Hot End Thermistor - Which Mounting Hole?

[Bill Havins]

I am in the midst of assembling the hot end for my Rostock MAX and my "perfectionism" has slowed progress to a crawl. In the assembly guide "geneb" indicates that the thermistor should be installed in one of the holes perpendicular to a heating resistor. That would be this location (see middle of photo):

Two things occur to me. In this location the thermistor's mounting is very precarious; it's going to take a lot of RTV/Kapton tape to hold the thermistor in place. More importantly (I think?) the thermistor will be measuring the temperature of the heating resistor, not the hot end.

There is another hole in the hot end where the thermistor can be mounted (see middle of photo):

This hole is much deeper (>5mm) and will easily accommodate the thermistor with PTFE tubing on its leads. The depth of the hole is such that the thermistor will contact a surface about 1mm from the threads on the nozzle. This allows the thermistor to meter the temperature of the nozzle almost directly (a more reliable measure of the temperature of the nozzle). With a nice dab of RTV the thermistor will mount very securely. I'm inclined to mount the thermistor in this second location.

Am I missing something?

Thanks!
Bill

[cambo3d]

I hadn't noticed this before you mentioned this. I thought the holes were the same depth and its too late for me to check mine since I already mounted my thermistor. I just picked a hole and mounted it.

I would think the one closer the brass nozzle would give you a more accurate reading of the nozzle temp. Like you say

edit: I just realized the other hole your talking about in your first picture ( that looks like a machining error or maybe a different version) . I dont have this on my hotend.
The one in your last picture is where i mounted mine. I have two of these holes, one on each side.

[TheMOX]

The holes that intersect the bores for the resistors is a relief hole. It allows air and silicone to escape from the bores as you insert the resistors. At the build event, it was suggested to inject the silicone into these side holes to insure there were no voids. These new openings are a relatively new addition suggested by Eric (?) to reduce the possibility of hot spots due to trapped air pockets.

The larger deeper hole is for the thermistor.

[cambo3d]

The holes that intersect the bores for the resistors is a relief hole. It allows air and silicone to escape from the bores as you insert the resistors. At the build event, it was suggested to inject the silicone into these side holes to insure there were no voids. These new openings are a relatively new addition suggested by Eric (?) to reduce the possibility of hot spots due to trapped air pockets.

The larger deeper hole is for the thermistor.

If these guys are suggesting this stuff above they should be putting this information into a manual or letting people know about these changes. in a technical update when you receive your order.

[Eaglezsoar]

The holes that intersect the bores for the resistors is a relief hole. It allows air and silicone to escape from the bores as you insert the resistors. At the build event, it was suggested to inject the silicone into these side holes to insure there were no voids. These new openings are a relatively new addition suggested by Eric (?) to reduce the possibility of hot spots due to trapped air pockets.

The larger deeper hole is for the thermistor.

If these guys are suggesting this stuff above they should be putting this information into a manual or letting people know about these changes. in a technical update when you receive your order.

This time I must agree, they are coming up with all of these changes and none is being documented and I don't think that Geneb should be responsible to keep up with these changes and incorporate them into
the "official doc" that he wrote. This responsibility should belong to Seemecnc, they are changing a lot of things and none of it is being documented. We should not have to rely on this Forum to learn about
the changes, it should not take that long for them to update the PDF that Geneb created. Cambo3D stated basically the same thing a while ago and I now see what he meant. Please Seemecnc, keep the
PDF updated with your changes.

[Bill Havins]

Here's a quote from geneb's assembly guide:

21.10 – The thermistor is installed in one of the two small holes drilled perpendicular to the resistor chambers on the Hot End. Fill one with RTV and press the thermistor in to place as shown below. Make sure you’ve let the RTV cure at least one hour before moving on to the next step.

One of the hot ends I received with my order (I bought a dual extruder kit) has a single deep hole drilled "parallel" to the two heating resistor bores (see second photo, above). It has holes drilled "perpendicular" to the heating resistor bores (see first photo, above) - that's two "perpendicular" holes. When I read geneb's assembly guide guess which holes I thought he was referring to...

This is not meant as criticism of geneb. And I'm not criticizing SeeMeCNC, either. But these "fiddly" little details can make quite a difference in success/failure of an assembly, and in printer function.

Bill

[MorbidSlowBurn]

When I mounted my thermistor I used the hole that intersected the resistor. I am on the fence of which would be more accurate. The one next to the resistor will relay the temp of the resistor back to the Rambo quickly. This would result in a stable temperture in the hot end and if you are concerned about melting the PEEK you could limit the max extruder temp easily. Downside to this is if one resistor fails you may be reading the side that is bad therefore the cold side of the hot end.

On the other hand using the deeper hole next to the nozzle gives an accurate reading of the nozzle. This could suffer from some slight thermal lag of the temperature of the resistors (probably not significant as the mass of the hot end is relatively small). If thermal lag occurred between the resistor and thermistor, based on the amount of thermal lag this could result in the extruder over heating. I am not sure of the programming of the firmware but I think the autotune feature is supposed to account for some of this lag.

Just adding my two cents, and the some of the pros and cons of each mount position. If someone has done a contrast study between the positions it would be helpful.

[Darth]

When I added my thermistor, I was concerned with the "fill the hole with RTV, and then insert the thermistor". In doing this I wondered if the thermistor was encased in RTV and would be insulated from the hot end. Haven't fired up my printer yet, so I don't know the effect, or if it makes a difference.?????
Same thing with the bed.

[Eaglezsoar]

When I added my thermistor, I was concerned with the "fill the hole with RTV, and then insert the thermistor". In doing this I wondered if the thermistor was encased in RTV and would be insulated from the hot end. Haven't fired up my printer yet, so I don't know the effect, or if it makes a difference.?????
Same thing with the bed.

I feel that putting arctic silver or alumina in the hole before the thermistor and put the RTV on the outside to hold it in place would give better results. The RTV does not conduct heat and I believe that by putting RTV in the hole is just a bad idea. Just my opinion. You only need a dab of Arctic on the thermistor. On the bed I would place a small piece of kapton tape over the hole on the front of the bed, another dap of Arctic silver on the thermistor and hold the thermistor in the hole with the RTV. You don't want the thermistor to extend above the bed, that's why the kapton tape.
Carl

[Darth]

When I added my thermistor, I was concerned with the "fill the hole with RTV, and then insert the thermistor". In doing this I wondered if the thermistor was encased in RTV and would be insulated from the hot end. Haven't fired up my printer yet, so I don't know the effect, or if it makes a difference.?????
Same thing with the bed.

I feel that putting arctic silver or alumina in the hole before the thermistor and put the RTV on the outside to hold it in place would give better results. The RTV does not conduct heat and I believe that by putting RTV in the hole is just a bad idea. Just my opinion. You only need a dab of Arctic on the thermistor. On the bed I would place a small piece of kapton tape over the hole on the front of the bed, another dap of Arctic silver on the thermistor and hold the thermistor in the hole with the RTV. You don't want the thermistor to extend above the bed, that's why the kapton tape.
Carl

Too late for me, but good advice for others.

[Av8r RC]

First off I used the hole in the second pic.

But I want to warn everyone about a potentially disastrous situation you could experience.

Today I was attempting to mount my Budashnozzle on my max. Before I took off my original hotend I had to heat it up to extract the filament in it. Once I reversed the filament I was taking the hotend off (while it was still hot) and the thermistor pulled right out almost on it's own. Once that RTV gets hot it doesn't hold very well. Try not to disturb the wires while it's hot.

FWIW, I used the same kind of RTV Geneb used from Harbor Freight. I already had it for my RC plane

Also. If you try to use the Budaschnozzle on the Max. You will need shorter standoffs. It barely fits thru the hole on the platform. The three standoffs, on the Buda, touch the hole in the platform.

[cassetti]

When I added my thermistor, I was concerned with the "fill the hole with RTV, and then insert the thermistor". In doing this I wondered if the thermistor was encased in RTV and would be insulated from the hot end. Haven't fired up my printer yet, so I don't know the effect, or if it makes a difference.?????
Same thing with the bed.

I feel that putting arctic silver or alumina in the hole before the thermistor and put the RTV on the outside to hold it in place would give better results. The RTV does not conduct heat and I believe that by putting RTV in the hole is just a bad idea. Just my opinion. You only need a dab of Arctic on the thermistor. On the bed I would place a small piece of kapton tape over the hole on the front of the bed, another dap of Arctic silver on the thermistor and hold the thermistor in the hole with the RTV. You don't want the thermistor to extend above the bed, that's why the kapton tape.
Carl

I don't think so. I don't have my RTV in front of me, but isn't it infused with copper? The label says 'sensor safe' - but doesn't say what kind of sensors. If it has copper in it, I suspect it will transfer enough heat to the thermistor to make it a moot point (just my $0.02)

Also, I opted to use the deeper thermistor hole parallel to the resistors. I felt this gave me a better mounting point to install the thermistor and guaranteeing that it wouldn't come out or short as easily. The holes perpendicular to the resistors didn't seem deep enough for mounting.

And I agree, if they are going to keep revising the plans, there should be some sort of change-log that users can review what has changed since the last iteration!

[Eaglezsoar]

81878.pdf

(88.12 KiB) Downloaded 473 times

If you go to Permatex and look up the material data sheet on Ultra Copper, you will see that it contains no copper. It is a high temperature silicone that offers resistance to heat before degradation, it does NOT conduct heat.

[cassetti]

81878.pdf

If you go to Permatex and look up the material data sheet on Ultra Copper, you will see that it contains no copper. It is a high temperature silicone that offers resistance to heat before degradation, it does NOT conduct heat.

Ah, my mistake, I assumed it had copper in it, again, it wasn't in front of me when I posted that. Hopefully it doesn't insulate against heat too much...