[Animats]

That's usually done with something called a rotary broach.[1] This is a clever trick. You first make a round hole. The rotary broach is a hex-shaped cutting tool. Both workpiece and tool are clamped in a lathe. But the center of the tool is slightly offset from the rotational center of the workpiece. Both spin, but the eccentricity makes it cut a hexagonal hole. Here's the process.[1]

A milling machine can cut a hexagonal hole, but the inside radius at each corner cannot be smaller than the radius of the cutter. A 4mm hex hole would require a tiny cutter to do a good job. Here's that process for a larger hole.[2]

If the hole goes all the way through, just get a hexagonal punch. Might need to drill a round hole first.

Or just buy a 4mm socket with a T-handle. Cost US$4.99.[3]

[1] https://www.youtube.com/watch?v=_AYEFjbGaL4

[2] https://www.youtube.com/watch?v=zOqSIRuBgCY

[3] https://www.amainhobbies.com/rc4wd-metric-hex-twrench-tool-4...

[throwaway81523]

Thanks, yeah, a separate 4mm socket with a handle is certainly a possibility, but I liked the idea of a 4mm hex hole in something like this, to accompany the holes that are already there:

https://knifeworks.com/crkt-9100kc-eatn-tool-black-oxide-fin...

I'll look at the rotary broach video. Yes the hole would go all the way through, so a punch sounds ok. Anyway it's not about making this one hole. It's more an example of the kind of stuff I'd like to do with metalworking gear if I had access to it and knew how to use it.

It does sound like hard materials are an obstacle as well. Aluminum is a start though.

[Animats]

Ah. Here's the tool for that job - a Roper-Whitney hand punch.[1] This is like a hand paper punch, but stronger. Costs $85. Made in USA. With a punch and die, you get clean edges on the hole on the exit side. Scroll down for how to order the specific punch and die you need. Hex punches are not common, but are available. If you fill out their form, and tell them you need to punch 3CR13 stainless, they'll tell you what to order. Might need something with more leverage than the small hand punch. Lubrication helps.

Most machine shops will have such a punch, but they won't have a 4mm hex die in stock.

Or you could drill an undersized round hole and file it out to a hex hole. Maybe use the existing round carabiner hole.

There are touch-up kits for guns, called "gun bluing", to make your shiny new hole black.

This is all do-able but way more trouble than it is worth.

[1] https://www.roperwhitney.com/our-products/no-5-jr-hand-punch...

[throwaway81523]

Cool, thanks, yeah for a single hole, doing it by hand with a needle file sounds more practical. Does something like this seem CNC-able?

https://sakparts.com/products/can-opener-diy-tool-part-for-8...

The idea isn't to duplicate an existing can opener, but rather to make custom specialized tools to fit into a SAK, or replacement knife blades out of specialty knife steels (i.e. very hard, so probably difficult to machine). It wouldn't just be for SAK's but also for other folding knives, each with its own special cutting pattern to work with its pivot and locking system.

Titanium is another material of interest, for ultralight gear.

All of this is probably impractical at the hobbyist level with limited work space, though. Oh well.

[Animats]

DIY knife forging is common. Search "knife forging". Look around for classes and forges with training. There's a huge amount of info on knife making and metallurgy. People obsess on this stuff. A good knife is a trick of metallurgy. The blade must be hard at the edge to be sharp but ductile in the body so as not to be brittle. How to do this is well understood today, but there was much mystery around it for centuries. If you're fascinated by metalworking, but don't have to make machine parts, that's a hobby direction.

Victorinox knife manufacture.[1] Stamp, heat treat, grind, polish. They're not exotic blades, just good manufactured stainless steel parts.

A primer on machining titanium.[2]

[1] https://www.popularmechanics.com/adventure/outdoor-gear/a351...

[2] https://www.harveyperformance.com/in-the-loupe/titanium-mach...

[throwaway81523]

Sure, knife forging is one thing, but the idea of making replacement blades for folding knives sounds like it takes machining, because of the weird shapes needed at the pivot end, especially for weird locking mechanisms. It would be nice to be able to do that at a semi-commercial level if one were to get into it at all. Victorinox is on a completely different scale, making millions of units of whatever. But forging is for making one or two of something, while CNC machining is interesting for making a few hundred.

In practice I don't have it in me to pursue something like that for real. It's just interesting to find out about.

If I simply wanted to make knives, then no machining would be needed, just some cutting discs and belt sanders. It's the specific thought of making replacement blades for existing folders that seems to want more automation. I guess there are existing shops that can do that type of thing from a CAD drawing though.

[WillAdams]

We (ob. discl., I work for Carbide 3D) have a video series on doing this:

https://www.youtube.com/watch?v=G5TYla5x-Nk

[throwaway81523]

Thanks! That looks like just what I want. I looked at the first few seconds just now, and will watch the whole thing later.

[bluGill]

Knifes are done with heat treating. you need to get the steel red hot and follow the proper cooling process. You machine soft iron to close then make it too hard to machine, finally grindit to the perfect size.