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I have a number of fairly large printed pieces. The parts, when assembled, form a very large sword. (The model is this: https://www.thingiverse.com/thing:4178060, and is not mine.) It is about 7' from tip to pommel, and individual pieces are fairly large. The pieces are already printed, so advice to modify the model geometry to add reinforcements at this point is moot.

I have printed the pieces in PLA. I'm not asking for glue recommendations, necessarily, but rather something that could help with adhesion in addition to the glue. I'm concerned that after gluing them together, they won't support their own weight very well. I'm not worried about swinging it, just holding it and hanging it on a wall.

If I was very clever, I would have modified the files to add a through-hole that I would've put a pipe through or something, but I didn't think that far ahead. I could drill holes in the printed pieces, but I don't have a good way to make sure they line up.

So, what is the best way to add some kind of support to help keep the parts from falling apart? Is there some trick I could use to figure out how to line up drill holes?

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  • $\begingroup$ Is the sword for static display or does it need to survive being carried, or does it need impact-resistance ? $\endgroup$
    – Criggie
    Commented Nov 7, 2021 at 18:18
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    $\begingroup$ @Criggie Thankfully, I plan on just displaying it - mounted on the wall, most likely. $\endgroup$
    – Tal
    Commented Nov 8, 2021 at 13:38

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Superglue - aka Cyanoacrylate can do the job quite well if given help.

It depends on two flat and clean surfaces. If your printer has any lifting in corners, glue won't save you. The two parts have to be flat as possible, which may mean sanding with a clean sheet of 120 grit glued to a backing board. Any coarser and it will rip strands out; too fine and it will take forever plus your released plastic micro-particles get smaller.

Also, toothing the plastic print is critical if you're gluing the bed-face to something else. The first layer tends to have an almost glossy surface, and this is hard for the glue to adhere to.

If you can, add "gussets" across the join line, to help give the glue more surface area to stick to.

Avoid using "kicker" because it gives a weaker bond, though it sets faster.

Other solvents can help too, but they depend on the plastic you've printed in.

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  • $\begingroup$ In your specific case, the handle looks like a weak point. I'd reinforce across the layer lines by gluing thin metal strips to the handle lengthwise, then whipping the whole thing with jute twine and knotting the ends. Then add more glue to the twine which will soak it in and set hard. Hide all that using thin leather or leather-looking grips. Bicycle handlebar tape might be a good source and comes in many colours. I still worry about the handle-crosspiece point - perhaps be creative, or don't swing sword into things at all. $\endgroup$
    – Criggie
    Commented Nov 8, 2021 at 4:02
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    $\begingroup$ The handle is certainly a weak point - the glue surfaces there are small, especially relative to the rest of the blade, with enormous flat sections for gluing. Your suggestion to reinforce the handle is a good one - perhaps I could even reinforce the full length of the blade with a thin metal strip, and hide it either at the back, (as it's a display piece) or underneath some modeling compound. $\endgroup$
    – Tal
    Commented Nov 8, 2021 at 13:38
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    $\begingroup$ I'm going to give you the check for the simple reason that I hadn't thought of post-glue-up reinforcing, and I think that'll be the best option for what I have already printed. Thanks for the idea! $\endgroup$
    – Tal
    Commented Nov 8, 2021 at 13:42
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Previous answer assuming not yet printed:

If you want to drill holes, print smaller guide holes to guide the drill bit. The best glue depends on the material. Printing structures like dovetails makes the connection less dependent on the strength of the glue.

Answer with the parts already printed:

The best way to align drilled holes is probably to glue the pieces together first, then drill the holes. Otherwise, you need a method to clamp the pieces together or do precision drilling.

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  • $\begingroup$ Sorry, I'll clarify the question - the pieces are already printed, and the model is the one linked - it has no dovetails, pilot holes, or other helpful geometry. I'm not concerned about what kind of glue to use - I can do some testing for strength if necessary. I'm looking primarily for a way to add extra reinforcements post-printing, if possible. $\endgroup$
    – Tal
    Commented Nov 25, 2020 at 19:58
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A stronger joint than glue can be had by welding, albeit uglier and best used in hidden areas. One way to weld is with friction, using a Dremel to spin a piece of filament so fast it heats up and melts with the two plastic pieces you are joining. You tack the pieces together with a couple spot welds, then move along the joint slowly, melting a bead of plastic that is melted into both sides. Like metal welding, penetration is key to strength. You need a straight piece of filament (heat gun works, and rolling it on a flat surface while it is soft), and 3mm is a better size than 1.75mm, if available. It also helps if the joint is a little “V” shaped, so you melt plastic in the crack between the two pieces. Search “filament friction welding” for articles.

You can also weld with some filament and a chisel tip soldering iron with variable temperature (lower the temperature to a little above the filament’s printing temp). Melt the filament into the crack between the parts. Do something to get rid of the fumes if using ABS, etc.

Both work, friction welding might get more penetration into the parts being joined, thus a stronger joint, but they are both a sturdy way to do a butt joint between two printed parts, that would be weak if glued.

The friction welding can also be used to “rivet” overlapping parts, if you dive in and melt the spinning rod through the two parts, then stop the motor and hold it in place while it cools, then snip the end of the rivet off flush.

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