Cost aside, would it make any sense whatsoever to design the mpcnc to be lead screw driven in all axis instead of just z? Or will I have finer control using belts?
I thought there was a response to this on the faq, I don’t see it now though.
I think the leadscrews that are similarly priced to belts are going to flex, not arrive straight, cost a lot to ship, and they are slower. Expensive screws and motors might fix some of these problems, but most users here aren’t interested in spending $3000. Belts also can be any size, and a few zip ties make the connections easy. For the low rider, I know Ryan has tried doubling up the belts, both by getting a larger size amd just looping two belts through everything and he says it’s great. I haven’t had any troubles with my single belt on my 4’x3’ work area low rider.
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The screws have finer resolution, and slower speed. Just imagine one rotation of the motor shaft, how far would the axis travel. With a belt, it’s more and with a screw, it’s less.
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The screws have finer resolution, and slower speed. Just imagine one rotation of the motor shaft, how far would the axis travel. With a belt, it’s more and with a screw, it’s less.
Resolution is actually what I am looking for. Was taking to an inlaw that owns an industrial router that is screw driven, so it got me thinking about doing the same with the mpcnc.
My other reason for wanting resolution is that I’d like to utilize the build volume of the mpcnc as a 3d printer. I mostly print in pla, so a heated bed isnt critical, but a 24"x24"x6" would open some doors for me (my daughter is SUPER into cosplay).
I’ve only sourced Amazon so far and it’s $30 for a 36" t8. I’m pretty sure I can find them cheaper without sacrificing quality as well.
The resolution is already better than you can use in wood and almost all 3D printers use belts and nema 17s. The first time you try to cut and it takes 6 hours to mill, you’ll care about speed. You can’t ignore a cnc mill like you can a printer.
There are also a lot of good reasons to either have an MPCNC as a printer or have it for milling. There arent many sizes that work well for both. 24"x24"x6" is going to be a stretch for milling. It’s fine for printing, if you can get the bed that size level (that’s 9x more area than a standard 8" bed).
Don’t let me discourage you. I would applaud your effort if you give it a shot. I’m pointing out where I see the problems. I am by no means a mechanical engineer, so feel free finding your way through these issues. I can’t predict the way these things will work.
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Properly sized belts are more efficient than screws, belts are actually the most efficient transfer of energy from what I remember. Think, Harley and the belt drive motorcycles. Screws, and chains require maintenance, lube and adjustments.
Belts and screws can be sized for any ratio, or resolution. The current designs theoretical resolution is 0.005mm, or 0.00025 depending on your drivers, you do not need higher resolution.
Whip. Horizontal (or even long vertical) screws have “whip” . to counter act this you have to support them at both and and have a diameter large enough it does not sag and whip, expensive, very very expensive.
Lubricant. all screws need lubricant, very bad in a dirty environment.
Backlash. Screws and belts have backlash, belts ave very very extremely small amount most of use can’t measure it. Screws have backlash, a lot of it, and every single time it gets used the backlash gets larger. To counter act this you need ball-screws, which are stupidly expensive and need to be adjusted. You can use a spring driven anti backlash but that adds to much friction. In either case you will need to up the power for that axis to overcome all this added mass and friction.
Adjustments. Ever decide you want precision over speed, or vise versa. All new screw and nut, supports and couplers. With belts, a new pulley.
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I made my first CNC machine before I had a 3D printer (or a brain.) 2” thick oak for all the parts, way over engineered, lessons learned. The Z axis was a standard lead screw (now in my MPCNC), the Y axis was belt driven, and the X axis was a Threadless Ball Screw. http://www.thingiverse.com/thing:125529
I loved that part of it, and only that part of it. The smooth shafts are much cheaper than treaded shafts, if their diameters are off it doesn’t chain anything, if they are a little bent you hardly notice it. It pushed all that oak hardware pretty darn fast and without slipping at all. A small 3D printer using those would work quite well. You can customize the pitch to adjust your trade off between speed and resolution by just reprinting those with different specs.
Have fun 
-Allsb
Looking back, I changed which axis were on belts and threadless ball screws at different times.
This video shows the Y axis, driving the gantry with motors on both sides, using the threadless.
Its moving slow because it was initial testing, but you can see how smooth it is even with the massive weight its pushing.
-Allsb
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Threadless-on-Oak-CNC.mov (4.42 MB)
Dude…
I love the idea of those thread less screws. I almost tried them on them original MPCNC. Watching your video it occurred to me if you used an aluminum rod, shaft, tube, it would basically thread it self and self tension. That could be really awesome. The hardened steel bearings will easily wear a groove, hopefully work harden it a bit and can only go so far as it would stop at the aluminum materials plastic zone…
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Once it has been broken in you might need to adjust the steps per mm but that is easy. That would be an awesome project.
If you wanted to be completely crazy, which I am apt to do, you could use the same EMT conduit as the rod. Heck, with some fun engineering we could have the same pipes act as both the support and the drive shaft with each mounted in a standard bearing block on both ends (where the legs connect) and a motor on each. Have the threadless take the place of parts Roller and RollerM. One day, after I actual finish my normal MPCNC, I’ll try playing that way.
-Allsb
Yes! We are on the same wavelength…the Mad scientist one…
That would be insanely awesome, accelerations, and emt concentricity would be a concern but would be offset by how cool it would be.
Hi there,
this thread less screws idea sound awesome. Did you boys get anywhere in the meanwhile?
Boris
Nope 
I put all my MPCNC dreams on hold after a mishap.
Warning, using a fly cutter is a bad idea. It cut nice, for 3 minutes. Then the vibrations rattled some bolts loose, the Z assembly crashed into the table with the fly cutter still spinning, bent the shaft on the cutter 45 degrees. Only damage to the MPCNC was just the Z Mounting Plate getting snapped in half from the shear pressure.
So out of fear, I’ve not touched mine in months.
-Allsb
Whoa! Glad everyone is safe.
I looked into those threadless ball screws a few years ago but stopped when I found that they have poor accuracy.
Here’s someone testing one: https://www.youtube.com/watch?v=NA_9GeFxV0o&t=347s
Here’s a blurb from a seller (https://www.zero-max.com/ce-rohlix-linear-actuators):
“The Roh’lix is a friction drive device and as a result, will show an accumulated error that is, in general, up to 0.002 inches per shaft revolution. Where greater accuracy is required, linear encoders can be used to indicate position.”
I’m not an engineer so I don’t really know what I’m talking about. Their linear encoder suggestion is interesting because it lets the machine provide position information back to the software. 3D printers and most CNC’s fly “blind” beyond simple homing switches.
Yup, that video is from 2014, I saw it before my initial build
He is going at a max of 400 mm/sec with no acceleration throttling. I was run 150mm/sec at most for travel, with acceleration limits. For most of my cuts, depending on depth, I was around 50mm/sec, and definitely not jerking it about.
Either way, it’s fun to think about.
-Allsb
If not going the linear encoder route, maybe you could add in re-homes during the cut to reset the accumulated error before it becomes an issue. You could calculate the approximate error ahead of time by analyzing the gcode and set a threshold for re-homing.
Ok, thanks guys, now I’m back to being excited about my machine
Getting the garaged cleaned right now. I’ll try to use the MPCNC to make the threadless ball screws (because my 3d Printer broken). Then I’ll cut my CNC down to a size that my linear rods can work with. I’ll check back in in a few weeks, hopefully
-Allsb