operation cnc

[Jonathan]

I think if we included the torsional stiffness in the calculation then the results would be a lot worse. Once this 3m gantry is drawn I should be able to do a FE simulation to find how much it twists, rather like the one in this post.

[irving2008]

I think if we included the torsional stiffness in the calculation then the results would be a lot worse. Once this 3m gantry is drawn I should be able to do a FE simulation to find how much it twists, rather like the one in this post.

Don't disagree, my point was to illustrate that the bearing sizes were OK for a gantry of that scale. Of course it will need to be stiffer in torsion and I'd consider plates both side of the box section and some perpendicular as well.

[kingcreaky]

Gents.

first of all, thanks you all so much for your input. You are all clearly much more astute than I. To be quite honest Im still pulling myself back together after reading Jonathons post http://www.mycncuk.com/forums/linear...ign-ideas.html about how one day he decided to manufacture the driven ball screw nut housings. That is on a complete other orbit to anything I have ever done / made.

Dont get me wrong. I have a metal lathe / miller of sorts


but ive never made anything close to the type of engineering displayed by Jonathon in that post. To be quite honest its now making me doubt my ability to persue the static ballscrew idea.

So Im reverting back to the idea of spinning screw static nut.

Also, I can see where I went wrong in my blogs and I have mislead everybody. To be clear forget about 3 meters. the bed im planning to make will be at best 1700mm by around 1100mm

[Jonathan]

Whilst I did make the rotating ballnut housing with CNC, that didn't really gain anything except aesthetics. If you just don't bother with the rounded corners and change the pockets for the pulley/motor to rectangular instead of having rounded ends then the design is much easier to make manually (since it doesn't require a rotary table) and none of those things will affect its performance.


The difficult bit is boring the 70mm hole for the 7207 bearing since the lathe needs a lot of swing to accommodate the plate and also a large chuck or faceplate to hold it. However it seems your MF42B machine has 420mm swing, which is plenty. To avoid buying those you could machine somthing in the extisting chuck to hold them using the pattern of holes already present.You will of course have to run it slowly. The other option is to use a boring head in the milling machine. You could also make the bearing housing and motor mount separate parts. If you clamp the two halves together whilst boring them this will ensure the bearings are concentric when it is assembled.
I made the shaft for the first one on the manual lathe out of two pieces, which was a mistake as it's harder to fasten them together reliably and ensure the face upon which the ballnut mounts is normal to the axis of revolution. Instead make it from some 3" aluminium bar and take it slowly. When you get close to 35mm diameter for the bearing use very small cuts and keep test fitting the bearing as it needs to be very accurate. Also thread-cutting the shaft and using a locknut to pre-load the bearings turned out to be a much better solution.

To be clear forget about 3 meters. the bed im planning to make will be at best 1700mm by around 1100mm

So just to be clear, am I right in thinking you want 1700mm*1100mm working area, but the available space for the machine is 3*3m? If so then that's very close to my machine - 1700mm*740mm and you would be best just using rotating ballnuts on the 1700mm axis as the ballscrews will be about 2m long. It's possible to do it without, but you'll have to spend a lot more money on the motors and drivers to get similar performance. With two bearings at both ends of the ballscrew the critical speed of a 25mm ballscrew will be about 760rpm, so you could get 7.6m/min with an RM2510 ballscrew which is an acceptable feedrate but it is rather close to the limit so may not be very 'stable' compared to the rotating ballnut which allows at least twice that feedrate. The other option is to use RM2525 ballscrews, but again these are much more expensive.

[blackburn mark]

To be quite honest Im still pulling myself back together after reading Jonathons post

take it easy man :)
be assured that most of us start pretty much where you are
i think some posters on this thread are being a tad unfair in flexing their intellectual muscles without remembering what its like to wade up to their neck in information and the inevitable burnout that causes... reminding you to chill should be on their list :)

you have a lath/mill so its safe to assume you are a practical bloke... if you have the gumption and time and a little help, nothing will stop you designing building a machine that will do as you require... it doesn't have to be rocket science and most machines are built without the in-depth mathematical analysis.

it wont be long before your the one sharing the love and helping some crazy arsed dude on a mission to build a cnc (just try not to melt his brain lol)

[kingcreaky]

Hi All.

Quiteness is due to a lot of thinking, and sketching.

Towards the end of last week, I was put in touch with a guy who was selling a used "branded" machine, capable of cutting 8x4 sheets, with vaccum bed etc. It was going cheap due to all the electrics being shot. However, cutting a long story short I dont think its still a viable option, he was initially talking about a lot less money than he is now, then there is the logistics of moving it etc, and I dont think its going to be exactly what im after.

Im thinking 1400mm x 800mm x 300mm. with the conventional setup (having x being the widest axis) and having a shorter y. Unlike what I previously suggested earlier in this thread.

Being Realistic, I simply do not have the skills to be able to pull off a spun- ball screw nut. Manufacturing all the housing etc is just out of my depth. (as previously suggested), so my first question is

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

[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