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The handle of a micro wave oven broke.

I can't just order a replacement part because I can't even attach the new one.

The problem is that the screw heads are somewhere on the interior side of the door, which cannot be disassembled (non destructively at least). I wouldn't even do it because of safety reasons.

I have access to the threads of two loose and captive screws to work with (indicated by the two red lines on picture one). The screws are not machine screws, but screws for plastic like in the attached picture.

The plan is to 3d print the plastic part of the handle and reuse the front aluminum cover.

I don't want the handle to be loose, so I'm looking for suggestions to attach the new handle. I have a lot of ideas, maybe I will share them later if they are not mentioned at some point. The main problem is: how to attach something when all I have to work with is a loose, non machine screw ?

enter image description here enter image description here

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  • $\begingroup$ Welcome to 3dPrinting.SE! $\endgroup$ Sep 15 '19 at 14:27
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    $\begingroup$ That's a long handle to attempt to replace with a 3D-printed part. You'll have strength problems. I would recommend instead drilling holes in the two (or more) parts of the brokend handle so that you can insert steel pins longitudinally through the break and then epoxying everything back together. $\endgroup$ Sep 16 '19 at 15:56
  • $\begingroup$ Yes, I actually realized that the largest dimension is 50-ish centimeters, and larger than the average hobbyist 3d printer. I also made some quotes on various 3d printing service websites, and the cost is around 30% of the device I'm trying to repair ( around 100€ for a 400€ + device....), so it looks like it won't be the way I will be repairing it. $\endgroup$
    – alecail
    Sep 16 '19 at 16:20
  • $\begingroup$ if the hole is too big change the size of screw, if you want to use the same screw then fix the hole. Carl describes the usage of epoxy and some inserts; this method works for many patches and non replaceable parts. $\endgroup$ Sep 16 '19 at 19:22
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You are facing 2 problems in this case:

  • Material choice
  • Design

Material choice

As far as I can see, this is an oven that can heat its interior to roundabout 250 °C, which means that the door will radiate a lot of heat. This needs special high-temperature plastics to withstand - PLA and ABS are not an option for this!

I would suggest looking for PEEK (stable at ~260°C) or a high-temperature resin print in this case, as there are some resins which can withstand much higher temperatures after curing. Note that PEEK is traded usually for above 100 €/kg and needs a specialty printer. Even the less stable PEI filament (works under 170 °C) needs a specialty printer: All metal hotend and 350-380 °C extruder are needed!

Design

The plastic screws you show are self-cutting plastic screws. if their head is accessible, they get screwed into holes that have a bore large enough for the center.

If the screw head isn't accessible, for turning but can be pressed down, then one could opt to use high-temperature resin to cure inside oversized holes over the screw. This is not an option if the screw can't be pressed into the part though, as then the handle always will rattle.

Repair?!

Note that you might be able to access the screws with a little work, if you have the right tools and manual.

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  • $\begingroup$ Unfortunately, the front door is one glued unit. It's not like there are hidden screws under a label or rubber pad or something. I believe it's designed so that it's not repairable by the end user. $\endgroup$
    – alecail
    Sep 16 '19 at 14:32
  • $\begingroup$ Thanks for the answer. I actually didn't realize that the picture was for a regular oven, when I was in fact looking for the picture for my microwave oven, which happens to the same control panel and overall look.. However, I didn't really think of that temperature problem. I will actually measure it when it's running, but I don't expect it to very different from the ambient temperature $\endgroup$
    – alecail
    Sep 16 '19 at 14:33
  • $\begingroup$ @alecail if it is only a microwave, then you have much less a heat problem. $\endgroup$
    – Trish
    Sep 16 '19 at 15:17
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If you have enough screw length exposed, clamp onto them via a 2-piece 'nut' that is itself bolted together via fasteners that run perpendicular to the screws. You can then fasten whatever to the 'nut' and maybe even hide the gaggle of fasteners that results. Another alternative for applying the clamping forces needed might be something based off a "toggle wing nut." I do not believe you will get good results via 1-piece print solutions the way the screws are.

Loads and nut material will determing how much screw you need inside the 'nut.' The more length you have to work with on top of that, the more room you have to hold the screws' rear ends w/ pliers/vice grip while getting your 'nut' on there.

If you don't have enough length, you will just have to use a different method of attaching to the door. Options may include adhesives and/or modifying the actual door - pretty annoying if you can't take it apart.

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  • $\begingroup$ This is exactly what I would do! Alternatively the multiple pieces could be glued with epoxy; e.g. when the screw length is short I would print a small cylinder (like a washer) with a hole a little smaller than the screw thread to fixate the screw, then glue on a larger cylinder as a fixation for the handle mount, and finally glue the handle mount onto the cylinders. $\endgroup$
    – 0scar
    Sep 18 '19 at 6:49
  • $\begingroup$ This answer does not give insight into the specific problems of 3D printing and the used materials. $\endgroup$
    – Trish
    Sep 18 '19 at 7:18
  • $\begingroup$ I don't know the exact term ( not a native speaker here), but I had in mind something similar to the mechanism used on bikes for the quick release. Really it's a wedge, with some material removed so that it can capture the threads, and it's in a cylindrical form factor. $\endgroup$
    – alecail
    Sep 19 '19 at 8:49
  • $\begingroup$ That wedge type of cam will certainly help remove rattling from your connections. They'll still need to be able to put the screw in tension via pulling on a 'nut' of some sort that is attached to those screws. About how many cm of screw can you get outside to attach to? $\endgroup$
    – Abel
    Sep 19 '19 at 23:44

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