This achievement is exclusive to the Claptrap's New Robot Revolution DLC. To get this achievement, you need to complete a series of four quests from Tannis that has you collect Claptrap parts. The missions are as follows:
1. Fight For Your Right To Part-E: collect 42 Claptrap parts.
2. Parts Is Parts: collect 75 Claptrap parts.
3. We All Have Our Part To Play: collect 100 Claptrap parts.
4. A Part Of Something Larger Than Yourself: collect 150 Claptrap parts.
In total, you have to collect 367 parts to unlock this achievement. I'd recommend picking these missions up when you're doing the story missions or other side missions, but if you've already completed the DLC, here are the best areas to farm for the Claptrap parts:
a. Hyperion Dump: this area is a good backup if you already beat MINAC, since you can knock out 40-50 parts per run, and since it's close to the Tartarus Station fast travel, it doesn't take too long to run back to this area.
b. Arid Badlands: this is the best area to farm for Claptrap parts since MINAC will infinitely spawn Claptraps for the remainder of the fight. NOTE: MINAC doesn't respawn after you beat him, so make sure to do this during the story mission.
c. Dividing Faults: this area is also a good backup if you already beat MINAC, as the factory area will continuously spawn a bunch of Claptraps, more than in the Hyperion Dump, however, it's a longer run back, so unless you're playing Lilith or Brick, i would stick with the Hyperion Dump.
In the process of unlocking this achievement, you might also see oil cans, bobbleheads, panties, pizzas, fish bags and 3D-glasses. These items will unlock other achievements, but keep in mind, these items are rare drops, so those achievements will take longer to unlock than this one.