Thread: operation cnc
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.
Last edited by irving2008; 18-10-2012 at 11:19 AM.
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
As irving has said, you need to use 10mm pitch (RM1610) ballscrews on the X and Y axis to get respectable feedrates without using a larger diameter ballscrew.
Clearly it depends on what the minimum feedrate you can accept is, but I wouldn't go above a 1500mm long RM1610 ballscrew without using a larger diameter screw or rotating the nut. If you're interested I can make the rotating ballnut mounts for you, let me know if you wanted the details. I wont be able to make them any time soon, but it looks like it will be a while until you're ready anyway.
The 3Nm stepper motor shafts are 8mm, so you would need 8mm to 10mm couplers. However I would strongly advise using timing belts and pulleys since this enables adjusting the drive ratio to get better speed or resolution with the added bonus of adding damping to the system to suppress resonance.
this really is like a mind boggler. ive spent pretty much every waking moment, reading posts, writing notes but mostly thinking. Oh, and getting frustrated with google sketchup.
I have made a final, firm decisions on a few things.
*It will be around 1.5m by 1m
*after reading the pros and cons, and weighing them up in my own head, Im using chai's supported bearings SKB16, maybe 20's for the gantry to slide along on(is that the X some people call it X some people call it Y)
*The first version, Il use the 1610 ballscrews I think that is what Irving is trying to suggest in his above posts) , and although maybe not recommended il spin the ballscrew not the nut. I may eventually re-think this once ive learnt from my mistakes. As Il be driving the ballscrews with timing belts I can gear it so they for optimal reliability not speed.
I am very confident with steel. I can weld well, (although I understand the issues over warping) so please see my gantry design.
Ive read thread after thread explaining the importance of keeping the gantry supported to stop flexing. I also appreciate the ultimate goal is keeping the cutting tip as close to the center of the gantry as possible through organising the Y and Z axis /ballscrew configurations.
With the comment over confidence in steel and no experience in extrusion or ali whatsoever, please see my current gantry plan (attached?)
as you can see, ive used two peaces of 50x200x6 I intend to triangulate the back for extra support and place the Y bearings one at top and one at bottom (top marked in red)
In terms of fixing the material together, I think im going to carefully weld it.
I need your thoughts now, Ive heard Jonathon banging on about steel box not being 'straight' and I should probably be using ecocast ali. or even extrusion like some of the builds here. Without entering into yet another full on discussion over gantry design. is carefully welding the steel going to distort it that much the z unit wont slide the length of the gantry without binding?
should I use extrusion?
With extrusion... how do you know the t-slots or slots in the extrusion are going to line up with the pre-drilled holes int he supported rail?
where do I buy extrusion?
totally consumed in this, and seeking some support!
I'm also new to this KingCreaky, so can only try help a wee bit...
For this size machine which is similar to the size I'm designing, choosing 1610 ballscrews seems spot on, but I think the gurus are likely to suggest 1605 only for the short Z-axis. And spinning the ballscrew rather than the nut is the default norm.
If you're gonna use supported round rail I think the gurus would suggest Chai's SBR20 rail.
Where peeps do use extrusion, a version of 45x90 extrusion is popular on this forum that hails from your neck of the woods... KJN in Leicester... Aluminium Profile with 10mm Slot
The KJN 45x90 extrusion has slots that are well suited to the precision-profile-rail that is terribly pricey, but not particularly suited to supported round rail.
Hope this helps.
I am working on a similar sized machine and have been through the same questions on ball screws - seemed to me there was a lot of different opinions floating around. I too decided that spinning ballnuts was a bit out of my league! I went for 2005 in the end, the larger dia will hopefully help with whipping and I am going to gear it 2:1 so as to get faster feed rates, with the option of gearing 1:1 for high res work...
I am coming round to the idea of using steel (ill have to learn to weld tho!) for the frame and x axis but Ali extrusion does seem the best option for the gantry. The L shape arrangement used in a few builds allows you to keep the gantry squat and rigid. Why does your gantry need to be so tall and thin...?
If you need any help with sketchup just ask I have been using it for years and have just about got through the pain in the ass stage !
If the L-shape gantry is reinforced by putting some box section on the back at 45° to make triangles it will be very strong.
OK but surely it depends on the speed you need. I remember reading on another post that Jazz gets no whip running his 1500mm 2005 up to 12meter/min... Plus you get greater mechanical advantage with a smaller lead? Well I have bought mine now so when I have built the machine I will be able to report - Just hope I will get enough speed to cut ply without burning it up!!
For a 1500mm 2005 screw the critical speed is about 1200 rpm with the standard bearing arrangement, so 1200*5=6m/min feedrate.
For 1610, same length etc, it's about 1000rpm, so 1000*10=10m/min feedrate.
From experience the formula used here seems to be conservative, so that is probably enough assuming the motors you have can actually output enough power to achieve that feedrate.
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