Second CNC table build, here we go again....

[Davek0974]

Yes, that could be a good idea, i'll sketch it up.

it would mean a small loss of Y travel or making the table a little bigger as the carriages will hit the support legs, no real issue there though.

[Davek0974]

A bit like this?


Might be better, Not totally convinced though, the rail is now facing the pierce splatter zone.

[Davek0974]

I have people advising I go with rack and pinion and I have people saying go with ballscrew - both with good reasons for and against.

How do i decide what method to go with???

[m_c]

Just done a rough sketch of what I was thinking.
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If you wanted to add support to the side rail, you could use a wider box section, and either move the rail nearer the edge and have the legs/supports mounted on the opposite edge, or use a double box section, with the rail on one and legs/support on the other.
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Dust could be mostly eliminated by adding a couple brush strips, or suitable strips of metal with minimal gap either side of the rail to further reduce the ability of dust to settle onto rails.
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And having just looked at my sketch and thought about how to protect the screw from dust, mounting it at the side of the rail so you only have one 'area' you need to minimise dust ingress might be an option.

[m_c]

Regarding the R&P v Ballscrew, both have their pros and cons.
As Lee mentions, a plasma doesn't really need the accuracy of ballscrews, but the biggest drawback is coming up with a design to keep dust out, as I'm sure you're well aware, plasma dust is pretty abrasive and will quickly kill ballscrews.
However R&P on the other hand, provided it's designed well enough, will be relatively unaffected by dust, but you lose accuracy, and have the issue of dealing with wear, especially uneven wear which will be challenging to adjust out to minimise backlash while avoiding binding. Off course the solution to this could be simply changing the racks when worn, building in some form of spring loading of the R&P, or using anti-backlash pinions.
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Both options have pros and cons, both will work, and both can be designed to have long service lifes. As with most things, it comes down to implementation.

[Davek0974]

Hmm, thanks, I have doubts now, not good;)

I wanted accuracy for small part cutting in thinner materials, so high speed without wiggle on the torch.

Putting the screw and rail under the beam is ok but is filling the gap I wanted left clear for large sheet entry, likely only a few inches clearance left and the covers will be at risk of taking a battering from sheets.

I see machines like PlasmaCam (which I think have a good name?) running simple plate/skate runners but they do use servo drive, they seem to work ok. Other production builds like Torchmate and PrecisionPlasma use various other simple systems ranging from belt to R&P. All mostly with not much regard paid to keeping the muck off of transport parts.

But there again, none of them using ballscrew either!

I'm not sure where this project is going now, back to stage 1.

[m_c]

Compromise.
Or just make a water table so you don't have to worry about dust :-)
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If you do go for ballscrews, then you are going to have to ensure they're well protected, whereas R&P doesn't have to be. Yes you'll get backlash using R&P, but you need to remember that Plasma is not that accurate a method, so even 0.5mm of play isn't going to have much effect on most items.
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Personally, I'd be designing the table so it sits above the side rails, that way the only thing in the way during loading would be the gantry. You then only have to handle dust from above, instead of spray back/dust coming up the way as well. You will still get some dust coming up the way, but hopefully a lot less.
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Have you had a look at the swift-cut machines for some ideas?

[Davek0974]

Thanks,
If i had the cash i would buy that 1250 table tomorrow! It is exactly where i want to be, has the full cut speed range I want, pity the pictures are not clearer ;)

I see that have R&P drive but use V-Rails all round, Can't seem to find that stuff?

I like the below-table idea too for the transport, places more stress on the gantry support but obviously works.

The motors are not much above what i have - they use 4Nm and I have 3.1Nm, both hybrid so thats a good sign.

Now, can i build that table with Linear rail or does anyone know where I can source the V-rails????