August 25, 2019 at 9:08 pm #110811
There are longer lcd wires. Ryan sells them too. It would be a quick swap.August 25, 2019 at 10:47 pm #110815
Oh cool. Good idea!August 30, 2019 at 12:59 am #111429
I haven’t got quantitative measurements yet, but it’s looking like I still have work to do. To my eye, just pushing with my hand, it is not as stiff as I had hoped.
To my eye, it looks like the two worst modes of deflection are legs bending and belts stretching. The leg bending caught me by surprise because I was expecting that constraining the upper part of the leg would keep it vertical, and the conduit resistance to bending would be plenty to keep the side rails from moving horizontally. This is in contrast to the standard MPCNC where the resistance to racking would be mostly dependent on the strength of the corners and feet, not the bending of the metal.
The remedy for this would be giant gussets or extra wood panels that fit under the side rails. I will still measure the deflection and only attempt this remedy if it is truly a major contributor.
If I grab the side rail and push on one of the rollers with my thumb, I can see the belt stretch. I am wondering if perhaps I have crap belts. My other machine has some deflection in the belts but it was in the zip ties and I cannot perceive any change in length in the belt at all. I’m going to see if I have extra, or I’ll remove the belts if I need to, and measure the amount of stretch of the belts themselves. If I do have crap belts like I suspect, then I’ll buy some good name brand ones and that should solve that.
Meanwhile I need to upgrade my tools to get some real quantitative stiffness measurements. I think I can use the same weight hangars I used before, but I have a little 3d printed stand for my dial indicator, which is too light and too small to confidently hold the dial indicator where I need it. Straightforward enough to build a larger, heavier one.
Also, I’m a little bit puzzled that my motor drivers (A4988) are getting quite hot. I had to turn them down well below 0.7 V to keep them from going into thermal shutdown. All three are behaving the same, and I even swapped one out with a spare and it did the same thing. With current turned down, the holding torque seems low, yet the drivers are still so hot I can’t keep my finger on them for long. One problem at a time, I’m deferring this until I get the stiffness better under control. Maybe in the end it just takes heatsinks and a fan.
Attachments:August 30, 2019 at 4:58 am #111454
If I do have crap belts like I suspect, then I’ll buy some good name brand ones and that should solve that.
It would be interesting to test this on belts outside of the machine and zip ties. There have been lengthy discussions about this on the forums and it’s hard to tell what is belt, zip tie, or just worry that is the root cause.
Also, I’m a little bit puzzled that my motor drivers (A4988) are getting quite hot.
You have a 24V PSU and all 4 motors are wired in series? You could wire up another driver to the same socket. It definitely seems like it should be doing twice the work, so get twice as hot, but I’m not sure where the heat is coming from. I could see the double motor resistance or double the supply voltage being the culprit, but for opposite reasons :).August 30, 2019 at 11:32 pm #111496
These belts I have, at 48 inches long stretch by about 4mm when under 5.5 kg of tension. I am not sure if this is typical. I’ll buy some name-brand belts and see if they are any different.
My test setup loops around one bearing and basically applies axial load to one of the side rails, which should have insignificant deflection relative to the 4mm I am measuring. By looping back to the anchor point I don’t have to try to watch both ends of the belt for deflection.
For some reason it didn’t occur to me that belt length is one of the contributors to decreasing machine stiffness as the build gets larger. I guess this is why the LR2 has 10mm belts. It’s obvious when you think about it, but belts were not one of the things I was considering.
Attachments:August 31, 2019 at 4:26 am #111506
You can also try doubling them up. Don’t mesh the teeth or anything, just put another belt behind the first, teeth pointing the same way.
That is more than I would have expected. I would assume that is linear so even 12″ would have 1mm of stretch. Hmmm.August 31, 2019 at 8:26 am #111520
Yup, exactly why the LR went to 10mm.
4mm is a lot more than I expect though. Another point would be The quick and dirty we have for aluminum is 1.9kg of cutting force and at worst that is distributed to at least 2 belts. So maybe at what point does the belt show stretch? I believe we are only supposed to tension them to 3-5lbs.August 31, 2019 at 11:13 am #111534
Today I measured another belt, left over from my first machine, and the stretch was the same, to within what I could discern with my eye, so I would say 10% difference or less. Maybe they ultimately come from the same factory in China and there are a thousand resellers. Seems plausible but I don’t actually know. These are the two products I ordered
It seems unlikely that I was unlucky, getting a particularly bad set of belts twice. I also have these name brand belts on their way, which I will measure when I get them:
They are pricey but I trust E3D not to cheap out with an “equivalent”.
For the heck of it, I also measured some Dyneema fishing line I have, with 10lb max strength. It has very low stretch compared to other string of equivalent strength (similar to aramid they say), and my string measured roughly 10 mm stretch under 2kg of tension, at the same 48″ or 1.2m length. This puts it at about 7x more stretchy than the GT2 belt. Of course the ultimate strength and the construction is different, but hypothetically if I cast my own rubber containing 7 strands of 10lb Dyneema fishing line, I would end up with about the same overall stiffness. (I have no intuition about cable or belt stiffness and this is me trying to get a feel for it.)September 17, 2019 at 12:10 am #114421
Finally got to cutting. This is Ryan’s crown gcode.
I measured the stiffness at approximately 0.01 inch of deflection per kg, and the best part is that it is not hugely dependent on the length of stickout, maybe about 50% worse at maximum extension compared to movement of the gantry rails.September 17, 2019 at 5:47 am #114440
That thing is a beast.September 17, 2019 at 7:21 am #114444
wow just thinking of the 3d relief carving this thing would be capable of doing…it might run for a week or more but wowSeptember 17, 2019 at 2:16 pm #114481
Now iis the time to start testing to see just how much more you can do. Try more depth and more speed to see where it starts to have issues…September 17, 2019 at 5:11 pm #114500
That is just nuts. I think if you use fusion you can build in the spindle and related geometry to make sure you don’t have a collision at crazy depth?
I hope Dui finds this soon….I can’t wait to see if he wants to try it.September 17, 2019 at 7:01 pm #114512
I’ll definitely be pushing to see how much I can do. I’m thinking aluminum might be next. The stiffness numbers are nice and all but cutting is where it gets real. I’m trying to keep my expectations from getting too far ahead… exciting, but we’ll try aluminum first.September 17, 2019 at 10:45 pm #114520
Wasn’t sure what to expect, but dang, breezes through aluminum.
1/8″ single-flute endmill
0.5 mm depth of cut
480 mm/min feed rate
10 degree ramp for plunging.
We’ve all seen MPCNC cut aluminum before, but with 10 inches sticking out below the center assembly?
I am so thrilled, it’s now official (according to me): I don’t have to choose between height and stiffness!September 19, 2019 at 9:56 am #114721
Wow, not bad at all. 🙂 Next step is to try the same but with 3mm DOC instead 0.5mm. Just pretend the aluminum is actually wood. And stand back so the broken bit doesn’t catch you (if it breaks).September 19, 2019 at 10:53 am #114733
You might even get more stiffness than a single-decker MPCNC. It is more constrained. I just really want to do this, but I don’t want it a meter tall.September 19, 2019 at 10:58 am #114734
TimParticipantSeptember 19, 2019 at 2:55 pm #114788
In my mind it is still ridiculous. I won’t say crazy ideas are my best ideas, but they are my favorite.September 19, 2019 at 3:08 pm #114789
That is pretty nuts to see at 10″.September 19, 2019 at 3:34 pm #114792
I won’t say crazy ideas are my best ideas, but they are my favorite.
That’s awesome.September 19, 2019 at 6:39 pm #114816
Dui, ni shuo de duiParticipant
Great thread and congratulations on this awesome success!
This machine looks sick and the results are impressive.
I really need to get myself a Dewalt router and try milling with a long Z axis, just to see if its possible without this dual decker setup.September 20, 2019 at 2:14 am #114830
Steel, why not?
Tonight I failed multiple times, broke/burned some tools, wasted some material. Had some serious fun, apart from being hit with tiny HOT chips.
I figured the rectangular shallow facing cut is really boring, so I would try for a pattern that’s a bit more interesting.
Two of my attempts were not totally awful. Here’s one:
September 20, 2019 at 2:34 am #114833
- This reply was modified 4 weeks, 1 day ago by Jamie.
Dui, ni shuo de duiParticipant
Awesome result, I think the only issue you have is plunging.
Who is this mysterious Tom S. ?September 20, 2019 at 4:58 am #114839
Heh, you should ship that to me or Ryan. We can put it on the table at mrrf next year.
1 user thanked author for this post.September 20, 2019 at 11:26 pm #114930
I hope this isn’t rain on your parade, but, Z limits are not the issue with Ryan’s design. Z limits are determined by the length of bit you can buy. 10 inches of Z travel with a 3 inch bit? I’ve melted a lot of plastic collets, and the nut on the chuck is burnt black from burning into the wood. That’s where I’ve found the limit to be.
September 21, 2019 at 10:12 am #114940
- This reply was modified 4 weeks ago by Tom.
Don’t worry, youre not raining on my parade. I agree, a super-deep relief carving for example needs to worry about tool collisions etc. and even if I had a 7″ drill bit, I couldn’t drill a hole that deep because the tool would need to raise above the gantry to start the hole.
The motivation for tall Z is to fit the tool changer and a 4th axis without having to punch holes in the table. Or to mill relatively shallow cuts on the top of taller objects.September 21, 2019 at 6:41 pm #114983
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