Pretty simple question. Does the Lathe blade upgrade and the mono upgrade stack?

No. This is specified in the erratta.

Edit: Or wait. Maybe that is power weapons. Hrm.

They're definitely not supposed to stack. IIRC, it wasn't possible to mono a weapon that isn't primitive in the first place.
The answer is somewhere in the Errata, but since I've just contracted an acute case of lazyness, you'll have to look for yourself.

It is in the errata, but like Cifer I suffer from the same acute lazyness so... You know the drill.

Madadh said:

The errata does explicitly state that the Lathe Blade upgrade cannot be combined with the Mono upgrade [Errata 3.0 p. 15].

No and it doesn't help power weapons either...

Dalnor Surloc said:

Although argubly you could upgrade the primative weapon that the power weapon is based on as you can for mono-weapons (but this has no effect when the power field is active).

I believe it also states that such is the case in the erratta, Khouri. Also, Force Weapons which are not held by a Psyker are mono weapons.

Khouri said:

The errata agrees with you.

I would allow it if you are freaking insane, why would you want to lower the penetration of a sword...

Santiago said:

Hey now, it's all about the skill of the swordsman, not...er..

Nevermind.

Hodgepodge said:

In fact it is!

Which is why I opted for a Lathe Blade in favour of a power sword for one of my PC's. A Lathe Blade looks pretty much like a primitive weapon. Just imagine the look on a power sword wielding opponent's face when the two blades meet and the Lathe Blade doesn't break!

I'm actually thinking of doing the same, for a similar reason: it attracts less attention, and on super-primitive worlds, doesn't make people think you are a witch. Which is doubly-useful when you are a witch, of course.

Also, you can Lathe-upgrade Great Weapons. Yummy!