Curious if any experienced MPCNC builders can provide some insight on if the MPCNC design would work for a large wall mounted drawbot.
1 - I’m assuming there will be issues with belt stretch (possibly there’s a steel reinforced belt that can be used).
2 - Speed is a goal. There won’t be any intense lateral forces on the machine. So something that can get moving would be preferable.
3 - Concerned about sag in the middle of the due to the top and bottom rails covering a large distance (8’) without the aid of gravity in the center where the bit would usually be on a table based machine.
4 - Curious what the most rigid pipe type might be and if pipes have been reinforced with fiberglass rebar and resin.
5 - The design will likely need a pulley based counterweight to not put a strain on the motors when pulling the gantry upwards.
I would only be concerned about the Y motors for vertical positioning because they will have the complete weight of the gantry while the x motors will have none. As designed, the weight of the gantry is held by the rest of the structure, not the motors. If speed is what you want, you probably want to put that core on a diet and get a much smaller z motor. Might make more sense to build a very large zen xy on the wall. You might actually be further ahead to use lead screws for the Y axis much like 3d printers use them for the z axis on like a bedslinger.
I’m not an expert, just trying to add some perspective in the hope that some of it might be helpful.
steel belt has been shown to be bad for stretch with these small machines
mass and acceleration and therefore top speed are not friends. you want a light small core. A core XY is fast. I mentioned zenxy or maybe the MP3DP v5 xy motion would be useful if you could get a linear bearing set that large that won’t cost more than an ebike. Still concerned → about the size and weight.
3 & 4. sag. weight is the enemy here. larger pipe diameter is going to be the most helpful, not filling pipes with more weight.
pulley counterweight could be a good option to help. Not sure if you will need to reinforce any of the printed parts to hold not just the cnc, but also the weights. The weights will likely slow acceleration and may be a possible source of skipping.
Have you figured out how you will deliver reliable ink or paint on horizontal surface without skipping or dripping? That seems like a hard problem.
Steel reinforced belts do not work on the MPCNC design. The turns are too tight and the steel fibers break resulting in uneven movements.
If you have some give on your accuracy, then many things become easier. Using pulleys with more teeth and/or changing the micro-stepping for the stepper drivers will substantially increase the maximum speed of your machine. And the minimal belt stretch will also become less of an issue.
For larger MPCNC machines, a mid-span support is recommended for the fixed rails. If you added mid-span supports, sag for the outer rails becomes a non issue.
There are a number of topics on this forum about filling the tubing with various substances. My take from reading these topics is that the impact of filling was generally minimal, and filling has both positive and negative impacts.
There is bit of information on the forum about the stiffness of tubing. For the same diameter and wall thickness, the generally accepted idea is that DOM tubing is a bit stiffer than stainless steel tubing. Conduit is notably less stiff than stainless, but it has thinner walls. Again, for the fixed outer rails, I would just plan on midspan supports to eliminate the sad rather than expenive, thick walled tubing, or attempting to fill the tubing. You only have to worry about the single, moving horizontal tube.
I side with orob in that, if this was my project, I’d be redesigning the core and picking the moving tubing solution to weight as little as possible. I think going lightweight for the parts will do more to address sag than trying to stiffen the existing MPCNC design.
With any kind of counterweight, I’d have some concern about backlash. This may not be an issue for drawing, but I’d still fous on making things lightweight and only add a counterweight if the machine has trouble.
If this was my project, I’d be tempted to just build the thing with cheap conduit and minimal changes from the standard Primo design, and see what happens with the full expectation that there will need to be significant changes to make it work. You could have it prototyped in just few days, and I think you will learn far more than our guesses about what issues need to be addressed. And even if the design has major issues, you will be able to salvage most of the parts.
The fiber reinforced belts won’t have any noticeable sag.
The Z acis depends on gravity holding it down. You may have better results with an antibacklash nut. But that can add friction.
I would start much smaller than 8’. Make a 24"x24" prototype and you will learn so much you will know exactly where the trouble is for the full build. Honestly, for a plotter, I doubt the sag is going to be the issue.
I would almost certainly do this as a CoreXY system, understand though that you need basically to be able to go around the perimeter of the area at least twice for belt length. Assuming you do not actually need 84’ (feet) and actually 84" (inches) then 10m of belt ought to be enough, but I might look for a longer piece just in case. (I get a bit over 9.1m for double the perimeter.)
The ZenXY design with a servo or small motor for Z ought to do the trick, and be able to offer some decent speed to boot. The conduit rails are inexpensive enough.
This design certainly looks the most stable. The wall presents a different gravity and stability profile than a table.
I’ve got two gantries off of flatbed printers I used in the past. They are about 14’ long each, belt drives (very quick).
It seems I could just start with bolting these to the wall then build to them.
I vertical x axis won’t be prone to sag.
Obviously, gravity is one of your biggest obstacles. Think vertical gantry that rides on horizontal rails which are supported at several points along the span (not just the ends). Any 8’ length of pipe, regardless of construction material, with less than a 2" diameter that is only supported on the ends will sag under it’s own weight, let alone a force applied mid span by a gantry. You’ll need a different truck than the MPCNC uses, but it’s definitely doable. Probably not your solution, but as an example: Linear Rails with Trucks on Amazon
Second obstacle: wall surface. Since I have no idea what you are doing (pen, laser, inkjet, cutting blade, etc.) this may not be a big deal, but there is no wall in the history of walls that is actually flat, so if the “printing/cutting/drawing/lasing/milling/routing…” device distance from the “paper/vinyl/wood/metal/lucite…” is somewhat critical, this may be something you will need to address.
Can you provide more details regarding the end product?
No. I’ve used inkjets for decades in my work. They’re high maintenance in a studio environment. If you don’t use them everyday (nonstop is preferable) they act up. Even UV inkjet does not like to be left for more than a week before problems arise.
FLATNESS
I think I’ll be making a shallow vacuum table out of T-slot using composite panel (Dibond) as the front and back. Those two materials will be as stable as I can achieve.
Some kind of leveling (pulling and pushing) away from the wall will need to be installed at about six point within the platen. I think with a laser and some adjustment, I can tweak the table flat.
PRODUCT
I’m not exactly sure what I’ll be using it for. I’ll likely be grooving acrylic medium or oil paints with custom silicone brushes. A lot of it will be semi-manual, making adjustments along the way.
I might mill EXP too. That would change the build a lot. I’ve done some huge projects using my 4x8 cnc with EPS foam and there was a lot of engineering the seams and such. But that will slow the machine down quite a bit to handle the larger z axis rather than just a pen or brush holder.
The reason I might go that way is I also have a C-axis head off of a cnc mat cutter. I’m somewhat interested in the ability to twist the brush, even if just as a manual function during operation. That too would cause the X axis to need to be a bit sturdy to hold that thing. It’s about the weight of a router.
The vacuum table idea solves a lot of problems if noise isn’t an issue. I looked into that but my wall plotter is going to be in a public space, so it needed to be quiet.
So you need to plan on a tool (C-axis head) with some significant weight. With the right stepper(s) and belt size, that shouldn’t be a problem.
I don’t know what EXP is.
I guess the obvious question then, is why does this need to be on a wall? I assume it is a space issue?
Edit: Also, if you are talking about moving a tool around that has the weight of a router what kind of speed are you hoping to achieve?
Sorry. The “EXP” was a typo. The later mentioned EPS is what I meant (extruded polystyrene).
I need the wall to be able to reach the middle of the piece during production (to work on and touch up). I want it to be artist and his easel type set up. Also that view of the piece is better for judging its progress. Down lighting can show you how the textures and such are going to look during install.
The speed thing is tricky. The faster I can go, the less drying time is an issue. UV curable thixotropics gel (maybe), retarded acrylics, and oils all can extend dry times can work. But then I might have a production jam if pieces dry too slowly and can’t be easily removed from the machine.
Also with the C-axis, something tells me it can be engineered way lighter than the CNC mat cutter head that I have.
My gut tells me that I’m going to want some speed, like 1000 inches per minute range. I could live with 500 inches per minute.
I have to agree with Jason. 1000 in/min seems a bit fast considering your other specs. Although, I also agree with your thought … “with the C-axis, something tells me it can be engineered way lighter than the CNC mat cutter head that I have.”
If you’re carving EPS, that doesn’t require a heavy motor with a lot of torque. Just spitballin’ but 500 in/min seems doable with a lighter “router”. And that will in turn require a smaller Z-axis stepper. Also, it sounds like precision isn’t paramount, so the occasional overstep due to over speed isn’t going to ruin your day.
Sounds like you have a good handle on things. I’m not sure that the MPCNC design is suitable for your project though. Maybe a hybrid?
I’m no artist. Maybe an engineer’s brain wanting to be an artist, but I like where you’re going with this. Please keep us posted on your progress.
Not a typo. They came off of EFI Rastek H700s. Huge aluminum extrusions with two Hiwin rails each with a belt that disappears into the interior of the extrusion which pulled the printer head left and right. The printer head was massive. I hated that printer. It was a belt feed. Very finicky.
