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  1. #31
    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.
    Last edited by Robin Hewitt; 29-06-2015 at 01:45 PM.

  2. #32
    I tried to add a picture but failed miserably...

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

    What happened in the end??

  4. #34
    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.!

  5. 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 ;)

  6. 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???

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  7. #37
    Quote Originally Posted by Davek0974 View Post
    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.

  8. 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.

  9. #39
    Quote Originally Posted by Davek0974 View Post
    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.!

  10. 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??

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