulling parts together) and is easiest to install the nuts. A downside is that you must have access on your part to the outer face of the pocket.

The second type is "slot style" as the right part in the intro picture illustrates. The nut in this design is retained by one detent resembling a speed bump. This style works in both tension and compression (pulling parts together and pushing them apart) and is great for hard to reach locations. You can make the slot as long as you need to reach your bolt hole location. The downside is it is a little tougher to install the nut.

Step 1: Measure Your Nuts and Bolts.

To begin you must measure the width and thickness of your nut. Also measure the diameter of your bolt. Write these values down. As previously stated I am using an M6 bolt with M6 nylock nut. The nut measures 0.389 in (9.88 mm) wide by 0.2275 in (5.78 mm) thick. The diameter of the bolt is 0.231 in (5.87 mm).

For slot style captured nuts you will also need to measure the point to point maximum width of the nut as shown in the 4th picture. My M6 nut is 0.4375 in (11.11 mm) wide from point to point.

Step 2: Pocket Style Captured Nuts.

Open up your favorite CAD software and begin to model. I modeled a square as an example, but you can use these principals anywhere on your part.

Tip: Pay attention to the coordinate system in your CAD. In Solidworks the top plane is not the same as the top plane on the 3D printer I am using. If I know how I want my part oriented on the printer prior to modeling it, I start the part in printing orientation on the Front plane so that the Z axis is normal to the plane and pointing out of the monitor as shown in the first picture in this step.

Model the clearance hole in the location you want. My M6 bolt has a diameter of 0.231 in (5.87 mm) so I added a thousandth (0.001 in or 0.0254 mm) for a tight clearance fit. Again, the printer I am using is highly accurate so you may need to adjust your numbers to suit your printer.

Since I am creating my part from scratch I extrude my sketch out. I use 1/8 in (0.125 in or 3.175 mm) as the minimum part thickness under the nut in order to provide enough strength between the contact surfaces of my parts. I also like to completely bury my nut in the part so that it doesn't stick above the part. Therefore I take my nut thickness (0.228 in or 5.79 mm) and add 1/8 in (0.125 in or 3.175 mm). I also add a few thousandths of an inch to make sure my nut is fully buried. In this case I extrude my part out 0.228 + 0.125 + 0.001 = 0.35 in or 5.79 + 3.175 + 0.0254 = 9 mm.

Tip: Keep your 3D print layer thickness in mind. For my current setup my slice thickness is 0.01 inches (0.254 mm). Therefore I rounded my calculation to two decimal places, as the printer will only print thicknesses to this level of resolution. You need to make sure your part thickness is divisible by your slice height with no remainder for your model to match the actual part.

Step 3: Make Your Captured Nut Pocket.

Select the outer face of your part for the sketch of your hexagonal pocket as shown in the first picture of this step. If this face is not flat on your part you will need to create a plane that is normal to your clearance hole and tangent to the outer surface of your part to start your sketch. I will not cover this in depth as it is more advanced than needed to grasp the underlying concept of these captured nuts.

Sketch your hexagon centered about the clearance hole as shown in the second picture of this step. Solidworks provides a polygon tool which allows you to sketch a hexagon (or any other regular polygon) in one drag. Set the width between parallel faces of your hexagon to the width of your nut. In my case it is 0.389 in (9.88 mm). In Solidworks I can do this quickly by specifying the diameter of the automatically created construction circle as the width of my nut. I then specify the top line of the hexagon is horizontal and my sketch is fully constrained. Depending on your CAD software you may need to add extra dimensions to fully constrain your sketch and ensure it is symmetrical.

Tip: The printer I am using is accurate to a thousandth of an inch (0.001 in or 0.0254 mm) in the XY plane so I can specify dimensions with a three decimal place resolution in this plane (widths) and expect my part to match the model.

Now make an extruded cut (or pocket depending on your CAD) that is one 3D print layer thickness greater than the thickness of your nut as shown in the third picture of this step. For my M6 nylock nut this is 0.23 + 0.01 = 0.24 in (5.84 + .254 = 6.1 mm). Notice that I rounded to two decimal places as this is the resolution of my 3D printers layer thickness.

Step 4: Adding the Detents.

Now it's time to pay attention as this is the main idea behind the pocket style captured nuts. We will be creating detents (bumps) at the corners of the pockets to hold the nut in place which are small enough to force the nut past them. I placed the detents in the corners to save time CAD modeling. This way you can select a face of the nut pocket to sketch on without having to create fresh plane.

Start a sketch on the flat surface of the pocket you just created as shown in the first picture of this step. I like to use a face that is normal to an axis of the coordinate system.

Make your sketch view normal to your sketch plane if it isn't already and set your part visibility to "hidden lines" or the equivalent as shown in the second picture of this step.

Sketch an arc from the top corner of the hidden line that was created from the outer vertex of your hexagonal pocket back to the same line. If you have a complex part you may need to rotate your view to determine which line is correct. Close the arc off with a straight line as shown in the third picture of this step.

Rule for dimensioning your detents:

If your bolt is 3/16 in (M5) or above make your detent's arc 0.02 in (.5 mm) thick. Make the width 0.08 in (2 mm). Remember that the thickness of the detent should be evenly divisible by your 3D printers layer thickness for the best results if the thickness of your detent will be created by layers.

If your bolt is smaller than 3/16 in (M5) make the arc 0.01 in (0.25mm) thick and 0.04 in (1 mm) wide.

Tip: In order to dimension your arc as shown in Soldiworks place a dimension between the arc and the line as shown. Click the checkmark. Now select your arc dimension in the sketch window and look at the left column property manager. Click on the "Leaders" tab and select the "Min" radio button under Arc condition as shown in the fourth picture of this step.

Once your sketch is fully defined you now extrude it so that it completely passes through the vertex of the hexagonal pocket and back into the body of the part. This is shown in the fifth picture of this step.

Lastly we will use a circular (or revolved) pattern to replicate our detent in each corner of the hexagonal pocket. Select your detent feature from the feature tree or sketch space and choose the circular pattern tool (it may be named differently depending on your CAD). Use the axis of your bolt clearance hole as the axis of rotation. In order to do so you may need to turn on the view of "temporary axes" under the view menu of your CAD software.

I chose to define my pattern using the equal spacing option, 6 instances, and a full 360° rotation. If your CAD software cannot do this you may space the detents every 60° (360°/6). Look at the sixth picture of this step for my settings. Accept your pattern. Your pocket style capture nut model is now complete. Save it!

Step 5: Save Your STL and Slice It.

In your CAD software click "Save As" and select the .stl extension. You may need to change your STL options so that it is saved in the correct units and with your desired level of detail. This is shown in the first picture of this step.

Now open your favorite slicing software and reorient your part if needed.

Tip: For parts such as my example no support material is needed if you orient your part with the clearance hole on the bottom (or detents on the top) as shown in the second picture of this step. Since I sketched on the Front plane with Solidworks my part is already oriented correctly when opened in my slicer.

Use your favorite settings and slice your part. In this case my extrusion width is 0.02 in (0.5 mm), my layer thickness is 0.01 in (0.25 mm), and I used five contours (perimeters) to provide enough strength for holding the nut so I can save material and print semi hollow. You can see what my sliced part looks like in the third picture of this step.

Step 6: Print and Install the Nut!

Print your part. Remove it and allow it to cool while you grab your nut, hammer, and punch.

Tip: If you don't have a properly sized punch you can use a socket, screw driver, rod, or anything that can be hit by a hammer. Your tool just needs to fit inside your hexagonal pocket without touching the edges, but it also needs to completely span the hole in your nut so you don't botch the threads!

Align your nut with the pocket, center your punch, and carefully hammer it in. If you are using a nylock nut like me, make sure the nylon is on the top. Take care to keep your punch centered after each strike so you don't crush your part by mistake! You will notice a change in feel of the strike when the nut has reached the bottom of the pocket.

Screw your bolt in and marvel at how well it stays put. Well done! You can now utilize the pocket style captured nuts for all your prints!

But what if you need a nut where you just can't hammer it in from the top? Keep clicking through the steps...

Step 7: Slot Style Captured Nuts.

So you need a nut in a hard to reach spot. This calls for a slot. In my example I again start with a simple square and create a clearance hole for my bolt that is 0.001 in (0.0254 mm) greater than the bolt's diameter.

For the pocket style captured nut we made sure there was at least 1/8 in (0.125 in or 3.175 mm) of material below the nut. The slot nut can work in tension or compression so we need 1/8 in (0.125 in or 3.175 mm) of material on both the top and bottom of the nut.

We must make our part at least 2*(0.125) plus the nut thickness. For my M6 nylock nut this is 2*(0.125) + 0.228 = .48 in or (2*(3.175) + 5.79 = 12.14 mm). I liked .5 in better so I chose to extrude to that thickness as it is greater than the calculated minimum as shown in the second picture of this step.

Step 8: Cut Your Slot.

Select or create a plane that runs through the axis of your bolt clearance hole to start your slot sketch as illustrated in the first picture of this step. As we did in the pocket style nut, orient the sketch view normal to this plane and change the part visibility to "hidden lines" or the equivalent.

Make a rectangle with the same width as between the flats of your nut and a height equal to the thickness of your nut. Again my M6 nylock nut is 0.389 in (9.88 mm) wide and 0.228 in (5.79 mm) thick as shown in the second picture of this step. Additionally dimension between the slot and the bottom of the part so that it is at least 0.125 in (3.175 mm) thick. Make sure your rectangle is centered along the axis of the bolt clearance hole and it is fully defined.

Exit the sketch and make an extruded cut (or pocket depending on your CAD). Remember when we measured the width of the nut from point to point? We need that measurement now. We will cut our slot in one direction by half this measure. My M6 nylock nut measured 0.4375 in (11.11 mm) wide from point to point (if you rotate your nut about it's hole in the jaws of your calipers this will be the widest measurement). 0.4375/2 = 0.21875 in or 11.11/2 = 5.555 mm. Note that you don't need this level of precision, but Solidworks has a built in calculator when dimensioning so I left it exact. Set your first cut direction and depth to this measure and your second cut "through all" as shown in the third picture of this step. Accept your cut.

Step 9: Make Your Detent.

Select a face that is parallel to the thickness of your nut as shown in the first picture of this step. Start a sketch on this face, orient the view normal to it, and change your part visibility to "hidden lines" or the equivalent.

Sketch a detent the same way as for the pocket style nut, but this time put it on the top or bottom of the slot. If you are using a nylock nut like me then you must put the detent on the surface that will contact the bottom (non-nylon) side of the nut as that will locate the detent at the maximum width of the nut (the nylon part of the nut isn't as wide as the bottom and is tough to measure because it is filleted).

Make the center of the detent half the point to point width of your nut just like the short side of your slot cut. For my M6 nylock nut this is 0.4375/2 = 0.21875 in or 11.11/2 = 5.555 mm. The same rules for sizing the detent apply here as with the pocket style nut as shown in the second picture of this step. That is:

Rule for dimensioning your detents:
If your bolt is 3/16 in (M5) or above make your detent's arc 0.02 in (.5 mm) thick. Make the width 0.08 in (2 mm). Remember that the thickness of the detent should be evenly divisible by your 3D printers layer thickness for the best results if the thickness of your detent will be created by layers.
If your bolt is smaller than 3/16 in (M5) make the arc 0.01 in (0.25mm) thick and 0.04 in (1 mm) wide.

Repeat Tip: In order to dimension your arc as shown in Soldiworks place a dimension between the arc and the line as shown. Click the checkmark. Now select your arc dimension in the sketch window and look at the left column property manager. Click on the "Leaders" tab and select the "Min" radio button under Arc condition as shown in the third picture of this step.

Extrude your sketch up to the opposite wall of the slot as shown in the fourth picture in this step. Your model is done! Save the model file, then save it as an STL just as in step 5 of this Instructable.

Step 10: Slice and Print Again!

Open your slicing software and orient your part how you want it to print. I printed my part with the slot facing upward so that I could minimize support material usage and also remove the material easily. If you can't print support material make sure you think about your print orientation. The bigger the bridges you need to make, the worse your part will turn out. This part would be fine without support since only the bolt clearance hole needs it, and all it does is provide clearance. If it doesn't come out clean you can always drill it instead. I decided to use support material to make my part look better for this Instructable.

Choose your favorite slice settings and print. This time I used a single contour instead of five, but everything else is the same as it was for the pocket style captured nut. That is: 0.02 in (0.5 mm) extrusion width and 0.01 in (0.25 mm) layer thickness.

Print that sucker!

Step 11: Install Your Nut in the Slot!

Let your part cool. If you used support material scrape it, pry it, and pick it off your part. You can see the support material on my part in the first picture of this step. It is unsightly. The part has been freed from it's deformities in the second picture.

Next, place the nut into the slot by hand so that the parallel faces are aligned with the walls of the slot as shown in the third picture of this step.

Use your punch (or a flathead screwdriver that will fit into the slot) and carefully start to drive the nut in as shown in the fourth picture of this step. Make sure that the nut does not rotate as it goes in. After each strike, look at the clearance hole and check the alignment with the threaded hole of the nut. This is important as the corner of the nut will butt up against the bottom of the slot and can crush it's way too far in the slot if you bash on the nut like a gorilla. The fifth picture in this steps shows when I was getting close and the last is all done! You can now thread your bolt in!