A few vacuum tubing rules:
A) Use the largest diameter tubing one can use.
This reduces vacuum resistance due to “friction” due to the tubing walls (departure from laminar (non-turbulent) flow).
B) Never reduce the tubing diameter and then increase again.
This creates significant turbulence and drastically reduces the effectiveness of the vacuum.
C) It’s OK for the tubing diameter to constantly increase - from the perspective of pick up to the dust collector.
The smallest diameter segment should be as large as practical (usually on the pick up end).
D) Use the largest diameter tubing - from the perspective of dust collector to pick up - for as long as you can.
Let the air flow as unimpeded as possible for as long as possible.
E) Minimize the number of turns, adapters and tubing segments.
Any turns/bends should use the smallest angle possible.
F) Transitions - from one diameter to another - should have smooth internal surfaces whenever possible.
Avoid abrupt transitions.
H) Keep the total length of tubing - from pick up to dust collector - as short as possible.
Less tubing, less resistance, better air flow.
I) Static buildup in CNC vacuum tubing is an issue.
It’s really easy to create conditions that can cause a fire or dust explosion. I’m not kidding! Use (and ground) static dissipative tubing or take the easy way out - run a bare, ground wire through the tubing.