I would like to know if some of my choices are valid to build an mpcnc (725x725x72mm, 150mm threaded rod)
What I’m still not 100% sure about:
-feet and side rails 25mm with a thick wall thickness (eg. 3mm) and cross+z rails 25/1.5mm (because steel tubes are mentioned to the emt condution as an upgrade and advised either a 0.049" or a 0.065" wall thickness, which are around 1.2 and 1.6mm-s). I guess the thicker sides would make the whole thing sturdier, but the cross and the head weight I should look out for because of inertia (thus 1.5mm, which is in the recommended range)
-Precision, welded, cold formed, calibrated, carbon steel tubes (I hope the translation makes sense)
Though I’m pretty sure about some of the things I would buy/do, but would like to hear your inputs:
-24v 6a psu (I want to go 24v for more torque)
-skr v1.3 board (integrated (so backup) sd card reader, more versatile, which makes it easier to repurpose or sell)
-lcd12864 (reprapdiscount) display with sd card reader and rotary encoder
-a4988 drivers (I’ve heard that drv8825s are more likely to skip steps, and trinamic would be an overkill)
-I wanted to use petg for the prints, but on facebook, 2 people already said they done it and it was a bad idea, because it flexes. So I will stick to some basic white pla (I don’t want too much dye interfering with strength)
It depends what you are cutting, whether speed is more important than stiffness, relatively speaking.
The side rails are loaded only vertically. They can sag, but even large spans can be supported to mitigate the sagging from weight. The gantry rails experience sideways loads and more complex vertical loads from torsion on the z axis. For this reason I would put the stiffer rails on the gantry.
24v won’t get you more torque. Torque is determined by the motor current which is limited by the motor and driver capability. 24v will theoretically give you higher speed but practically you are unlikely to see the speed difference. There is no downside to higher voltage if your boards can handle it, so your plan is fine.
I wouldnt expect a problem with petg if that’s what youre accustomed to, nor would I expect the dyes to make any difference in pla strength. White vs. natural might affect your part quality. I’ve experienced some weirdness in the past with white behaving different from other colors (although it was ABS). I assume they have to add junk other than simple dyes to achieve the white color.
That’s pretty big. Have you looked at the low rider?
Your talking about doubling the wall thickness and so doubling the weight of your rails, and using softer carbon steel rather than stainless. I’ll be curious to learn how stiff it will be and how well it works out.
@Jamie:
Tubes: Those tensions would transfer to the sides, so, it’s not just the sagging I would avoid with it, but also cross would run on stiffer rails.
Stiffness is more important for me, but as I said, I would stay within the recommended thickness range for the cross and the Z, because I don’t want to have issues because of intertia, I want the motors to be able to move it
24v: https://youtu.be/GVs2d-TOims?t=1290 (21:30), at lower speeds, he got higher torque. Even if this is a trinamic driver, I read a few places that it pushes the max rpm, and - not by much - raises torque
Filament: Since 2 people already sad that they used petg and it was a bad idea, I will go with pla. I had the idea to use it, because it’s a stronger material, but not nearly as stiff, and I want to avoid this flexing.
@kd2018:
Size: how is 725x725mm considered large and low-rider territory? I often see people having 1x1m, and even the kit already gives you 610x610mm.
Tubes: Doubling only on the sides and feet, because those won’t move. About it being carbon steel: I have no knowledge how it compares to a stainless steel. I don’t know how it compares to the EMT conduit that’s often used. This my main concern what to use for it (Hungary, EU). I wouldn’t want to give an arm and a leg for them of course, but this was something, that is steel, has the size in the specification, and it is calibrated, so the OD is quite accurate and consistent.
Of which there are twice as many tubes for support.
Torque never has been an issue. We have two steppers per XY axis. Max speed also does not matter.
Yes.
A lot goes into material properties other than the base material. How it was made also matters.
I understand you are trying to make the best possible machine, but I/we really have put a lot of effort an brain power into the recommend specs over the years. If it was as easy as getting thicker tubes I would recommend thicker tubes.
I have not watched that video but I can say trinamics can not handle as much power as the DRV…another reason I recommend those if you are using something other than an Ultimachine board (with drivers properly mounted for heat dissipation).
I apologize, I was thinking in terms of cutting area when you said 725mm x 725mm. You’re only proposing an extra 4 inches or so in overall length so I wouldn’t think deflection would be too much worse.
The lateral forces from cutting are not supported by the side rails, they are teansferred through the gantry rails to the belts, then to the corners. The center assembly cannot push axially (tension or compression) on the gantry rails because it rolls. The lateral cutting forces make a lateral bending force on the gantry rails that transfers to the belts. This is the reason I would stiffen the gantry rails first but you are right that it carries a penalty to inertia and weight. But perhaps if you are more interested in stiffness then you are less interested in speed because you’re not cutting foam.
First, thanks for all the help guys! Didn’t think I will get so much.
I was talking about footprint But guess I will go lower. Footprint is almost 1x1m for that work area, and wanted to make 60x60cm things, and I thought it would work and that I’m still safe with that. Is it so far fetched to reach that size with the mpcnc?
I wasn’t going to use trinamics, it was just an example, that higher voltage can result in higher torque (at least it doesn’t hurt, so why not). The A4988s are already ordered, will I be fine with them? Or DRVs are strongly recommended?
“Cold formed seamy / welded calibrated precision tube” - this is all that I know of the material, and they said it’s carbon steel. It costs 2.69 usd/m for 25/1.5mm. Should I go with it or try to find something else? Or what properties should I ask? They said this has an accurate and consistent OD (calibrated).
So it turns out what I was looking at is E220, so non-alloy steel, and I’ve compared it to 304 and 316L stainless steel, and although the stainless has a 1.7-3.6x hardness, bending stiffness is essentially the same. This E220 tube that I can buy is accurate and cheap. Should I worry about hardness, or is it fine that the rollers might make some grooves? I was more worried about stiffness TBH.
I wouldn’t worry about the hardness. Ryan’s build at mrrf had some horrible flats worn in and it was doing perfect repeat drawings between homing. No one should ever tighten the bearings that tight.
I was trying to figure out if the emt conduit is made out of e220 non-alloy steel as well, but all I could figure out that it’s steel and zinc plated to avoid corrosion (which I guess is there because it’s not stainless steel)
Hey there, did you progress on your build?
I live in Hungary and found stainless steel 25mm with 1.5mm wall for 1980 Huf + áfa per méter. I can give you the contact info if you are interested.
Whereabouts are you in Hungary?
