The Great Machinekeeper:
1 Tuner + 1 or more non-Tuner Machine monsters
All machine type monsters gain 350 ATK. This card cannot be targeted for card
effects while you control another machine type monster. Once per turn; You can
tribute 1 machine type monster; this card can attack twice in your next battle phase.
If the tributed monster is level 5 or higher, this card gains the following effect:
- this card negates all monster card effects during the battle phase.

ocg fix

1 Tuner + 1 or more non-Tuner Machine monsters
All machine type monsters gain 350 ATK. This card cannot be targeted for card effects while you control another machine type monster. Once per turn; You can tribute 1 machine type monster; this card can attack twice in your next battle phase. If the tributed monster is level 5 or higher, this card gains the following effect:
• this card is unaffected by monster card during the battle phase.

350 ATK, though? Am I reading too much into it or does 'next' really mean next turn's battle phase? if so, why wait? If not, maybe the wording could be

Once per turn during your Main Phase 1, you can tribute 1 other Machine-Type monster you control. This card can attack 1 additional time this turn.

or 

Once during your Battle Phase, if this card has attacked, you can tribute 1 other Machine-Type monster you control to attack again.

it meaning you use it during main phase 1 then that turns battle phase other wise it would be your battle phase next turn

ok. how i saw it left it open to tributing during main phase 2 and being able to use that effect next turn. i really need to take my ass to sleep lol

Alrighty, I updated the card.

The card seems okay, but do you have any reliable way of summoning it? Perhaps you could make a Tuner or an archetype that can easily or reliably bring this guy to the field.

Fortress + Birdman alll day errr day.