screw type for high temperature ground? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: June 29, 2024, 05:20:54 am
content: The muffle furnace I have has badly oxidized screws. I cleaned them up, but they are part of the grounding, so I feel that I should replace these.They are behind the insulation but they might have a elevated temperatureNaturally I want to use stainless steel, but I am wondering if its electrically suitable.I.e. if it gets beat up by heat, does it conduct electricity better or worse then rusted carbon steel? It won't look nice and form some invisible electricity barrier on it will it? Nickel and chromium oxide im not familiar with.I know that nickel crimps are used for high temp.Do I maybe need... pure nickel screws?My concern stems from the fact about how god damn rusted the inner chassis was. The outer chassis looked great. IDK if its chemical exposure from its previous owner that maybe caused the damage.actually I think I will also run a lug connected to flange of the enclosure down the back of the unit so its not grounded with sheet metal screws. I am reading electrical code and grounding though those sheet metal screws seems like a bad idea

author: JJ_023
author_rank: Frequent Contributor
timestamp: June 29, 2024, 02:06:54 pm
content: You should look into Inconel 718 fasteners. Designed for high heat environments.

author: coppercone2
author_rank: Super Contributor
timestamp: June 29, 2024, 06:11:15 pm
content: is there any high temperature fastener that might not be suitable for electrical grounding that is otherwise suitable?I am worried about something behaving strangely, like aluminum, at high temperatures. Exclude might be better then include. Like it turns out something has particularly non conductive oxide formation because of heat, which while thin, and invisible, prevents electrical flow.essentially im worried about a 'booby trap' behavior from some new fangled materialIt made me paranoid because the kiln door, which was not as badly oxidized, when I cleaned it, I thought "this shit has to be painted stainless steel". I am wondering if the rest of the body used to be painted stainless steel, that corroded so bad it looked like the regular steel I deal with.Maybe I can shave off some of the body with a deburring tool and see how it reacts in acid

author: jpanhalt
author_rank: Super Contributor
timestamp: June 29, 2024, 07:57:58 pm
content: Quote from: coppercone2 on June 29, 2024, 06:11:15 pmPlatinum?

author: David Hess
author_rank: Super Contributor
timestamp: June 29, 2024, 09:18:04 pm
content: Stainless steel would work fine and stainless hardware is commonly available. I would be more worried about the screw backing out and becoming lose because of temperature cycling.

author: coppercone2
author_rank: Super Contributor
timestamp: June 29, 2024, 09:34:17 pm
content: Ok yeah but it has like a lip near the lid, I can ground to that with a nice thick ground wire with lugs and stainless. That bit should not get hot because its on the edge.The resistance reads OK but I am suspicious that once its all hot and stuff it might get dangerous in a long time when I forget about it years from nowIts built.. bad. The bottom chasis is mesh steel, and the front plate where the ground connects to is aluminum (it is grounded through a aluminum stud that mounts the temp gauge . I am going to ground the top heater chassis, the front plate and the mesh chassis and the mesh bottom to a star ground #10 bolt that connects to the mains earth terminal on the cord.Suprizingly, the lugs that are in this unit, despite the age, are basically beautiful. I assume it must be solid nickel or something. I was expecting them to be black but its shiny. They are the nicest looking lugs I have seen in something this old.

author: coppercone2
author_rank: Super Contributor
timestamp: July 05, 2024, 09:17:56 am
content: Another question I have is about the interconnects in this little furnace.I burned out the heater, and when I took it apart to remove it, I wanted to measure the resistance. I found that the nichrome wire was basically totally oxidized. I could not get a probe on it without sanding the wire end with 220 grit sand paper. This was only after a few hours of operation.While it does have ceramic studs, I though that this really dodgy.If I got nickel lugs, and soldered them to the wire, would it be more reliable? Because the nichrome is 80% nickel. A plated lug is 100% nickel.Are they more resistant to oxidation? Or is this just how things are with high temperature interconnects? I really feel that they should stay.. conductive. It just seems like a really unreliable junction.Maybe ceramic wire nuts would be better then a brass screw (thats what it comes with).?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 05, 2024, 01:52:33 pm
content: At kiln temperatures (baking clay, steel heat treatment, etc) everything becomes "special". Resistance wire for this purpose usually has a few percent of aluminum in it. This aluminum oxidizes in the first few heat cycles and the oxidized aluminum protects the rest of the wire against oxidation. Even "regular" stainless steel stops being stainless and rusts at oven temperatures. You can attempt to use whatever materials you have lying around but it's likely that they will not last long. If you use your kiln very occasionally it may still be acceptable, but if you use it more often, the wire will likely fail at a moment that is most inconvenient to you.Also, the resistance wire is regarded as a consumable. That is part of the reason the wire is almost always put into an open groove. It makes it easier to replace the wire.For connection to lugs, I think it's customary to double up the last section of wire. At the very least this makes it mechanically stronger and reduces it's temperature because of the increased surface area. If it has an electrical connection then the current will also be divided over the two wires, which reduces the heating a lot more.Making the lugs big and from heavy metal also makes them act as a heatsink, which also lowers peak temperature, and thus expansion / contraction with each heat cycle.I have not looked too deep into details, because I do not have a kiln (yet). I recommend to do a bit of research and then just stick with common and proven practices. Your kiln will last longer and with less maintenance.

author: coppercone2
author_rank: Super Contributor
timestamp: July 05, 2024, 08:29:04 pm
content: This one is nichrome. For chemistry they use nichrome because its more resistant. For well defined loads they use kanthal wire which has aluminum in it but its touchy. Even for defined kiln loads, nichrome has benefits. If you splatter a bit of kiln wash on nichrome its OK if you do it on kanthal it gets reduced by the silicon (like thermite !). The kanthal gets a little higher temperature.so if you start a kanthal kiln dirty with a bit of glaze on the elements, it will corrode it on first firing so bad it will break on the second, from what I read.I remade it to how THEY made it but it makes me feelmy idea was1) make stainless steel bracket attached to the equipment body side (rather then bolts going to the kiln top directly attached to wires2) use a ceramic terminal block on this bracket3) braze on nickel ring lugs of the correct size to the nichrome wire4) connect it with other side of terminal block instead of stacking hot crimps (4 bolts in the ceramic terminal block vs 2 bolts in the default unit). That way the terminal block is not attached to the kiln floor and the ceramic lugs act like a heat sink. Then there are less hot crimps in the system. The ceramic terminal block should act like a heat sink to dissipate the heat from the nichrome wires, so the two power crimps have cooler junctions.Since the terminal block is not attached to the kiln floor (which gets fairly hot even if it has insulation above it) it will generally run alot cooler. It wil only be heated by contact resistance and the nichrome wires leading to it. It will also be floating on a bracket so it will have convection to cool it. I think then the joints will all run at a much cooler temp .

author: coppercone2
author_rank: Super Contributor
timestamp: July 06, 2024, 10:25:56 pm
content: the ceramic terminal block is interesting. its built in a way where the metal is essentially loose in the ceramic (not like a normal terminal block where the metal is bonded to the plastic). It rattles around even if you tighten down the screws. I guess for true high temperature operating you need the conductor to be lose.A little unusual !I hope I can fit it in there on a bracket, I would feel alot better then to use those hot brass screws on the bottom of the heat chamber.