operation cnc

[Jonathan]

I don't like the bed being unsupported over a 3m length. You can at the very least mount a piece of steel box section vertically at the back and the front, but the front one would have to be removed when you make the axis longer. A third piece of 3m long box section in the middle wouldn't go amiss either. Also I would make the bed bolt on to the 4 verticals on the other side, again to make it easier to extend the bed in the future. Currently the frame can 'skew' quite easily, since there are no triangles - so add some pieces to make triangles! You can do this in all 3 planes which is a good way of using up the left over box section since no doubt you'll be ordering in 7.5m lengths. Incidentally how much did Adey Steel quote you for the box section, presumably 60x60x3mm? Their pricing seems to be a bit random...

The way you have mounted the ballscrews is good since it allows adjustment of the height via moving the 4 pieces of box section, which is nice and strong. You can make the end mounts fasten to the box section via holes slotted horizontally to get adjustment in the other axis, which will help when aligning the ballscrews.

[irving2008]

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...

[irving2008]

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.

[r0bsk1]

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.....?

[m.marino]

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

[kingcreaky]

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!

m@

[WandrinAndy]

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.

[JoeHarris]

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 !

[Jonathan]

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

Not really, you'll end up worse than with 1610 regardless of the drive. Critical speed is proportional to the diameter, so going from 16 to 20 means the ballscrew can rotate 25% faster. However to get the same feedrate with 5mm pitch (2005), you need to spin the screw 100% faster, but can only go 25% faster... so you end up with 62.5% of the feedrate attainable with 1610.

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.

[Jonathan]

I'm not keen on how far the Z-axis overhangs (parallel to X). Presumably you've done that to position the Y-axis ballscrew more optimally than having it on the back, a long way from the cutter? That's sensible, but I would at the very least try and add pockets to compact the design, or add a second piece of extrusion above the existing one such that the ballscrew can run between the two, i.e. directly between the rails which is optimal.