
Grimcleaver |

So I want to customize a Clockwork Soldier my players are set to face off against in the next session or two. I know some Clockwork constructs have spells--the various Clockwork Dragons breathe fire, flaming tar, sleep gas and all sorts of other spell-like effects so conceptually I know it works. I'd like my Clockwork Soldier to be more of a mage--more Burning Hands and Magic Missile instead of chopping folks in melee.
I'd normally just consider swapping out its +1 polearm for a couple of built in wands to reflect it being able to cast the spells I want--but wands are spell completion items and I'm pretty sure a Clockwork creature can't be a spellcaster. Then again a Clockwork also can't breathe fire...but the Clockwork Dragon does. Can I just swap gear? That would really make things easier.
My question, really, is if I add some spell-like abilities to my construct, what does that do to the CR and what's the underlying math for doing that?

Bob Bob Bob |
Here are the rules for creating monsters. Unfortunately, those are pretty much useless. It starts "Creating a monster is part science and part art." and the section you want says:
Monsters are different from characters in that they can have all sorts of different special abilities and qualities. Each of these is tied closely to the creature's concept, allowing it to fill a specific role in the game. For examples, look at monsters in this book. Monsters should use abilities from the Universal Monster Rules whenever possible, instead of creating new yet similar abilities—when you do create new abilities, use the Universal Monster Rules as a template for how to present and create the new abilities.
Most special abilities that cause damage, such as breath weapons, give a save (Fortitude, Reflex, or Will depending on the ability). The DC for almost all special abilities is equal to 10 + 1/2 the creature's Hit Dice + a relevant ability modifier (usually Constitution or Charisma depending on the ability). Special abilities that add to melee and ranged attacks generally do not allow a save, as they rely on the attacks hitting to be useful.
Special senses and resistances to certain energy types are common in creatures of CR 5 and lower. Damage reduction, energy immunities, and regeneration are more common in creatures above CR 5. Spell resistance and immunities become more common above CR 10. As a general rule a creature's spell resitance should equal its CR + 11.
So, use the Universal Monster Rules if possible, how to make more if you don't, and the vague ranges that general ability types appear in. Not how they affect CR, not how many to use, not even what monster roles should have what, nothing about actually including them on a monster.
That's the rules answer for changing a monster's special abilities. It's pretty useless. Now, depending on how long this opponent is supposed to last I can give you a decent alternative suggestion with actual rules support. Rules here. The monster's role is combat so you can add two levels of sorcerer (use the one with wisdom casting) and it will add +1 to the CR and get 2 spells it can cast 4 times a day.
If you want to go beyond the rules, honestly, take the stats (ability scores, AC, saves) and sorcerer casting of this and then convert it to a construct then to a clockwork. A bit of work but should give you something close enough to the original CR.

Grimcleaver |

Yeah I've been reading through that same section. I don't know, man. For a game that's notorious for being incredibly rules dense and game balance obsessed it seems like there's some questions that should be really easy that just become a mess. You end up like the crazy guy sitting on the floor with every wall covered in scribbley notes.
It seems like adding spell-like abilities to creatures is something that comes up enough that there should just be a rule somewhere--add this many first level spells cast 3/day for +1 CR.
Like there's necklaces that create fireball effects. Demons and devils have tons of spell effects they can do. Adding spells to things seems like such a big part of how Pathfinder works. It's crazy there isn't just a simple rubric for doing it.

Grimcleaver |

I want to say that since a 6th level character is supposed to have 3,450 gold worth of stuff (it says so here). Roughly half offense, half defense. That I just find a number of wands in the book equal to that cost and call it a day. Just assume that while the object isn't a wand per se, rather a component built into the construct, that the process for creating it and its effects are so similar that the difference is negligable.
So it has a bunch of "spell guns"--two first level attack spells, two first level defense spells that it can use 50 times each until it needs reloaded. It also has 450 gp for limited use items so I can maybe huck, say four fuse grenades and a tanglefoot bag at folks.
Presto, instant magic casting robot.

Bob Bob Bob |
Oh, I forgot about the simple class templates. Here's the sorcerer one. It's not as useful as actual levels but that's because it's only meant to last like four rounds, at most.

Grimcleaver |

Or just use a Clockwork Mage. Maybe a bit of a high CR, but that can be accounted for.
Looking over that write-up I feel even better about the idea of giving my construct wands since there's another wand using clockwork out there. Yeah a 9th level CR would wipe my 5th level party. A 6th level soldier is looking to be a tough enough fight what with it hurling grenades, flamethrowering folks, and magic missling left and right. Then it can also slam folks around with its spider legs for 1d6+14 twice a round with fat +18/+13 bonuses to hit. CRAZY! Lots of fun stuff it can do. Plus its AC starts as 20 (22 with shock shield up) AND it has a damage reduction of 5!
It should be a fun fight.
The idea is that the construct will be like a clockwork spider tank that drops down from the ceiling of this alchemical/necromantic weapons lab they're investigating. And cue chaos.