P_11.3.740: I've a neat OBJ drop/tower bolt, with the slide+turn rod/bolt carefully extracted from case.

I load case (0.05%), position and parent to swinging door, copying bends from parent. It stays attached when I swing door.

I load bolt, adjust its xyzOrigins to axial to allow slide/turn dials, position in case. Parent to case, it either flies off across scene, or mutates, or both...

Tried parenting to case (parented to door.)...

Tried parenting directly to door....

Worse, using edit to undo wonky parenting did not work reliably. I'd sometimes be left with a misplaced mutant. So, delete and try with duplicate...

Drove me to distraction in a scene I was building. So tried in a much simpler test-scene with flattened box as door, cylinder as post...

Am I missing something really, really obvious ?? 

Bake the scales before adjusting the origins. Don't use inherent bends for mechanical rotations that do not bend.

"Bake the scales" ??

I can't really tell from your images what's going on but it appears, by your description, that it's a drop bolt with the housing for the bolt attached to a wall and you want the bolt to slide down and turn in place to lock within the housing.

There's no bends happening here so do not check inherit bends. It's a translate Y rotate Y.

Before you adjust origins/centers you need to bake any scale adjustments you made, you can do this by spawning a prop from the group of the scaled object or exporting wavefront obj of the scaled object and reimporting. This will bake any scales as 100%

Once the scales are 100% default you can adjust the origins without things flying all over the place.

The inherit bends is probably causing the deformations.

So as I read it the parts are flying off (unbaked scale origin editing) and deforming (inherit bends)

Once these things are done the recommended parenting hierarchy would be parent the drop bolt housing to the wall and then parent the bolt to the housing. I would recommend setting limits on your translate and rotation relevant settings via dial settings menu, but that's not absolutely necessary, it just makes your dial settings and mechanical action less sloppy and exact to what they need to be for the action. 

When someone refers to baking something in it basically means it is default.

So if a specular highlight is baked into a texture it is part of the texture

If a scale is baked it is a non 100% scale transformed to default 100%

If 5 morphs are baked down it means it is no longer 5 morphs but 1

And be careful to not make any bumps or loud noises near a baking scale or it might collapse and crash Poser.

Just kidding lol ;)

NikKelly posted at 4:27 PM Tue, 4 October 2022 - #4445840

"Bake the scales" ??

Maybe it's easier for you to use this:

https://adp.spdns.org/#TrueSize

Select a prop, leave size dials untouched and start the script. Enter new Y-size in PNU, inches or cm, select if the object should be placed on the ground and/or centered in X and Z direction. After pressing "Ok" the props vertices will be changed accordingly (all dials are left unchanged).

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My Profuse Thanks, every-one. Elucidation dawns. Several useful 'Lines of Enquiry' to explore....

( My secret dread was quaternions: I can sorta hack MMD/PMX rotations thanks to an online math app, but prompt migraine ensues... )

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"...not make any bumps or loud noises near a baking scale or it might collapse and crash Poser."

Hey, we have a clan of 'Poltercats': If not stuck down or in a clip-lock lunch-box, Consider It Moved...