If your main goal is to keep the arms square while you assemble a frame, the best jig is usually one that gives you a fixed 90-degree reference and holds the arm ends and center plate in the same plane. For most builders, a simple frame alignment jig or assembly plate works better than a fancy tool. The key is not the brand name so much as how well it locks the arm spacing, angle, and motor-mount positions before you fully tighten the hardware.
A good jig for this job usually has a flat base, hard stops for the arms, and slots or clamps that let you set the frame in a true X or dead-cat layout. If you are building a freestyle or racing quad, even a small twist in one arm can change the stack alignment and make the frame feel off in the air. A jig helps prevent that by letting you tighten the center bolts while the arms are still held in the correct position. Some builders also use 3D-printed alignment blocks or laser-cut acrylic fixtures, but those are best if they match your exact frame dimensions.
If you want something practical and easy to use, look for a jig that references the motor screw holes or the arm root rather than just the outside edges. Outer edges can vary a little from carbon cutting tolerances, but the mounting points matter most. For 5-inch racing frames, people often use a flat assembly board with right-angle blocks, or a purpose-built FPV frame jig with adjustable arm stops. For custom builds, a machinist square, a flat plate, and a couple of clamps can work surprisingly well if you take your time.
The most important thing is to check alignment before final torque. Put the frame loosely together, seat everything on the jig, snug the hardware just enough to remove play, and then verify that both front arms and both rear arms are mirrored. Look down the centerline and check that the motor mounts are not toe-in or toe-out relative to each other. After that, tighten gradually in a cross pattern so you do not pull one side out of square.
If your frame uses thin carbon and stack screws that can bite into the material, a jig will not fix a warped plate or poorly cut arm, so inspect the parts first. It also helps to use threadlocker only where appropriate and not on carbon contact faces. For many builders, the best setup is a flat surface, a right-angle jig, and a dialed-in method more than one expensive tool. If you are buying one, choose a jig that matches your frame size and layout, because a universal jig is convenient but not always as precise as a frame-specific one.
