Want to make some large wooden screws? Need to thread a large wooden dowel? In this Instructable I show you how to make a 1-1/2" diameter 6 TPI wooden nut and bolt with a router and some 3d-printed jig parts.

With a similar approach and modified 3d-printed parts you can cut both interior and exterior threads with even larger diameters and different TPI, right handed or left handed. You can buy tap and die sets for large diameter wood screws; but they are very expensive and don't give you the flexibility to change diameter or TPI.

Tools:

Router
Router table
3d-printer (or use commercial 3d printing service)
1-1/4" Forstner bit for nut
1-1/2" Forstner bit for bolt
Double-angle cutter bit (60 degrees) for interior threads
V-groove bit (60 degrees) for exterior threads
3/8" collet chuck (for the router, depends on your bit shaft size)

Supplies

1-1/2" hardwood dowel
Hardwood board for nut and hex head

Step 1: Internal Threads for Hex-Nuts

The basic idea is to 3d-print a hex-nut holder with threads on the outside and a thread box with matching internal threads per inch (TPI), left or right handed. Then you essentially transfer that TPI to your hex-nut workpiece. The smallest internal thread possible with this jig is 7/8" due to the size of the chosen angle cutter bit. You could buy a smaller bit if you wish, but at smaller sizes it is more economical to buy a traditional tap and die set. The rendered images and cross sections as well as the video should explain the set-up.

The 3d-files included are for making a 1.5"- 6 TPI nut. If you want to make a 1" hex-nut with the same TPI you can 3d-print a reducer-insert. If you wish to cut a larger size or different TPI thread you will need to 3d-print a different hex-nut holder and thread box. Everything else on your setup stays the same.

Step 2: Make the Jig for Internal Threads

3d-print the parts and make sure the parts fit and thread together well. Make a base for the router from 1/4" or 1/2" plywood or MDF, 10" by 10". The template shows the hole pattern for my Bosch 1617EVS router. Cut a couple of wooden/MDF strips at 30 degrees to make the dovetail sliders. Attach the sliders to the router base so that the 3d-printed thread box slides in/out easily with minimal play. Cut a couple of wooden or MDF blocks at 30 degrees for stop blocks.

Attach your router to the completed base. Clamp the router/base to a worktable and install the double angle cutter bit.

Step 3: Prepare Hex-Nut Blanks

Now prepare a hex-nut blank with the following dimensions: 2-3/8" across flats, 1-1/4" center hole, 3/4" to 1" tall. To do this, take a piece of wood (maple and cherry work well) and drill a 1-1/4" hole through it. Then use the 3d-printed hex-nut template to mark the outline of the hex-nut. Cut the outline on a bandsaw and sand the outside to the pencil mark. The nut should fit snug into the 3d-printed hex-nut holder.

Step 4: Cutting the Internal Thread

Insert the nut blank into the hex-nut holder; and screw the hex-nut holder into the thread box. Install the thread box into the dovetail slide on the router base.

Raise the router so that the angle cutter extends into the hole of the hex-nut blank. Slide the thread box until the edge of the cutter bit just touches the inside of the hex-nut blank. Fix several 1 mm thick spacers and the right side stop block in place with a clamp. The spacers allow you to move the hex-nut holder assembly for progressively deeper cuts. Lower the bit (or unscrew the hex-nut holder) to clear the hex-nut blank. Then remove one spacer and put it against the left side stop block. The spacer determines the depth of the cut, i.e. the depth of the thread groove. Make sure the thread box is fixed in place between the two stop blocks and spacers.

Turn the router on at medium speed and slowly turn the hex-nut holder. I push down on the hex-nut holder and use a small clamp for a turning handle. This will lower the hex-nut blank into the cutter bit to cut the thread. Once you are thru the hex-nut, slowly back out while the router is still running. Make several cuts at increasing depth by moving spacers from one side to the other. Remove the hex-nut/holder and try it on your bolt. Adjust the depth of the thread groove as necessary. Make very light cuts (especially for the final one) to prevent burning the wood threads.

Sand the hex-nut to take off the sharp edges and corners. Then finish with linseed oil or butcher block conditioner.

The video shows the entire process.

Step 5: External Thread for the Bolt

As you did for the hex-nut, you will need to 3d-print a thread sleeve with 6 TPI threads on the outside and a thread box with internal threads. The thread sleeve firmly slips onto the 1.5 inch dowel, and both are screwed into the thread box. The threads are then cut into the dowel on a router table.

Step 6: Make the Jig Parts for External Threads

3d-print the thread box and thread sleeve and check that they fit together and turn easily. Make a 2.5" by 2.5" by 1.5" guide block from MDF and drill a 1.5" diameter hole through the center with a Forstner bit. Alternatively, 3d-print the guide block. Mount and clamp the guide block on the router table just to the right of the v-bit. The bit should be aligned with the center of the guide block hole/dowel.

I have also included a file for a 1.5" - 6TPI thread gauge you may find useful.

Step 7: Cutting External Threads

Take a 1.5" dowel, 12" long and drill a 5/32" pilot hole into one end; then insert a 1/4" lag bolt. This will help with turning the dowel when cutting the threads. Insert the dowel into the thread sleeve and make sure it is firmly seated. You don't want it to slip during the cutting process. Insert the dowel/thread sleeve into the thread box, and clamp the assembly to the router table fence. The front of the dowel should be just to the right of the v-bit. Adjust the v-bit height for a light first cut, e.g. 1 mm deep.

Get a 7/16" socket, 10" extension, and ratcheting wrench ready for turning the dowel. Double-check that everything is clamped down securely! Start the router at medium speed, put the socket wrench over the lag screw, and slowly turn the wrench to advance the dowel into the router bit. This will make a first shallow cut. Keep turning the wrench but make sure that the thread sleeve does not exit the thread box. You should be able to cut about 5 inches of thread. Reverse the ratcheting wrench to back the dowel out. Turn off the router. Raise the v-bit by a very small amount and repeat the cut. Make several cuts at increasing depth. The final depth of the thread groove should be approximately 3 mm, but it is hard to measure. Try the thread gauge from the prior step. You can also unscrew the dowel/thread sleeve assembly and try a hex-nut on the dowel to check for a proper fit. Just don't remove the dowel from the thread sleeve.

The video shows the entire process.

Step 8: Completing the Bolt

Once you have a well-working thread you can cut the dowel to complete the bolt. You may include some unthreaded dowel length for a partially threaded bolt.

The hex heads for the bolt are made just like the hex-nut blanks in step 3. The only difference is the hole is 1.5" diameter, and does not go all the way through. Glue the threaded dowel into the hex head to complete the bolt.

Sand the bolt hex head to take off the sharp edges and corners. Then finish with linseed oil or butcher block conditioner.

You can tell from the pictures that I made some of my own dowels from laminated maple and cherry boards. I also used commercially available dowels made from hard maple and poplar.

If you want to make smaller wooden nuts & bolts see my Instructable at:

Wooden Threads with Your Router Table

The nuts & bolts make nice little toys or decorations; but you can also use them to make large clamps or build a workbench vise. For my next project I will scale the jigs to make a 2" or 2.5" nut & bolt set!