1. Some simple calculations can give you a better feel.

By way of example (and i'm not suggesting this is how to do it necessarily) lets say your gantry is a couple of 60 x 60mm x 3mm 2m long steel box sections joined by 3 bits of 10mm ali plate 200mm x 200mm at each end and the middle. The overall weight is 15.5kg and the deflection in the middle under its own weight, (ignoring the extra stiffness from the ali plates) would be ~0.1mm. A 10Kg spindle/z-package would cause a further 0.1mm sag, for a total of ~0.2mm. The max load on the bearings would be 255N which is nothing for a 25mm SBR. Increasing that to 100 x 60 box section with thicker walls would stiffen it further. Box section has its own issues with resonance but there are ways round that. The Y-rails would add further stiffness. All calcs are approximate and a more thorough analysis could be done.

So its doable... just need to get the detail right...

2. 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.

3. Originally Posted by 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.
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.

4. 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

5. There's nothing wrong with that lathe/mill for aluminium as long as you're gentle with it (at least for the final cuts). Its a little harder to make by hand what Jonathan made by CNC (I suspect) but not impossible with some care. Like all things planning is key, understanding the order of processes etc. The engineering itself is relatively easy. What I find it best to do is draw it myself to help visualise the process. I know there are people that can look at a CAD drawing and machine it straight off but I find if I draw it in Sketchup the same way I would make it, i.e. start with a block and then remove bits, it helps visualise how.

6. 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.

Originally Posted by kingcreaky
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.

7. This is excatly where i'm going with my machine simply because it'll suit my workshop more. My latest design has the longest axis of 2m on the gantry using 2 or 3 lengths of 60x60x5 stacked on top of each other with an ali back plate and using a rotating ballnut to drive it on a 2510 screw.
But then am i sacrificing rigidity of the machine just to make it easier for me to access.....?

8. 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)

9. 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
Last edited by kingcreaky; 18-10-2012 at 09:07 AM.

10. Have you thought about going vertical with this build? It would allow a good bit of what you seem to be wanting to do. It can limit the amount of part depth (item that you are working on) but there is always trade off's. As far as having pieces that need to be milled/machined, more then a few of us here are more then willing to help folks out in that area.

What do you want realistically for your machine?

What are you going to be using it for in the 1st yr (projects you can point to and have planned and are progressively more complex [trust me starting with really complex stuff does the head in])?

What do you want to do with the machine long term wise (upgrades and production levels)? This includes the budget for now and as well as a long the road of use.

Have you started costing out the software and tools to do work with? This includes the bits and such you will be using on the machine.

I am not trying to put you off or be a jerk, the information on this forum is staggering for many folks. Also there are a lot of folks who look at it and say "I want" without really counting the cost of getting there (trust me, why do you think I am on my second machine?). It is better to step back a bit design and work the design over hard and through away more then a few of them in the process, then to build with half an idea and end up costing you a lot more then it needed to.

My current machine does everything I need and when I upgrade the spindle (control or outright upgrade of the unit) then I will be able to cut materials beyond any need I really will ever have. The ting is my machine is part of business and Jazz/Dean helped by kicking into touch so many design ideas, as did other folks. Think the design out, not just use and parts but maintenance, where and how/wen upgrades or replacement parts can be dealt with. In doing so you are building a much longer usable life cycle into your machine and will get a greater return for your investment. To the folks who helped me and keep helping others thanks and I hope this pays forward a small part of the debt I owe.

Best of Luck and ask me if you need any help,

Michael

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