CNC machining and fixturing to accurately cut a wooden remote
Step 3/4 in the “Everything you need to build your own Turn Touch smart remote” series
This is part of the full guide on how to make your own Turn Touch from scratch. This is the story of the design challenges faced when trying to make a seamless remote and how to overcome them. If you follow this guide, using the accompanying open-source design files, then you will be able to build your own Turn Touch that you can use to control your smart devices and apps on your phone and computer.
If you want to get your own, Turn Touch is on Kickstarter.
Now that we have our models, we need to figure out how to machine them out of wood. Workholding is the principle concern here. In order to consistently machine the wood on two sides, the wood needs to be held down. This is much harder than it sounds.
The source code for the CAM toolpaths explored below is available on Github.
There are two central issues to consider: one is that each piece needs to be machined on both sides, and second is that the workpiece needs to be held down both facing up and facing down so that it can be machined on both sides. In our case, we want to scoop out the insides of the remote out to fit the circuit board and button arms while contour cutting the outsides of the remote to fit the desired shape.
Designing the CNC machine toolpaths to sculpt the contour of the case while pocketing out its insides, assuming the workpiece is held down, is not terribly difficult. There are only a few strategies that you need to use and they mostly involve answering the question of how fast to cut while maintaining as clean a cut as possible, which we’ll get to later.
Above, I’ve designed the fixture that gives us 15 pattern-matched remotes at a time from a single board of wood 24" long and 7" deep. That’s just about 1 board-foot (bf) of wood. A single board-foot costs anywhere from $5 to $50, with maple at $5/bf, mahogany at $10/bf, padauk at $15/bf, and rosewood at $40/bf.
Pattern matching the wood
The top half of the fixture is the inside of the remote, the bottom half is the outside of the remote. We’re machining both the top and the bottom next to each other at the same time because we want the wood to pattern match.
Since we want the top of the remote to match the bottom of the remote, they must be machined next to each other. Our fixture reflects this by placing a top half next to a bottom half, and always in the direction of the wood grain. That way when the top case is flipped over like a book and closes on the bottom case, the grain lines match up.
How to align the wood for two sided machining
Let’s start with the first fixture design because it was simple and was designed to only make a single remote at a time.
This animated gif beautifully illustrates the basics of machining the remote on both sides. This design uses a vise clamp that will quickly prove to be haphazard and inaccurate.
Unfortunately, the truth is that using a vise clamp leads to needless suffering.
The hardest part about using a vise clamp is ensuring that the registration and orientation match when flipped upside down. Not only do points on the x, y, and z axes have to be the same, but they need to be the same all the way down the workpiece. Any accidental tilt, turn, or shift when flipping the wood blank over results in a mis-alignment, and when that shift is greater than the width of the wood, we’re left with a work piece that abruptly breaks during milling.
And to make matters worse, errors are doubled when the piece is flipped. A 0.005" shift (less than the height of a fingernail) is a proportionate shift in the other direction on the opposite side, resulting in breakage happening more often than such a slight shift would imply.
The good news is that it’s easy to design a better fixture. The vise clamp suffered from multiple issues. Having to hold down the vise clamp itself was problematic, as it could subtly shift during machining. There are constant orientation and registration issues with a vise clamp unless you are fortunate enough to have a CNC track that can securely hold down the clamp.
We can clean these issues up with a fixture cut into a ¾" plywood board.
This is fundamentally the same fixture as the vise clamp with a few important differences. See those alignment pin holes in the top image? Those are going to be responsible for keeping our x and y axis consistent when the piece is flipped over. And the z axis is kept consistent with the use of a flat plywood board that is level to the CNC’s spoilboard.
Note that the fixture itself is not registered to the table, it’s simply attached with screws. So when we put a blank wood work piece down, we drill the alignment holes directly into the wood and into the fixture. When one side is completed, we flip the wood blank over and use dowel rods to ensure we’re still registered in the same spot.
So this fixture addresses alignment and orientation issues in all three axes, but because it is not itself registered to the table, we cannot reuse the registration holes, as the fixture is never going to be put into the exact same spot again next time it’s used.
More importantly, there are still tabs holding the wood remotes in place during machining. Tabs take time to machine around since you can’t just have your toolpath wrap continuously around the contours of the remote without stopping. The CNC machine now needs to pick itself up and climb over the tabs for each rotation around the remote.
Using tabs also means that the remotes cannot be placed right next to each other. There are these bars of unused and uncut wood between each half of the remote. These untouched bars allow the wood to be held down with clamps, since the remote isn’t sitting on anything. But they are a waste of wood!
And to top it off, once the tabs are cut with diagonal cutters and you pull the individual remote halves out of the larger workpiece, you have to manually sand down the tabs, which is error prone and dangerous.
How to hold down the wood for two sided machining
Here’s where it gets interesting. In order to get rid of tabs, we have to figure out a way to hold down each individual remote as it is being machined.
The cutter is contouring around the outside border of the case, so there’s no way to hold the case from above without having it in the path of the cutting tool. Ideally we’d hold it from below, but that requires messy adhesives or a complicated locking system. And that brings us to vacuums.
Vacuum tables are outside our price range expensive, but we can build our own for the cost of plywood and glue. Above you can see that we’ve built a hollow box about 3" high. It’s just ¾" plywood cut into the shape of a box and glued tight. On top we’ve glued additional wood, which is then machined into the inverse of the remote.
For a fixture, alignment is everything
Zooming in we see that the top case (left) is too thin to use a large hole. So we drill a few dozen 1/8" holes around the perimeter of the top case.
On the right we have the bottom case, which is solid in the center, so we can use a large 1/2" hole to have the vacuum hold it down in one spot.
You may also notice 5 metal alignment pins on each half. The vacuum holds the case down in the z axis, but the alignment pins are needed to hold the case down in the x and y axis as the wood is being machined.
Above you can see how the 1/8" alignment pins handle the slight variance in the buffer between the case and the inverse fixture. They do a great job of holding the remote tight while it’s being worked over by the cutting tool. Before I used these alignment pins, oftentimes a case would become dislodged by the cutter, flinging it across the room.
The fixture itself sits on a spoilboard that is aligned to the machine, which allows for reuse of the fixture without having to recut anything. To pre-align the fixture, simply cut holes into your own spoilboard that sits on top of the machine’s spoilboard.
Use those holes as reference guides for the dowel rod that aligns the fixture to the spoilboard. You can see the dowel rods sticking out of the fixture and into the pre-drilled alignment holes at the bottom.
I’ve also cut out handles into the fixture, which makes carrying these heavy fixtures around much easier.
Behold the vacuum
Just attach your vacuum hose to a $3 flanged inlet fitting attached to the side of the fixture. You’ll need to cut a hole out of the fixture. I used a $17 hole saw on a drill press to poke a hole for the vacuum to attach to.
This is your standard 2½" hose, attached to an inexpensive ShopVac. You don’t need more than a couple horsepower to pull enough air to hold the wood down in place. This vacuum seal isn’t going to do anything for movement in the x and y direction, so if all it has to accomplish is offsetting the minor pull in the z direction due to vibration, then you won’t need an industrial pump or expensive vacuum to do the job.
In other words, it shouldn’t hurt when you place your hand over the hose and seal the vacuum.
Because the fit is so tight, we need to add tiny ejection pockets underneath the wood case so we can wedge a lever in and pop the remote out. You can see this space at the bottom in the above photo.
You can see the secondary spoilboard on the button vacuum fixture, which follows the same principles as the case vacuum fixture. There are holes beneath each button, and three alignment pins holding the buttons in place as they are machined.
Using off-the-shelf tools and cutters
It’s important to keep as many tools and parts of the assembly off-the-shelf. There’s something to be said about being able to order more components and tools from a trusted vendor rather than having to source new custom tools. All of the tools used to machine Turn Touch are relatively inexpensive and will last you long enough to machine thousands of remotes.
The tools above are much of what I use in the wood shop. Starting with the milling and drilling bits, left to right:
1/8" shank bits
- $3, 1/8" brad point drill bit — used on the vacuum fixture to provide a vacuum channel for the top case
- $12, 1/16" flat downcut 2 flute end mill — used cut out the buttons without leaving a trace
- $19, 1/16" bull nose 0.01" radius 2 flute end mill — used to sculpt the two north ridges on the buttons
1/4" shank bits
- $36, 1/8" flat 2 flute end mill — this pokes small holes that alignment pegs fit into inside the top and bottom case
- $33, 1/4" flat downcut 2 flute end mill — pocketing out the insides of the case without tearing out wood from the edges
- $24, 1/4" ball 2 flute end mill — contouring the outsides of both the top and bottom case
- $12, 1/4" flat upcut 2 flute end mill — surfacing and planing the wood for the buttons
1/2" shank bit
- $17, 1/2" flat 2 flute end mill — used to dig tracks and layout for each of the fixtures
Accessories
- $1 Square head power bit driver — square heads don’t strip as easily as phillips screw head does
- $30 1/8" ER-25 collet — size used by Shopbot CNC
- $30, 1/4" ER-25 collet
- $30, 1/2" ER-25 collet
- Composite plastic brad nails — used to attach the remote fixture to the CNC’s spoilboard without damaging the spoilboard
- $1, 4 square head screws and washer — long enough to go through the fixture and the spoilboard
- a pencil — don’t use a pen on wood, it smears and it kills the pen rather quickly
Miscellaneous lessons learned
Apart from the construction of the vacuum fixture, there’s many other lessons to learn from machining these remotes.
Figuring out the feeds and speeds for cutting into the wood
The feeds and speeds took a bit of trial-and-error but eventually I settled on values that meant an entire remote could be machined just under 10 minutes. With some changes to the cutting tools it is possible to bring that down further, but custom cutting tools cost on the order of $1,000. So the workholding fixtures we have here use off-the-shelf cutting tools, like flat end mills and ball end mills, which only cost $20 each.
My rule of thumb for mahogany is 120 inches per minute with no more than a 0.15" step down. For rosewood that number is cut in a third, so 40 ipm. Finding that number is simply a matter of running the machine and interactively increasing or decreasing the feeds until there is no chatter and the CNC machine sounds like it’s under control.
How do I know if I’m moving too quickly?
Apart from the horrible sounds of chatter, this may happen.
Different hardwoods need different cutting speeds
Some harder woods need to be machined at a slower speed than less dense woods. Rosewood needs its speeds cut into a third of what mahogany would use. Instead of having to update the toolpaths, I wrote this python script to automatically copy the mahogany toolpaths and cut the speed in a third and then re-save them as rosewood toolpaths.
The inverse fixture needs to compensate for differences in the wood
Below is a picture of what the fixture looks like without the wood on top. Notice that the fixture is red at parts. That denotes that the wood is about 0.005" inset. This means that there is a slight gap between the inverse fixture and the wood workpiece that will fit over it.
The reason for this slight gap is that if there is no gap then the workpiece will either not fit perfectly onto the fixture or it will get stuck, as it is holding the fixture on all faces. By adding a 0.005" gap between the two, they can better fit together. And the alignment pins and vacuum holes protect the wood from shifting around any axis, so the buffer could possibly even be bigger.
With that, we now have perfectly aligned wood pieces that are ready to be adhered together. All we have left is to inlay mother of pearl on the bottom of the remote to complete the process.
This is part three of a four part series on everything you need to build your own Turn Touch smart remote.
Next step: Laser cutting mother of pearl and laser engraving wood
If you want to get your own, Turn Touch is on Kickstarter.
