When you are scaling, you are asked for the reference point. This point will not move in the scale process.
So when you scale up, you may use an edge of the bounding rect.
When you scale down the single parts again, use their center as reference point.
I assume that your gears already have circles in their center. Then use the Snap Center function when you are asked for the reference point. This will snap to the center of the circle which should be the same as the center of the gear.
If there are parts without a center, you can add circles on a separate layer for reference point snapping. For production this layer can be deactivated.
I hope this works for you.
Armin
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