3D Printed Mod Corners - For 2D Panel Construction

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If there is an issue with cracks forming at the neck, you could actually improve the mechanical performance by removing material to form in internal rounded fillet to reduce the propensity of cracks forming and therefore mitigate a major mode of failure.

 

[wywywywy]

One more thing - the first 2mm or so of a self tapping screw has a smaller diameter (or even no thread), so a 7/8mm self tapping screw will not be able to grab enough "meat" on the plastic.

 

[wywywywy]

For machine screws and nuts, M3 nuts are too thick (2.4mm) for the thickness of the plastic (3mm). So the plastic may break as soon as a nut is pressed in.

M2 is too small and fiddly.

I personally think that M2.5 is the best compromise and also more available than M2. Nuts are 2mm thick which is probably ok but will need trying.

 

[wywywywy]

If there is an issue with cracks forming at the neck, you could actually improve the mechanical performance by removing material to form in internal rounded fillet to reduce the propensity of cracks forming and therefore mitigate a major mode of failure.

Unfortunately there's no internal material that can be removed because the neck is only a ~4.25x4.25x4.25mm triangle!

 

[Thehack]

The M3 self-tapping screws that you can easily buy have about 6.3mm of full threads, which is enough to grab 3mm of materials, and 3mm of the corner piece. In my own case, the M3 self-tapping is very secure.

I find 3mm of 3d printed part a good point between size and strength, at least for SLS. How does the 3mm FDM part feel?

It seems like I need to increase the size a bit to accommodate an M2.5, however I'd rather not. Have you tried pulling off the corner piece?

In your picture, the panels didn't sit flush. Is this due to the printing tolerance?

 

[wywywywy]

I find 3mm of 3d printed part a good point between size and strength, at least for SLS. How does the 3mm FDM part feel?

It completely depends on the filament, infill, nozzle size, and layer height. For a small PC case personally I think 3mm is enough. But bear in mind the stress point (hole for screw) is very close to the corner. This will be the weakest link by far.

It seems like I need to increase the size a bit to accommodate an M2.5, however I'd rather not. Have you tried pulling off the corner piece?

Stuck in the office right now

In your picture, the panels didn't sit flush. Is this due to the printing tolerance?

It's mainly because the screws aren't screwed in. But also the surface that had support underneath isn't totally flat.

EDIT - By the way, given 3mm screw diameter, I measured the max length to be 7.5mm with the original design.

 

[wywywywy]

Ok, so I tapped the holes for M3 screws, and used 6mm long ones to screw the panels together. And this is how it looks.

Notice that the left panel doesn't sit flush, because that side had support on it and the surface finish isn't good at all.

I tried very hard to break the corner with my fingers, and couldn't! It's stronger than it looks.

What should I do now? Should I break it with pliers?

 

[|||]

Unfortunately there's no internal material that can be removed because the neck is only a ~4.25x4.25x4.25mm triangle!

If done correctly, it should not remove much material and the minimum cross sectional area should be the same.

 

[Thehack]

Ok, so I tapped the holes for M3 screws, and used 6mm long ones to screw the panels together. And this is how it looks.

Notice that the left panel doesn't sit flush, because that side had support on it and the surface finish isn't good at all.

I tried very hard to break the corner with my fingers, and couldn't! It's stronger than it looks.

What should I do now? Should I break it with pliers?

Good stuff. The next issue to resolve is the finish of the surface for FDM printing. Is it possible to orient the inside corner as the base?

I'm not sure how that would affect the neck... Otherwise we'd have to compensate by trimming the materials.

 

[Thehack]

If done correctly, it should not remove much material and the minimum cross sectional area should be the same.

I don't think we need to remove the materials as plastic is not brittle like acrylic. But it may be useful to remove material to give it a cleaner corner for FDM printing.

 

[|||]

I don't think we need to remove the materials as plastic is not brittle like acrylic. But it may be useful to remove material to give it a cleaner corner for FDM printing.

It's not just brittle fracture; there are failure modes in bending, as well. Sharp internal corners are large stress risers.

Just trying to pass on proper mechanical design advice.

 

[iFreilicht]

Good stuff. The next issue to resolve is the finish of the surface for FDM printing. Is it possible to orient the inside corner as the base?

I'm not sure how that would affect the neck... Otherwise we'd have to compensate by trimming the materials.

I think the problem with this orientation will be that the holes will be far from perfectly round and the angled faces will be rough as well. The normal of faces should ideally be perpendicular or parallel to the normal of the printers bed with FDM. But with the right settings, you might be able to print this without supports and even if you need them, they're not in critical places.

 

[Thehack]

I think the problem with this orientation will be that the holes will be far from perfectly round and the angled faces will be rough as well. The normal of faces should ideally be perpendicular or parallel to the normal of the printers bed with FDM. But with the right settings, you might be able to print this without supports and even if you need them, they're not in critical places.

The holes do not have to be round as it'd be tapped or use a nut. I'm more concerned with a consistent height for the mating surface. I know it's be more rough but it'd require less finishing work. So we have to make a trade-off.

The goal is to make it feasible on FDM. SLS tech can handle it.

Looking to buy a small 3d printer myself.

 

[Thehack]

It's not just brittle fracture; there are failure modes in bending, as well. Sharp internal corners are large stress risers.

Just trying to pass on proper mechanical design advice.

Thanks. This is good advice. I will remove a little bit material in the next iteration. It's to help FDM printers handle the corner as well.

 

[wywywywy]

This is the cross section showing how the neck is joined.

As you can see there's very little plastic in there already. Removing 2 perimeter lines will make it very weak. Also 3 perimeters is the minimum for a reasonable quality print.

 

[wywywywy]

Good stuff. The next issue to resolve is the finish of the surface for FDM printing. Is it possible to orient the inside corner as the base?

I'm not sure how that would affect the neck... Otherwise we'd have to compensate by trimming the materials.

Kind of but not quite. There will be near zero bed contact surface with it like that.

The design needs to be changed to have a flat bottom, like this -

But of course then you won't be able to use nut traps.

 

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This is the cross section showing how the neck is joined.

As you can see there's very little plastic in there already. Removing 2 perimeter lines will make it very weak. Also 3 perimeters is the minimum for a reasonable quality print.

Yes, but as I mentioned above, the minimum cross-sectional area does not change, so the amount of material there does not change. The thing the changes is instead of those being sharp 90 degree bends, they are then radiused arcs.

 

[wywywywy]

I see what you're saying, but the corner on the outside can't have a fillet because the panel won't sit flush, and the corner on the inside can't either unless the outside one is

 

[jottwehh]

( CPU for scale )

Costs me about 2min (+3 days of waiting) and 10$.
Sadly I will be on vacation starting tomorrow, so I wont be able to test them, or give some practical input...

 

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I see what you're saying, but the corner on the outside can't have a fillet because the panel won't sit flush, and the corner on the inside can't either unless the outside one is

I wasn't proposing the outside corners be filleted at all, as that would create obvious gaps. Is the inside corner needed for stability? I would have assumed that the nub would have been there more for guidance and alignment with the friction of the larger surface preventing rotation through friction, as is the case with a lot of screw/bolt affixed things.