Talk to Mother in the Necralisk on Deimos and start an Isolation Vault bounty. The tier of Isolation Vault does not matter.
Follow directions to enter the vault area and destroy the enemy Necramechs found inside, They now can take damage anywhere on the mech, with damage attenuation instead of just hitting weakpoints to do damage (Weakpoints also charge weapons with headshot charge requirement), but weakpoints are of course the most efficient areas to hit (for increased damage). Destroyed mechs will drop Damaged Necramech Parts, which are used to build whole Necramech Parts. The blueprints for the whole Parts and the Complete Necramech can be bought from Loid in the Necralisk after reaching rank 2 with his Syndicate (operator mode required to access this area, operator mode unlocked by finishing The War Within).
The Damaged Parts can also be traded between players.
Applicable for both Voidrig and Bonewidow.