This thing is a simple jig to allow you to grind a groove on a plain roller bearing using an abrasive wheel and a Dremel or other die grinder type equipment.
One the reasons you might want to do this is for your extruder idler bearing, this will help keep the filament on axis. One of the problem I have experienced with my (poorly made?) hobbed bolt extruders is the filament wandering up and down the hobbed portion of the bolt.
So I was inspired by ebay seller you2bay http://myworld.ebay.co.uk/you2bay/ who sells grooved bearings to have a try at this and it works great. My first attempt was just to grind the bearing by hand but I think this is a much more repeatable method.
Please take care and use safety glasses as a bare minimum safety equipment when ever you are grinding anything.
The openSCAD file is dumbly parametric (I.e you need to adjust all the parameters) and I heartily recommend that you spend a little time tuning it before printing out a jig. The 2 stls I have uploaded are based on the dremel slitting disk I had to hand.
The most important dimensions are the diameter of the wheel (this will shrink) and the diameter and length of the boss beneath the wheel. Also the clearance of the grinding wheel is important, too little and you are going to melt you guideway.
Print 100% infill but be aware that this is a limited use item unless you mahine one from steel.
1) Bolt the bearing in place on the jig using a suitably sized bolt and a washer each side of the bearing. Make sure it spins freely.
2) Wipe any oil of the outside of the bearing
3) Adjust the abrasive wheel so you can slide the wheel down the slot freely, using the back of the fixing boss as a guide on the jig
4) Grind the bearing in a pecking motion, don't let the bearing spin too fast
5) Take you time and dont overheat the bearing