This isn't your grandfather's Affinity deck. In fact, it's barely an Affinity deck at all. I'm calling it that because it shares a few cards with traditional Affinity. There's Arcbound Ravager, Arcbound Worker (a staple in the Standard variant for those only familiar with the more recent Modern iterations), Mox Opal, and a couple others.
But this deck is significantly different than previous Affinity decks on a structural level. Rather than go wide with cheap creatures like Signal Pest, Ornithopter, and Vault Skirge, we're trying to go big. How you ask? Hardened Scales, of course.
As it turns out, Hardened Scales has a lot of synergy with various artifact creatures in Modern. It's great with modular creatures, especially Arcbound Ravager. It supercharges Steel Overseer, and we saw in Standard what it can do with Hangarback Walker, so it and the similar Walking Ballista also make good additions.
This deck isn't going as explosive out of the gates as traditional Affinity, taking a turn or two to set up, but once the engine gets rolling things get out of control almost immediately. The ability to put a bunch of counters on a Hangarback Walker and then sacrifice it with Arcbound Ravager can give you an army of fliers to pump with Steel Overseer or a pile of artifacts to sacrifice with Arcbound Ravager and make every attacker lethal. You can also go all-in on Walking Ballista to fireball your opponent if combat isn't an option.
With these synergies as the main plan and no Springleaf Drum to enable, the switch away from zero-mana creatures and toward Welding Jar is a heady one, as that card has always overperformed in my eyes anyway. I also like role player Throne of Geth, which naturally enables all the synergies in the deck even if you just sacrifice it to itself as a one-shot effect. But at its best you get sequences like the following:
T1: Hardened Scales
T2: Hangarback Walker (2 counters)
And unlike traditional Affinity, this deck doesn't have you drawing Memnite on turn 6 and questioning all your life choices. Almost all your draws are good topdecks, especially Ancient Stirrings and Evolutionary Leap. You're going to be worse against the fast combo decks, but much better against heavy disruption, which is where Modern sits right now.