Hello everyone, it’s been about 1 year since I had to disassemble my MPCNC 525 because of a change in appartments. Since then all the parts have been lying in a box gathering dust.
This fall I’ll have some spare time and I thought about rebuilding it as a Primo, and with a smaller workspace. Around 320x220mm would result in a footprint that will just barely fit in the last open spot in my workshop.
Since I’ll use it for building combat robot parts it will mostly be used for HDPE, some aluminum and maybe eventually some steel.
Please give me all your advice on building this machine as rigid as possible. The updated Primo parts and the small size should already help a lot, and I’m also planning on using a CF PETG filament. Is there anything else I could/should do?
Also, do I have to get any new hardware or can I reuse all the parts from my 525?
Sounds like a fun project. The primo should be very rigid at that size. Nothing is perfect, so you’ll still need to learn CAM. But it should be a great tool for you. Keep the Z to a reasonable height.
There are some differences in hardware. I don’t know off the top of my head. But most of the fasteners changed to M5 and the M8s have different lengths. There was a conversion kit in the shop at some point. But that was a while ago.
Then I’ll have to sort out the hardware I already have and order the rest
If I remember correctly, the belts used to be 5mm wide, now they are 10mm?
I know filling the tubes with vibration dampening material comes up from time to time in this forum, has anyone tried PU construction foam? It’s supposed to dampen vibrations while also being lightweight.
Just make sure you have dimensionally accurate parts and you don’t gorilla tighten the trucks and core bearing mounts. I have found on my Primo, that the z axis lead screw can be a source of slop in movement if worn. If you fail to print proper infill or don’t do the 3-4 perimeters on the printed truck or core, you can crack them during use and those cracks will be a point of flex and subsequent loss of rigidity. Also be aware that extreme temperature swings where the primo is located may contribute to printed part failure as well. Keep the bearing tubes clear of debris, tighten your grub screws, space off your pulleys… all this wisdom is painfully discovered over the last year. I just reprinted and reinstall the trucks and core on my primo and it works well. I cut a lot of foam and the foam shavings can be charged and like to cling to the tubes. This is bad news over time. Hopefully with your aluminum and HDPE cuts, you won’t have much issue with that.
Just finished sorting out the leftover 525 hardware.
I have enough steel pipe and M8 locknuts and the motors, electronics etc. are all usable.
The bearings all have too much play in them, I’ll have to get new ones, and the bolts are also all different sizes than what is needed for the Primo.
Don’t sleep on the lowrider at a small size. I got to work with a local builder on his 1x2 LR3 amd it outperformed my similar-sized primo. The LR4 is also looking pretty sharp.
The LR is slightly more expensive because of the linear rails and aluminum plates (if you go that direction), and it’s slightly more complex to build because you have to partially assemble it to cut pieces for the final assembly, which requires a disassembly.
But it has much better workspace access, integrated dust collection, and is more rigid near the spoilboard, which is what you’ll want if you’re going to occasionally cut plate aluminum and steel.
Soooo after quite some time contemplating this build I think I have decided on a plan:
Work area of 300mm x 220mm x 81mm will result in a machine that just fits under my workbench for storage and can be set on top of the workbench when in use.
Question, does the torsion box need more ribs? It’s the first one I’m building and I have no idea what the best practices are…
The empty chambers will be filled with PU foam to help quiet down the machine.
Torsion boxes are surprisingly rigid. I build one 60" long using 3/4"x3" plywood and only 5 spars across that length and it was very rigid. The other direction was 30" and I only had the outside edges.
The thing with torsion boxes is that you can get an idea of how rigid they are by considering each cell as a separate box.
If you’re going for 500x500 then that as a box by itself with a decently thick top will already be massively more rigid than you need.
I have my MPCNC on a 1m x 1m piece of 25mm triboard and I can prop one end of it up easily without any deflection. I could stand in the middle of it and likely only see a couple of mm.
I would save yourself the hassle and try starting out on just a single sheet sitting on some other work surface. If you really wanted to go ham and keep the space for mounting things, a simple box will be more than enough. At the absolute most I would do a single divider in each dimension. At that point you’re likely at ‘using it to park a car on’ levels of strength and potentially making the flatness more difficult to achieve rather than better.
