Rotating Ballnut - design ideas

[Jonathan]

You can ignore the stuff about critical frequecy/deflection of the screw with the spinning nut. I had a dim moment and forgot you were supporting the nut with bearings. Its late or at least that is my excuse!:cry:

Interestingly I've since found that when you rotate the ballnut the critical frequency still has an applies - the end fixity is just better which, in my case, gives 1250rpm. So 12500mm/min. I think I'll be lucky to get that much out of my motors anyway so I'm not going to worry.

I just got prices for the screws:

2 of RM1610-2000mm with nut and end machined(both ends machined same as A type )
238 usd
1 of RM1610-900mm with nut and end machined(standard end machining)
72 usd
Sub-total
310 usd
Air express shipping
127 usd
Total
437 usd
-----------------------
Offer tow:

2 of RM2510-2000mm with nut and end machined(both ends machined same as A type )
269 usd
1 of RM1610-900mm with nut and end machined(standard end machinings)
72 usd
Sub-total
341 usd
Air express shipping
168 usd
Total
509usd

I'll go for the second option - the price difference is not as much as I expected. The smaller screw is for the Y-axis. I've decided to get both ends of the big screws machined to fit the BK type support ... that should enable me to tension the screw using the nut on either end.

I have also acquired a pair of 7206 FAG bearings cheaply, so I'm hoping I can use them instead of 7207. The obvious problem is that the bearing bore is only 30mm, and with the screw passing through it being 25mm the shaft will be very thin - about 1.5mm.

[Jonathan]

The ballscrews arrived today - I paid for them on 08/06 and I requested different end machining (longer portion for pulley and same machining on both ends of screw) so I think that's good service. He also charged me $447 in total even though I ordered the 25mm screws, so $72 less than originally quoted. The ballnuts backdrive easily and they seem nice and smooth.

I will start working on the rotating ballnut mount soon. I'm currently doing one last 3D drawing. I will use 5/8" aluminium for the plates that hold the bearings with some aluminium 'posts' to hold the plates together parallel at a fixed distance.

I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

[Robin Hewitt]

I made a mistake with the bearings and bought open not shielded so I'm going to have to put some sort of enclosure round them. I got them on eBay, one SKF and three SBC 7207.

Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

[Jonathan]

Don't bust a gut searching for single row angular shielded, I don't think it exists :naughty:

That explains a lot!

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.

[Jonathan]

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:

[Jonathan]

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!)

[Jonathan]

Video here:

http://www.youtube.com/watch?v=mSNFD...=youtube_gdata

Whipping caused it to stall at 15m/min. That be less of a problem when everything is mounted properly.

Some pictures:





At 8m/min I couldn't stall it by pushing on the gantry. Above that I
can, but only by bracing myself against the wall and pushing with both
hands...otherwise it just pushed me along the floor.

After the video I tried adjusting the acceleration. It went up to 3m/s^2 quite happily.