I use a magnet on a 1 sec delay timer when the smelter triggers to suck up the bar and the nuggets that fall out. After a second, they drop onto the conveyor below.

Then the conveyor routes around the setup (one magnet and exit conveyor per smelter), kicks the bars out and recycles the nuggets back into the sorter for the smelter.

This method doesn't give super accurate bar sizes, but they're within a couple nuggets weight usually. I thought this was way easier than stopping the belts every time the smelter dumps, just to prevent nugget jamming. I DO get the occasional stray nugget (one an hour or so) that the magnet just can't get, but nothing compared to the gem jams.

Now, I just need an automatic bar stacker. :)

Magnet setup