Then you need to get all four bearings very flat at the same time or they will be riding on a point load as well. You still need steel though. I might use a small box for the Zenxy, but not for the LR at any time soon.
I have been doing testing on the old Burly and New gantry on my red and black build. I measure no noticeable deflection at the corner tops, literally none. I guess the looks of them throw people off? People are very determined to redesign the corners but I have never found them to be an issue in any way, even from version 1. I would rather spend my time on the parts that do move.
I measured 0.005mm with 1 kg of load on the end mill, so practically none indeed. The corner pieces just don’t look that beefy, but I agree they are very effective.
The corners have another tube connecting them though. You only get a racking movement (which we agree does not really exist, at least for short legs).
However, if you put an additional straight leg under the center of a frame tube to halve the effective size of the MPCNC, I think it will be less effective at fighting the tube flexing in the XY plane, because there is no tube in the XY plane to help out. I have no idea whether just a straight leg is already good enough, it may very well be.
Wait… I don’t think extra legs will work… it’s the gantry tubes that you need to stop from flexing, not the frame tubes. Extra legs don’t help with that. Though extra clamps for the belts will help with reducing belt stretching, but you can just mount that on the frame tubes, no need for legs.
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I have legs that are 27mm of metal showing between foot and lock. With a 4.2kg load on the end mill I show less than, 0.02mm (0.001") at the corner. My dial indicator just shows a hint of a wiggle so I am assuming less a third of that.
With that same load the new center gets 0.2mm (0.01") deflection at the endmill, the burly center takes 2.6kg to move the same amount.
Yes the side rails are under no load axially, and no load side-to-side. The only load is vertical. Additional vertical supports can help reduce sagging in Z and this can make a difference for very large machines.
An extra belt clamp for a reduced workspace is interesting. The belt contribution to deflection could be reduced. Hold the belt and attach to the table, or hold the belt and attach to the side rail (now the rail does have axial load), or hold the belt and attach to the belt elsewhere, like clamping a 2mm rack on the entire unused length.
But more important than all of that is to measure the deflection because if the gantry rails deflect with the cube of the length and the belts deflect linearly with length, you have a good chance of getting no practical improvement even with perfect belts. It is still a fun academic exercise but not a realistic improvement.
True. But a 1 meter 304 stainless steel tube deflects around 0.040mm per kg, while the belts stretch 0.160mm at that 1kg load (assuming both belts take equal load). This is assuming you have a 1x1 meter build, and that you have minimal stick out. At larger stick out, the belt stretch becomes less. But the tubes will also deflect less.
At different sizes there will be different ratios, but 1x1 meter is already a pretty large MPCNC. For a lowrider with a very long Y axis, the difference can be very large with extra clamps.
All that said, I haven’t tried it so it might indeed be academic 
Ah ok, fair enough. I still stand by my statement that measurement is most important, exactly because of stuff like this! 
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I’ve tested belt clamps and they work extremely well.
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Quick and dirty round 2
Slightly less dirty this time. I just checked SS and 1" EMT…interesting.
4.345kg (9.58lbs)
71" span, simply supported
72", SS 25.55mm OD (the wrong size) , 1.55mm wall thickness (0.060"), 1.65kg = 5mm sag
72", 1" EMT OD (29.6mm) , 1.42mm wall thickness (0.056"), 1.76kg = 3mm sag
So that is not bad, so much cheaper, more rigid and not a huge weight difference. It is still much more rough, but to save that money a little sanding is not a huge ask if needed, but at my local store they all seem to be pretty non-straight and this piece is not too bad at .2mm oval, stainless is about 0.1mm oval. If any of you remember take a look at 1" EMT next time and see how it looks.
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What about the “rigid” 3/4" emt? I would guess it is more consistent, since it has outside threads on one end. The OD is 1.05". Is it just too heavy?
Good idea, I should go get some.
Quick check, this site, https://steeltubeinstitute.org/steel-conduit/types-of-steel-conduit/electrical-metallic-tubing-emt/
Lists 3/4" RMC 26.7mmOD, as 8% heavier then 1" EMT, Very interested to see how it handles the super scientific test, might need to grab a fresh stick of 3/4" emt just to test it on this setup as well.
Having a hard time finding any other international standards page like that though.
I had a look at this stuff last week. The surface finish was horrible, which i feel makes it pretty much useless for what we’re doing.
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Darn, this thread got a lot of love just after I ran the numbers to decide what tubing to buy for my lowrider 
Too bad I didn’t post my calculations a few weeks back. Being a true believer in a scientific method, I wanted to run an experiment. I just got the test tubes yesterday …
So here we go, QD #3
Length: 72" - span ~71-71.5" : specimens were supported as close to the end as possible without slipping off the supports however exact contact points are hard to determine.
Specimens:
- 3/4 EMT - 5.5mm
- 1" 0.049" 304 stainless - 4.4mm
- 1" 0.065" 304 stainless - 3.5mm
- 1" 1/16" 1010 low carbon steel - 3.2mm
Load: 4635g or 10.2lbs
Note that these are deflections due to load. I have no good way to measure deflection due to gravity.
This lines up pretty well (within 10%) with the results I got from a google spreadsheet I created. Many calculators on the internet are using incorrect formulas. I guess they can put stuff on the internet that is not true …
I would call that claim BUSTED To be perfectly honest, I don’t recall Ryan saying that stainless is stiffer, he said that 1" stainless tubing is stiffer than 3/4 EMT and that is indeed the case.
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All steels have a young modulus in the range of 190-210 GPa. Fusion 360 simulation defaults are 193 for stainless and 210 for steel. Looking around the internet, 316 stainless is claimed to be around 193 - 200 GPa, 1010 mild steel 190-210 GPa so stainless can be slightly softer, but doesn’t have to be. Conduit manufacturers don’t really specify what alloy they use besides calling it “mild” which is supposed to have 0.05 - 0.25% carbon content. 1010 is comfortably in the middle of that range having carbon content of 0.08 - 0.13%. It’s also an alloy that I’ve seen manufacturers disclose as a material for round tubing e.g. this one. There is research suggesting that modulus of elasticity is about the same for high and low carbon steels and nickel steels (aka stainless) should have a slightly lower modulus of elasticity. To cross-check from another place, AISI 4340 heat treated steel has been tested to 196 GPa. The most reasonable explanation I’ve found so far comes from this heated discussion:
The modulus depends on the chemical bond and the crystalline structure of a material. In steel it is confined between 190 and 210 GPa while the strength can vary between sau 350 anf 2000 MPa. Thus, the ultimate properties are strongly affected by thermal histories while the elastic properties remain almost constant (or slightly vary)
Thus the actual grade of steel seems to matter very little given the elastic properties are not affected much by treatment and carbon content.
Interesting. Theory suggests about 3.5mm for this one even with young’s modulus at the lower end of the spectrum. Weights are spot on though.
+1 to that
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To toot my own horn a bit, here are some graphs from that google spreadsheet
Feel free to copy ( File -> Make a Copy ) and play around with it to your heart’s content.
Base parameters are 72" long, 1" OD, 0.065" wall, 200 GPa modulus.
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Next time, break up awesome results like this into ten posts, so I can give you 10 
s
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Guessing the oval shape has something to do with it? Not sure, real world can be a beast sometimes.
I just bought all fresh material to test everything from 1/2EMT and including 3/4" IMT. But your numbers work for me. I had some ideas for new rails but after spending all day I would rather continue to make this work worldwide with Conduit. Just need to find a solid size reference for other countries edit-Aus. So I can finalize some numbers with the new build. I want to make as many pieces overlap as possible
Side note windows screen frame is surprisingly rigid for it’s weight and price. Might have to make a Zen out of it.
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