There are many different styles of air assist, the two most common being "conical", which directs compressed air perpendicular to the material and directly down the cut, or "tubular" that directs air down the cut. An angle towards the rear of the machine.
There is also an air curtain design where compressed air is drawn from the bottom of the X-axis frame through small holes, creating an air curtain effect across the width of the work area. Each type has its own advantages - the cone style minimizes residue on the surface and provides a cleaner finish, especially on wood, and since the cone covers the bottom of the lens, it also provides a little extra protection of.
The downside of the cone is that you have to use compressed air 100% - even during engraving - or you can damage the lens. Also, during cutting/engraving, there is limited clearance between the bottom of the cone and the material, so if your material is not flat or the part slopes up during cutting, there is a higher risk of collision or difficult to engrave into recessed areas.
With a barrel style, it provides extra clearance for engraving and cutting uneven materials or recessed areas, and allows easy inspection and cleaning of the lens without removing any parts. There is also no need to use air assist during the engraving process, which provides cleaner results in most engraving applications.
