Delta-Pi "Flexing" hub

I have my printer fully assembled but am seeing some odd behaviors I wanted to run by the other reprapers.
Firstly, my hub-tri in the center does not lay flat, pretty much ever. It is always tilted to one or another angle and will angle upward as it approaches any of the towers on the side of that tower. I am thinking this is somehow due to the clevis/arm assembly, but unsure exactly what I should be looking for.

I would print new parts and try for better dimensions/accuracy but as I posted in another thread, my Printrbot Simple is temporarily bricked

I did notice that when I detatched one end of one of the arms, and rotated the hinge, resistance increased dramatically as the arm turned toward the inside of the carriage. I also noticed that the ends of the arm joints seem to be flaring out in order to fit the washers inside the hinge.
Any tricks to get these to turn smoothly across their full range of motion?

Hi Beamsaber
Those joints do need to be free to move but also need to be free from play.
To achieve this can take a bit of fiddling with the clevis bolts and their nylock nuts.
Make sure you make up the arm assemblies and adjust them before you try attaching them to the carriage or the hub.
Cheers,
Mike P

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Thanks Mike. I did a lot of work on these today and found several over-tightened joints as well as one rod where the epoxy did not take. I managed to rework two of three assemblies and will be reassembling the final one tomorrow. Fingers crossed I may be able to run my first test print or at least a solid dry run.

OK, I have a new problem. After I have repaired the arms, the hub is a LOT more level. I am however having a problem still. The Z floats downward as I issue movement commands. So if I send a +50X, then -50X repeatedly, by about the third back and forth, the hotend has lowered 2mm. Is this most likely an issue with motor voltage?

Have you got fans and heat sinks on your stepper drives?
My first guess would be your stepper drives are overheating and dropping steps.
Try dropping the motor current if you can't add fans and heat sinks straight away.
When they overheat and shut down it only takes a fraction of a second for them to cool enough to start working again.
It looks just like slipping.

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[www.makershop.co.nz]
Makershop - Reprap parts, adding new stock almost daily.
+++++++++++++++++++++++++++++++++++++++++++++

I am using a Printrboard for the drivers, so they are integrated. I do not currently have a fan / heatsink attached to them. I suppose that would make sense.. it does seem to be happening fairly quickly after starting the printer though, would think it would take a bit longer for the steppers to overheat... hmmm

My multimeter has a thermister attachment, I could see what the heat is on the stepper drivers. Not exactly sure what the optimal range is though.

Edited 1 time(s). Last edit at 01/06/2014 07:37PM by beamsaber.

Just put your finger on the heatsink. If you can hold it there comfortably without burning it, it should be ok. I would think its a software calibration problem.
From: [reprap.org]

Quote
reprap wiki
If your print head is too high or low in the middle of the print surface, adjust DELTA_SMOOTH_ROD_OFFSET by half mm and try again.

EDIT: See Johann's blog post for the solution to the head moving up and down as you move it across the bed: [deltabot.tumblr.com]

Edited 1 time(s). Last edit at 01/11/2014 05:06PM by TheTechnicalNoob.

So, it isn't heat. The stepper driver chips are cool to the touch immediately after powering down, as are the stepper motors.
The problem is not that the hot-end moves up and down as it crosses the bed, that would clearly be firmware / settings causing problems. The problem I am seeing is that if I jog the hot end, say, 50mm on the Y axis four times, the fourth time it returns to the center of the bed, the hot end will have dropped a millimeter lower. It's like the motors can't counter the gravitational force, but they feel like they have plenty of torque if I attempt to move the carriages without disengaging the motors.

Still not sure what to check next. Could be the belts skipping, or maybe I misjudged the amount of torque I am getting and need to turn up the voltage.

I tried turning up the trim pots until the motors were very hot to the touch when running for a couple of minutes. Same problem, so not a low voltage issue.

I have a new hypothesis.
The motors I am using are VEXTA 2 phase 5V stepper motors (got them cheap).
I did my best to find the wiring for the steppers that best worked as I am stepping down from a 6 wire to a 4 wire configuration. I did not have a multimeter when I started this, so I may have paired off the two phases incorrectly. I may have hooked one pair to the common and ground and the other to the primary and ground. That would, I think, throw off the accuracy of the rotation by a good bit. I am going to try testing the pairs I have on each motor to make sure that both pairs are throwing back the same resistance. Wish me luck!

Who is the manufacturer of your printrboard? There's a dodgy version that uses a modified design and only 2 PCB layers.
The proper design is a 4 layer board.
To drop the extra 2 layers they have replaced the power plane with a single thin track that winds its away around the board and supplies power to everything.