Intro to CNC Milling a Custom Watch

This project is the culmination of several projects centered around restoring & upgrading a Proxxon MF70 manual miniature mill that had been converted to CNC but left unfinished and unusable. It took months to convert it to use Open Source Hardware, and it runs using GRBL as its firmware. Much thanks to everyone involved in the GRBL project, and to those keeping Open source alive!

Wristwatches have been a valuable possession for many decades now, whether they’ve been used for function or for style. Perhaps their importance has been diminished in the recent years with the spread of mobile phones & alternatives to checking the time, but that also allowed them to be used for expressing ones’ self as well.

Recently, my sister got married and I decided I wanted to make them a matching set of wristwatches to commemorate the occasion. While horology could have dozens of Instructables just for its main subjects, this instructable aims to guide and show you my process for how I decided to make the watches, from design to milling them in POM. I’m still in the process of finalizing the flood coolant system, so I haven’t machined the final watches out of aluminum yet.

Given the nature of the topic, I've tried to include details while keeping it concise, so I hope my CNC explorations here can be of use to you, and please let me know if there are any suggestions or questions you have.

Step 1: Research

I started by visiting my local watch repair store, and discussed my project with the proprietor. He suggested I use an ETA F05.111 quartz movement, 3 hands and a date window. He also gave me a lot of advice regarding the specifics of the watch, such as how wide apart the lugs need to be, glass lens makers, etc.

There’s a number of parameters you need to keep in mind while designing the watch, and I’ve listed some of them here. I’m still a complete novice to horology, so please rely on your own research to avoid expensive errors. I find watch parts suppliers like Esslinger are handy for reference for certain off-the-shelf parts.

• Case back type- snap off, screw off, and 4 screws.
  • Snap off uses pressure or snap fits to keep the back in place.
  • Screw off has notches to help rotate the entire back to unscrew it
  • Using small screws to secure it. I used M1.6 screws in my design.
• Lug spacing, this depends on the width of your bracelet, are usually ranges from 12 to 24mm+ in even steps.
• Spring pin diameter, usually needs a 0.8mm or 1mm hole to fit into.
• Crystals can be purchased in a variety of diameters and materials, and usually you can find stores to custom cut glass crystals for a reasonable sum.
• Dial size and thickness
• Gasket size
• Plastic Movement Ring, which usually comes with the movement or from your local watch repair store.

It’s a bit daunting, but making the project a collaborative effort with your local horologist or watch repair store can help fill in most of the details you’d need.

Step 2: Modelling a Detailed Mock-up

Making the mock-up is one of the most crucial steps in the entire process, as any measurement errors here will cost you a significant amount of time later down the line, so take your time and go slowly. I was able to use a datasheet for the F05.111 I found on a watch repair website & a digital vernier caliper to accurately measure all the major dimensions and measurements of the watch.

A dummy model isn’t strictly required, you could just directly attempt to model the watch’s case itself, but having model helps clearly visualize the look and fit of the body. It would also help with the tolerances to avoid tight fits or rattling, but sometimes iteration allows you to improve the design, so don’t be afraid to scrap an imperfect piece and do it over again.

Step 3: Sketching

This is something you can begin before you even start the project, since all you need is a few sheets of paper and a sharp pencil. In fact, it might even be better to start with sketching rather than rushing into 3D modelling, so let your imagination run wild, observe the changing styles over the history of wristwatches, and stay inspired. Let your pencils loose and try to imagine what kind of shape and form you want your watch to have, and the colours or materials. Consider the surfaces and its textures, the way it links up with the bracelet, the sheen of its crown and the numbering on the dials.

Horology and wristwatches have become more than style and functionality, they’re jewellery, art work that show off your personal aesthetic and world view. They may mirror your personality, or just serve as reliable workhorses to keep pace in a world obsessed with time.

Speaking of time, this is another step that will save you a lot of time if you can experiment and settle on the looks and shape of your watch. It helps to have multiple drawings from different angles to reference while modelling the watch’s case.

Step 4: Designing the Watch: the Reality Check

Now’s the time to blend the sketches with the mock-up of the model. Here’s where you have a reality check and try to settle on what’s possible with your CNC milling setup.

For instance, I would have liked to include a slight slope to the side of the bezel as shown in the above image, but I couldn’t find a tapered endmill that would fit the mill, so I decided to remove that feature. Perhaps I'll use a handfile to add it in later.

Step 5: Designing the Watch: Visualizing Layers

I find that the best approach to designing a watch like this is to think in vertical layers, so I created several construction layers to define the case depending on what was supposed to be on that layer.

This also helps you break down the shape and form of the watch into separate shapes, like imagining a series of cylinders that form the seats for the crystal, movement, and watch back. This is vital to keep in mind because once again this depends on your machine’s capabilities, and to avoid creating features that are impossible to mill. You’ll also want to think of how many orientations you would need to print from in order to finish the part.

These lines of thought may seem a little excessive at the moment, but later it'll help clarify the process when you're setting up for milling.

Step 6: Designing the Watch: Visualization Aid Ft. the Other Popular CNC

An important part here is making sure your clearances and tolerances are correct. Deciding on factors like bracelet width or having the physical parts to measure really helps ensure you’ve left enough space to insert and seat the components, and leaving enough space to keep them in place.

If you have access to a 3D printer, you can also print out the watch to check its dimensions and feel in your hands, and if the other watch parts are suitable for it. I’d advise against testing fit or tolerance though, as FDM parts will differ significantly from a milled part in terms of tolerances & accuracy.

Step 7: Setting Up for Milling

The process of milling starts with the process of setting up your computer CAM program with your machine’s specifications, here Fusion 360.

It starts with switching to the Manufacturing interface, creating a new Setup, and either selecting or adding in your machine. It’s a fairly straightforward process that differs from person to person, so I won’t go too much into detail here.

Once the machine’s added and selected, the next step is setting the WCS (Work coordinate system). This is vital so that the machine’s axes are aligned with the G-code that Fusion outputs, or you’ll crash the machine. I usually set the orientation by selecting the model’s Origin (NOT the model’s orientation. Origin is the centre of the workspace, and has all 3 YXZ axes and planes defined. The below picture illustrates the selections specifically for the Proxxon MF70 as I have set it up, so make sure your settings match your machine.

Step 8: Setting Up for Milling

Model by default assumes all bodies, so I prefer to select the specific body I’ll be working with, here the watch case. In the next page, I setup the Stock settings to match the 16mm Delrin (POM) slab I’d cut out beforehand. I used offsets in the Stock section to place the body where it wouldn’t take up too much of the model.

Remember to leave clear sections to place hold-downs!

Step 9: Tooling: Meet the Tools

Here be Dragons, Tooling, Feeds & speeds, Carbides and coatings, end-mills of all shapes and sizes, and Excessive Reading. It’s quite daunting to someone who hasn’t the faintest clue what most of it means, but understands it’s probably important. Let’s break it down a little.

Your endmills are what do most of the work, and there’s plenty of them in all different shapes and sizes. The ones you can use are limited by your budget and the capabilities of your machine, and here I’ve used a 3mm flat endmill for most of the work, and a 3mm ball endmill for the finishing operations. Both are tungsten carbide & made by Totem-Forbes, so a quick tour of the manufacturer’s website and we have the datasheets, which gives us two vital sets of information. The tool’s dimensions, and the tool’s recommended Feeds & Speeds in different materials.

Step 10: Tooling: Delicious Formulae

Fusion’s tool setup wizard is both a form to enter data into, and also a calculator that allows you to skip the math behind calculating the feeds and speeds yourself. However, understanding the math can help you improve the efficiency of the processes as it goes along, so I've included the chart from totem-forbes is included here.

Basic terms-

• RPM- Rotations per minute
• mm/min- feed rate, how many millimeters per minute it moves.
• mm/tooth- this is also called chip load, or how much material each ‘tooth’ of the endmill must remove with each rotation.
• VC- Surface velocity, or how fast the cutting edge of the endmill is moving in relation to the workpiece.

Step 11: Tooling: the Tool Library

We need to open Tool Library on Fusion’s toolbar, create a new library, and follow the wizard to enter all the endmill’s dimensions so that the match up.

Once the endmill’s dimensions are setup, you’ll also want to make a model representation of your chuck to go with it. An accurate endmill & chuck help you properly test out the simulation feature to prevent any collisions before they even hit your machine. Pun intended.

Step 12: Tooling: Feeds & Speeds

Now for feeds and speeds, we need to understand a concept.

The endmill’s rapidly spinning, and moved into the stock to turn it into little ‘chips’ that fly out and make a mess everywhere. The endmills’ sharp edges are what do the cutting, and so their speed in relation to the work determines how cleanly they cut. This is called surface speed, if it’s too low then the endmill can’t cut fast enough, too high and the endmill will be rubbing against the work piece heating it up. Using the manufacturer’s recommended speeds for our material, here Delrin, we can optimize our process.

I strongly suggest starting with an engineering plastic like Delrin or nylon, simply because of how easily they machine and how forgiving they are, which gives us newbies a lot of leeway.

Step 13: Tooling: Feedrate

Next up, Feed rate, or how quickly the spinning endmill is moved into the stock. Once again, too low and the endmill heats up excessively, too high and ping goes your expensive endmill as it gets snapped off at the shoulder.

The manufacturer’s have a chart that lists recommended feed rates depending on your tool diameter, and it’s always helpful to be a bit more cautious and run it a little slower than their recommendations, until you become familiar with your machine’s responses and rigidity.

The proxxon has rigid cast iron ways & fine pitch leadscrews, so it’s extremely rigid but also not very fast. Your Machine may vary.

Step 14: Creating Milling Operations: Tool

Now, we’ve configured the machine, created a tool library, and configured the speeds and feed rates, and we’re ready to setup machining operations. Fusion has a wide arrange of operations, each suited for a specific task that it is optimized towards, such as Adaptive Clearing for removing large volumes of material as fast as possible.

Now’s where your modelling earlier gives fruit, if you spent the time to consider how each feature should be machined. I started by mentally breaking the model into sections that could be milled sequentially.

When creating an operation, you are confronted with 5 tabs to configure, Fusion’s online CAM course is really helpful here to highlight every single detail. If you have gone over fusion's CAM videos, then feel free to skip this section.

Fusion CAM Course

Tool

Here’s where you select which of the previous tools that you entered you want to use, and its Feeds & Speeds. As long as we diligently entered the details in the tool library, we don’t need to change any of the settings here except if we want to customize it for a specific operation.

Step 15: Creating Milling Operations: Geometry

This tab allows you to define what work needs to be done. If we’re using a pocket operation, then we define the holes or pockets we need milled.

Machining boundary allows you to select which area of the overall model gets milled, and how to contain the tool to the required area.

Stock Contours is useful if you have some parts regions you don’t need machined, like areas outside the stock or those already milled.

Rest machining is very useful for operations that succeed other operations, like an Adaptive Clearing op after a set of pocket operations. This helps the machine only remove stock that’s still remaining, instead of wasting time cutting through the air.

Touch/Avoid surfaces is quite straightforward, it tells fusion to either avoid or only work on particular surfaces, very handy to avoid milling already finished surfaces.

Step 16: Creating Milling Operations: Heights

Remember how we split our model into layers? Now imagine splitting the operation itself into layer. As the tool starts following the toolpath, it’ll move down from Clearance height, which is a height set well above any fixtures or stock.

Retract height is where the tool moves to during the operation between cutting two areas so that it does not collide with the stock or workpiece.

The tool begins a cutting move downwards from Feed Height, where it gets ‘fed’ into the workpiece as it starts removing material between Top height and Bottom height.

Top Height allows you to set the top of your cutting operation, which can either be the stock’s upper surface, or inside the model itself if you’ve already cleared an area for it.

Bottom Height is the floor, or the lowest point that you want cut. I’d suggest you never try cutting directly through your stock unless you’ve got a spoil board and tabs setup to hold your workpiece in place.

All my heights here are set to rather fine values in the millimetre range because the mill has fine pitch leadscrews, which keeps it very accurate and reduces backlash. For larger or less rigid machines, the heights & clearance sizes should be different to suit.

Step 17: Creating Milling Operations: Passes

Passes are the machining operations themselves, how the tool moves through the workpiece. You can really customize how the tools behave here, and it’s an instructable in itself to do so. Primarily we’re interested in the Multiple Depths section, where we can set the number of passes we want our machine to make in the workpiece, and the depth of cut (DoC).

Since we’re using small 3mm endmills, we want to keep the DoC low to avoid breaking our tools, and this is done by removing smaller amounts of material at a time. Since I’m working with Delrin, I’ve kept the roughing step down to 2mm, as the sharp carbide makes quick work of the plastic. If I was milling aluminium, I’d have to reduce it down to 0.25-0.4mm at best, along with coolant to lubricate the cutting edges.

Of course, this is all subject to experimentation so that you can reduce the amount of time it takes & optimize tool life.

Step 18: Creating Milling Operations: Linking

Finally, we come to linking, we’ve thus far selected the tool, the sections we want cut or avoided, the heights and depths, the paths and passes, and to tie them all together is linking. Linking tells fusion how to move between different paths in terms of speed, movement patterns and angles, and the like. Here most of the settings can be left at default, you’ll want the lead-in & lead-out turned on, which results in gentler entry and exit into the milling path. Helix is also important, as plunging vertically down into a part is not great for its lifespan, and helix instead makes the bit move in at an angle to engage the bottom and sides, distributing tool wear.

Once again, operation settings are a beast unto themselves, and I’d recommend you check out Fusion’s CAM tutorial series for an in-depth tutorial.

Fusion CAM Course

Step 19: The Milling Operations

I setup a number of operations to first mill the internal pockets, then used adaptive clearing to rough out most of the remaining material. With most of the material removed, I then applied contour operations to cut out the outline of the wristwatch, leaving the it held to the stock by the lugs. We don’t want to cut it completely free just yet, as we’ve yet to mill the reverse side. To reduce the amount of hand sanding required, I also added in a parallel & spiral milling operation.

But wait, when we select the operations all I see is colourful spaghetti, how do we know the paths we’ve programmed are working as intended?

Step 20: Simulation, Doublechecking, and Exporting

That’s where Simulation comes into play. Once the tool path is generated, you can hit the simulate button to view a preview of how the tool will move, and how it will remove stock. The menu allows you to see the material being removed, and you can also see how much time the operation would take. This allows you to doublecheck, optimize, and most importantly prevent collisions without damaging your machine.

Using the simulation screen to continually check for errors, collisions, and incorrect material removal, I managed to ge the mill cutting properly on the first time itself.

After this, I just exported the G-Code using the Post Processing option. Remember to select the appropriate machine in Post Configuration, I used the built-in GRBL option as I’ve found it mostly reliable. However, it does have some issues so once the .nc(gcode) is generated, I go in manually to remove the RPM & Tool changer g-code.

Step 21: Return of the Mill

Now this is where the fun begins. I already had my stock cut out to fit the bed, and I began by using a simple bump tool to align the workpiece, it’s essentially a bearing press fit onto a dowel pin that can be used to either align stock to your bed by running it along the edge, or to act as an edge finder by moving it closer to the side of the stock while it’s running until it makes contact, at which point it’ll either slow down or stop rotating.

When you have fresh stock, you can also eyeball the XY plane to make sure it’s roughly aligned. However, Z height is very important to have it perfectly aligned, so the easiest way to do so is to insert your endmill, take a small gauge block, raise the bit a little above the surface and try to slide the gauge block under it. Once you get a slight drag from both the bit and the surface, you know you’ve got a very accurately set Z height, so just subtract the height of the gauge block to the machine’s work coordinates to perfectly zero it.

The Machine’s 0 should perfectly match the WCS we setup earlier in Fusion, or you’ll have problems. If you don’t have a gauge block, a drill bit is nearly just as good, since they’re also very accurately machined.

Step 22: Sending G-Code

At this point, the code is ready and the mill’s loaded and zeroed. Since GRBL requires the G-code read to it, I use a PC program called Universal GCode Sender. It's fairly handy to jog the machine to align with the XY zero on the WCS & stock.

Just load the file into it, doublecheck everything for good measure, start the spindle and hit send!

Step 23: Finally, Chips

Once you hit send, the mill whirs to life and beings to spray chips. I’d set the machine up with an aquarium pump for flood cooling, but wasn’t able to get aluminum cutting fluid in time so I went with Delrin instead. Coolant also serves the vital role of evacuating chips from the area after they’re cut, to prevent gumming up the tool and reducing lifespan. I just manually used a vacuum to clear out the chips as they piled up every few minutes.

Here’s where you can see the actual progress happening, and because I spent the time watching the simulation and triple checking my work, it managed to work on the second try.

Step 24: The Postmortem

So originally I was going to include double sided milling here, but I ran into an important lesson instead. That lesson took the shape of ramming the endmill diagonally through the stock, leaving it look like a lunar probe crash landed on the surface of the Moon. Remember when I said WCS was important? Turns out that I hadn't taken that to heart myself. This was the second crash I had, the first time everything looked good but I had the WCS setup exactly backwards so it started drilling the alignment hole, then charged off into the opposite direction of the part and hit the endstops.

The point I'm trying to make is that even though I spent hours optimizing every last process, sometimes you still miss out on things. Often the only way to learn is to persevere through these happy little accidents, and improve on them next time. I think this is a good time to end this particular exploration/guide, next time I'll cover aluminum milling and 2 sided milling.

Thank you for your patience in getting this far, and please let me know if you have any questions, comments, or suggestions.