Borderlands The Pre-Sequel – Why Can’t Three Purple Shields Be Grounded?

According to the wiki, you need to input specific recipes into the grinder. The recipes, are as follows, and require items of the same rarity in every case but the legendary case:

• Legendary of desired type: 2 legendary weapons + 1 purple weapon of the desired type
• Up one rarity level (specific type): 2 weapons of the desired type + 1 other weapon
• Up one rarity level (any type): 3 weapons of different types
• Up one rarity level (non-weapons) 3 items of same type (grenade mod, shield, etc)
• Assault Rifle: 1 SMG + 1 Sniper Rifle + 1 Shield
• Rocket Launcher: 2 weapons of any type + 1 Grenade Mod
• Shotgun: 1 Pistol + 1 SMG + 1 Shield
• SMG: 1 Assault Rifle + 1 Pistol + 1 Shield
• Sniper Rifle: 1 Assault Rifle + 1 Shotgun + 1 Shield

For all except the legendary recipe, these recipes work for anything up to and including blue inputs, with the potential to output a weapon of the same rarity or greater. If fed moonstones, the grinder will always output an item of increased rarity, and has a chance to add a luneshine effect to the weapon.

I made a video:

there's three touchplates. One by the air dome generator by the terra madre treasure quest, one by the garbage where you search for the map and fight the thresher, and that one thing there.