Gallery, Projects and General > The Design Shop
3D Printed Ball Nut for the Insane
Joules:
Revisit time for this mad project as I now have the 0.3mm nozzle. First things first, find out what the real hole sizes are that I need for these bearings. It's one thing doing a design in CAD but another doing the prints, so a ball gauge is needed with holes in Z and Y from 2-2.5 in 0.1mm steps. Why Z and Y, because printers don't print true, they normally do a good round hole in Z, but you end up with a droopy ellipse in X and Y. This will impact on the raceways within the nut. As can be seen in the pictures, balls in the Z pass through at 2.4mm and pass at 2.3mm in the Y. I was expecting the opposite result, however that sets my minimum hole size at 2.4mm. That means the nut needs redesigning to take this into account. I will also need to make some test nuts that just have the spiral raceway in to set them, it could be they have a different diameter or spacing from the holes depending on how the balls peen the printed surface.
I looked up ball options as per PK's original observation that I needed different size balls. The nearest I can find are 2.45mm and 2.5mm, but in a printed nut you can't hold that close a tolerance so we go ahead with full contact balls of 2mm.
Joules:
Well that was interesting. I printed my first test nut and found I printed a mirror of what I needed. Doh.... So I tested the nut for fit on the lead screw noting the threads running in opposite directions and marked the nut as bad. Then I had a thought, well a single bearing should cycle through nut with no motion. It did, and I could feel the quality of the race within the nut and it was good, with no tight spots. I will cycle this a few times more to see how it beds in. The nut was tight on the shaft so the next MIRRORED nut has 0.3mm extra clearance but same thread profile and position within the nut. No play between the nut and the shaft with the bearing inside, looking good.
Doesn't mean a whole bunch of balls won't lock up in the correct nut, but we should know in another hour or so.
Well bugger me !!! loaded 13 balls in the test nut and it seems to be working, I wasn't expecting this.
Joules:
It works !!! Loaded the balls and they are recirculating. At the moment the ball nut is still a little rough, but the bearings are making the track smooth. I have enough clearance using a 2.3mm track throughout. No backlash in the nut, yet.
S. Heslop:
That's pretty awesome! Interested to see how it holds up in testing.
Joules:
After hours of running the ball nut along the shaft it has flattened out all the internal imperfections. The nut isn't as free running as a metal nut, it was never going to be. It does have zero backlash and should maintain that in low load applications as the nut contains around 70 balls, lightly greased so they slide against each other, but don't seem to slip in the nut or on the screw. I have published a couple of STL files on the GrabCAD site listed earlier in the thread. They are this new nut and the ball gauge. Don't just expect to print this nut and it to work. The gauge was used to set the raceway dimensions based on this printer and filament. The nut is scaleable if you have the CAD skills to draw the raceway, I think my 2mm balls are too small as they sit in the lead screw and they should ride on the edges. But hey, it works for me, don't underestimate the importance of using a 0.3mm nozzle with balls of this size, that blob size will catch you out.
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