One of my players has a really high Trade: Armour smith skill.
Not really knowing what we where doing, he ended up making best quality Carapace Armour for less than the price of Average Carapace Armour.

Now he wants to make a Thunderhammer.
After days of back and forth, Iv given in for the simple fact that any more arguing over it will result in me not DMing anymore, its that much of an issue.

The problems arrise when my player aruges that it should be vastly cheaper to make an item than it is to buy one and the rules on cost for crafting referance creating a building for the 'making from scratch' section which he argues isnt what hes making and therefor hes right.
Then for the duration comes into play. The example for 'Days' duration is making custom armour, which he syted as proof that you can make Power Armour in days and there for since hes 'Just making a hammer' while canablizing a power weapon he already has, it shouldnt take him any more than days to do. My idea of years (which can easily be reduces to weeks with a -20 to the roll, or -30 for days) because its Tech that revered as holy and difficult to make therefore it takes a long time to make. But alas and alack, he refuses to agree to this, calling bull---- and that Im just trying to stop him from making it.
But with his skill and quality of materials at hand, he could make a Thunderhammer is MINUETS with about a 50-60 stat to roll against.

Someone, please tell me there is a detailed errata for the crafting that fixes all of these issues before I hang up my GMing badge for good. I didnt sleep last night I was so stressed out about this. (Sad I know but true.)

Even if there is no erratum for this (I haven't checked yet), I'm sure your player will understand that if he crafts this thunderhammer and so puts his DM out of the game, the hammer itself won't be very useful.

I'd use my godlike DMpowers to rule that a thunderhammer is just too hard to craft, that his current power weapon can not be used as a base for the hammer or that he misses vital information to create this thing...

If this would mean the player invested a lot of XP in the trade skill that would then go to waste, I'd allow him to retreive the XP and then let him buy other advances.

Acolytes that have even days to spare are not working their primary purpose, rooting out threats to Imperium. Period.

Also keep in mind that Crafting allows a character to make what he or she already knows how to make. It doesn't magically impart Lores that gives them total understanding of the ultra-rare, closely guarded secrets of ancient technology which they have never encountered before. For that matter, how does this character even know what a 'Thunderhammer' is? They are not commonly spoken of; the tiny minority of people who have heard of them (never mind seen one) are in the highest echelons of Imperial society, with very high security clearances and whatnot.

Heretical as hell.

Tell your player no.

Blatent powergaming aside, a thunder hammer isn`t even a conventional power weapon, it`s more a hybrid of a power maul and a shock maul with a massively amed up power source.

I`d consider the ver simple rule of allowng the plyer to only craft items of a base common or better availability for free. any rarer item he has to "sacrifice" one to learn it`s secrets. (i.e. he has torn it into little bits and tried to rebuild it a fewdozen times.)

Me? I'd go "Sure! - if you have the proper blueprint."

And then make sure he never got the blueprint. Ever.

Aren't Thunder Hammers incredibly large, incredibly heavy, and generally only wielded by Space Marines? I'd let him get the blue prints, make the hammer, and then realize after the fact he just made a weapon too big and heavy for him to even carry around, let alone use.

I agree with the DM's here talking about lore, he would need to know how to build one already.

For example, I might be really good at working on car engines but that doesn't mean I can automatically fix a jet.

Well, not much to add there. Unless he has a Mechanicus-Sanctioned Blueprint and Pattern, whatever he'll build will be obviously a new model, likely FAR inferior to every Thunderhammer out there and obviously heretical to many a Tech-Priest who knows how to build those things.

Furthermore, the Power Field of the Thunderhammer is far more advanced then that of a Thunder Hammer (which also has, as mentioned before, a shock-module as well as usually some sort of concussion-gizmo) so the most he'll probably get to build would be a Hammer with... probably 1D10 + 4/1D10 + 5, a Pen of 6 or 7 as well as the "Unwieldy" quality - certainly a stylish hammer with a Power Field, but far from gamebreaking in any way, if they have access to power swords, in my opinion at least.(Hell, I let one of the players in my Rogue Trader game build a 9-71 Energized Entrenching Tool, basically a Power Spade - then again, it's Rogue Trader)

I would quote a text about why technology in the Imperium works the way it works (or don't) but I can't find it currently, just be always aware that whatever the Imperium is doing with Technology is the most it can ever hope to do, it also compares the Adeptus Mechanicus to a single librarian trying desperately reconstruction some of the text out of a library which burned down to the ground... twice. If someone knows where it is, can you link it please?

Edit: Found it: http://1d4chan.org/wiki/Adeptus_Mechanicus#Why_Everything_is_so_Grimdark

Heavily influenced me on depicting the AM and Technology in the Imperium.

And if all of that fails... talk to him about it. Directly say "I think you having a Thunder Hammer at this point would be detrimental to the fun of the group and it'll probably break my story." If he still doesn't understand, tell him to either look for a new group or a new GM, depending on the dynamic in your group. (Yeah, I know, 'GM is nothing without his group and should GM so the players enjoy it', but whatever, it's still the GM creating the setting and doing the work of preparing plot you can enjoy, so ultimately whatever the GM says, goes. Something I always exaggerate in my groups when I GM, as it saved me a lot of trouble a few times )

It's all been said already: You can't make that which you don't know how to make!

But more importantly, I think you need some assistance (or tender love and care, if you prefer)

Having one of your players have a Thunderhammer, should not make you want to quit GMing! You just need to realise, having a massive hammer of Uber Ouch, does not make a player untouchable! It makes him devastating in melee, certainly, but everyone can see this hammer from miles away, and nobody in their right mind would engage him!

Let the cowardly enemies snipe him from afar! Let them unleash webbers on him, then flamers, perhaps some suppressive fire to keep him still while he is flames, webbed, and grenaded into oblivion?

Put him on a boat! Then sink it! Swimming with a big, 2 handed hammer? Sounds like a swim-30 test. Read the swimming rules! Laugh!

Invite them to some formal event. Have the doorman point out that 2handed hammers of excessive doom are not appropriate for the venue. Laugh as he collects his "wardroom chit" for the priceless artefact that will be stolen in 10 minutes.

Have members of the Ad.Mech show up, and insist upon seeing the formal writs of authority from the Priesthood of Mars for manufacturing such holy items. He doesn't have them? (Then how did he make them in the first place?) Oh, that's not good. Think Apple vs Samsung "I sue you for a crapload of gazillions of Thrones" not-good.

There are countless ways of countering such a heavy hitter! Don't be discouraged!

Just to be clear: Nothing "breaks the game".

It's a big galaxy out there, and anything your players can do, your NPC can do better.

If your players are getting powerful, that's just an excuse for you to break out the big bad guys. And to make their gear work against them.

Trying to infiltrate a lower hive gang? That big, power-hammer is going to look pretty conspicuous. Going to war? I'll see your hammer, and match you a pair of regular guardsmen using an Autocannon.

According to the Inquisitor's Handbook, modifying or creating power weapons is a Very Hard test so it has a -30 penalty.

Also note that Trade (Armourer) and Trade (Smith) are two different skills. One is used to create/modify weapons and the other to forge metals into shape for example to create a blade or a suit of armour.

Taking the above into consideration, canibalising a power weapon that he has to "just create a hammer" is too simplistic in my opinion. Think of all the different circuitry that a hammer would have compared to a blade for example. Let alone that building the hammer is not easy as he would need to have a workshop and the proper materials to do it, which includes the appropriate blueprint as others have already said.

I would flatly say that he cannot do it.

Agree with all the observations above. To summarize in roughly the right order:

1. It is not always easier to make than buy. The Forges of the Adeptus Mechanicus deal in literally unimaginable bulk and are controlled by artificers with decades or even centuries of focused experience. Does he, "oh, but I've spent XP on it" comments aside, really think he's better than that?
2. Building a building is creating from scratch, which is what he's doing. The scale may vary but not the nature of the task.
3. Just how high is this guy's Trade (Armourer) skill?
4. You cannot be considered to have good quality materials to work from if you are 'cannibalizing a power sword' - you've got an underpowered disruption field generator (because it's a physically smaller weapon) and it's circuity is designed to project a field that's the wrong shape. Which - given what happens when a power field interacts with a solid object - has predictable results if you get the jerry-rigging wrong.
5. "Just making a hammer" from scratch - at least, one made out of the kind of stuff high-end imperial military hardware is made out of - ceramite, adamantium et al - is not the work of hours. Nor are raw ingots of this stuff purchaseable over the counter.
6. The 'pattern' for a thunder hammer is going to be exotic, rare and hard to obtain. There is a reason that only Inquisitors and Astartes have ever been shown in the background as wielding them. Simply being 'A Tech-Priest' is no garuantee of knowing this. If you don't know it, you can't make it.
7. Customising armour in days is okay - modifying a suit to fit, or air-sealing it, or adding in a preysight mask layer (maybe). His argument is flawed because building powered armour from scratch (and yes, if you start with bits other than something labeled "powered armour, some assembly required, it's "from scratch") does not fall into the same category as, say, carapace plate - powered armour's servos and support systems are inherently something completely different from just "plates of armour", it's more akin to making a suit of carapace plate PLUS four augmetic limbs PLUS a rebreather and vox.
8. Have him prosecuted for tech-heresy. If he's doing it in hours he can't have properly blessed it, if he doesn't have an STC for a thunder hammer, he can't be following one, and if he's buying the proper component materials, someone is going to want to know what he's using them for.

I've never actually had a problem like this, but I have heard the "the only good player is the one who doesn't know the rules." thing before

Quoth I think you need to stop thinking about Thunderhammers and start thinking about the whole situation instead. You have a player who is sufficiently engaged that he's sat down and spent a not inconsiderable amount of time understanding the system, but who has unfortunately somehow gotten the idea that the pay-off for achieving system expertise, should be the ability to game the system.

Your group needs to talk about this, because you can't fix stuff like this via the game. If you try to address it through the fiction - sinking his boat, denying him blueprints, or whatever (my personal fav would be gravi-gunning the hammer, but please don't do that. And shame on all you deliciously vindictive GMs ) - all you do is refuse him a pay-off. If you address it via rules, then at best you're going to have a player at the table who is more interested in throwing the rules at you as you GM, than he is in enjoying the fiction the rest of you are creating. If you try to use the game to solve this problem, whether it's the rules or the fiction, you're only going to make the experience increasingly bad for the both of you.

Also, shake off any resentment & irritation you might be feeling before you have this discussion. It's no more right or wrong to suggest you're a crap GM for not knowing how to handle a god-like PC running amok in your campaign, than it is right or wrong of your player to think breaking the game is his rightful due for taking the time to get a good handle on the mechanics. Campaigns like that can totally work and be a blast for everyone involved. I certainly can't run such a campaign, and I wouldn't even want to try. I've never even run a game where the Acolytes had access to Power Armour, because I dislike so high-powered games. But plenty of people enjoy the craziest, over the top loot fest campaigns with zero issues.

Almost-finally, if you're not having fun as the GM, chances are it's more than just one problem player making your life miserable. So cancel the next session and take a step back and examine all the issues you have. Almost any sort of issues can be worked out between friends by simply talking, listening and being willing to compromise. And hey, the point of playing is for all of you to have fun. You no more or less so than your players.

Finally, the car engine analogy is a pretty good one, except in this case there's no chance the PC knows how a car engine actually works, and he doesn't have the high-tech robotic fabrication line necessary to make even a moderately crap one. And in 40K it gets even worse, because the bare minimum facilities necessary to create anything like a car engine are not something the Imperium is capable of constructing any more, and access to an existing facility cannot be bought, and certainly cannot be appropriated by a lowly Acolyte with nothing but a feeble Writ of Authority.

I would say that building a thunder hammer from scratch is about as realistic as building the Space Shuttle from scratch.

Plus I would imply heavily that the player is an ass.

I agree with everything Simsum has said. Simply say to the PC that 'you might be right RAW that a Thunderhammer can be created. However for the game I want to run a Thunderhammer will unbalance the dynamic' If he won't agree to that then he is basically saying he doesn't trust you to run an interesting game without him having a Thunderhammer. Then there is an issue.....

If however you do want to go down the ingame route then.....

Me? I'd go "Sure! - if you have the proper blueprint."

And then make sure he never got the blueprint. Ever.

And then when he did somehow finally get the blue print have the first Ad Mech Official he comes across say 'And do you have the correct blessings of Holy Mars and All the Martian Priesthood to produce such a sacred weapon? No?'

BLAM! multi-melta to the face!

The other point that he is overlooking is that Thunderhammers are very very rare. They are also very very expensive. They are also a weapon of status and power. Look at the requisition and renown requirments that a Space Marine requires. They are also in the grand scheme of things small and easy to carry (compared to say a dimond encrusted sports car).

Look at the Table Top game. Are there any troops who carry such weapons that are not high ranking members of the Imperium?

And this PC's plan is to carry such an item into some of the brutal seedy areas of the Galaxy such as an Underhive or a Frontier world? Oookay...well the response from the denizens of such places would likely be that every crook, cut throat, ganger, recidivist,bandit desperado, cutter and scum within a days travel is going to be descending on the PCs position to try and kill and loot the corpse of this lunatic who is openly carrying a weapon which will set a man up for life.

It might be worth giving him the Hammer just to see the look on his face when he comes out of an underhive bar to be met with a street deserted of civiliians but absolutly full of gangers, armed to the teeth led by some notorious Crime Lord (according to Wiki the yakuza have 103,000 members so for a decent sized Crime Family in a Hive, mustering together 80 or 90 to take on some agents of the Imperium doesn't seem excessive) .

'Hello. My Name Thurkoz Headripper. I'd like to 'borrow' that fancy hammer of yours.'

In fact if you want to treat this in a seriously 40K way make the Hammer like the Number 1 and Numebr 2 headbands in Afro Samurai.

And this PC's plan is to carry such an item into some of the brutal seedy areas of the Galaxy such as an Underhive or a Frontier world? Oookay...well the response from the denizens of such places would likely be that every crook, cut throat, ganger, recidivist,bandit desperado, cutter and scum within a days travel is going to be descending on the PCs position to try and kill and loot the corpse of this lunatic who is openly carrying a weapon which will set a man up for life.

It might be worth giving him the Hammer just to see the look on his face when he comes out of an underhive bar to be met with a street deserted of civiliians but absolutly full of gangers, armed to the teeth led by some notorious Crime Lord (according to Wiki the yakuza have 103,000 members so for a decent sized Crime Family in a Hive, mustering together 80 or 90 to take on some agents of the Imperium doesn't seem excessive) .

'Hello. My Name Thurkoz Headripper. I'd like to 'borrow' that fancy hammer of yours.'

In fact if you want to treat this in a seriously 40K way make the Hammer like the Number 1 and Numebr 2 headbands in Afro Samurai.

Hmm... I think it'd actually play out slightly different. I'd think whatever cultists they try to snuff out will disappear pretty much instantly the moment they realize "Hey, there's someone with a Thunderhammer around here... they might have business with us...", then drugging him, slitting his throat, taking his hammer.

But, yeah, an all-out attack isn't unthinkable either.

And this PC's plan is to carry such an item into some of the brutal seedy areas of the Galaxy such as an Underhive or a Frontier world? Oookay...well the response from the denizens of such places would likely be that every crook, cut throat, ganger, recidivist,bandit desperado, cutter and scum within a days travel is going to be descending on the PCs position to try and kill and loot the corpse of this lunatic who is openly carrying a weapon which will set a man up for life.

It might be worth giving him the Hammer just to see the look on his face when he comes out of an underhive bar to be met with a street deserted of civiliians but absolutly full of gangers, armed to the teeth led by some notorious Crime Lord (according to Wiki the yakuza have 103,000 members so for a decent sized Crime Family in a Hive, mustering together 80 or 90 to take on some agents of the Imperium doesn't seem excessive) .

'Hello. My Name Thurkoz Headripper. I'd like to 'borrow' that fancy hammer of yours.'

In fact if you want to treat this in a seriously 40K way make the Hammer like the Number 1 and Numebr 2 headbands in Afro Samurai.

Hmm... I think it'd actually play out slightly different. I'd think whatever cultists they try to snuff out will disappear pretty much instantly the moment they realize "Hey, there's someone with a Thunderhammer around here... they might have business with us...", then drugging him, slitting his throat, taking his hammer.

But, yeah, an all-out attack isn't unthinkable either.

I agree. I was going to add an extra bit to state that to add injury to insult the cultists they were tracking or whatever would leg it soon as they heard about an Imperial retinue turning up.

The criminals I'm talking about are just extra mooks who otherwise would have basically ignored the PCs.

What would be funny would be to have Imperial authorities keep mistaking the Acolyte for an Inquisitor (Thunder Hammer=Inquisitor). Sounds like a benefit except he will need to keep correcting them otherwise he is guilty of impersonating an inquisitor which is a very serious offence.

Try to cut down on combat and run more roleplay, mystery and research and players will have much less ideas about making "ultimate" weapon. Even if you prefer more combat heavy games try some undercover missions and explain to them that thunderhammer, even IF he knew how to make it would not be good weapon to take to such mission.

My players tend to stockpile cheap and disposable guns for undercover missions.

Have to shot citizen for consorting with heretics when you are not able to arrest him as you don't trust local local enforcers? Shot him, get rid of gun. Much less fuss to get rid of stub-automatic if needed then bolt pistol you just got.

My 2 cents: your player can't cannibalise power axe to build thunderhammer, if he disagrees with that then ask him to craft Lamborghini from ford focus in real life, then he'll probably understand

About blueprints: I don't actually see that as necessary if you know how a piece of technology works, nevertheless I'd recommend you to ask him to pass tech-use test on a thunderhammer with penalty equal to penalty for crafting it resembling him disassembling the weapon to learn how it works with the next result - 3DoS is a straight success and goes not give any penalty on tests to craft thunderhammer plus any DoS extra will lower the difficulty of tests to craft but lower than 3DoS on this tech use test will result in -10 penalty for full ten that is under 3DoS, while failure will result in -20 penalty as DoF will, 3DoF mean that you not only fail in understanding how it works but also destroy the item in the process (possibly even in a spectacular manner like explosions and stuff) , then roll again to see how much components did he break while doing that. These house rules may be hard to understand (like why not just give extra -30 penalty on techuse test and do all the counting on DoS/F) but they are made especially to blance the game (simple items are easy to craft, genius in trade armourer and smith still struggling to craft power Armour In the field)

The next thing is that without a proper facility it is hard (sometimes even impossible) to craft some advanced tech, secondly you need components to build anything, thou mind you - crafting should be profitable to characters who are good at it at the cost of time (that's why most pc's only craft in downtime - you can't craft something, then leave it for a year and then continue - as it will simply rust and stuff, but shorter pauses and they possibility depends more on the task in question and, of course, you as a gm)

So all in all "never say never", and if your pc's are ready to march through hell and back while doing the task in question then let them go for it

First, you need to talk to the group and figure out what they want out of this game. If one player’s power gaming is causing a problem and doesn’t match the group you should address that player. If it is a group consensus that they want a ‘higher’ level of play you should work with them. One player not having fun is a problem, the GM wanting to quit is a tragedy.

Ok, I have had crafting in my RT game since its inception. The Rogue Trader has a 63 Int, Unnatural Intelligencex2, Trade(Armorer)+10, Talented (Armorer), and a Best Quality Labratorium that gives an additional +20 to crafting single items. The Explorator/Magos is better. Does they make crazy stuff, yep. Does it break my game? No. But, RT is a much higher power game and he has access to things your player's character doesn’t even know exist. I don't have my book to quote from, but I will try and do my best. It sounds like you have a few problems, so first some questions:

Days to customize armor...Customizing armor isn't crafting it. It is modifying it with padding, fitting the webbing, and generally repairing problems with it. I believe that you can only upgrade any item one category. Crafting Power Armor takes months or more. He isn't better than the Magos in charge of such things, no matter what he thinks. I would have used weeks as a basis for crafting Carapace Armor and he would have needed a forge and specialized equipment to do so. If he isn’t fixing or modifying an existing piece of equipment, he is making it from scratch...PERIOD. He isn’t ‘just making a hammer’, he is creating a weapon that wields arcane forces to disintegrate matter as he hits it without an external power supply or eating itself. If he disputes crafting time, compare it to similar items in fluff if you can.

Components...Does he have the pieces salvaged from a destroyed Thunder Hammer, or all the proper components provided by a Forge World? If not, he isn’t using Best (or even Good) components. The power field generator from a power sword is not the right one, and will not work for the hammer without serious rebuilding that will take as long as making a new one. Unless he has access to the right components to make something he shouldn't be better than common quality components, and may actually be using poor components. Good or Best means the right parts to build the device, made to a high standard of quality.

Facilities...Does he have a personal lab? If not, then he is borrowing one (probably common quality) or using an improvised one (Poor). A good lab is either a personal lab with all the right tools to do this particular job, or an excellent one he is borrowing. A Best lab is a personal lab with exceptional equipment and shouldn't exist except on a Forge World or as a Magos personal lab. Best quality labs are a reason to commit murder on Forge Worlds.

Does he know how to make it? I use the rule that Tech-use lets you make Scarce and below. Trade skills allow Rare and below. For everything higher you need a STC or Manufacturing Schematic. They are usually 30 to 500 times more expensive (IE, a Very Rare item requires an acquisition check at -30 for my RT characters). My players buy the things constantly, and at Rank 7 (Rogue Trader) have amassed sufficient to make some Forge Worlds take note. The Mechanicum guards rare STCs religiously, because they are holy relics. You need to prove you are worthy of it, as well as make them willing to give you a copy of something so precious. For a lowly Tech Priest to ask for such a hallowed STC, he is going to have to make some senior Tech Priest or Magos very happy...

Complexity... It is a specialized power weapon, which is normally only used by elite members of the Inquisition/Astartes. Base difficulty of -30. A 60 stat (with common parts and lab) has only a 30% chance of succeeding the check. He is going to be very lucky to get even a common quality weapon.

Do you make him make every roll in front of you? There are a lot of rolls in the crafting system. They can easily derail even the most skilled of crafters. Are you subtracting failures? I believe there is a penalty for too many degrees of failure as well (I know we use one, but don’t have a book in front of me). Remember that he has only so many time segments to finish the device and get at least one degree of success to make it at common quality. He needs 15 cumulative degrees of success to make it at best. If you aren’t making him roll and tracking how many attempts he has made or degrees of success he has, then you are letting him cheat. You have to hold the player accountable for following ALL the rules for crafting or the system will break. From your statements it is obvious that you are giving him too many bonuses. That will break the system. If it is too much hassle, then you need to talk to your group about it. If you decide to remove it from the game, remember to reimburse the player for xp spent on a skill you made obsolete.

These are the purely mechanical issues. The roleplaying ones are going to be more crippling. Does he have enough time before he has to do something to craft the weapon? Can he get the rare components to make it? They aren’t available most places, and converting similar parts means he is using lower quality components. Others have touched on the problems of owning it, so I won’t. Remember that crafting in 40k isn’t like manufacturing today. It is as much a religious endeavor as a technical one and he probably doesn’t actually understand why he does half the step (or more). He isn’t Girl Genius or a Star Trek Engineer, he is a priest performing a religious devotion to the Machine God. If he is a Heretek, then he is still working from limited information and without the guidance of holy ritual to warn him against using gold for ornamentation on the access hatch to avoid magnetic resonance interference that can cause fatal feedback...

Sorry for the long post. I like the crafting system, but I had to really understand it before I allowed it and enforce it closely. My players have had no problems with how I run it, even though it limits them somewhat. They agree that it keeps it closer to the feel of Warhammer 40k by making it complicated and difficult. That makes those custom items they have even more valuable. Unfortunately, it is a bit of work for the GM and requires that the players accept your rulings. If they don’t or you can’t spare the time then you should just drop it. Or use Black Crusade rules, but the cost of such items is about 1/5 ^th of the final product.