Laser mirror and lens allignment


I’ve few doubts about about the mirror alignment and laser focusing. The reason why I’m asking this is that , the edges of my cut piece are not square. After searching through lots of forums I found that if laser do not hit the exact centre of the lens, result will be beveled cuts. Some posts says that beam should hit all the mirrors at the exact centre, and some other says It should be on the centre of 3rd mirror other wise beam would be slanted.

I’m not using a factory made machine, Mine is a modified X carve and It’s placed inside a home made casing.

I’m really confused about this. Should the beam strike all the mirrors at the dead centre or it doesn’t matter as long as the beam strikes same spot at every position (inside mirror not exact centre). It must be important for the 3rd mirror but what about others. Most post I’ve read have fixed 3 rd mirror but mine is adjustable one(3 screws).

can I use the screws on 3rd mirror to make the beam strike exact centre of lens even if it doesn’t strike the centre of 3rd mirror.

In a perfect world, you would like for the laser beam to be perpendicular to the work surface and follow the shortest path to get there.

This would give you the best edges and deliver the most power to the work surface.

In the real world you are going to have losses along the way. If the light path gets longer then you lose some power in the beam. Whether the loss is significant or not is a different question.

To get the best performance you would like the laser beam to enter the lens perpendicular to the lens and in the exact center (this is for a simple lens - compound lenses are different). However you manipulate the laser beam prior to entering the lens to get it perpendicular and at the center of the lens will just result in loss of power. Again, it may or may not be a significant loss.

The beam is actually an hourglass configuration with the focal point at the narrowest point. If you are above or below the focal point, you will have a bevel effect.,658192,659206#msg-659206

My laser has adjustable nozzle also (I cange the length of nozzle)

I’m really confused about this whole alignment process,
If the laser falls at the centre of lens at the shortest distance of nozzle ( 3rd mirror in default position -no screw adjustments ) then by adjusting screws or without any adjustments on 3rd mirror I could strike laser at the exact spot (centre) when head is at the longest distance and for that to work laser should strike at the centre of 3rd mirror (by default 3rd mirror is at 45 degree, so it will deflect incident beam to 90, and I think without any adjustment laser will be perpendicular and it would strike the centre of lens at both shortest and longest distance ).

What if laser falls not at the centre of 3rd mirror, it will be deflected at 90 degree but wouldn’t fall on the centre of lens (say lens at shortest distance ), and by adjusting the screws on 3rd mirror I can make it fall on the centre of lens (but deflection here will not be 90 degree ), but when I move the lens to its longest distance obviously beam would not stay at the centre , it will at some other spot. By adjusting screw even if I make the spot centre, It won’t be in the same place when lens move to the shortest distance. simply I can make the laser fall at the same spot at both distances, but it won’t be at the centre of the lens.

This is my conclusion,

  1. For the beam to strike at the same spot (shortest and longest distance ) it should be perpendicular to bed.

  2. If it strikes at the centre of 3rd mirror then beam would strike at the centre of lens at both distances.

  3. If it strikes any place other that centre of 3rd mirror, the postion of beam on lens will vary accordingly, and by adjusting screw I could make it fall at the centre of lens , but only at one postion.

Correct me if I’m wrong , Please…

Also I will get bevel effect if my beam if not striking at the centre of lens

My comments are for the general case, and applies to the characteristics of lasers and optics.

Since I don’t know the geometry of your mirror/lens system, nor the types of adjustment your system uses (move mirror back and forth, or rotate mirror, etc.) I can’t really comment helpfully on how to adjust your system.

As the person in the referenced link said, with the lens removed if the laser beam enters the support tube for the lens in the center and exits the support tube for the lens in the center then the beam is oriented correctly.

If you then put the lens back in and the laser doesn’t hit the center of the lens for the length of travel it can move in the support tube then the laser beam is not really aligned with the support tube, or there is an issue with the support tube holding the lens correctly.

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Okay, I understand that. But in order to hit the beam at the centre of lens at both shortest and longest distance( to enter the support tube for the lens in the centre and leave the support tube for lens in centre) shouldn’t beam strike the centre of 3rd mirror (3rd mirror don’t have any horizontal , vertical or rotating adjustments only 3 screws for fine tuning, my remaining two mirrors have all 4 adjustments)

I’ll try to explain what I mean by some pictures

Same situation happens if beam hit at the bottom of lens or any other part(position of beam on the lens and mirror alignment can vary)

So, If I’m right, I can adjust the beam to hit the centre of lens even if the beam doesn’t reflected from centre of 3rd mirror by adjusting screws on the 3rd mirror mount but position of laser on lens will shift from centre as the tube that holds the lens move up or down (see picture) i.e., beam will only be centre at one point not for the entire length of lens tube. But If it hit the centre of 3rd mirror that won’t be an issue.

correct me if I’m wrong?

@Athul First, as your image shows in Fig 1 you will be entering the lens at an angle. The lens cannot compensate for angles. Normally the “3rd” mirror as you are calling it is normally fixed and that is the end point of adjusting the mirror system.
The document I linked to has all this information. If you are still not understanding the principle of the mirror adjustment I would recommend asking your question in the Google+ group K40 LASER CUTTING ENGRAVING
There are a great number of people who have been working with this type of laser for years and might be able to put it in terms you understand.

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What does that mean and what will happen if beam enters the lens with an angle, wouldn’t it be focused to the focal point?

Hi Athul,

If the beam is entering the lens at an angle, the lens will not correct the beam path. Yes it will be some semblance of a beam but not optimal.

A quick look around the web I found two articles on laser beam lens principles. First one is very basic and the second ones goes very in depth. I hope this helps you with your issues.