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

[Davek0974]

Hi all,
I am now starting my second table build, this time a bit bigger at 1250x1250mm cut area, I have also upgraded to a Hypertherm Powermax 45 cutter. My first table is now too small and cannot handle the speeds needed by the PM45.

I have just about decided to go with ballscrews all round, 16 or 20mm lead with anti-backlash nuts, motion is on HiWin 15mm rails and carriages with scrapers and wipers fitted.

Here's a couple of sketches of the gantry ends, just ideas at present but would seem to fit well..


The main table will be 50x50x5mm box section steel, fully welded, the motion supports will be bolted on so I can shim them up and avoid welding warp etc.

The top rails or Y-axis will be above the bed so I can slide a bigger sheet in if wanted, this will also keep the tracks and screws above the pierce splatter.

Drive is with my existing Nema23 3.1Nm steppers, if that does not play nicely then I can upgrade easily to Nema23 servos.

I am aiming to get a speed range of 100mm to 9000mm/min so I can use the full range of the PM45 without messing around.

Feel free to offer any advice :)

[m_c]

A plasma table has reappeared on my potential project list, and I do have a couple thoughts about mitigating dust.
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I wonder if mounting the rails on the underside of the side frame would help dust from settling onto the rails, then have the screw mounted below the rails/gantry/carriage? On the inside edge you can then have a bit plate hanging straight down to stop dust coming of the cutting area, and arrange a couple brush strips on the outside to minimise dust blowing around and in.

[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???

[Lee Roberts]

All the ones I've seen have been RP, share their thoughts with us, I would have thought ballscrews would be abit overkill given it's contact free cutting? I don't know but RP just seems like it would be a better fit, I guess it could get pricy if you got really good stuff but dose a plasma based machine need that kind of acuracy ?

.Me

[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?