Z axis is binding

[lasershark1]

My Z axis is binding - most noticeable when I move a large distance (up 3 inches to change a pen). I put a dab of paint on the nema17 shaft so I could be sure that the shaft wasnt slipping in the coupler - nope - the shaft stopped turning with a horrible grinding noise. It happens even doing the Shapeoko 'hello world' - pen comes up, moves, goes down, makes the S, comes up, goes down - and doesnt go far enough to touch the paper to draw the H.

My allthread is not in straight - and damned if I havent re-seated it 10 times trying to get it straight - every time I tighten it down, it winds up skewed so it whips about when the Z motor turns (obviously before it's in the delrin nut). My guess is the whipping plus speed = binding in the delrin nut?

I'm wondering if slowing it down would help? Whats the best way to do that? Change the stepping? I'm at 2x stepping now - would changing stepping help? Or is there a better way to slow it down?

[lasershark1]

Nope. Set it to 8x stepping, upped $2 to 1280, and it still made a terrible noise and stalled.

[potatotron]

Try changing to full stepping (for maximum torque) with $2 set to 320, and running slower. That did it for me.

I've been looking at this and I think the problem may be the coupler, skewing the center line of the threaded rod. I'm not convinced the delrin nut is entirely to blame.

[lasershark1]

But wont full steps, and $2=320 throw all depth adjustments off? $2 is a direct correlation between pulses & the thread pitch of the rod. At 1/2 steps on a 1.8 degree motor, it will take 400 pulses to rotate the shaft 1 time.

400 / 1.25 (how many MM in 1 turn) is 320 pulses for 1mm movement.

If I go full steps and 320 pulses, I wont move 1mm, I'll move 2. Cant do that.

As for the other post and thinking the mount is throwing the shaft off center, it absolutely is, you can see that just by looking at it. That being said, even without the mount, I cant get the shaft into the coupler perfectly straight - it is off a good few drgrees and whips like crazy.

Tonight I'm going to try removing the bearing and let it run without it. The bearing is whats pushing the rod off center - no bearing should mean no being thrown off.

And I'm going to try laser cutting a new mount tonight as well with the spacing fixed. Gonna have to make it out of wood though - acrylic is too brittle and I dont have any hdpe like plastic to try.

[edwardrford]

It's easier if you refer to them as steps, rather than pulses. It's also important to remember that the number of steps per revolution is exactly what microstepping is manipulating. If we take away microstepping (2x, 4x, 8x, 16x), then we're only dealing with the motors basic specs. 200 steps per revolution.

We're interested in the number of steps required to rotate your threaded 1 full rotation.

For a 1.8 degree motor without microstepping that number would be 200 steps. So, 200 full steps turn your motor's shaft 1 full revolution. OK.

Now, the pitch on the threaded rod is 1.25mm, which means that if we rotate 1 full revolution (200 steps), we will have moved the z-axis 1.25mm. We want to find out the number to only move 1mm. Not 1.25mm.

so, 1/1.25 = .8

.8 * 200 steps = 160 steps = 1mm.

Code: Select all

$2 = 160

-Edward

[lasershark1]

Right - but he suggested $2 = 320 at full stepping - I guess that'd work with a .9 degree motor, but, mine's not

Still gonna try the other methods tonight and see if I can get it to work. As it stands now, my Z axis is completely unusable.

[potatotron]

Sorry, my mistake. I did use 320 for a 400 step/revolution motor. You do indeed want 160 for a 200 step/rev one.

[edwardrford]

If it were me, I would remove microstepping, so the motor has full torque and change the $2 = 160. This will give you a baseline to work with.

Once that's done, through manual gcode entry do some tests.

Z10 (moves 10mm up)
z0 (moves back to 0)
z-10 (moves down 10mm)

When you run those commands, they will run at your default speed settings. That might be too fast.

If it stalls running at the defaults, add some feed parameters before issuing the commands.

(re-zero your z-axis first)
F300 Z10 (moves up 10mm at 300mm/min)
F300 Z0 (moves Z to 0 at 300mm/min)
F300 Z-10 (moves Z down 10mm at 300mm/min)

Increasing and decreasing the F command will increase/decrease the speed at which your z-axis is moving.

Let us know the results.

-Edward

[lasershark1]

Ok so heres the steps I took tonight...

First, I made a new mount for the Z-axis motor. Mine was not lined up right - when the allthread was in the bearing, you could see the coupler bending - the hole for the bearing was not centered right. So I made a new one (laser cut wood) and that solved THAT problem (coupler was straight once all mounted).

But I'm still binding. So I tried speed tests... I didnt mess with the F command I just set the $5 (default seek rate) down to 200 from 1100 - and it didnt bind, but the motor stuttered. You can see it in this video:

http://www.youtube.com/watch?v=WpkNmMMUi6U

What the video shows is a stutter going up, then a stutter going down (at lower speeds the motor does this even when not in the mount - bad motor?).

The third movement in the video is after I set the speed back to $5=1100 so you can see the binding. It accelerates and hits top speed and binds.

When turning the allthread by hand, you can feel it get hard to turn as it reaches a point in it's off-center-ness (if spinning free of the delrin nut it whips - and I cant center it to save my life - may try buying a new rod).

Any thoughts on both the stuttering, and the binding?

(Edit: And yes, the mount plate is NOT on tight - thats why it rocks. I thought loosening it might help the binding as the motor had room to wiggle. Nope. Still bunds. But you can see by just how much it's wiggling just how badly my rod is off center, whipping, causing the binding).

[moino]

I had a similar problem, could hardly turn the z-axis at all, the flexible coupler was really bent. Then I disassembled the z-axis, unscrewed the threaded rod from the delrin nut (some debris came out, the is was better).
Then I noticed that the threaded rod did not go into the flexible coupler deep enough - one of the set screws hit the tapered end of the rod, making the coupler bend. When assembling the z-axis again, I also found that the motor mounting plate was in a slighlty different position than before. Now I can turn the axis by hand, however the resistance still varies with the position. Today I will test it with the motor.

Could it help to to have another bearing at the lower end of the threaded rod, to keep it better in place and have everything straight? I imagine that would also slow down the wear-out of the delrin nut (if this has ever been a problem at all).