In the construction of my leg vice, by the grace of the cosmos I came into possession of this huge wood screw. I estimate it to be about 100 years old; it looks like it's seen enough action to have earned a few purple hearts and a postgraduate degree. But the requisite nut, that component needed to turn rotational force into amplified lateral force, was nowhere to be found. I had the option of making the complimentary nut on a wood lathe, but I also have access to a CNC router. I could machine a nut to precise dimensions and, if I'm careful, I can do it right the first time, wasting no wood.
"Pass me the calipers" I said.
I make all my wood nuts at TechShop.
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Step 1: You Will Need
- 3D modeling program (I used Autodesk Inventor)
- a CNC router (I used a ShopBot)
- 1/8" router bit (ballnose)
- 1/2" router bit (I don't care what shape, whatever makes you comfortable)
- Wood glue
- Basic woodworking tools
I have opted to use poplar wood for this project. Though it is a bit softer than maple (the typical wood of choice for threaded parts) the very coarse threads with a 2" diameter make for a robust shape that requires minimal edge-holding properties.
Step 2: Taking the Dimensions of the Screw
Using calipers, carefully measure the height and width of the threading. These threads are triangular, making the dimension a simple function of height and width. Here I focused on the most intact areas, as they have the truest dimensions and the nut must accommodate the widest part of the screw (and then some, but more on that later). Next, measure the diameter of the screw, both inside and outside, and count the threads per inch. These dimensions are constrained by each other, so theoretically knowing the width of the thread and the threads per inch will give you the diameter, or knowing the diameter and the width of the thread will give you the number of threads per inch. But our world is not an ideal one, so take those measurements.
Step 3: Modeling the Nut
- Start by sketching the profile of the thread
- Extrude a block that will make-up the body of the nut
- Define the center axis of the coil
- Enter the values of the coil (the threads)
- Fill-in the coil with an extruded circle
- Alter the features of the block extrusion to render the two halves separately
- Now fillet all the interior corners with a 1/16" radius, since that will be the radius of the tooling for the final part. This won't change how the part is finished but will give an accurate rendering of it.
Herein lies the guesswork. The negative space in the nut has to be slightly larger than the screw to allow the parts to move freely what since we live in a universe of friction and not idealized Platonic structures. I settles on adding an extra .1" to the diameter, giving the screw an extra .05" of wiggle room in every direction.
Step 4: Machining the Parts
For the rough cut I used a cheap 1/2" x 3" straight router bit from the hardware store. For the finishing pass I used a 1/8" x 3" carbide ball nose endmill from McMaster-Carr. I used a feed rate of 1 inch per second.
Step 5: The Moment of Truth
I clamped the two parts together to make sure the screw would mate proper. Success! Now it's time to formalize their union.
You can go fancy with this part, using any manner of dovetails and tenons, but really a little bit of wood glue is all you need. The nut will fit into the leg of the workbench tight enough that structural integrity shouldn't be an issue. I planed it down a bit more to better fit its nook in the bench.
The screw spins freely but tightens-up and stays tight when clamping its business.