CNC Milling for Watch and Clock Makers

A new series looking at modern methods of making missing or obsolete parts.

Introduction

CNC (computer numerical control) machines have been around for a long time now but so far they have been too cumbersome and expensive for use in a watch or clock workshop. Not that it hasn’t been done before, but few of us have a CNC controlled lathe or mill in our workshop.

With the parts supply situation as it is and prices of entry level machines being very low, the Guild Board feels that now is the time to make a push to get the horological industry to adopt the use of CNC milling into their workshops.

If you want to have a look at what sort of parts you can make with a CNC mill go to the Guild’s Watch Parts Database CNC Section at watchparts.org.uk/cgi-bin/watchparts?action=cad, and you will see what can be done.

With an investment of under £1,000 you can get a usable entry level CNC mill. No, I didn’t omit a zero! Go to ebay and search for ‘800w cnc 3020’. These little machines cost around £600, have a 24,000 rpm watercooled spindle, and you can control each axis to ¼ of 1/100 of a mm. You will find models in different sizes and the smaller 3020 is the right machine for watchmakers. Clockmakers might want to go for a larger 3040, or the largest model, the 6040. Make sure you don’t go for anything cheaper – you need ball spindles on all three axes and the 24,000 rpm water cooled spindle motor.

Pictured top right is the one in my workshop. I suggest going for the model with the USB port (rather than the parallel port), as it’s hard to find a computer that still has a parallel port. You need an old computer with windows XP on it (the software to control the machine only works on 32bit windows operating systems). The one you see in the photo was off eBay for £44, so this isn’t a big problem, either.

A nice machine vice, as you can see in the photo, is also needed (the clamps come with the mill). Optional is the manual remote controller (search on eBay for ‘mach3 manual remote controller’) and you are in business. You can see on the side of my spindle a spray cooling device, but I would leave that for later. You can learn the basics and make your first parts without that.

Some end mills are also needed and you can find cheap and cheerful ones at www.shop-apt.co.uk/micro-diameter-end-mills-2-flute-altins-coated-ultra-.... As you get more advanced you can spend on more expensive end mills, but to start us off, these are fine.

CNC milling machine, controller, manual remote controller and laptop with Mach3 software.

CNC milling machine, controller, manual remote controller and laptop with Mach3 software.

Get a 0.3mm, a 0.4mm, and 0.6mm and a 0.8mm to start you off (maybe two of each – you will find out why later). If you are into clocks get a 1mm, 2mm and 3mm end mill.

Set up your milling machine on a table, connect the computer, and install the Mach3 software that comes with it and see if you can operate it through the software. This isn’t an easy process, and it can easily take you a day before you move your first stepper motor. Don’t give up, google, read up or, as a last resort, pack it into the boot of your car and drive over to my workshop and I’ll help you to set it up.

Once that’s all done, you are ready for Chapter 1 of our ‘CNC Milling for Watch and Clock Makers’ course, see opposite. In the first chapter we operate the milling machine without a gcode program, eg ‘by hand’ and then slowly ease you into what gcode is and how it works. After that it’s on to producing CAD drawings of the parts you want to make, producing gcode from those drawings, making those parts on the milling machine and finally how to make screws and other funky bits and bobs. The series will probably run for well over a year, but it’s worth hanging in there if you are interested.

So get yourself a CNC milling machine off eBay (only order from sellers within the EU or you will be stung for duty) and get it set up. If you want to install FreeCad on your computer you can also watch our video ‘FreeCad for Watchmakers’ (search on YouTube for the title) and get familiar with the CAD software we will be using.

Chapter 1: Using the CNC milling machine in manual mode

Now that you have set your CNC milling machine up, let’s try and move it around manually. Make sure the controller is switched on and your computer is connected to the controller.

Mach 3 controller software.

Mach 3 controller software.

If you have your emergency stop pressed, or have just switched on the computer, you will have a message next to the ‘Reset’ button on your screen saying ‘Emergency mode’. Press the big red ‘Reset’ button, and the message should disappear.

Now we want to display the MPG (manual pulse generator) mode as we want to move the x,y and z axis manually.

MPG controller on right side of the screen

MPG controller on right side of the screen

Press the TAB key, and the MPG screen appears. There are a set of buttons below the ‘Button Jog’ label. Click on those with your mouse, and you will hear your CNC machine moving along the axis that you clicked. Get familiar with what X and Y and Z are, and which direction is positive (+) and which one is negative (-).

You will notice that the spindle moves along very quickly - much too quickly to do any manual machining.

Jog rate, X, Y, and Z position controls.

Jog rate, X, Y, and Z position controls.

Click on the red downward pointing triangle next to the ‘Slow Jog Rate’ window, which should show 100%. You will see the percentage going down if you press repeatedly. Take it down to 10%, and move the machine again. You will notice that it moves much slower. Now to down to 1%, move again, and then to 0.1%. This should give you a good feeling for operating the X, Y and Z axis manually. Please get familiar with this, as it will otherwise cost you end mills, which aren’t cheap.

Broken off dial screw head missing.

Broken off dial screw head missing.

Our first project is to drill out a broken off dial screw from a Rolex 3135 plate.

Often, broken off screws can be coaxed out, but this one was so rusted that the head actually came off when trying to remove it, and the rest of the thread is rusted into the plate. We could of course use alum to dissolve the screw, but that takes its time, and can also leave discolouration. So today, we will mill out the broken screw.

Firstly, we need to know the size of the screw. By measuring the one that’s still fine, we find out that this is a 0.7mm thread. I’ve put up a handy screw thread size table at http://watchguy.co.uk/cgi-bin/cnc?action=threads, and you can see there that a 0.7mm screw has a minimal internal diameter (Int. min minor) of 0.518mm, so we can use a maximum end mill diameter of 0.5mm, and that should get our broken screw out without a problem.

As we have to make sure that we are perfectly centred, we won’t go straight in with a 0.5mm mill, but instead use a 0.2mm end mill first, check if we are centred, and then drill the screw out with our 0.5mm mill.

Clamping the plate.

Clamping the plate.

Having clamped the plate so that the broken thread points perfectly upwards (using cardboard or thin wood to prevent damage to the plate), we can now start centering our mill.

Centering the Y position of the end mill.

Centering the Y position of the end mill.

Here is our 0.2mm end mill above the hole. As it’s not easy to see this with the naked eye, I use my camera on macro setting, and take a photo from the front and from the side. Looking at the photos, I judge which way the mill has to move to be centred, and then I take another set of photos, and so on, until I’m happy with the position of the mill.

Centering the X position.

Centering the X position.

I’m happy with the position of the mill, and I can now set X and Y position on the Mach3 software screen to 0. This is very important, as it will allow you to find your 0 position again when you change your tool from the 0.2mm mill to the 0.5mm mill.

Click the ‘Zero X’, and ‘Zero Y’ button as shown in the photo. Z will change anyway if we change the tool, but feel free to set it to 0 as well.

Now we will take the jog rate down to about 5%, and lower the end mill close to the broken screw. Start up the spindle motor by entering ‘24000’ into the ‘Spindle Speed’ field (visible under the MPG window by pressing the tab key again), and starting up the spindle by clicking on the ‘Spindle CW F5’ button. Make sure your cooling pump and water are connected.

When we get close, lower the jog rate down to 0.1%, and take it easy until you can hear the mill starting to remove metal from the broken off screw. Now we can slowly go down by about 0.3mm or so with the Z axis, to make a small hole with the 0.2mm end mill. This is just to check our 0/0 position in case it still needs correcting. Once you have done that, move the mill up, and then move the whole spindle away to the back so that you can get a clear view of the plate. Take the jog rate up to 50% or so that you don’t have to wait forever to move the spindle to the back.

Moving back the spindle to control the centering.

Moving back the spindle to control the centering.

The spindle is moved towards the back (don’t go as far as you can), and we can inspect our hole.

Spot on!

Spot on!

The hole appears to be perfectly centred, and that’s what we want. If it’s off, you have to guess by how far in which direction, and re-correct your X and Y zero position, which is easily done. Use your digital camera in macro mode, and you can easily judge distances by looking at the photo, as you know the diameter of your broken off screw.

Once you are perfectly centred, remove the 0.2mm end mill from the spindle, and fit your 0.5mm end mill. Move back to X and Y zero (don’t use the automatic move to 0 button, as your Z position is wrong, as you changed the mill – also, the move goes in a straight line, meaning that you will break your mill off at the rim of the plate). Once you are at X and Y zero, slowly move your end mill down until it touches the top of your broken off screw, put your Z position to 0 so that you know how deep you are drilling, and then drill down using a jog rate of 0.1%. You can measure on your plate how far down you have to go.

Drilled out screw with lead-in still left in the plate.

Drilled out screw with lead-in still left in the plate.

Here you can see the drilled out screw, with a tiny bit of screw left at the end, which can just be pushed out.

Perfect result!

Perfect result!

This photo shows the thread after having pushed through the leftover bit of the screw, and it looks perfect. Just what we wanted, and it shows that we worked with a precision of 1/100mm or better, as the thread didn’t get touched by the end mill. Remember, the internal diameter of the thread was 0.518mm, we use a 0.5mm end mill, and we also have a bit of runoff in the spindle, so there wasn’t much room for error.

Now your little CNC machine has already earned 1/10 of it’s initial purchase cost, and, in my book, the machine is already worth it’s cost just as a super precise pillar drill!

In the next chapter, we will look at manually programming the CNC Milling machine to move the spindle around. In the meantime, try to use your milling machine for manual drilling operations so that you get familiar with it.

For mor information or any questions, please email christian@watchguy.co.uk.

About the Authors

Frank Boswell

Frank Boswell, a member of the Board, has spent a lifetime in engineering and enjoys bringing ideas from other disciplines which can be modified to solve watch and clock making problems.

Christian Dannemann, also a Board member and  regular writer for TimePiece, is a watchmaker with modern innovative ideas and runs a very successful watch restoration business. He is also the person behind the Guild’s highly successful watch parts database.

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