The physics for lofting - dust pick up - is complex but in the end one can greatly simplify (the approximations are more than good enough). For “perfect” collection one needs smooth 5 inch tubing, all the way to the dust head. Since there are losses along the way due to friction - non-laminar flow - and turbulence - bends and turns - man shops have 6 or 8 inch piping for distribution.
The infrastructure for 5 and 6 inch CNC dust collection is expensive. The alternative - widely used - is 4 inches. Why? Because a 4 inch system, properly designed and implemented, is very close to the efficiency of a 5 or 6 inch system… with much lower cost.
Dust collection is not simply a matter of using a vacuum - pressure - it is also a function of the air speed. Both are only possible using large diameter piping/tubing. The bigger the better.
Under no circumstances is 2.5 inch tubing satisfactory for larger CNC machines. For small CNC machines (e.g. Nomad 883 Pro) a properly designed 50 mm or 2.5 inch vacuum system is “pretty good”. The spindle or router tend to be smaller, generating less dust, and with a proper enclosure design one can be safe.
Large machines (e.g. 48 x 96) are often installed without an enclosure, due to the cost and need for accessibility (large sheets need to be fed). This means that there is more of a chance for dust to escape - particularly the very small, lung dangerous particles. This is why it is critical to have a high quality, properly designed vacuum system. One must ensure that as much dust as possible is collected.
Other downsides of having an “open” CNC machine are noise and safety. The noise - which can go on for hours - is very damaging for hearing. One must wear high quality hearing protection… which isn’t a lot of fun. The safety aspect touches on two areas: things going flying and hands going where they shouldn’t.
A dust separator - typically a cyclone - is necessary to reduce the amount of dust sent to the dust collector. The dust separator introduces friction - reduces the pressure and air speed - and so has to properly calculated into the system.
The same losses occur with a HEPA filter. Everything in the system matters. Also, keep in mind that HEPA filters are expensive… especially for large scale systems. They become “full” after a while and have to be replaced. Some can be cleaned and reused a few times…but there are even more expensive.
In factory scale CNC, the air is often dumped outside without a HEPA filter - just a 5 micron filter… which creates quite a noise issue for those nearby. One can solve this… again at cost.
There is no simple solution nor is there a trivial cost solution. The good news is that with a good understanding of the layout of the large scale CNC machine and the room it is in, it is possible to implement a “more than good enough” vacuum system at a reasonable cost. It’s all about risk management - air, ear, and mechanical safety.
Again and again I see a rush to get a CNC machine, only then to discover that the true cost involves a vacuum system… and the vacuum system is compromised. Better to get a smaller CNC machine with a proper, safe vacuum system, than a larger one with an inadequate vacuum system.
The noise from a vacuum system is often LOUDER than the CNC machine itself. The same principals that apply to CNC machines apply here as well plus a few additional issues. A bit of care and this can be solved easily.
Again and again, I see little concern given to the noise. This is a HUGE issue, affecting the safety and performance of all those in the vicinity of a CNC machine.
A cautionary note about CNC vacuum systems and especially about HEPA filters - virtually all vendor LIE. There are no testing standards; they are all testing under different conditions. Many only work as advertised when the filter is near the end of its life. Caveat Emptor!
For cost reasons I’m a big advocate of dumping the air outside whenever possible - no HEPA filter. Dumping the air inside - often the only practical way of doing things - requires a great deal of thought… and don’t forget that the HEPA filters have to replaced (often if the machining volume is high).
I use and highly recommend the KentCNC dust heads.
I’m always willing to talk to people and help out. Far too few people are concerned, exposing themselves and those around them to preventable dangers. Solving these issues isn’t rocket science.
P.S. I do not work for any of the companies whose products I recommend.