operation cnc

[irving2008]

...[stuff]

1,)What is the maximum length I can have, with a conventional 'spun-ballscrew' setup (keeping the nut static)?

I intend to have two screws for the x axis, (one on either side)

The ballscrews descibed in the ebay auctions (linearmotionbearings2008) detail as being 'end machined'

4 pcs of anti backlash ballscrews RM1605-350/800/1450/1450mm-C7(4screws+4ballnuts+4endmachined)
4PCS NUT HOUSINGS/BRACKETS FOR THESE 3 BALLNUTS
4 sets of BK/BF12 with locknuts and circlips+ 4pcs 6.35mm*10mm flexible couplers
Linear slide rails SBR20-600mm(2 supported rails+4 SBR20UU bearing blocks)
Linear slide rails SBR20-300/1400mm(4 supported rails+8 SBR20UU bearing blocks)

so with the 'flexible couplers' also supplied in the kit, I assume will fit my steppers ?
Nema23 Stepper Motor 3.1Nm x 3

Given the BK/BF mountings you have a fixed/supported arrangement (supported BF at the far end, fixed double BK near the stepper) the critical speeds are:

350mm:10000rpm (lots of m/min)

800mm: 1965rpm (9.8m/min)

1450mm: 598rpm (3m/min)

To get closer to 6m/min on X you either need 10mm pitch screws, or you need to increase diameter to 25mm but then the inertia of the screws is going to be limiting on acceleration.

Another option is to upgrade the bearings to be double angular bearings (BK) at both ends. This will increase the critical speed to 880rpm (4.4m/min), better but still not great.

Here is a good PDF that shows how these are assembled: https://tech.thk.com/en/products/pdf/en_a15_314.pdf

And the critical speed calculator is here: http://www.nookindustries.com/engine....x=0&reset.y=0


Couplers are fine: 6mm stepper shaft to 10mm end machined screw.

[kingcreaky]

now I am confused! I think im going to have to read your post another 100 times slowly to understand, then study the pdf. perhaps im not asking the right questions? or perhaps relying too much on telepathy
Essentially you have succeeded in making me aware there are different 'types' of ball screw housings. I think what your saying is the ones ive detailed above (which incidently I havent yet bought anything) are for a fixed screw arrangment, and if I want a spinning screw I need to re-think my choice of screw housing? once again, one question results in ten more :D sorry

[irving2008]

now I am confused! I think im going to have to read your post another 100 times slowly to understand, then study the pdf. perhaps im not asking the right questions? or perhaps relying too much on telepathy
Essentially you have succeeded in making me aware there are different 'types' of ball screw housings. I think what your saying is the ones ive detailed above (which incidently I havent yet bought anything) are for a fixed screw arrangment, and if I want a spinning screw I need to re-think my choice of screw housing? once again, one question results in ten more :D sorry

Nooo... Imagine the screw is a long bendy pole... the faster you twirl it the more it flaps around. To stop it flapping you need to constrain it. The way to do that is to put bearings at each end. The bearings are fitted into bearing housings. The one at the far end has a single bearing in it, the one nearer the drive end has two bearings. The single bearing just supports the screw, it doesnt stop it moving end to end (axially). The double ones stop the screw moving end-to-end, hence 'fixed'. Both types allow the screw to turn! By putting a 'fixed' one at the far end as well controls the whip a bit more but it needs a different end-machining of course.