It depends on how much error will be built in to the finished item and how much error is allowed for the finished item.

Check the tolerances for the item being fixtured.

I'd say whatever allows you to keep the assembly points positioned within a certain tolerance loaded and unloaded.

as above, the tool needs to achieve (at least) the tolerances of the installation. For example, I imagine that the plane of the tool was assumed to be that ... a flat plane. I imagine (I'm pretty certain) that points on the part have co-ordinates (like on the airplane), so the tool needs to accomplish these points to their tolerance.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

sarclee,

A crude rule of thumb for fixtures is that they must be ten times as accurate as the tolerances they are maintaining. If the fixture is flexible, the deflection in use should not exceed a tenth of the fixture tolerance. All sheets, plates, and plywood sheets are flexible.

--
JHG

Your fixture looks problematic.

The plate seems to be the sole provider of "plate" stiffness to the worktable surface.
Imagine the weight of the workpiece pushing down on this fixture. The legs are on wheels, therefore free to spread out. Each leg spreads away from the centroid. That applies a moment where each leg is attached to the table surface.
Any workpiece weight will deform the table making its top surface concave.

I think I might see a tiny little horizontal surface a few inches below the worktable surface - that is completely inadequate to resist the spreading of the legs.

To fix your problem, put a plate between the caster and the leg, spanning between all of the legs. This will prevent them from spreading and increase the stiffness of your fixture by about 10x.

Next...

How flat is the floor of your shop?

If the jib is only just able to make the part within tolerance that means everything else needs to be perfect every time.

I would 2nd sparwebs comments except that I would make a square support out of box section rather than adding more plate. Do consider what happens when the techs push it flat out down the facility and it comes to a sudden stop due to a bolt jam under the wheel.

I can't understand why so many people ask "What standard controls my special situation?"

As others have already pointed out, it's your product being assembled on your jig. Therefore, it's your job to design the fixture so that your product works the way it's supposed to.

Quote:

May I know if there is standard for the allowable deflection? I understand for some beams design, there are rule of L/360 for instance. What about a plate? Thank you!

Ask your designer what the assembly error budget is; the as-built product design should have a build tolerance that includes fabrication and assembly tolerances.

TTFN (ta ta for now)
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there should be a required tolerance of several control or datum points, to ensure the item fits in the plane as intended.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

L/360 is to keep the drywall/plaster ceiling from cracking, I think.

Are all those green angle bracket part of the fixture?
How accurate will their lengths be controlled ?
A couple are offset with clamps. I'm thinking they will look

Is the pink item a monolithic component ? How accurate will all the interfaces with green brackets be?
How stiff stiff is the flange on the pink item? Are you expecting the fixture to position all the flange sections to be co-planar for the assembly process?