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

[Davek0974]

Ideally, it needs a structure that resists torsion moments - this is where my current machine fails - the gantry is a length of 44x44 structural aluminium which is ok in the beam but poor in tension as the centre box is only 18mmx18mm.

The torch and axis are not balanced and when accelerating it twists one way and the opposite on decelerating - causing ripples in the cuts.

What is ideal is a simple, light structure that ca withstand torsion or twist as well as support 3-4kg in the centre.

What that is, I have no idea, most machines just use box section because it's there I guess.

It needs a thick enough wall section to take 5-6mm screws firmly so I guess 5-6mm thick.

Without getting a structural engineer on the job, what stock item would be best suited here?

[Robin Hewitt]

When I started making one I got a piece of 4"x4"x10g aluminium box and made the gantry as the long axis. Across the front I bolted a linear rail, using the bolt from behind kind so the dust could not get inside the block via the bolt holes. I used 4 1/2" bolts and spacers so I could bolt everything from behind without crushing the box. I cut the box ends dead square. I inserted 1/2" square plates with 8mm steel nut inserts dead centre from either end. The gantry end plates had 4" x 4" recesses and an 8mm bolt hole. When I tightened that sucker up it became incredibly rigid. The gantry end plates got double round linear blocks running on 30mm round linear rails which held the assembly in place simply by being bloody heavy. I liked my design but it all went horribly wrong because the 5mm pulleys were such a crap fit on the 5mm belt. I lost heart.

[Robin Hewitt]

I tried to add a picture but failed miserably...

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[Davek0974]

That looks ok, the 5mm belt is very light, more suited to 3d printers i think.

What happened in the end??

[JAZZCNC]

Dave I'd stick with belt drive on Linear bearings and just protect it good. V-pulleys are a pain in the arse on routers and with Plasma dust etc I'd think they would be a just has big a mare.

Belt with give you best of all worlds. Higher resolution, Higher efficiency, less maintenance, Smoother action. R&P is ok and works but more to building it to do properly and it becomes expensive. Belts will be cheaper and easier to replace if damaged.!

If was using a Ballscrew (which I probably wouldn't) I'd put it down the middle and run it thru a tube tunnel with slit in bottom. I'd use profiled linear bearings on underside like you have shown.!

[Davek0974]

Thanks, I have opted to try a direct drive version now, linear guides, rack and pinion with 15T pinion.

Part of the issue I have now is caused by the belts so I don't want to add that back into the mix.

I have also got some larger motors coming - 4Nm with DSP drivers and a higher voltage PSU, that all means i can build more of the new table before removing bits from the old one.

Ballscrews are out for the present.

If the direct drive fails then an easy upgrade to belt reduction boxes is made. Rack and pinion seems to be the industry standard, a few are direct drive, we will see what happens I think.

The gantry is looking like 50x75x3mm steel, with a strengthening strip inside where the rail and rack screws on.

Aluminium was looked at, and is lighter, but stiffness suffers as a result and I want it rigid this time ;)

[Davek0974]

Frame bracing???

First frame idea, overall size is around 1500mm x 1500mm x 1000mm to bed level.

Should i be looking at bracing or gussets anywhere???

[Robin Hewitt]

Aluminium was looked at, and is lighter, but stiffness suffers as a result and I want it rigid this time ;)

Aluminium has the huge advantage that it is usually quite straight. Steel box section can be quite horrendously bent, as far as CNC building goes, but still well within tolerance. Even slightly bent can make it hard to align a rack. I agree with Jazz, belt for a plasma cutter.

[JAZZCNC]

Part of the issue I have now is caused by the belts so I don't want to add that back into the mix.

Very much doubt the Belts caused the issue.!! Belts are super accurate and very close to ballscrews for efficeincy so if you had any issues they probably came from how it was designed or put together rather than the belts.

One thing I do know is that an old mate of mine builds Plasma Machines and his first test machine used Direct drive R&P and didn't work very well. Low resolution and low efficeincy needing large motors, think it used 8Nm or 12Nm Nema34.?
Anyway it was crap.! So then went to sprung loaded into rack with 3:1 ratio using timing belts with Nema 34 motors. This worked but he couldn't reach high feeds as the ratio and slower spinning motors worked against him. He was also having issues with R&P binding etc at higher feeds.

Last machine I seen was using gearbox's and what looked like Nema23 motors.!