The extruder bumping (lightly tapping)  

RE: The extruder bumping (lightly tapping)

You're not alone, it's a classic by now. It's either the belt tension being off, the gantry not being square (identical belt Hz, but front Y-endstops not being touched at the exact same moment). 

But that's only when there's no heating! As soon as the nozzle is being heated, the heater can interfere with the load cell (the black, smooth frame where basically the whole print head is mounted to). 

When the controller board detects weird loadcell signals, homing and probing can become unstable. 

First thing to test: Display - > control - > auto home.

That does the banging (+ bed touch for Z) without the heater!

If that is not 99% of the time constantly short (2 side bangs, 2 front bangs, then 3x double-hit side-front), you need to tune the belts to exactly the same Hz.
Disable motors, move the print head manually to the front, while in the middle of the X-axis and check for a little gap on the left or right Y-axis end-stops.

Depending if/where the gap is, loosen the belts completely and bend the gantry with the "spanner trick", like in this video:
Beware: While doing this, one of the belts will become tensioned! If you didn't loose that belt enough, give it a bit more slack now, to not stretch it!

After that, the Gantry should be perfectly square and hit both stops at exactly the same time, when you're holding the print head in the middle and against the front.
Note: the slacked belt will push the end-stops away. Just slightly keep pulling the extruder towards you at the front and check for the gaps left and right.

If it's "perfect", tune the belts both to exactly 85 Hz. (Tightening one, makes the other one tighter too, but not as much.) 
Then grab the print head again, pull it against the front and check the gaps.

Repeat, until it's "perfect".
Then do the "auto home" without heat about 5x and check, if it's quick. I was so lucky to have my print head banging itself to infinity...

So I simply tightened the right tensioner screw a tiny bit and at some point, it became reliably quick.
Didn't need much tension. My belts are basically at 84.5 / 85.5 Hz now.

THEN, if the banging is worse, while starting a print, you need to look up how to edit the custom Start G-Code in expert mode and put some "M104 S0 ; disable hotend heating for clean loadcell signal" before homing and before nozzle cleaning.
And maybe copy the heating commands (M109) before the MBL part.

RE: The extruder bumping (lightly tapping)

"Thanks for the reply, Raaz.

I must have spent 6 hours on this today, plus all the time I’ve put into it here and there over the past few weeks. I managed to get zero spacing between the blocks, but I still can’t get the right tolerance for the belts!

With 0.00 spacing, I still have at best an 8 Hz difference between each belt! I felt like that was far from the 3 Hz target, so I opted to get the belts very close to 85 Hz, but now I have a spacing of 0.026" (0.66 mm) on the right side. I was thinking of adding shims on the right side — do you think that’s a good idea?

What shocks me the most is that it came like this! It was doing this on day one…

I’m open to any advice you can give me."**

RE: The extruder bumping (lightly tapping)

With 0.00 spacing, I still have at best an 8 Hz difference between each belt! I felt like that was far from the 3 Hz target, so I opted to get the belts very close to 85 Hz, but now I have a spacing of 0.026" (0.66 mm) on the right side. I was thinking of adding shims on the right side — do you think that’s a good idea?

I feel your pain... But definitely NO shims! The frame and the end-stops should be very accurate, so you can take them as a given, perfectly square element.

I also found that just because the gantry hits both end-stops perfectly in sync, with slack belts, it doesn't mean it will be by in sync without a gap, when both belts are at the same Hz. 

It's not just about the gaps being identical (none), when the belts are slack. It's about the gaps being 0, when both belts are at the same Hz. But syncing the gaps with slack belts is the starting point. 

What I would do: tune both belts to exactly 85 Hz, then disable motors, put the extruder in the center and pull it against the front. If there's a gap on the right: make the belts very loose, push the extruder a few cm back, put the little multi-wrench in the gap on the left and pull the right side towards the front (I grab the X-axis, where the angle-bracket is screwed onto the steel bar behind the linear rail).

One belt will become tensioned. Make that belt more slack, then pull the X-axis on the right again.

If you think the right side is closer to the front than before the bending: take out the multi-wrench from the left, tune both belts to exactly 85 Hz and check the front end-stop gaps again.

Repeat the loosening of the belts, bending and re-tuning, until the gantry is perfect with both belts at 85 Hz. 

Then try the Auto Home without heating and if it doesn't work, just press the X on the control panel to reset the printer. 

While it's rebooting, turn the right belt tensioner 1/4 turn clockwise.

Then Auto Home. 

If it still won't be quick, reboot and make the right belt tensioner another 1/4 turn tighter. 

Auto Home again. 

If it still doesn't run correctly, reboot and turn the right belt tensioner a full turn anti-clockwise. 

Basically, put both belts to 85 Hz, repeat the bending, until both front end-stops are hit at the same time, then play around with small adjustments for the right side tensioner, until it runs perfectly. 

And if that still doesn't work: maybe the nozzle has too much tension inside the extruder! I had this happening, after a nozzle change. Simply slightly loosen the two thumbscrews, until the hotend starts to slightly move. Give it a very gentle, little wiggle, put just your finger tip against the nozzle tip to very slightly hold it against the end-stop inside the extruder and tighten the thumbscrews again.

Don't make the thumbscrews too tight, they just need to hold the nozzle in place, not put pressure on it. 

RE: The extruder bumping (lightly tapping)

Hi,

Thanks for your reply, and sorry for the delay — I had a lot going on...

I still have the problem! The next step will be to try setting the endstops on the blocks, but with belts that aren’t necessarily exactly at 85 Hz. I’ll see if I can get good print quality and if the knocking issue is still there. I might also try putting the endstops on the blocks and getting the belts to 85 Hz (which feels like mission impossible). I’ve already spent several hours on this... sometimes it just comes down to luck! This problem is really wearing me down — very disappointed with this crap.

The idea of “twisting” the frame will be the last resort... I’ve already done calibration work (at Airbus) with a Leica 960 and have good experience in that field. Twisting something is a 50/50 chance: either it works or you break something... 🙂

Thanks again for your advice, it’s truly appreciated.

RE: The extruder bumping (lightly tapping)

You're not "bending the frame", as in bending some thick, integral parts. You're only bending the brackets from X-axis to Y-axis a few degrees. They aren't really stiff and are "happy" to slightly bend into position.

Just make sure to look out for the belt that's getting tensioned, while you're bending and make it more slack. 

RE: The extruder bumping (lightly tapping)

Over the past few days, I’ve had some time to work on improving the knocking issue with the Core One. I set the spacing between the carriage blocks and the belt tensioning blocks to zero, and it helped a lot. But I noticed that the knocking is more on the X axis rather than the Y axis… could it be a different problem? It knocks twice on the Y axis, but 6 to 8 times on the X axis currently when I run the automatic homing routine.

What do you think?"