What skills for building machines?

[Inky]

Some of the "New Inventions" section in Basic Set is a bit confusing, and there isn't much about things that aren't new inventions.

If B is helping A to build a machine that A already knows how to build, what skill does B need - does it have to be Engineer or will Mechanic do? And what skill does A need?

[Kaslak]

Originally Posted by Inky

Some of the "New Inventions" section in Basic Set is a bit confusing, and there isn't much about things that aren't new inventions.

If B is helping A to build a machine that A already knows how to build, what skill does B need - does it have to be Engineer or will Mechanic do? And what skill does A need?

I think for building it depends on what you wish to build, and you might need to refer to the skill description. Building seems to me in a gray area between design and repair skill groups (e.g. Electrician explicitly allows building, while electronics repair does not).

My approach would be : in general, note that that engineering defaults to the corresponding repair skill at -6.
You could argue that building under somebody else's direction (that must have passed his own engineering roll), when you have all the parts and tools, is a routine engineering skill task (+4) resulting in Mechanic -2 (including the default -6 from the technician's skill). You can take extra time to offset the malus, if the engineer is not too impatient.

Things might change (or require additional tests) in the following cases, where dedicated technician expertise might be needed:

If you need to create tools (something that would fit in a workshop), as part of or as prerequisite for the invention, or mechanically modify parts you might also consider Machinist.

Note that it is also mentioned explicitly in the skill description, that Machinist rolls might be required "before attempting an engineer roll to assemble the gadget", In my opinion, that could also apply to existing inventions, if you cannot simply use standard size pipes, flanges and screws, or whatever is needed for the invention from the market, for whatever plot reason.

If it requires also to work raw metal to create the individual machine assembly parts you might need Smith, and a reasonable amount of time.

Armoury is the specialized skill for weapons and armored vehicles.

[Refplace]

What Kashlek said.
I would say that Engineer is about designing and designing is more than just inventing. Want to build something to specific expectations or specifications? That requires Engineer (even if at default).
Need to retool parts? Thats Machinist.
Need to tweak an existing item? That's more like Armory or Mechanic.

[malloyd]

Building a machine you are familiar with is just like any other craft skill to make something, which admittedly GURPS doesn't go into a lot of details for.

My rule of thumb is it costs you 1/4 to 1/2 its price (for whatever quality you want) in parts or raw materials (if you go cheap, it may require a second skill roll, often against Machinist, to get them into usable form), around price/$10 hours (less if you had very specialized tools, but you don't if you aren't in a factory that makes these things), and a roll against the same skill you'd use to repair major damage (it's completely in pieces is pretty major). So, yeah, Mechanic (machine type) would usually be fine for a particular machine. If you want it faster, or in better quality without taking extra time for the higher costs, just use the usual rules for taking less time, fine is usually x 4 cost, and completing it in 25% of the usual time is -7 or 8, so you could roll at -8 in the normal time, which seems about fair.

This usually works OK - yeah sometimes the numbers aren't perfect, but they are never too unreasonable and building stuff is never such a big focus that it's a problem. If for some reason you were planning a game where most of the play time was about making stuff, you'd probably want to do some research on the specific stuff, and want some more interesting rules.

[Anaraxes]

Not only do the design skills (Engineering, etc) default to the repair skills at -6, the repair skills typically default to the design skill at -3 or so. (Mechanic is Engineer(same) -4; Electrician is Engineering(Electrical) -3.) Note also that engineering is typically Hard, while repair is typically Average. So, with equal point investment and stats, the engineer has a skill number in Engineering one lower than the tech (Mechanic).

In other words, all technicians have at least a little ability to design (-6). And all engineers have a somewhat better ability to build (-3). Techs are better builders than engineers in the sense of actually handling the tools and correctly executing the procedures to solder, tighten a bolt to a torque spec, etc. Engineers are noticeably better at understanding the theory and design principles behind what they're making.

No one absolutely must have both skills, because they default either way. A character (especially a cinematic character) that's supposed to be good at both probably has points in both skills and doesn't use the defaults.

So, to be specific about the way I interpret the OP's question:
(1) Inventions use the design skill (engineering of some sort). If all you have is a tech (say, Mechanic), then they can still invent -- but they're using their default skill in the design skill (Engineer(Mechanical)) to do so. Not recommended, but possible.
(2a) B is working under the direction of A-the-engineer, so only needs Mechanic. (Note the paragraph on B346, Long Tasks, about directing others instead of directly working. A can either give B a bonus, or just work directly. Supervising probably isn't worthwhile unless A isn't as good as B and you need skill to succeed in a tricky builds more than you need twice the man-hours put into building a big job.) An engineer can hack together their own prototypes, but they'd rather have a tech to do that. The engineer can help with the building, but the tech is better at it.
(2b) Both A and B would roll against their repair skill for the actual build of a prototype. Someone has to already have done the design and produced detailed plans at some point. This might have been A as engineer, since "A already knows", or A might simply be a tech that has those detailed plans for the "know how to build" part.

So, an engineer can invent and build prototypes with nothing more than their engineering skill, defaulting the tech skill necessary to actually build their prototypes. The team will be better off with one of each. A team that has access to existing plans from a third party to build a well-known machine is better off with two techs.

Those two techs will also be better at building a new invention prototype than two engineers or one of each, but they'll struggle to get the bugs out of the design. (Techs do diagnose in order to repair, but there's a difference between diagnosing faulty components in a typical device and finding the design flaws in the prototype of a new device. A prototype may be flawless from a component and construction standpoint, and yet still not work because the design or theory is flawed.)

[Varyon]

Generally speaking, I'd go with using the same skill for building and repairing. To my knowledge, LTC3 is the only rulebook that actually has rules for making things (rather than designing or repairing them), and it generally uses Repair skills as Build skills (in cases where neither skill exists, it tends to use Professional Skill).

Low tech machines kind of break the mold here, where it seems one uses Mechanic for repairs/maintenance and Machinist to actually make them. That feels a bit off when considering Machinist doesn't have required specialization. Part of me feels like Machinist works for simple machines as-is, and also allows you to build anything for which you have the appropriate Mechanic skill (probably using the lower skill... or maybe the lower of (Lower Skill +2) and (Higher Skill)). Maybe that should be a general trend for other things? Someone with Armory (Melee Weapons) knows how to repair and maintain swords; someone who also has Machinist knows how to make them.