[gregmary]

2nd Generation CNC Router Design 32.5” X 14” X 6”

I am working on a second generation CNC router design based on John’s popular JRGO router. The range of motion is 32.5” in the X dimension, 14” in the Y and 6” in the Z. The differences I have incorporated include:

1.) ¾” hardened shafts and roller linear bearings in place of skate bearings
2.) 3/8”-12 Acme precision lead screws in place of threaded rod.
3.) Stiffened the gantry

It is commonly stated in this forum that people will build a first machine, like the JRGO design, to expose themselves to CNC, giving them a platform to learn about the capabilities and limitations of hobby CNC. Then they will move on to a more accurate and capable machine. It is this second machine that I am targeting in this design.

Even though this machine uses higher performance components where required, minimizing cost is a driving requirement. To this end I am looking at utilizing common materials for things like bearing support blocks and lead screw nuts. Attached is a rendering of the proposed machine. Comments are welcome.

Greg

[Rance]

Greg,

Nice design.

If you're going for precision (and stability), then one thing that stands out to me (one who still has yet to complete his FIRST machine) is that you might have room to widen the spacing between your Z axis rails. Your alignment blocks have space between them and the Z axis trolly. The wider these are, the more stability you 'should' have. If you've got the room to widen them (without widening the outer wooden structure of the Z axis itself), then do it. You don't want to widen the wooden part because that would narrow your Y axis travel.

Just my 2c worth.

Rance

[Kammo1]

Hi Greg that looks very impressive and is a really nice design, I especially like the gantry sides they look very sturdy. Am I right in thinking that making the sides like you have you can use thinner MDF making it lighter but also more importantly much rigid ?
I too am in the learning curve as regarding making a small CNC machine and I am still contemplating costs as well but I would like to probly make one to start and learn and then move on to a more suitable one like you are doing, still I would love to see the process that you make with yours and post updates as these are always great to see and learn from. Kammo1

[buscht]

Greg, Have you checked the deflection of the 3/4" shafts? I think at 32" you will need supported shafts or alot larger diameter.

[ynneb]

The thing that confuses me is why do most designs have long gantry supports at the side and the linear bearings underneath. To me this introduces lots of potential unwanted movement.

If the gantry was closer to the linear bearings, and the bearings were raised and supported higher, you effectivly get the same Z movement but better side ways support.

See this design


Here is your design for those who dont want to download the pdf.

[skippy]

In reference to Ynneb's comments, the one and only advantage of using a design with long (high) gantry support arms is that when the gantry is fully back, a job can be slid off the bed in any direction (forward/left/right). With the other design (as per Ynneb's picture) the bed is lower than the side rails therefore the job must be lifted out or slid forward. Start to look at the leverage factor exerted on the bearing rails (in this case unsupported) by the long arm version and in my case at least, I wouldn't go near it. That's not to say that all long arm versions are no good but simply to say that by opting for a short arm/high rail design you can build you machine a lot lighter to achieve the same rigidity. I also agree that 3/4" shaft is way too small and that the spread between front and back bearing blocks (trucks) along the X axis is too small. More spread will mean less X travel or longer shafts required but it's worth it.
Skippy

[maxxgraphix]

Shafts will sag big time. Must be supported. Any hardened shaft over 24" will sag. I have 60" 20mm shafts and I was shocked. I added supports with split bearing blocks to fix the issue. Really should have bought THK rails and been done with it. But I guess $$$$ is the real issue. I'll gladly share my design with any that would like it. $2500 is the cost to build 50 x 50 x 5 with T-Slot.
I don't like MDF designs. I think they will fall apart within a month.

[gregmary]

Max,

How far apart are your linear bearings in the X and Y dimension? Say from the end of the bearing block to the end of the other. Since you added supports to your shafts after the fact I take it your shafts are not secured to the supports, or did you drill and tap them?

If you did not drill and tap them, do you ever get any 'negative' flex when you plunge into the part?

If you did drill and tap them, how? That stuff is hard.

Thanks,

Greg

[maxxgraphix]

The bearings are 9" apart. I couldn't drill and tap it. Too hard for my tools. I used T-nut, a flange nut, and a hex cap screw to make adjustable rail supports. The weight of the x axis doesn't allow for negative flex on the plunge. My machine is not designed for precision machine work. It was designed to make BIG signs. How ever I have engraved acrylic with an accuracy of .01 using a dremel instead of the Porter Cable router. The shafted are mounted in 2" block with lock pins. My X axis sags a little since it's not supported. There's much less weight and it's distributed across 2 rods.

[gregmary]

Thanks everyone for your input. I have made the following changes to the design:
1.) Seperated the Z axis support shafts as far as possible
2.) Used supported shafts for the x axis (The long axis)
3.) Seperated the bearing blocks on the x axis to 12"
Attached are some views of the CAD, click on the drawing for a larger view.