Help fixing sheared heat bed wires
 
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rob.s5
(@rob-s5)
New Member
Help fixing sheared heat bed wires

Hi all,

I've had my MK2 up and working for over a year now with moderate use - yay.

But recently, I started to get intermittent "MINTEMP" errors. After some investigation, the plastic cover that fixes the GRN and VCC leads to the heat bed was loose ( http://manual.prusa3d.com/Guide/7.+PSU+%26+Heatbed+assembly/297?lang=en#s5052 ). No problem - let's tighten it back up...

... but then the leads immediately sheared off (see picture in attachement) 🙁

Any tips for reattaching these? I have solder so I can try that, but there's probably a fair bit of power/heat that goes through these, so that may be a bad idea.

Also, any ideas about the cause of the shearing?

Any help here would be appreciated!

Thanks in advance.

...
Posted : 25/02/2018 8:34 pm
alexander.s27
(@alexander-s27)
Estimable Member
Re: Help fixing sheared heat bed wires

The cables are soldered to the bed, aren't they?
Why not just remove the old pieces and solder the wires to the bed again?

Posted : 26/02/2018 12:27 am
rob.s5
(@rob-s5)
New Member
Topic starter answered:
Re: Help fixing sheared heat bed wires

I guess I was looking for someone to confirm this is a reasonable repair. I'm probably overthinking it, but I was concerned that the resulting wires might be too short, that a special solder was used, or just that if I fixed it without knowing why it broke in the first place, then it will happen again...

Anyone willing to share any experience with this problem would be much appreciated!

...
Posted : 26/02/2018 3:41 am
alexander.s27
(@alexander-s27)
Estimable Member
Re: Help fixing sheared heat bed wires


Anyone willing to share any experience with this problem would be much appreciated!

That I already did.
There are tons of threads about broken cables and how to repair out there, anyway.
That cables are long enough to afford the loss of some 7-10 mm strand.
And even if it were to short, this being not exactly rocket science every cable with the same thickness would be suitable.
Same goes with the soldering. If it is soldered initially, soldering as repair would not be exactly wrong.

Having had some problems with broken cables myself, I decided to get rid of those spiral plastic sheaths and
use some small zip-ties instead. One of those every 4-5 cm evenly allocated along the cables looks as neat as those spirals.
With those the harness is less rigid and the bending evenly spreads over the whole lengths instead concentrating at at the bed...

Posted : 26/02/2018 7:27 am
richard.l
(@richard-l)
Moderator
Re: Help fixing sheared heat bed wires


And even if it were to short, this being not exactly rocket science every cable with the same thickness would be suitable.

And this is how fires are started. Not every 16awg wire is rated the same.

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Posted : 26/02/2018 3:56 pm
BunchaHooey
(@bunchahooey)
Eminent Member
Re: Help fixing sheared heat bed wires

Don't cut off more wire than you have to. If it's too short you'll need a length of wire with the same or better rating (when in doubt, get a wire with a higher rating than you need).

Also make sure your spiral wrap is pretty loose. I've had my thermistor get pulled off more than once, and realized by watching the printer that it was the spiral wrap that was causing the tugging.

Be sure to check out the newest instructions on cable management for the heated bed wiring. If I recall correctly, there are some pretty small changes that seem to help mitigate this problem. Also, make sure the zip ties that go nearest to the bed are tight.

Good luck!

...
Posted : 26/02/2018 5:36 pm
BunchaHooey
(@bunchahooey)
Eminent Member
Re: Help fixing sheared heat bed wires


Having had some problems with broken cables myself, I decided to get rid of those spiral plastic sheaths and
use some small zip-ties instead.

I too have noticed that the spiral wrap seems to cause this problem, especially if it's wrapped too tightly. The instructions aren't specific on how tight to wrap them.

I too have tried the zip tie method, but went back to loose spiral wrap after having the zip ties snag on my filaments (mmu has lots of stray filaments dangling in the back). I'm thinking of using some shrink tubing or some of that flexible mesh. Do you see any downside to either of these?

...
Posted : 26/02/2018 5:40 pm
alexander.s27
(@alexander-s27)
Estimable Member
Re: Help fixing sheared heat bed wires



Having had some problems with broken cables myself, I decided to get rid of those spiral plastic sheaths and
use some small zip-ties instead.

I too have noticed that the spiral wrap seems to cause this problem, especially if it's wrapped too tightly. The instructions aren't specific on how tight to wrap them.

I too have tried the zip tie method, but went back to loose spiral wrap after having the zip ties snag on my filaments (mmu has lots of stray filaments dangling in the back). I'm thinking of using some shrink tubing or some of that flexible mesh. Do you see any downside to either of these?

You are right. That I forgot to mention, not to pull them to tight. I have them not exactly loose
but just so that they are holding the harness together without pinching the cables.
Those spiral wraps are not bad as such but the used one, although right in dimension, kind of rigid for that purpose.
Shrink tube could be a solution but somewhat awkward with slipping on and in kind of repairs. (which sooner or later will be needed)
Flexible mesh would be nice with the same downside to slip them on.
Once getting rid of the spiral I have been happy ever since...

One might check on the clip in the attachment which I made for my heatbed.
Apparently I forgot to upload it to Thingiverse for some reasons.

Posted : 26/02/2018 6:03 pm
BunchaHooey
(@bunchahooey)
Eminent Member
Re: Help fixing sheared heat bed wires



You are right. That I forgot to mention, not to pull them to tight. I have them not exactly loose
but just so that they are holding the harness together without pinching the cables.
Those spiral wraps are not bad as such but the used one, although right in dimension, kind of rigid for that purpose.
Shrink tube could be a solution but somewhat awkward with slipping on and in kind of repairs. (which sooner or later will be needed)
Flexible mesh would be nice with the same downside to slip them on.
Once getting rid of the spiral I have been happy ever since...

One might check on the clip in the attachment which I made for my heatbed.
Apparently I forgot to upload it to Thingiverse for some reasons.

Good point about removal. I'll continue thinking about this. Spiral wrap is working for me ATM, but it too is a pain in the put to remove/attach. Probably less so than shrink tubing.

Regarding the stl, that is EXCELLENT. Great job. Put that on thingiverse (or better yet send to Prusa)! I'm going to print it out next time I remove my bed, which should be as soon as my 2.5 upgrade comes in.

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Posted : 26/02/2018 6:40 pm
rob.s5
(@rob-s5)
New Member
Topic starter answered:
Re: Help fixing sheared heat bed wires

Thank you all for the pointers. I'm not super comfortable with a soldering iron but was able to de-solder the old broken connectors, strip the wire a bit, and resolder the new bits back on. From the feedback here, I've done some experimentations to make sure the wires are not too taut when the y axis is fully extended. Bottom line, after a complete recalibration, everything seems to be back to normal again.

Thank you all!

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Posted : 28/02/2018 7:28 am
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