Lowrider cnc accuracy

Hello,

 

I am thinking about building a Lowrider cnc but I have some concerns with accuracy. The first thing i will use it for is to engrave acrylic (no laser) and cut acrylic, will it have good enough accuracy to create a detailed engraving like this one: https://www.thingiverse.com/thing:3197486

I also would like to do my own PCB:s, I know the lowrider is not the perfect tool for making PCB:s but would it be possible?

The MPCNC is probably a better choice for these two applications but i see more use of the lowrider in the future, for instance my wife wants new kitchen doors.

Accuracy on CNC machines is a funny thing. The theoretical accuracy on the Low rider is very good (in the fractions of a mm), but it is dependent on how you use it. More specifically, how the toolpaths (CAM) are doing when cutting. If you want very high accuracy, you’ll want to do a finishing pass, where the machine goes through and cuts the pattern leaving a small amount of material, like 0.1mm and then does one final grazing with the bit to clean up the last 0.1mm and in that pass, there will be basically no load on the bit and it will be very very precise.

The other hard part is that the bigger the machine, the harder it is for it to stay rigid. If you double the length of an axis, you generally 1/4 the rigidity. Keeping your Z low and making it smaller than a full sheet size will help you a lot. Having a more rigid machine means the errors you can make in CAM can be a lot bigger and you’ll still achieve great results.

Engraving jobs like that are also dependent on having a consistent Z height. If you’re good at 3D printing, you can think of that as bed leveling. If you’re working on a small work piece, the size of a piece of paper, and engraving, the LR will do fine. If you want to carve a 4x8 sheet of plywood, you’ll probably have too much error in Z to do the whole thing in one go. If you’re through cutting a 4x8 sheet of plywood, you can just set the job to cut an extra 10mm deeper and it will just cut into the spoil board when it’s not at the right Z height.

All that said, I have carved plywood designs that were about 14" across and had good results, and my gantry is 60" wide. I have also cut out 24x36" or so pieces and they ended up better than I could do with a band saw and various sanding devices.

I will also say that I started with an MPCNC (the LR wasn’t a thing then) and then cannibalized it for parts for my LR. The MPCNC is very forgiving by comparison and it’s a great rookie CNC. The LR is OK and there’s a lot of tutorials and things on it. But it is more difficult than the MPCNC. Having a 24"x24" MPCNC for a year will teach you a lot, including how much you really want to have a full sheet CNC machine.

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Wow, thank you for a very detailed respons Jeffeb3.

hmm… maybe I should start with a MPCNC, the problem is that the acrylic i want to engrave is 440x440mm so i need quite a large working area of the MPCNC. The project I am working on is the Prusa IKEA Lack enclosure, I want to engrave some cool things on the acrylic walls (as if 5 meter of rgb strip that light up the printer like a christmas tree wasn’t enough)

I watched Thomas Sanladerer series of the MPCNC, that is what got me hooked on this project even though it didn’t turn out as he wanted it. He used stainless steel rods of 2mm thickness i think, those are very expensive and my budget is quite limited so i will have to go with 1mm thick tubes, will that suffice for a MPCNC with a working area of 500x500mm?

Ryan recommends 0.049" thickness SS tubing, and that’s 1.2mm, so you’re not far off.

I haven’t tried or even touched 1mm tubing, so I don’t actually know, but my guess is that it will be fine for 500mm.

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From the LR2 parts page: “I recommend minimum .065″ wall thickness”

.049" is the MPCNC SS tubing recommendation: “an upgrade would be .049″ wall thickness stainless steel tubing”

That isn’t to say that the thinner wall won’t work, it should be fine. The extra deflection caused by the thinner walled tube will add an additional 25% or so to the sag of the tubes so that needs to be taken into account.

EDIT: Didn’t realize the conversation had shifted to an MPCNC build. In that case .049 is correct.

In theory it will work. In practice, I doubt you will be satisfied. Rigidity is the main problem.

I hope you find a great solution for your situation.

Actually the standard size for an MPCNC is 24" square work size, and 440mm is only a bit more than 17", so you should be great with your 500x500 thought.

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For a laser, it’s fine. EMT tubing is more rigid than the 2040 extrusion commonly used for DIY co2 lasers.

FWIW, after doing some modifications like tensioning my belts and adding wheel tracks I was able to mill aluminum parts to better than 0.1 mm accuracy. That is all the accuracy I needed not sure where the limit is. Where I had more challenge is Z accuracy although with some work I think that can be solved too.

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Do you happen to have any pictures of your creation? I need some inspiration, i can’t decide if I should build the MPCNC or the lowrider.

Another question, should I use UNF or UNC bolts?

Doesn’t matter, I use coarse.

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