Is there a reason this doesn't tap but instead has the "once per turn" thing?

I think that was because of our common Wrench, since it can untap artifacts.

Are we sure we want the interactions with this an modular? It's seems like the board state could get really complicated really quick.

I think this is going to get some players confused. Imagine the questions: "The cost is colorless! How could I not spend colorless mana?"

So you take Thran Dynamo, add this...and a second copy of this. The "once per turn" restriction isn't such a hard limit after all.

Even better, have this, Dynamo, and an uncopied Sculpting Steel, so that each of the copies can become something different. Then you can make three copies of Dynamo and one copy of this each time, and the whole process is a bit faster. It also gets around the attempted drainplug of changing the targeting condition to "target artifact not named Assembly Line".

Since I think this card might be broken beyond repair, what about changing it back to Mirrorworks? I seem to recall that being what this was originally. Thoughts?

I agree this appears pretty massively broken. Copying artifact creatures with modular and sacrificing artifact creatures with modular does also sound like a recipe for... making certain Johnnies very happy and confusing everyone else.

I think the idea of an assembly line is good, just this is too strong. I think we could make a better factory. What if it made 3 assembly worker tokens via etb, and then allowed you to tap assembly workers to copy a noncreature artifact you control other than assembly line?

The classic factory card either make small artifact tokens, or pump out mana (possibly spendable only on artifacts)

So maybe just get back some mana every time you feed an artifact to it?

Something like this...

Assembly Line
Artifact [Rare]
When ~ enters the battlefield, put two 2/2 colorless Assembly-Worker artifact creature tokens onto the battlefield.
­, Tap two Assembly-Worker creatures you control: Put a 2/2 colorless Assembly-Worker artifact creature tokens onto the battlefield.

I was thinking a 6 drop with.

Steam powered: When Assembly Line enters the battlefield, if only colorless mana was used to cast Assembly Line, put 3 2/2 colorless assembly-worker artifact tokens onto the battlefield.
Tap 3 assembly-workers you control: put a token onto the battlefield that is a copy of target noncreature artifact other than assembly line.

But then it's likely be useless if you didn't Steampower it. Yeah we have Assembly-Workers at common, uncommon and rare but what are the odds you'll have three on the field at once normally.

Plus I kinda like the image of Assembly-Workers assembling Assembly-Workers in a sort of ever growing cycle. Not to mention the word play on Assembly Line.

The pun convinces me.

Are we ok with assembly workers not tapping to give +1/+1 to another assembly workers? That's kinda their thing.

The Assembly-Worker token from Urza's Factory didn't have the tap ability.

Good point, changed.