40 wt thread is usually used for machine quilting when you want the thread to show up. It's too thick for piecing.
The thread weight and ply determines what size needle to use. 50 wt 3 ply is thicker then 50 wt 2 ply. But fine piecing thread is much different then regular run of the mill thread in thickness and smoothness even if the same wt. Thread education is key to successful sewing and machine quilting.