CNC Milling for Watch and Clock Makers

Making your first watch part

In this series Frank Boswell and Christian Dannemann provide a step-by-step guide to CNC milling which will give watch and clock makers a chance to produce their own difficult to obtain parts.  Here’s part three …

By now, we hope that you have got to grips with the manual operation of your CNC machine and that you can confidently break end mills. Today, we will move on from the ‘glorified pillar drill’ stage and do some proper CNC work by making an instruction code to enable the machine to do things on its own.

There are many software packages available from which you can produce CAD drawings and we will need one if we want to make parts on our mill. I use FreeCad, not only because it’s freely available (there is no licence fee!), but it’s also a good piece of software capable of producing the drawing and the instruction code for the CNC mill, the so-called ‘tool path’. FreeCad is available from: https://www.freecadweb.org/downloads.php. Once you have downloaded and installed the software, you are ready to go.

Don’t expect to come up with your first drawing within an hour of installing the software. It takes a while to get your head around how things work as it is quite different from producing a drawing with a pencil in hand.

To get you started, I’ve put together a tutorial video that you can view on YouTube. Either search on YouTube for ‘FreeCad for Watchmakers’ or go to https://youtu.be/lQtIHmOm0A8. The video is fairly fast-moving, so you may need to pause it, go back and view again, and do this whilst producing your own drawing of a part.

Again, don’t expect this to work the first time around. Patience is needed, but once you have your head around how to produce a drawing, you can generally draw up most watch parts in 20 minutes or so.

Figure 1. A good photograph of the part you want to make is essential.

Figure 1. A good photograph of the part you want to make is essential.

The starting point of any CAD drawing is a good photo of the part you want to make. You also need a ruler in the photo, as you won’t know the size of your part otherwise. Figure 1 is a photo of a broken setting lever spring on a Smiths movement.

I have put the setting lever in its correct winding position, as we will need the exact location of the pin which engages with the setting lever spring, as we don’t have the broken off piece.

Figure 2. A CAD drawing of the required part.

Figure 2. A CAD drawing of the required part.

Watch the video and see how to scale the image and how to draw the part. You can see that I made up the spring part of the setting lever spring as it’s missing.

Once the drawing is complete, Figure 2, you can produce the tool path, eg the instructions for the CNC machine, which is also explained in the video.

Some parameters for spring steel: Your maximum horizontal speed should not exceed 30mm/min or 0.5mm/s. Your step down (eg how far you go down per run) should not exceed 4% of your end mill diameter. As the pin on the setting lever has a diameter of 0.5mm, this will be the largest our end mill can be, so our

vertical step-down can’t be more than 0.02mm per run. Spindle speed should be on max, so 24,000 rpm is fine.

As for the steel, if it’s too hard (I started off with feeler gauge, which is too hard), it will blunt your end mill very quickly, and if it’s too soft, you will have to harden and anneal at the end, which adds to your workload.

Figure 3. Superglue your spring steel onto a piece of acrylic to hold it in place.

Figure 3. Superglue your spring steel onto a piece of acrylic to hold it in place.

Choosing the right spring steel saves you a lot of work and end mills. I suggest you just search on eBay and buy it from there.

A big problem when CNC milling is how to fix your workpiece to the machine. You can’t just clamp down a piece of spring steel. What I find works best is to super glue your spring steel onto a piece of acrylic, Figure 3.  Clean both the acrylic and the spring steel with acetone, apply your super glue generously, and press the two parts together between sheets of wood in your vise.

Now it’s easy to clamp the acrylic to your CNC mill bed with clamps, and you have a solid connection between the bed and the spring steel.

Next put your 0.5mm end mill into the spindle, tighten the collet and start the spindle motor. Manually go close to the top surface of your spring steel and lower the end mill by tiny increments (jog rate 0.1) until you hear the end mill touching the metal, Figure 4. It’s really important to set X,Y and Z to zero now, or you will wave good-bye to your end mill. Once that's done, apply some cutting fluid (I use a 4% solution of Bennis Supplies Soluble Oil 2470) and start your program.

Figure 4. Our end mill just touching the metal – ZO.

Figure 4. Our end mill just touching the metal – ZO.

Figure 5.The outline of the setting lever spring appears.

Figure 5.The outline of the setting lever spring appears.

Figure 5 nicely shows the outline of the setting lever spring. The machine will take 20 minutes for both holes and the shape, and then the raw spring will pop out. 

Usually, the part separates itself from the superglue whilst it's being milled through the vibration and the cutting fluid, but if it’s still sticking to the acrylic, dissolve the glue with a bit of acetone.

Figure 6. The part in the rough.

Figure 6. The part in the rough.

And here it is, Figure 6. Not pretty, but bear with me …

Figure 7. Brushed and good to go.

Figure 7. Brushed and good to go.

We now just have to make the chamfer on the two holes,

polish to de-bur and we are done, Figure 7. You’ve successfully made you first watch part.

If you want to practise the machining before making your own drawings and tool paths, go to: https://watchparts.org.uk/cgi-bin/watchparts?action=cad

Here you will find plenty of watch parts complete with the drawing and a tool path. Please feel free to use the drawings as the Guild makes them available to the public free of charge.

If you have drawn your own parts and tested them, send the drawing and tool path to christian@watchguy.co.uk, and I will add them to the database.

To get you started check out Christian’s tutorial video on YouTube.

Search on YouTube for ‘FreeCad for Watchmakers’ or go to https://youtu.be/lQtIHmOm0A8.

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