Aligning a laser cutter

[Things]

Hey guys, as osme of you may know, I've built my own laser cutter from scratch. It works well, but I'm having problems getting the CO2 laser aligned. The CO2 laser sits along side the table, and I'm using optical mounts to get the beam to the X carriage. I can get it aligned at one point, but as you move the table, it slowly drifts out of alignment. I can align it at that new point, but then it goes out of alignment when you move it back again.

Is there any specific method I can try?

Cheers,
Dan

[MechEng3]

Most of the industrial light machines employ a small set of workpoint source collimators. The beam is partially collimated to a almost non-divergent beam so that it can be directed off 2 morrors to the delivery point on the table arm or quil. Then the beam is focused from there to a point or with a change of final collimators into a tiny beam for thick materials like foam insulation. Most of the >2000 watt units {metal cutting} have fiber delivery heads that I have been around lately.

[Things]

This is what I have at the moment:



The laser stays pretty close to the same spot on that block of wood across the whole table, but it still moves too much to hit the ZnSe lens reliably.

[MechEng3]

Sounds like an alignmet issue.....squareness is paramount here...the beam and the target spot alignment. The Z axis adjustment for the lens will control the spot elevation? Any adjustment of the laser tube CL and the table Y/X axis micro adjustments? The kinematics may not have enough travel.... Maybe the addition of a small <20mw red spotting laser will help here with the addition of a bit of fog to help align things.....I think Sinolaser of fleabay has the dichro for this.

[Things]

Yes, getting everything nice and square is the part I'm having troubles with. I think buying a dichro and combining in a red laser is probably the best option for alignment, since I'm sure i'll have to realign it many times more.

[dave]

It might be a bit more work making, but wouldn't it be better if the cutting head remains stationary, and the table below moves? similar to a mill?

[mixedgas]

The ones I've worked on, upcollimate the beam right after the laser, and have limiting holes precisely adjusted and spot welded into the frame. Getting the laser beam through the holes gets you into the ballpark, you then slide the lens and fold mirrors around until your parallel across the table. Visible aiming lasers help a great deal with the problem.

The carriage and and rails are made to a high degree of precision and the factory adjusts the rails using something like a autocollimator, They then lock or weld the rails down.
I've never seen a tabletop one where the user adjusts the rail positions. I have adjusted the beam paths on 60 and 400 watt units, and its not a easy task at the first go.

All of them I have seen have L folds, the laser is at the back, and is turned through a set of mirrors that set its height post collimation. The laser is first bent upwards, and then the second mirror sets the height as soon as it leaves the collimator. The laser is always below the work table level. The beam then travels along the shortest edge of the table, and a pickoff mirror on the carriage routes the beam to a fold mirror which shoots it down into the lens. The lens has adjustments for centering. Usually every mirror has course adjustments for height and position and centering.

Adjustment is time consuming, and often involves tiny movements of the laser housing.

A typical alignment aid is as follows: A ring is lathed to set in the alignment holes. The Ring is the same diameter as the mirrors. It is also the same diameter as the alignment hole above the final focusing lens. A slot is milled in the back of the ring to hold a stiff piece of paper. A pair of steel wires, orthogonal to each other, forms a cross, just in front of the paper in the slot. The wires are sized to take the power of the upcollimated beam. As you align the laser to the path, mode burns are done at each alignment location and each mirror holder. You may center the beam by imaging where it burns the paper behind the cross.

The really big machining lasers move the table under the focal point, instead of moving the focal point to the work.

Steve

[Things]

Yea, some cutters have a moving table design. I tried to avoid that, as it requires a fair bit of excess space around the machine, but it does make alignment much easier.

I am thinking about upgrading the whole system to a steel rod based design, instead of v-groove bearings on tracks. It'll be much sturdier then.