I understand that pipes are recommended because they are less expensive and lighter.
If I use solid 1045 Steel (Turned, Ground & Polished) instead of a pipe - I guess it would make the machine more rigid and precise?
It would also make the shafts much heavier. Is that a problem (i.e. for motors)?
Looking for your thoughts. Thanks!
Yup, heavier = slower accelerations. This machine does a lot of little tiny accelerations/decelerations so it has a huge impact. Heavier can also mean more sagging and less accuracy depending on length.
You will have virtually no induced flex in your pipes. Overkill for sure but if you want to, the slower accelerations are the only issue. If the rod is heavier but has more cross section, does it sag more on its own weight? hmm…Kind of unrelated but I wish carbon fiber worked. Wonder if you could take 2 rails where the CF tube has an OD the size of the ID of a thin walled stainless steel tube.
Solid rod is for tension or compression loads, not beams. If you want to add rigidity it is best to add to the OD not the ID.
So you’re saying a bigger rod entirely vs adding wall thickness?
Yes, ID is diminishing returns. The further in the less it does.
Moment of inertia do hickey whatever. Good to know because after seeing that miniture sized mpcnc I would really like to one day make a mega mpcnc that uses bigger size conduit.
LowRider CNC. Not all things scale easily.
Yes, it can make quite a difference, but it will still have a tiny bit of flex, no matter what you do.
I honestly don’t think so. The MPCNC is not designed to face crazy accelerations anyways, so inertia is not really a big concern.
The biggest issue will be to attach your tool on the Z axis, some hardened tubes cannot be drilled, so You’ll have a hard time to attach anything. I had this problem on my machine, which uses hardened, very thick tubing.
Yes, it it best to add to the outside diameter whenever you can. However, it cannot be done here, so the only solution left is to increase Internal diameter, even if it is not the best option. Still better than nothing. There was a link to a strenght calculator on the forum, increasing the inside diameter actually increased strenght quite a lot, even if it was not linear. At some point the deflection becomes too small to really matter.
From discussion above I understand that the rod weight on X and Y is not important on slow accelerations.
What about the rod surface? The Turned, Ground & Polished rod is much more precise, smooth and straight comparing to any pipe. I thought it may help.
So on the outside rails you could use heavier material with some support. Ground and polished would make for nice outer rails although I would imagine its quite expensive. However I believe Ryan is correct here that you don’t want to use heavy material like that for your inner x, y, and z axis. Keep them lighter so they are easier to move around. Now if you intend to always move the machine and cut very slow then maybe it doesnt matter. And depending on how long your x and y are will depend on the deflection. But solid bar does bend under its own weight. Just cause its solid doesnt mean it will bend less. Its a function of size vs weight x length vs type of material. If you dont believe it go to the hardware store with some rebar and pick up a piece by one end. It will bend like crazy under its own weight. Then pick up a piece of conduit the same way. Much less bend. But it still bends or bows.
Thanks for all replies!
I understand that OD has more impact on the structure. Are there pipes larger than 1" ?
(in States/Canada)
And if yes, can we have another set of MPCNC parts matching that diameter?
No, and not anytime soon. It takes a lot more work than you would think to adapt this design to a larger diameter. This works everywhere in the world as is, and that has been verified in some crazy places. If you want a larger machine I have made the LowRider CNC for that.
Thanks Ryan
How does the rigidity of the lowrider compare with the mpcnc? More specifically, will a lowrider have a significant increase in rigidity over a smaller and shorter mpcnc? Of course a same sized mpcnc will be so much more flexy, the conduit stretching 8ft. yikes.
A lowrider can be built as small as you want, it will be more rigid in general than an MPCNC at the small working envelope. The Lowrider is just not capable of speeds (because of its greater mass) you might need for 3D printing, or laser etching (but great for laser cutting speeds), so it is a bit less multi function.
Hi, hope this is the right point to ask this question. (I am a total noob, and haven’t even got started with the build and have no experience at all yet.) Regarding the flex that you’ll get in the pipes: The sagging is a cubic function of the pipe length, so might become appreciable when you have long pipes (and a heavy router on top of that). But in principle, the sag can be calculated as a function of the x and y position. I was wondering if and where one could built in (z-)corrections for this.
An interesting idea. I think there will be too many other effects, like for example the corners apply some moment like a cantilever instead of being idealized pivots.
If you have a surface plate you can measure the sag as a 2D function and try to use mesh bed leveling. This is more data driven and not a mathematical model, so its less mathematically elegant but should be pretty practical if you want to go to that level.