Spanish newbie (with a couple of CNC projects for photography)

[blackburn mark]

For the slider, I will use a couple of 1m rails with bearing blocks.

bear in mind that the type of bearing blocks and rails we use in cnc are going to be pretty heavy to be lugging around in the field

it may be cheeper to buy carbon fibre rods, maybe 30mm or 40mm di and twist someones arm to make you some wheels with the 30/40mm di profile in them and use them in pairs to pinch solid onto the rod (i havnt got a radius turning attachement for my lathe,sorry)

I may control a servo with a servo tester, but not much control over the speed, etc.

im not sure what you mean ? a servo tester has a dail to control the speed from very slow to flat out ? unless your refuring to top speed?
if your using a 7D 5D and shooting some standard 24/25fps then id agree that you may not get the speed you need to do run a tracking shot unless you opt for a larger servo

The pan & tilt movement maybe done with a couple of servos. There's no need for full 360º, so no need to modified the servos.

i cant see how you would control the speed for single frames ?

To move the dolly I'm thinking of a pulley and belt system like those you may find on desktop scanners.

sounds good , you can buy small "endless" timing belts by the meter
you could fix a single length and attach your timing pully and drive to your sledge?

are you looking to shoot video as well as single frames ?