The short answer:
For a smaller wood and plastic cutting machine (and maybe – maybe! – some light aluminum), several people on CNCzone seems to have had success with 100-200 oz-in (0.7-1.4 Nm) stepper motors direct driving a 5 turns/inch (5 mm lead) ACME (trapezoidal) screw.
The long answer:
... hopefully you can find in the attached file. Please, if you spot any errors or have trouble viewing the file (like missing characters or fonts), write a comment in this thread: http://cnczone.com/forums/showthread.php?t=5639, or use the email address in the file to notify me. Other comments are also welcome to my email address.
I've made up an Excel Spreadsheet based on Arvid's excellent PDF file.
The spreadsheet let's you enter values for Motor Torque & Speed, Drive Ratio & Effiency, Screw Pitch & Effiency, Cutting Force (plus any additional weight), plus Desired Speed. Most of these measurements can be entered in Imperial or Metric units, and will be automatically converted to SI units.
It will then show results for Desired Screw Speed, Actual Screw Speed, Actual Screw Torque, Required Screw Torque, Desired Drive Ratio (Speed or Torque based), Actual Movement Speed, Actual Movement Force, and Required Motor torque, depending on what values have been entered.
Usage - Enter your known and/or desired values into the relevant pale yellow cells (ie Motor Torque in Nm, enter it in the yellow box with Nm in the column before it, or if oz-in, the yellow box with oz-in in the column before). The relevant SI value will then be displayed in the blue box in the next column, and the red text will tell you what value the SI value is based on.
Once enough values have been entered, the blue boxes at the bottom will change from NED (Not Enough Detail) to the calculated figure.
None of the cells have been locked or protected, so be careful that you don't accidentally delete any of the equations.
Note - The drive effiency isn't covered in Arvid's PDF, but it allows for any ineffiency in the drive system. If the drive is direct (ie motor coupled direct to end of screw), then enter 100%.