Thread: Rotating Ballnut - design ideas
I've just estimated the moment of inertia of the rotating bits of the assembly, so pulleys shaft, ballnut and bearings. Surprisingly it comes to slightly more than the inertia of the 2085mm, 25mm diameter screw! That's annoying since one of my main reasons for doing this was because I thought this value would be much lower. I've not included the inertia of the bearings I would require to rotate the screw, and I suppose I would use pulleys with it anyway. The inertia of the ballnut and shaft is still about two thirds of that of the screw.
This is going to make a big difference to the rapid speeds I can get since the torque required to move the 50kg gantry, due to the low coefficient of friction of the bearings, is very small.
Now I'm going to try modelling it with the stepper at different ratios - not 1:1, and see what happens.
I have at last started making the rotating ballnut assembly. I've started with one stepper motor mount, it's 100x245mm:
The reason for having such a long cutout for the motor is to allow me to use any reasonable size pulleys without changing the belt, which will obviously be difficult. I CNC milled all except the bearing bore which was bored on the lathe. I had to remove the gap bed on the lathe to do it - the join was painted over so it looks like this might be the first time it was removed! The six holes closest to the bearing will be used to push a ring against the outer ring of the bearing to preload it. The 4 bigger holes are for posts which link this to the other bearing mount.
I'll make some more bits when I've found my caliper :whistling:
Last edited by Jonathan; 30-06-2011 at 01:58 PM.
I've now made all 4 bearing/stepper mounts and one shaft. I will machine the pulley (30 tooth) directly on to the shaft.
When I machined the shaft on the lathe I got only small pieces of swarf, not the long stringy stuff you normally get - see picture below of drilling it for what I mean. It was almost like cast aluminium...but it's not. Must be a strange grade?
Lots of photos, hopefully pretty self explanatory:
It got 0.01mm bigger after drilling...so now 34.99mm which fits the bearing nicely:
(Hopefully nobody without broadband is trying to view this!)
Last edited by Jonathan; 02-07-2011 at 09:49 PM.
Very nicely done
I'm currently making a mandrel from 35mm steel bar to hold the shaft in the rotary table/4th-axis to mill the pulley teeth on to it.
I'm using XL belts, purely because I already have lots of XL timing pulleys which will enable me to experiment with the ratios. HTD would be better in theory.
I will use a form tool to cut the pulleys. The profile is a simple trapezium which I have ground on to a piece of HSS. You can make HTD pulleys accurately with a ballnose cutter ... or some people use a drill but I think that's a bit crude.
If you do decide to try it you might find my program here useful:
Today I cut the pulley teeth into the first shaft, and whilst that was running made the second shaft.
The belt fits (yay!):
I will cut the pulley teeth into the second shaft tomorrow, then I just need to make the part that connects the shaft to the ballnut, and the ring that pre-loads the bearings...and maybe some bits to act as bearing seals.
I have now cut the second pulley, and made the remaining parts for one of the assemblies except the bearing pressure plate. I immediately stuck it in the milling machine vice, put a stepper on and tested it:
Will post a video tomorrow. With me just holding the 2092mm ballscrew in one hand it did 14m/min. Almost certainly more - I didn't try.
Might be while until I finish off the other one since the tip on my parting tool became an Unexpected Flying Object. It might still be ok, but my lathe is buried in so much swarf I can't find it. It took me 5 mins to find the 60mm diameter part I had parted off, let alone a tiny tip...maybe I'll tidy it tomorrow.
Looking good fella!
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