Solid Wood Clock Gears

I love mechanical clocks with exposed gears. Most of my designs are 3D printed, but wooden gears can look incredible. This Instructable describes a method of making solid wood gears that remain stable over time. The first image shows a 7" solid cherry gear built using this method. The reference clock and small gear is built using flat cut gears.

Wooden gears are often made from plywood due to its inherent stability. Baltic Birch is recommended as the best commonly available plywood for gears. It is flat and void free, although a bit bland. I prefer the look of solid wood gears.

The simplest method for making solid wood gears is to cut them from a flat board. This works great for small gears but can cause problems in larger gears. Expansion from humidity will change a flat cut gear into an oval. A small gear can tolerate this change, but larger gears may have issues. Anything larger than 4-5" could see enough movement to prevent them from meshing properly.

Several builders on the internet have made solid wood gears using pizza shaped wedges glued into a circle. The gear diameter is stable since it is defined by the long grain direction. However, humidity changes over time will affect the gear. Most builders that report back say their gears eventually cracked or warped.

This Instructable describes a method for building large solid wood gears with minimal risk of warping. So far, I have only cut a few sample gears. They remain perfectly flat after several seasons. My plan is to make larger more impressive clocks using these gears.

As with most woodworking projects, there are many ways to make something. These are the supplies and tools I used to make the sample clock gears.

Materials:

• Fine grain clear hardwood
• Wood glue

Tools:

• Miter saw
• 12" Quick-Grip clamps
• Drum sander
• CNC router
• 1/8" downcut spiral router bit
• Bandsaw with 8-10" depth of cut
• Random orbital sander
• Router table
• 1/8" roundover router bit

The first step is to draw an involute gear with a 20-degree pressure angle. The gear will be cut on a CNC router so use a tooth size that is large enough for your router bit. I use gear profiles generated using Gearotic with a 45% tooth width for extra clearance. Extra modifications are done in CAD. The tips are extended to round them over and valleys are cut deeper so the router bit can cut them easily. The spokes are also optimized for this process.

Draw three spokes with rounded corners that will be smoothed later using a 1/8" roundover bit. The inside fillets should be large enough for a small spindle sander and the bearing on the roundover bit. The clamping method used to make these gears works best with three straight spokes.

Draw a hexagon around the entire gear with 0.15" or more clearance outside the gear teeth. Draw another inner hexagon inside the spokes. Add the remaining lines to define the sizes of the glued-up gear blank. There should be at least 0.15" clearance inside and outside all cut lines. Add dimensions to define the sizes of the wood segments to be cut.

Repeat the CAD process for different size gears. A 32-tooth gear will have a different size blank than a 36-tooth gear.

Prepare some flat stock in the widths defined by the previous step. At least 0.5" thickness is needed. Thicker pieces are easier to work with and you will be able to cut several gears from each blank.

Each gear blank uses three inner segments and six outer segments. Everything is cut using 60-degree and 90-degree angles. I use a stop block on the miter saw to cut each piece to a consistent length. I built a small jig to clamp the small pieces while cutting so I can keep my fingers away from the blade. You could also use a crosscut sled on the table saw.

The outer trapezoid shape segments can be cut from long stock by flipping the piece after each cut. The inner "house" shaped segments are easiest to cut both 60-degree angles first, then make the 90-degree cut at the proper length.

Cut enough segments for all your gears plus a few spares.

Clamping from three directions is easy with a simple clamping jig. Start with a scrap piece of plywood or OSB. Add two side rails at a 60-degree angle and a flat bottom rail. Cover the plywood with 2" wide packing tape to prevent glue from sticking. The usage should become obvious in the next few steps. The size depends on the size of your gears.

Start by gluing the three inner segments and three of the outer segments. Add glue to all the touching surfaces.

I find it easiest to glue the inner "house" segments to an outer segment first. Press the pieces together and create a rub joint so the pieces stick. Then apply glue to the center surfaces and place everything into the jig. Slide the pieces around until the center tips are touching and the three outer segments are approximately centered.

A single 12" Quick-Clamp can be used to squeeze everything together using this jig.

Wait for the glue to dry after the first clamping step. Then add the remaining three outer segments one by one. Dry fit each piece before adding glue. Sand the angles slightly if needed.

This last glue-up requires three clamps. The clamping surfaces are parallel, so the clamping jig is not needed anymore. All three clamps can usually fit at the same time if the lower clamps are placed on an angle.

Add a couple of tabs around the perimeter so the gear blank can be attached to the CNC router. These tabs position the hold-down screws well out of the way of the router bit. A simple rub joint should be sufficient.

Flatten both sides of the gear blank in a drum sander after the glue has dried.

Screw the gear blank onto the CNC router spoil board and cut the gear profile using a downcut spiral router bit. Cut at least 0.1" deeper than the desired gear thickness. A downcut spiral router bit leaves a clean top surface without chipping. The lower surface will also be clean after it is cut away from the blank later.

I cut the gear profile using a pocket cut defined 0.002" and 0.2" away from the line. Take several passes to get to the full depth, then finish the cut with a full depth pass on the line. A downcut spiral will pack sawdust into the cut, but a pocket cut has room for the sawdust to escape. It takes a bit longer, but bits last longer and rarely break using this method.

Tabs are not needed with since the blank is held down by three screws outside the gear perimeter and we are not cutting all the way through.

The gear can be separated from the blank on the bandsaw against a tall ripping fence. Try to cut near the bottom of the CNC routed depth. Take appropriate precautions since the blank is slightly circular and will try to rotate when the blade first touches it.

Remove the bandsaw marks from the backside of the gear on the drum sander and sand it to the desired thickness.

Both sides of the teeth should be clean and sharp using this method to cut gears. The downcut spiral bit leaves the top surface clean. The bandsaw and drum sander leaves the bottom surface clean.

The spokes look a lot nicer with a small roundover on a router table. Finish cleaning the surfaces with a random orbital sander working up to around 320 grit. The gear above was cut using this method. The only sanding was done with the drum sander and ROS. I think it will look amazing in a clock.

This is the new gear placed over the top of my tiny wooden gear clock (actually it is bamboo, but it looks and behaves like wood). The new gear is completely stable. Expansion from humidity will make the spokes wider, but not longer. The rim will also get slightly wider, but not longer so the shape should not change. The gear was cut a few years ago and is still perfectly flat and stable.

I got side-tracked with a bunch of 3D printed clocks and never finished a large clock using this gear cutting method. Hopefully, it is coming soon and will be the topic of an upcoming Instructable.