Two questions: thermocouple temporary fix, and X-axis stop microswitch

[John Armbruster]

First, the important question. The corollary to Murphy's Law is that when one thing goes wrong, everything starts going wrong. After switching out my x-axis cable (simple repair), I immediately started getting Heater Errors #4 and #5. Now, I had actually noticed during the last two prints I did before the cable change that, briefly during the prints, the extruder temperature was N/A, so I don't believe the cable fix is related to the errors. I've been following this thread 

https://groups.google.com/forum/?hl=en&fromgroups#!searchin/makerbot/thermocouple$20repair/makerbot/6g4swDbf4oE/UMUtS67tbpQJ

for a temporary fix while I order an upgrade replacement from BilbyCNC. However, I have a question. When I finally managed to pull out the twisted wire, it looked like this (sorry it's not a better picture) and it actually looks like the wire was never stripped before being twisted. This can't be right, no? Also, the brass insert seems to have some white threadlike material in it that apparently was helping to hold the wires in (insulation material?). I think I will need to remove this before reinserting the newly stripped and retwisted wire. Should I try to drill it out gently by hand reaming with a small drill bit? And how best to recrimp the brass nut without using the insulation material?

 

Second, when I was replacing the x-axis cable, I noticed the small steel lever arm missing on the x-stop switch. I priced a replacement switch, then sent a support ticket to see if I could just get a replacement. I also found a hack from WingCommander using a bent tie-wrap and epoxy. Interestingly, Makerbot support replied that this stop doesn't use the lever arm, that the stepper motor actually comes in contact directly with the switch. Again, does this sound correct?

[Jetguy]

Been covered hundreds of times. The lever is removed on purpose because it rattles when installed and would drive you mad. It is not necessary and that's why they remove it.

[Jetguy]

Also, I generally do not recommend self repairs on thermocouples. If it fails mid print but isn't detected, the heater could get stuck on and overheat. This is not an area of the bot to play around with.

[Dan Newman]

> Second, when I was replacing the x-axis cable, I noticed the small steel
> lever arm missing on the x-stop switch.

It's not supposed to have the metal lever arm: that's by design.

> Interestingly, Makerbot support replied that this stop doesn't use the
> lever arm, that the stepper motor actually comes in contact directly with
> the switch. Again, does this sound correct?

They are correct.

Dan

[Jetguy]

Again, on your tc repair, just about everything is wrong there. You should spot weld or otherwise permanently connect the 2 wires. If you don't have the correct tools or skills, stop right there. Twisting the wires is a sure fire way to get failures mid print. The white stuff inside the threaded end was high temp potting compound. Probably something you also don't have on hand. Twisting the wires and slapping this together is a waste of time. Just purchase a correct thermocouple rather than causing yourself to take the bot apart more than once and risking even more failures.

[John Armbruster]

Thank you both for your quick replies. Makerbot support is shipping a new cable Monday, and I have the BilbyCNC cable upgrade on order.

Should I be concerned that I've used the stock unstripped cable for almost a year now?

[Jetguy]

A thermocouple is a JUNCTION (AKA  electrical connection) between two dissimiliar metal wires.

That means it's IMPOSSIBLE for it to have worked at all if the ends were truly insulated as you are describing. Obviously, there is a junction buried in the thermal potting compound and the wires broke from flexing or stress above that junction.

Again, it could NOT have worked had there not been a junction.

http://en.wikipedia.org/wiki/Thermocouple

[John Armbruster]

I understand how it's supposed to work as I use Omega K type thermocouples for temperature data collection at work, but I thought I would just throw this out there as a data point on the Makerbot construction. For what it's worth, I did take a microdrill and gently cleaned out the remaining potting compound, although there was very little of it and it really did seem to be more like the white ceramic insulation tape than an epoxy or resin. If there were a twisted end to the thermocouple lead, it wouldn't have been longer than 1/16", as it was, I found nothing. Again, since all this was different than David Kessner's experience in disassembling a thermocouple, I thought I would add it here.

As an aside, I do have some spare Omega thermocouples. With this end, it fits snugly in the brass fitting, and the lead is long enough to reach the motherboard. Why would this not be a suitable replacement, no soldering required?

[Dan Newman]

On 14/12/2013, 2:13 PM, John Armbruster wrote:
> I understand how it's supposed to work as I use Omega K type thermocouples
> for temperature data collection at work, but I thought I would just throw
> this out there as a data point on the Makerbot construction. For what it's
> worth, I did take a microdrill and gently cleaned out the remaining potting
> compound, although there was very little of it and it really did seem to be
> more like the white ceramic insulation tape than an epoxy or resin. If
> there were a twisted end to the thermocouple lead, it wouldn't have been
> longer than 1/16", as it was, I found nothing. Again, since all this was
> different than David Kessner's experience in disassembling a thermocouple,
> I thought I would add it here.
>
> As an aside, I do have some spare Omega thermocouples. With this end, it
> fits snugly in the brass fitting, and the lead is long enough to reach the
> motherboard. Why would this not be a suitable replacement, no soldering
> required?

You do need to electrically insulate the t/c bead from the brass housing/thermowell.
Otherwise, you will get incorrect temp reads as the static buildup in the extruder
assembly will induce additional current into the t/c. And that will translate into
incorrect temp readings. Put differently, the t/c setup is intended for ungrounded
operation: MBI didn't design it for grounded t/c operation.

Dan

[Jetguy]

Sure, just keep in mind some requirements. The thermocouple should be electrically isolated from the heater block but needs to be connected thermally.

This is the purpose of the white compound, it's an electrical insulator but a thermal conductor. If you change the thermal resistance then this throws off the PID tuning of the hot end resulting in over and undershoots thermally.

Being so many folks complain about jams, printing with a hot end that is unstable temperature wise seems like a bad idea.

Let's be clear here, that I am trying to state the possible problems and what the "correct" engineered way this is supposed to all work together. Sure, you can swap in thermocouples, mount it in many different ways to the heater block and maybe print something. But, as far as being accurate compared to before it's all a blind guess. Again, given the problems people have with a good hot end and sensing, a rigged one is very questionable.

[Jetguy]

Since I see that Dan answered, also be aware he is one of the folks who tried mounting a bare thermocouple to the outside of the heater block, much like the way MK7/ Replicators mounted them.

We've long known there is a significant temperature difference between the external mounted thermocouple VS the internal kind being used on the 2 series and he was the first to flush this out.

For example, It's generally agreed for PLA on an external mounted thermocouple, around 195C seems right on a MK7/Replicator hot end, where as on a 2 series, 230C is more the accepted normal set temp.

Making your own joint and thermal connection may stray you from the normal temperatures.

Again, I'm not saying you cannot print, I'm not saying that you cannot adapt many commercial thermocouples and methods of attachment. What I am saying is that average Joe/Jane here in the forum who can barely keep from air printing normally, and is not experienced or skilled in thermal calcs and construction is probably better off waiting for a proper replacement thermocouple rather than attempting to mod something or repair one. Using any other mounting method, any repair job, or a different thermocouple, is highly likely to result in either unstable temps or different temperature setpoints may be required to achieve the same printing results as before.

There are some folks who did write-ups on repairs and have printed using them. The big question is how reliable is that connection long term, how much experience did those folks have and most likely, they understood the implications.

I think it's a stretch given many of the folks around here do not have such technical backgrounds to advise them to attempt such a repair. That's all I'm trying to say is that there is skill involved here. Any deviation in methods or material results in a thermocouple measurement that reacts differently than stock. If it's supposed to be a precision instrument, repairs can take the precision right out of it.