Quantifying Frame Flex

C Peter Lotz wrote:

With regards to raising the anchors to eliminate Z axis force- if the frame can be flexed by the belts’ tension, then any arrangement where the anchors are attached to the frame will transfer the same net force to the frame- and the greater the anchor offset, the greater the moment arm- more leverage, more force on that point on the frame.

see Possible super-simple frame - #39 by bar for a
picture of what I’m talking about, that arrangement avoids the lifting force on
the frame.

David Lang

Except it doesn’t.

OK, let me qualify that dogmatic statement. When we’re talking about flex in the frame, we’re talking about torsion (there’s that word again). The belts of the M4 are non-planar- with the work surface and one another- and no rearrangement of the anchors can change that- the forces will still be applied.

At the beginning of the noted thread (which I had read at the time but moved on as I didn’t intend to working on the floor) you propose attaching a minimal frame to a concrete floor. The rigidity of the concrete would have assisted the frame in resisting- but not avoiding- the forces. But given that the wood and plastic frame is far less rigid than the concrete, and attaching the anchors to the frame transfers the forces through the frame- the overall system is far less rigid- and thus more susceptible to deformation- than attaching the anchors directly to the concrete.

The thread then moves into avoiding the attachment to the floor entirely- which eliminates the rigidity afforded by the concrete.

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if the belts are level with the workpiece, they are not pulling in the Z direction on the anchor. If the anchors are braced at the level of the anchors rather than below the anchor, they are not twisting.

the minimal frame I’m talking about would not need to be anchored to the floor, the forces are transmitted through the 2x4s at the level of the anchors/arms

yes, the different anchors are pulling at slightly different heights on the frame, but that is a minor difference compared to the height of the arm to the back of the spoilboard

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I loaded 0.74.1 and I think it indicates my frame is flexing, so I will stiffen it up and rerun. Results were:
Maslow-serial (2).log (3.4 KB)
Thanks for all your continued work on this project.

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That looks a little flexy but not too bad. I would be curious to see the results after stiffening it up

Hi Bar,
I am using CPL’s hourglass frame design and was having some issues with it flexing. I have skinned the top and bottom with 3 mm ply which has made it much better and have finally got an acceptable config values. The following is the calibration comparisons after the changes. It might be of value to compare deviations at other points, maybe the corners?
[MSG:INFO: Proceeding to take a measurement with calibrationCurrentThreshold set to 1500]
[MSG:INFO: Measured waypoint 0]
[MSG:INFO: Center point deviation: TL: 0.107 TR: 0.160 BL: -4.599 BR: -5.240]
[MSG:INFO: Center point deviation: TL: 0.107 TR: 0.160 BL: -4.599 BR: -5.240]
[MSG:INFO: Proceeding to take a second measurement with calibrationCurrentThreshold set to 2000]
[MSG:INFO: Measured waypoint 1]
[MSG:INFO: Change in TL: 0.019 TR: 0.016 BL: -0.703 BR: -0.604]
[MSG:INFO: Total change: 0.336]

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0.336 looks great! That’s about what I get or maybe even slightly better

after you release .76 today, can you please release a .76.1 that has the frame
flex testing in it (assuming you don’t have it in .76)

David Lang


hi @bar

I just had this experience Found some frame problems - Troubleshooting - Maslow CNC Forums

Is the frame flex test performed as the first step of calibration?

I think this would be a really good idea. That way if frame flex values are off the charts the calibration can be pre-emptively stopped and the user advised of the issues.

Probably yes. But as a part of the whole calibration process. So someone has just remade their frame so that it flexes less (they hope), now they can test that specifically on it own.

I don’t think so, but…
This is an issue that needs to be surfaced as part of calibration. So it’s not really a ‘take slack’ problem
This is an issue that needs checking for every now and then. So maybe it is done as part of ‘take slack’ on occasion

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I agree it seems like that experiment showed that it is a valuable feature and we should integrate it

I’ve eliminated most of my anchor point flex as per 3D Print: New Reinforced Corner Mounts - Hardware - Maslow CNC Forums

And calibration now passes.

Frame/Anchor Point Flex I believe is the single biggest cause of failing calibration.