UGH! So many good ideas in this thread... It's impossible to really read them all and comment on every one, but I was thinking about things I would like, and went searching for a thread like this to add them to.
While it's been said before on numerous occasions, here, I would like to mention conveyer belts again. I've been thinking about the way that they might be implimented, and I think I have a good idea regarding that (apologies in advance if a repeat):
Conveyor belts can be run alongside (or possibly through) workshops, and set to dump or haul items automatically.
Workshops, as they stand now, essentially rely upon a "dump" method of item sorting. As far as I can tell, workers gather "input" materials, and dump them on the floor. When they craft a new item, they dump the "output" on the floor, and go looking for a new input material for their next job, leaving it to the haulers to clean up after them.
What if, instead, "inputs" were assigned to a single tile in the workshop, and there might be multiple "output" tiles. Conveyors could run next to or through the "input" tiles, and if something that the production orders of that workshop would require, and was not already present in sufficient quantities for that workshop ran past the workshop, the workshop (if manned, if we want to be really realistic about it) would automatically pull that material off the belt, and into the input bin. Outputs, meanwhile, if they were sorted into different categories (I'm thinking a slaughterhouse, for example, would want two seperate output lines, one for the meat and fat, the other for the chunks and bones) would have seperate tiles, so that seperate conveyors could carry off their products, either cycling past workshops (potentially forever), or just dumping them into a warehouse (potentially controlled by a switch that might go to a swinging gate, or just a single conveyor stretch that might be reversable.
All this should also include some vertical "bucket" conveyors (dumwaiters / elevators), as well. Such conveyors should probably just dump things at the top (potentially to other conveyors), since if we wanted things dropping down, you just need to convey them into a pit.
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Other ideas for mechanical power-requiring workshops:
High-temperature furnace: The high-temp furnace would be capable of firing reactions normally out of reach of normal dwarven technology. This idea is based upon the actual ancient chinese use of technology like this, so it should be well within the "flavor" of dwarven tech. This furnace, instead of using the irregular billows, can use a screw fan to supply a constant flow of a larger amount of oxygen than a normal forge does, allowing for a hotter fire. (As it stands, magma smelting is probably plenty hot, but we can just look the other way, and agree that a mechanically more complex system should drive greater benefits.) This allows for burning more fuel more quickly, and burning a hotter fire.
(This means that it would require rotational power, as well as a giant screw and probably a tube, just basically being an "air pump")
I was going to detail a "porcelain" craft, but you know what, I'm going to make this a seperate suggestion since it gets very long and detailed about how to make an entirely new kind of commodity.Powered Looms: Real-life, when mechanization took off, mechanizing the textile industry was one of the easiest things to do...
In any event, at its most basic, looms can be mechanically powered, so as to help make them, at the very least, work faster. It would actually be best if it were combined with a seperate idea, however, to try to make dyes a little more versitile.
Powered looms could, perhaps, be able to take in thread with multiple dying colors, and use them at the same time to make cloth that is multiple colors, or rather, patterned. In much the same way that a reaction that takes in four ores would produce four metal bars, two threads used at the same time would produce two cloth units, both with the doubled dye pattern.
This would also be able to potentially lend assistance to the poor, much maligned Hide Root, as every additional dye color improves the value of your clothing, even if they are not the most valuable dyes.
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While I wanted to put a few more examples on this list, I have actually been so distracted by forum crawling while leaving this post open that I've actually kept it up for several hours, and I think that it's plenty long for today.
When I remember what the other ideas I had were, I'll post again.