Depends on the #3 head. Some are spot on, others are not. Too many assume that the incoming hole to the #3 head is lined up wit the center of the mirror. Sadly not all engineers think that is important.
It is another factor in getting the larger mirror and lens size head. The lager mirror to compensate for a possible offset in the mirror to incoming hole to avoid beam clipping, and the larger lens to get the M22 nozzle thread for easy focal length changes.
I suppose that means Leader Jeff got one of the well engineered models!
Looks like he did. Many do hit the mirror center very close to the center of the entrance hole, but it is not a sure thing. I believe Russ hits the center of his gold head a little high, and I hit my black one 2.5mm high. I’m right on the edge of beam clipping. It’s was one of the deciding factors in switching from the stock head to his MK2 lightweight aluminum head. The mirror is fully exposed, no chance of beam clipping. Add in the ease of lens tube / focal length changes it became a no brainier.
Hey Stroonzo,
thank you for sharing your way of calibration. I’m curious about the sensor on your head, what is it? I have seen this on several machines and assumed it is some kind of capacity/inductive/IR sensor for referencing Z but I don’t get how this could safely detect all materials at the exact same distance to the sensor and have never seen such a sensor to buy as an upgrade. So, what is it and how does it work? 
Cheers,
Brian 
To clarify, just as @Dave01 mentions, I too was hitting mirror 3 slightly high prior to my last 3 hour alignment overhaul that included painstaking height adjustments on every point upstream of the lens holder.
I was having beam clipping in my furthest extensions of x and y, but this wasn’t necessarily because of where I was hitting my mirrors. The clipping was a result of never spending the 2 to 3 hours to truly do an alignment of the optics in the year I have owned this new machine and not being parallel.
The point is true: the larger the mirrors, the better. Hitting center isn’t important. But, consider that the raw / unfocused beam width exiting the laser tube is maybe about ¼" (6.3mm). This means you need to be hitting a 25 MM mirror as such (anywhere on or interior to the light green ring shown in this horrible graphic of a lens holder and lens): 
(single pulsed unfocused laser beam illustrating the beam width):
See how my old molybdenum (Mo) mirror was affected by the high clipping where the edge of my beam on mirror 3 was heating up the housing of the mirror holder.
And Shhhh… don’t tell anyone. I started to use my stock pile of Si mirrors on this machine. Yeah, they’ll be short lived with all of the 90% power layers on my W6 tube. But I have about a dozen of these.
I have a stack of polished copper mirrors coming.
Man, I hope I am seeing those push button switch pics wrong. Is that a block to space your work away from the nozzle for your 4" lens?
This may help:
Different blocks for different focal lengths / lens positions. I am going to switch to a universal focus block. Within Ruida’s auto focus settings, I can define the focus offset specific to the lens / lens holder I have installed. With this, subsequent to the switch closing, the offset adjusts from there.
Okay, I guess that work. I’ve never used the auto focus, just a step gauge between the nozzle tip and the work. My material rarely changes in thickness more than 2mm, so it’s easy to bump the bed up or down a tad… or loosen the lens tube and let it drop to the notch. I keep between 5mm to 8mm between work and tip for maximum air assist.
Took me over a month to explain to a guy on a different forum why him popping a 4" lens into his 1.5" holder and lowering his bed was not getting him the same results a friends 4" set in the tube properly was. He had his air assist cranked as far up as he could and drilled out the nozzle tip to keep it from overheating and still didn’t understand why he had problems. He al but accused me a faking my pics on my Google Mod Page at the end showing I had engraved and cut 1x6 clear pine with my 80W and a 4" lens.
How do i post a picture?
thanks rick
i will try
Hope you guys see the pic.
Hope ya’ll under stand the pic.
This is hitting my third mirror.
No on the understanding.
Stroonzo, Why the 4" in it’s normal position and another with a wide nozzle tip down near the nozzle? Some special cutting set up?
what you don’t understand and maybe i can explain
The bottom right is laser at position on table and pulse shot, same for the others.
the mm measurement is the distance between the beams at corners of table.
Mainly for safe Z clearance for situations such as:
Yeah, I was just reviewing the picture again and realized that was the representation. That result (the variation in movement in all directions) is indicative of your beam / mirrors being out of parallel in all directions.
Huh, never thought of doing rotary engraving with a 4" lens. Or is that strictly for the distance?
