Hi Goodwood

I have just re read my post and I have given you the wrong impression about spindle speed - sorry about that. The higher speeds are needed for end milling with small diameter end mills (typically 2mm or less for crossing out clock wheels for example or engraving).

The speed you would need for running typical six tooth form cutters for pinion or wheel cutting would be 500 rpm for a 14 mm diameter cutter to 250 rpm for a 26 mm cutter diameter at 250 rpm. Higher speeds are required for using fly-cutting with a profiled cutter to generate the gaps between the teeth.

However when getting started do not get tied up in the search for higher rpm as this is only important when you need to achieve higher feed rates. The work done at the cutting edge is a function of the amount of material to be removed. That is only another way of saying that traverse speed and rotational speed are related. This simple relationship is a bit more complicated as the machinablity of the metal and the need if any for lubrication are factors as well as the shape of the cutting edge in terms of relief and clearance angles. There are calculators for feed rates for different situations but again these are a starting point and experience cutting metal is the best way of appreciating the way speeds and feeds are related. I use this program to give a starting point to define the maximum when inputting speeds and feeds for a new part and I often start at 1/2 on each and then work up if necessary. Small parts don't take long so high traverse (feed) rates are not of much importance but high quality surface finish is the main requirement. Suggest you look at Machinist ToolBox™ as this will give you a starting point. There are other programs and here is a link to another well respected one CNC Machinists Cookbook: Software and Information. Both have free trial editions.

As far as CNC is concerned it is the drive to the axis positioning that determines the accuracy as rigidity of the machine is not going to be stressed by watch making. A typical low cost bench top machine would use ball screws and stepper motors to drive them. The drive is operated open loop which means that errors in rotation step and lead screw variations contribute to the co-ordinate inaccuracy. Unfortunately closed loop drive to the axis position increases the cost by around ten times as feed back control systems require expensive scales and more complex motor and associated controls. However a lot can be achieved on a machine operated by stepper motors and this makes for a good starting point for learning both the control of the machine (CNC) as well as gaining experience in making quality parts.

It would be practical to make your own cnc controlled mill given that you have a lathe. Parts are readily sourced on the web if you feel inclined to have a go! That said a bench top mill drill or a suitable milling table for a lathe would aid a DIY build. Don't lose sight of the fact that watch making evolved over the years to reach the expensive mass production machinery now in use.

You are right to be concerned about the extra bits and pieces required. For example you would need some form of dividing head be it a mechanical one or an electronic one. Don't forget you need to allow for the jigs in this case a dividing head as part of the machines operating envelope. As a rough guide the tooling will cost about he same as the basic machine. Unfortunately the second used accessories command a higher price compared to buying new than the machine.

IMHO start with the basic machine and learn by making the necessary fixtures. For example a dividing head is relatively easy to make particularly as you would only require a limited number of ratios determined by the going train of the watch or clock. For repair work unfortunately there are variations which will require a wide range of division ratios but you would have covered this in your horology classes.

Can I suggest you draw out a typical watch plate and one of the wheel pinion pairs on paper and think about the dimensioning problems in terms of X,Y&Z i.e no radius dimensions used. Accuracy has two main classes of error. Firstly making a part on one machine might be different when made on another due to absolute measurement differences, this comment also applies to parts made using a different position on the same machine due to errors in lead screw and stepper motor. (I am assuming you are going the open loop stepper motor route on cost grounds. Do a web search on closed and open loop servos if you need more information.) The second class of error is due to the spread in errors that exist between parts made at the same time as in a small batch run. These would be due to tool positioning errors with respect to the work piece. This simple exercise will show you how close a tolerance is actually required. Don't forget the gearing operates in one direction and the need is for low friction not high speed operation so that the mesh of the gear teeth needs to be loose rather than approaching the full depth engagement. There is a lot on the web about the tooth forms for gearing BUT going trains do not require uniform motion nor high speeds BUT do require low and constant frictional losses as seen by the escapement. The web also has a lot on clock escapements that decouple the pendulum from the effects of friction in the going train.

Hope this helps - regards - Pat