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Comment by s1artibartfast | Hacker News Reader

> I can't work with plastic.

Why not? If you can machine it from metal, it is easier to machine it from plastic. I fabricate plastic replacements often with a drill, files, and saw.

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It's because plastic injection molding has different constraints and trade-offs in parts design compared to metal machining. E.g. injection molding, after the mold is done, doesn't really care about machine time, complexity, or the availability of specific cutters and drills. So sometimes the geometry and tolerances of an injection molded part is a pain in the arse to replicate manually -- it's just not made to do it, unlike metal machining, which at scale is still a rough approximation of the manual process.

I get the manufacturing Tradeoffs. What I was responding to was not an issue with part complexity, but part material.

They said "I can work with metal. I can't work with plastic."

Sure, nobody is going to machine a plastic replacement complex injection molded housing. You probably werent going to re-create a complex press-formed metal part either.

IT seems like it is more of a design complaint than a material issue.

How often do you encounter mass-produced consumer goods that include parts made of machined engineering plastics?

I have a pocket knife in my pocket (perhaps not quite as mass produced as what you were imagining) whose handles are made of G10 which is a composite material made partly of epoxy. It has been 3d machined into its current shape.

That's not what Im saying. Machined plastics are rarely encountered, but many molded plastic products can be trivially machined.

Im saying that the fabrication difficult is driven by design, not material.

almost nobody is going to home fabricate a spline gear at home, and it doesnt matter if it is metal or plastic. If something is like a plate or flange, it is trivial to fabricate and doesnt matter if it is metal or plastic.

For any given design, I think refabrication is the same or easier for plastic.

Not sure why we're arguing, but I think we are on the same side. We would prefer that products have parts which are easily replaced by, in order of preference:

1) easily-sourced commodity products like standard screws, washers, bolts, etc. 2) barring that, parts that could easily be fabricated by realistic home production methods (hand tools, FDM printing, possibly simple machining) 3) barring that, parts that the consumer can have easily fabricated by a third party (maybe it requires a 5-axis CNC but all the CAD/CAM files are available to upload somewhere like Shapeways) 4) barring that, easily-ordered at-cost OEM parts

...and in all cases the user manual should require all relevant drawings with dimensions.

The problem is that if you tell an industrial designer to keep costs down, and that they can use injection-molded plastic parts, they will almost certainly NOT design parts that are conducive to 1-3. They could, but all the incentives run the other way, so they probably won't.

I was mostly just curious about the person who said they absolutely cant work with plastic, but can work with metal.

I like some things that are repairable, but don't think everything needs to be. My product choices rarely are willing to compromise cost, function, or aesthetics for repairability.

That's the idea (I assume, not the same guy): you're right that is mostly the form of the part that makes it easy/hard to work, not the material, but nobody designs machinable plastic parts. they design injection-moldable ones. So in practice "plastic part" is synonymous with "impossible to recreate in small quantities". Yes, in theory plastic parts can be of the same shape (and therefore ease of boutique manufacturing) as metal ones, but they aren't, not in the real world.

It's an... unusual skill to have, I'd say. Maybe it's the issue of education or culture, but I'm with GP here: in my mind, metal parts can often be repaired by hand, or an improvised replacement can be made; plastics break too easily, and you can't make new ones without a 3D printer or something.

Yup. Same with wood and fabric. Those kind of parts or components I can replace and work with. Plastic? That's a whole different ball game due to the potentially low tolerances in terms of dimensions and the nature of the type of plastic used. With wood, metal, and fabric it is much easier to gauge the correct replacement material.

I'd say education and culture might be right on the money. I'm not sure it's occurred to me to take a small block of some type of plastic and cut it to shape using knives, planes, chisels and files like someone could with wood, but now that I'm thinking about it, it seems like it might be considerable easier to work with than wood in some cases, especially with how easy it is to join two parts afterwards with some epoxy or maybe even through heat.

Additionally, it looks like you can possibly re-melt the shavings into another block (I'm not sure if specific plastic types are required).[1] That's like woodworking but being able to easily gather and compress your bits and ends and sawdust into more wood.

1: https://www.youtube.com/watch?v=34wPmcgDRmg

Yeah, I was gonna say. Not that I'd necessarily want to cut a gear with hand tools even in nylon, considering how exacting the profile would be to get right, but it's doable.

I'd rather hand cut a gear in nylon than one in steel. If you arent doing it by hand, who cares what it is made of.

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