Assuming you are following our FFXV walkthrough to complete all quests in the video game, you already know that Cid can upgrade your equipment if you bring him the weapon you wish to improve, and the materials he needs.
A Better Circular Saw side-quest is no different than the rest, and the following walkthrough explains what you have to do if you wish to obtain the Circular Saw Plus machinery, that can be used by Prompto.
First you need to get the Circular Saw by completing the second chapter of the video-game developed by Square Enix.
This item is dropped by the MA-X Cuirass, the last boss you need to defeat in order to beat the Declaration of War main quest.
After you get the Circular Saw from Loqi, if you show it to Cid, he will tell you that he can upgrade it for you when you bring him a Sky Gemstone.
Objective: Find a Sky Gemstone for Cid
Just like the Cactuar Needle (A Better Auto Crossbow side-quest), the Sky Gemstone is a rare treasure, which can be obtained later in the game.
The good thing though, is that the Sky Gemstone is in fact a quest reward, so you won’t have to fight anyone for it.
The get the Sky Gemstone for Cid, you need to progress through the game’s story and reach Cape Caem in Duscae.
Just keep playing story quests and the Imperial blockades will be removed. When Cape Caem outpost in southern Duscae is unlocked, check the crates behind the lighthouse to find the friendly cat you have encountered in Galdin Quay.
The said cat should have a side-quest available for you (A Feline Feast), and if you complete it, the cat will reward you with a Sky Gemstone.
What you should know is that the cat will spawn at Cape Caem, only if you complete A Kitty Catering side-quest, and Living off the Land). Look for it during Chapter 8 after you finish those two side-quests.
Objective: Give the Sky Gemstone to Cid
When you pick up the Sky Gemstone x 1 from the friendly cat, travel back to Cid in Hammerhead, give him the treasure and the Circular Saw, and he will start upgrading your weapon.
Remember that Cid can upgrade one weapon at a time, so make sure you complete all active side-quests from his quest line, or you will get stuck.
Finish two hunting quests or two side-quests, then send your team to sleep, and when they wake up, Cid will call Noctis to inform him that the Circular Saw was upgraded.
A Better Circular Saw Quest Info
Description: “A master mechanic, Cid can modify mechanized weapons to unleash their full potential. If Noctis can obtain a sky gemstone, Cid will upgrade the Circular Saw – but he might take a while.”
Reward: Circular Saw Plus x 1
Post Quest Suggestion
Complete A Better Valiant side-quest.