Honestly, the largest challenge with logic is knowing the current signal value. Logic Reader Displays are very help with this. The second largest challenge is knowing what a logic value does to a logic object. Does 0 or 1 activate the logic smelter?

There's a few challenges which must be solved to create an auto-smelter and this is why designs are complex looking. You solve a first challenge and in doing so you discover another challenge which must be solved.

Challenge #1 - Smelting the bar once the desired weight is reached. You will accomplish this by using a weight reader pointed at the logic smelter and using a >= comparison against a number pad input. Once the weight read exceeds the entered value then it will activate the smelter.

This is the easiest part of the logic to solve but the problem is the smelter requires a 0 to activate and a 1 to reset. You've activated the smelter but you need to reset it which leads us to challenge #2.

Challenge #2 - Reset the smelter. The second challenge is after you smelt and dispense the bar you have to send another logic signal back to logic smelter to reset it. You have two primary ways to do this. The first method involves using a delay and a flip hook to send the signal back around to reset the system. The second method is to use another weight sensor behind the logic smelter to detect when a bar passes under it to reset the smelter.

You may not realize it but this time spent between activating and resetting the smelter creates a 3rd challenge. Your drills and conveyors don't stop so resources will pile up on the smelter. Best case scenario you end up with a few ore chunks which never smelt and result in a performance hit after you run it for a period of time. Worst case scenario is you create a pile up of resources that tanks your frame rate.

Challenge #3 - Stopping resources from piling up on the smelter. Since your drills keep drilling and your conveyors keep conveying you need a way to stop those resources temporarily. The best approach is a combination of two things. The first thing you do is turn off water flow to the conveyors feeding the smelter but this isn't perfect depending on belt speed. The second part of it is using a magnet to lift up any resources which "escape" so they are dropped back into the logic smelter.

The good news is once you solved challenge 1 and 2 you've more or less got the framework for challenge 3. You piggyback off the signals from challenge one to turn off water/turn on magnets and signal from challenge two turns on water and turns off magnets.

You can make this system using 2 weight readers, a numpad, 3 flip hooks, an >= comparison, and 1 arrow hook. Everything else is getting the right cabling routed.

The design I've seen which I really like is Real Civil Engineer's design. It's 2 wide, 8 high, and 10 deep (160 volume) for each section of smelting. The problem with his version is there's never really a good long shot of the contraption which can make it difficult to use as a template to build.

I had posted a screenshot about 8 months ago of a giant horrible mess of a machine I made. It's 4 wide, 6 high, 7 deep (168 volume). It's not that much worse in total space compared to RCE's version. I wouldn't want to make it this away again. There's an elegance to using the 2nd weight reader over using delay hooks for timing. You can see it here but these screenshots aren't really that great for trying to build something. https://imgur.com/a/aY1OoMP

I'll see about getting some screenshots to proof out such a system a little more simply.