Is it weird to build stuff from scrap while waiting on the next job?

Every shop I've been in has a scrap box and somebody messing around with it, so you're in good company. The trick is making stuff that's actually useful so the boss sees it as practice with a payoff. The grinder stand you made is perfect becuase he'll probably end up using it himself without ever admitting where it came from.

The unwritten rule I've learned: keep it small, use the scrap pile, and have it done and cleaned up before anyone starts asking why you're not standing around. The second you're cutting up decent stock or making something daft, that's when you catch grief. A bead on a corner of angle iron takes what, two rods? Nobody's costing the company money on that. Things I've made or seen made during dead time: a torch stand that actually stops the leads getting kinked, a heavy door wedge, a little anvil-style table for tacking small stuff at a comfortable height instead of hunching over the bench, and a rack for clamps that was just badly needed. My favourite was a simple frame to hold a bucket at waist height so you're not fishing it off the floor. All out of offcuts, all got used.

If your goal is keeping your hand in, mix it up. Do one joint flat, one horizontal, try a vertical if the offcut allows. Rotate your rods too, especially if you've got an open pack of 7018 sitting around going stale, though that's more about the rods than your practice.

As for the boss not smiling, don't read too much into it. I've had foremen who'd moan if you breathed wrong and then quietly take the tool you made home. If he never says a word but keeps walking past your little projects, you're fine. If he actually tells you to knock it off, then you just keep them out of sight and make smaller things.

Been doing this since the 70s and it never went out of fashion. Back on the pipeline we'd burn scrap between joints just to stay warm in the hands, old-timers called it keeping your eye in. I keep a chunk of thick plate on a stand in the shed and run a bead or two every morning before I touch anything that matters. Costs me next to nothing and the first real weld of the day is alredy dialed in. If the dead time hits regular, maybe ask the boss if there's some maintanence job he's been putting off, gates, brackets, a cart that squeaks. There's always one. Half the time he's just glad someone offered.

I'm not on a crew or anything, just got a little MIG setup in the garage, but I get the same thing with dead time. My problem is the opposite though, I've got nobody around to tell me if my stuff is garbage or not. So I started keeping a running list of little jobs for the house, brackets for the hose reel, a holder for the garden tools, that kind of thing. Sounds daft but it works out well since the wife thinks I'm being productive and I'm really just running beads. One thing I noticed, the first couple welds after a break always look like porridge, so now I warm up on the same ugly offcut I keep chained to the bench before I touch anything I actually care about.

Gotta disagree with the last bit of advice in the first reply. If the boss tells you to knock it off, the answer is knock it off, not 'make smaller things and hide them'. That's how you turn a nothing conversation into a write-up. I had a supervisor years ago who let one guy slide on little projects for months, then the same guy got walked out over a bigger one because he figured the rules were flexible. They weren't. One quiet warning usally means the next one costs you. Also on the stale rods thing, once that open pack has been sitting a while no amount of running beads on scrap fixes them, so don't burn your dead time practicing with rods that'll crack on you anyway.