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Comment by krasin | Hacker News Reader

My biggest concern is that they use screws over plastic as fasteners: https://github.com/PCrnjak/PAROL6-Desktop-robot-arm/blob/mai...

Quote from the manual:

> Screws are in this example M3 screws and holes are undersized to 2.7-2.8mm that means that when we screw in the screws we are tapping holes in 3D printed parts.

> There are multiple benefits to this:

> ● Connection is strongest compared to tapping holes with a tap or using brass inserts

> ● It is simple and fast

> ● No need to prepare the hole, it can be printed undersized

> Cons are that you can’t disassemble it a lot of times. In case you feel screws slipping in the hole. Put some super glue in the hole and wait for it to cure. After that re tap the hole.

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I've used this technique on several of my projects. The formed threads are strong and reliable. It cuts down assembly steps and BOM. The tight fit means the screws won't back out (like locktite or nylocks). The only downside is they can only be reassembled a few times before they start getting loose, at which point you can either add material (superglue), or just reprint the part.

If you have something you want to reassemble frequently, use inserts. If you're putting it together once and intend to use it that way for a long time, threadforming works fine.

> I've used this technique on several of my projects.

Did they have any moving parts? Did they experience continuous vibrations and frequent mechanical shocks? For DIY robot arms, fasteners are very often the issue #1, if arms operated more than just for demo purposes.

Just print it and find out. If there is a problem put in heat set threaded inserts. I don't think it'll be a problem though

> If there is a problem put in heat set threaded inserts

You would be surprised how many screws will go lose after a a thousand of hours of operation, even in this case.

I'm a mechanical engineer, product designer and technical leader that has designed many products, some of which have been implanted in people and are in the Smithsonian, etc. I know screws can come out, that's why they make loctite.

What I'm saying to you is to make sure something is an actual problem before saying it is. In this case, the fact that the screws are used as self tapping, the screws themselves create the threads, likely combats this. Like a nylock nut.

This isn't what I would do for say, a surgical robotics system, but that's not what this is.

If given the choice of having the design as is, or making it more expensive, more difficult to assemble, and less accessible. I'd choose it as it is.

There are always tradeoffs. I believe the designer made the right ones here.

Before saying they didn't, maybe build one.

This is a case, where appeal to authority actually makes sense. Thanks, I accept your point.

If you like, you can always slightly modify the parts in CAD before printing or even after printing with a drill bit and then put one of those heat set threaded inserts in with a soldering iron. Not a big deal. Do what you want

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