Which material for CNC hold-down clamps?

[curious aardvark]

nylon would probably be your best bet.

That said - pet-g is weird stuff.

I've got a scraper I made from esun white pet-g. And it's phenomenally strong,
The miniture gears I made from blue colorfabb xt - were incredibly strong.

Everything else I've made from pet-g has been crap.
And for the life of me I can't remember what settings I used for the gears and scraper :-(

I think the trick is to print very slowly and in very thin layers. The scraper was printed at 0.1mm layer height. Even my dad only managed to break off the leading edge up to the 0.3mm thickness mark. We were trying to change an awning on a motorhome, made the scraper to remove glue.
My dad is one of those people who looks on the phrase: 'virtually indestructable' as a challenge :-)

So yes pet-g can be superstrong, IF you get the print settings right. And given how much of a bastard printing in nylon is - it's probably worth persevering with the pet-g.
Or any of those I mention below.

Mechanical parts - depends entirely on what you're making and what your printer is capable of. I love pla.
But if you have a heated bed enclosure and a high temp extruder - then things like poly carbonate and PEEK are incredibly strong.
Bear in mind that PEEK runs at around $800 per kilo.
Polycarbonate is a 'slightly' cheaper at $40 for 750gms :-)

Now pc is also a right bastard to print with. you need the heatbed very hot, the extruder around the 260-280 and it doesn't stick to anything that well.
Oh yeah except the sheet of buildtak you get with the polymaker pc-plus. It sticks to that like it's been epoxied. I never did get the test print off the buildtak. I managed to seperate the buildtak coating from it's base layer. But not the print from the top coating.

It's also worth looking at ninjateks armadillo. That's a hard polyurethane that prints like pla and is incredibly tough.
Also bloody expensive. https://ninjatek.com/products/filaments/armadillo/
Around £60 a kilo

This stuff sounds good - I've never seen it in the real world or had any to test. But I'll see if globalfsd have any. http://www.polymaker.com/shop/polymidecopa/

Nylon 645 sounds interesting.
Just ordered 10 metres. Finding a nylon that will print easily would be fantastic.