MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.

Download Now

Report as inappropriate

Honestly, I typically just pull a few candidates out of my box of assorted springs, and try a few until I find one that feels right. After building a whole lot of extruders, I can kind of eyeball the spring and estimate whether it's stiff enough or not.
Probably more stiff than it needs to be: http://www.thingiverse.com/make:200447
Looks about right: http://www.thingiverse.com/make:176763
Looks a little weak: http://www.thingiverse.com/make:156324

Err on the side of longer/stiffer when buying unknown springs. If it's too stiff, you can cut off a wrap or two from the spring to loosen it up a bit.

I would guess that a ~25mm long spring rated ~8 lb/in should be in the right ballpark once compressed to ~15mm. The force applied at the idler is roughly double the spring force, so that would give somewhere around 6 lbs force on the filament. I would think that's acceptable...? If not, you can always print one of the adjustable bases and tighten it down.

It's on my to-do list, but I haven't yet created a calculator to relate idler pressure to drive gear stripping :-)

B'Struder: Universal Bowden 1.75mm Extruder
B'Struder: Universal Bowden 1.75mm Extruder
B'Struder: Universal Bowden 1.75mm Extruder
by Heffa