Those numbers don’t seem quite right to me. I had just the smallest amount of bend in a couple of the 34" X axis pieces for mine. Like maybe 1/32" out of true. To straighten them out, I supported both ends and stepped on it in the middle (I am over 200lbs) and I had to use most of my weight to get it to deflect enough to straighten it. I just can’t see how a 5lb weight would deflect almost half an inch.
Well, that would be good news. How much deflection would you estimate? The model says you would be seeing over a 2" deflection with 100# applied to a 34" span, and over 4" with 200# applied.
I believe that calculator is based on the tube being fixed at one end and the force being applied to the other end without support. I ran a couple of simulations in fusion and a 48" one fixed side SS with 5lbs resulted in deflection of 0.351". (Somewhat in line with the results you reported). When fixed on both ends it was only 0.003". Also fixed at both ends with 250lbs in the middle calc’ed as 0.126" which is pretty much in line with my real world experience.
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Great! That’s a huge difference. Thank you for the correction.
The calculator didn’t provide any context: https://www.engineering.com/calculators/beams.htm#tube
If it’s not terribly inconvenient, would you mind posting the fusion file for that? I’d love to learn how to do that.
It would seem that for the size MPCNC that you’re building, the weight of the router is practically a rounding error as far as deflection is concerned.
Don’t forget to add in it is not a point load and it is supported by a minimum of two rails on the gantry.
The file is attached with the 250lb study. I think the force is set up as a point source, but I would think that would be worst case scenario. Lars Christensen on youtube has a good basic walkthrough of using simulation in fusion.
As far as the weight of the router (or rails or vacuum shoe, etc) it isn’t so much deflection as it is moving mass when the steppers need to accelerate or decelerate. The more mass they have to move, the slower you have to go to maintain accuracy. (Not sure if accuracy is the right word, but you get the idea.)
Tube-Bending-v5.zip (141 KB)
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