Is there any rule of thumb ?
Probably, but it depends upon:
Cutter/drill type: HSS, carbide etc and the number of flutes.
Cutter/drill size.
Material: Even if you are cutting aluminium you should be aware of the different grades.
Available spindle speeds and if you have sufficient torque available at any given speed.
The alignment limitations of your machine and how well it will hold tolerance over the time it will take for the job.
The capabilities of your CnC machine: Rigidity and speed for each axis given all of the above.
Useful reading... maybe even some some useful software:
http://www.cnccookbook.com/CCCNCMillFeedsSpeeds.htmhttp://www.hsmworks.com/docs/cncbook/en/#Ch03_CuttingSpeedsAndFeedsFormulashttp://www.practicalmachinist.com/vb/cnc-machining/online-cnc-speed-feed-calculator-242061/ - this is the thread for ...
http://zero-divide.net/index.php?page=fswizard - this could be very useful, I've not tried it.
Okay, please read, or at least scan through all of those links. It's a lot to take in but I'm not asking that you know it all - I most certainly don't - just that you are aware of what you are trying to achieve and more importantly what you should try to avoid when you start up your machine.
Now, the important questions:
How much time have you spent playing with your machine?
What did you do to determine the accuracy and repeatability of your machine?
How did you determine the safe maximum speeds of your machine?
I was lucky, my machine started as a simple manual milling machine so I was able to spend hours listening to the noises it made and actually feeling the vibrations as I cut and drilled holes in stuff. No plans, no design just playing around and squaring up reality with the little machining theory I knew. Time well spent.
Get a rough idea of the upper and lower limits you should stay within. Make some test cuts with the tools and materials you are using. To this day I hate CnC drilling, but drill some holes remembering the basic rules of peck drilling (eg,
http://www.practicalmachinist.com/vb/cnc-machining/peck-drill-depth-rule-thumb-124486/ maybe start with 1xdiameter to be safe-ish). Never be tempted to use a milling tool in a drill chuck, ever! Listen to your machine, feel the vibrations lol.