Measure distances in mm

You might be better with a mechanical dial kind of tool with digital output. Not sure what that is called.

You mean one of these
http://engineerblogs.org/2012/02/awesome-tools-for-every-mechanical-engineer-by-sam-feller/

No, I mean something digital like one might find on an incoming-inspection table, or perhaps a calibraton table, for automatic recording of results.

what about a webcam (properly affixed with screws or so... connected to some stable USB box... like an OLinuXino...) that looks at the piece from the side?

You mean something like this

?

A webcam to measure thicknesses??

nope
not capacitive but optical...
or is it impossible to look from the side?

yes i can be measured from the side if that is what you are asking.
But your are saying that the capacitive is not going to work for me?

Thanks

CrossRoads:
No, I mean something digital like one might find on an incoming-inspection table, or perhaps a calibraton table, for automatic recording of results.

This sort of thing?
http://www.tormach.com/store/index.php?app=ecom&ns=prodshow&ref=31761
plus USB interface
http://www.tormach.com/store/index.php?app=ecom&ns=prodshow&ref=31865

I can't use that type of things you just mentioned because the wood is always "runing" on the machine...

Something like this (but better) perhaps? http://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/GP2Y0AH01K0F.pdf

Yes, that's the kind of thing I was thinking of.

nmlfreitas:
A webcam to measure thicknesses??

yup - u would need to do some image processing...
the machine parts (in the background) should have a much different color than the wood pieces...
how fast do those pieces pass the sensor? less than 25Hz?

That Sharp sensor would work form me the minimal distance is the problem, i need 0.8 mm and for what i've seen they don't have that kind of sensors.
Do you think that this would work?
http://www.baumer.com/int-en/products/product-finder/presence-sensors-measuring/#pfinder_sensor/scripts/level2.php?language=en&header=&country=CH&cat=CONFMyAppli&psg=madynGrundfunktion2|1993482997,madynPGGrundfunktion|Optisch&pid=&ptk=&viewmode=&filterViewMode=&perpage=

Check the ZADM 034
(ParCon, analog output)

Please

the specs look good...
the accuracy is much better than u need: 50um...

Yes i know but do you think it is hard to connect it and make readings with arduino?

it seems to b easy:

  1. connect 12V to it...
  2. connect wires for the output like this:
    GNDsensor-----------------------wire#1-------------------100R(or similar)-----------------------wire#2-------------------OUTPUTsensor
  3. calibrate the arduino (i would use the INTERNAL ADC ref, which should give u reproducible results at the same temperature/voltage):
    analogReference() - Arduino Reference

It has a current loop output so you need to calculate your load resistor cairfully. Also there is a 4mA output minimum so you will get a fixed offset voltage. Given that you might have to do some messing about withan op amp to give you the full range of the A/D.

However have you looked at the price of this? I can't see one which is never a good thing. I would expect this to be in excess of £200.

I've send email today requesting price still waiting for response.

nmlfreitas:
Check the ZADM 034

I was about to mention these - they are basically close range LIDAR devices. I can guarantee you they -will not- be cheap. Grumpy Mike is probably right on target (if you can even find a distributor that will sell to the public and only sell you one of them)...