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

[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.

[Davek0974]

I find the thinner wall box to be pretty straight usually, especially down in the 2-3mm section.

I have belts on my existing table and they cause issues, not solely down to the belts but as 90+% of commercial builds use rack, I am going that way first this time.

[Davek0974]

Reality check time...


How do you decide what table is the right one?


I know the 2'x2' i have now is too small, it works ok for most of my jobs but i can only cut with the plate square on the bed so no quick twisting it to get another cut out etc.


I can physically fit the 4'x4' (1250x1250) into my shop, thats ok.


I cannot lift a sheet of steel that size, have little possibility of lifting gear etc. as its only a double garage. A sheet of 3mm which is my most popular is about 36kg so only just about ok but the other one i use a fair bit of is 4mm and would be 50kg, no way i can lift that.


Would it be better to go 3'x3' instead?


Cost difference in build is minimal, but what makes sense - having the real estate and not using it or saving floor space etc??

[Davek0974]

View from front looking at current gantry design.



This is for the open-bed style table with the longer gantry arms. The linear guides can be easily covered from the top.

The gantry beam will pass through the side support plates and be welded up and gusseted inside with some nicely cut parts, the outer face of the vertical supports can be strengthened with a couple of ribs to resist side-sway.

Mounting the support rails is harder on the open bed style though, I was going to bolt them on but this style really wants a box frame at bed level.

[Robin Hewitt]

Every action has an equal and opposite reaction. If you apply 300N accelerating force to the cutting head, the gantry will push sideways with 300N.

[Davek0974]

Yes I can remember that from school ;)

But those 300N can be countered by applying gusset plates and ribs where needed, most tend to throw a slab of alu at it but that might not be totally needed i think.

[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.!