Laser alignment tolerance

[Ronald Erickson]

I discovered I was having an alignment issue with my new Trotec Speedy 300. When I cut thicker materials (1/4" is the thickest I've cut), I was getting a parallelogram shaped cut on the left and right cuts with both sides slanting from the upper right to the lower left. The top and bottom cuts were also slightly tapered from upper right to lower left.

245725

As part of my troubleshooting process, I found that if I left the laser head in the upper left corner and moved the Z-axis down, the red dot (and CO2 beam) was moving to the left 1/4 inch and 1/16" or so towards the front of the laser for every 4 inches of Z-axis travel. When I put tape over the mirror 3 hole, I could see that it was firing up and right of center.

After spending 8 hours yesterday fiddling with the alignment, I came to the conclusion that I'm far too anal :o, and that I may be having an issue with the Y-axis.

For the most part I corrected the cutting angle problem. Now the cuts in 1/4 inch acrylic are subtle hourglass shapes. I spent today making a bunch of domino shaped cuts to test different power, speed, and Hz combinations. It seems that 70 power, .3 speed, and 1kHz produced a pretty good cut in 1/4 inch acrylic without making the acrylic too hot. Having the air assist off prevented any hazing on the acrylic. The only thing I would like to correct is the start/end of the cut. If it starts or ends on a round object (circle, corner, etc), it leaves a very small dimple. It seems the start of the cut is dimpled and the end of the cut finishes slightly higher than the start. If I start/end the cut on an angle, you don't notice the defect. Any advice on how to fix this would be appreciated. :)

245727

The remaining problem I am having with the alignment is when I move the gantry along the Y-axis from the upper left corner to the lower left corner, the red dot will consistently stay centered until I reach the 10-11 inch mark on the Y-axis ruler on the table. At that point, the red dot will start to raise up until it is 1/16" of an inch directly above center by the time it gets to the end of the Y-axis (17+ inches). At this point in time I've been keeping my cuts within the 12 x 24 area for better exhaust, but I do have concerns that the deviation may produce issues if I use the last 5 inches of table space. (In the picture, the red dot is coaxial to the CO2 beam so where the red dot points, the laser burns)


245726


The summary of the issue is: If I don't pass the 12 inch mark on the Y-axis, the beam will stay centered. Past that it will start to raise up in the lower left corner. Would a 1/16" deviation cause much of an issue if I were working on a piece that took the entire 17 inches of Y-axis travel? It does also raise up on the lower right corner just past 10 inches, but it is much more difficult to measure/see.

Any suggestions on what to check?

(7:18 pm update): Just to be clear, all the alignment testing was done by burning tape. I just have the laser set so the red-dot is coaxial to the CO2 beam and I used the red dot in the pictures because it was easier to show what's happening. So everywhere you read "red-dot", think "CO2 laser beam". :)

[Scott Shepherd]

I'd forget about the red dot. That's a pointer and it may or may not be where the actual laser is located. You should test everything with the actual burn of the tape. I've seen crazy things in those pointers before. Some of them are inserted a various points along the path, so it's not always a reflection of the actual beam. Only real test should be burning tape, in my opinion.

I know mine is off too, but I've been putting off aligning it as long as I can. I suspect the shipping from Austria to the USA isn't an gentle a ride as they'd like it to be.

[Ronald Erickson]

I reread my post and I guess I wasn't too clear and made reference to the red dot a lot. I did all my testing by burning tape. The CO2 beam is mirroring what the red-dot pointer is doing and deviating during the last 5 inches of travel on the Y-axis. (I've spent a total of 12-15 hours working on the alignment and have probably made a few hundred marks on tape).

If I keep the laser head within the 11-ish by 27 inch area, the CO2 laser doesn't deviate. But if I move the gantry past the 11 inch mark on the Y-axis ruler, the beam starts to rise.

[Scott Shepherd]

Gotha Ron. I can say that after this post, and the last run of the day, I made the mistake of starting the alignment at 8:15. It's 9:30 and I just put the covers back on. That was one of the most difficult alignments I think I've ever done.

A couple of things that you might help. You may very well have learned this already. If so, I apologize for repeating what you already know.

I had the two marks in different places. I'd do it at Y-0, and then Y-17. It was 1/16" or so between them. I moved it while at Y-17, did another burn, moved it to Y-0, burned again. Off. I moved it again. Burned at Y-0, moved it, Y-17, burned again. Off. It took me literally 25-30 tries and chasing it all over before the lightbulb came on while watching an adjustment. I figured out I need to do ALL adjustments while in the same place. So I burned at Y-0, then at Y-17. I moved it over some. New tape, moved it back to Y-0, burned, then to Y-17 and burned again. AHHH HAAAA! Progress. So I made sure ALL adjustments were always done at the same place, rather than tweaking it at both places. After I did that, it took me about 5 minutes. I will say that I actually slide my #1 mirror block, and I also loosened my entire tube and raised it up. So I made sure that I got every single thing possible out of whack :)

Once I got that #1 mirror adjusted, I adjusted the red pointer and never could get that perfect, but it's close.

Went on to Mirror #2 and it was out about 1/16" as well. Adjusted it a couple times, then moved it across the table and it was still out a little. Went back to original adjusting position X0-Y0 for this case, and was able to tweak it in fairly well. I don't think it's perfect but it's a lot closer than it was prior to me starting it. I'd say it's within .020-030" across the table.

One note in the manual, it said if you can't get that last bit right, you need to go back and fine tune mirror #1, which was something I really didn't want to do again.

Whew, that was a mouthful. I hope it makes sense and helps you in some way.

[Ronald Erickson]

I think I follow exactly what you are saying and I believe I've found out the same thing. I first adjusted mirror 1 to hit the center of the hole of mirror 2 in the Y0 position, then ignored making adjustments based on Y0 position. All my adjustments for mirror 1 were done at Y17. Since Y0 is closest to mirror 1, any adjustments needed at Y17 would be nearly imperceptible at the Y0 position. At Y17, the adjustments were easy to see.

Once mirror 1 was adjusted, I went on to adjust mirror 2. I'd burn a mark at X27/Y0, then move the head to X0/Y0 and burn another mark. Make the adjustment while at X0/Y0, new tape, and repeat until satisfied. Once both marks were the exact same for adjusting mirror 2, I'd put it at X0/Y0 and very carefully slide mirror 1 to center the mark. Sliding mirror 1 was a frusterating experience due to the slop in the milled slot that the mirror 1 housing slides in. Every time I touched the mirror 1 slide screws, it would change the alignment in all directions (up, down, left, right, as well as sliding back and forth). I found that I had to go back to the slide adjustment a few times because the "sliding" action would sometimes pivot the mirror a bit when I locked down the screws. I ended up removing the mirror housing and cleaned the nail polish off the housing, screws, and washers so it wouldn't affect the tightening of the screws. It's helped a bit, but it was still a bit tricky dealing with the side to side slop. I'd rather not have to touch the mirror 1 slide, but when the burn dot was the same at X0Y0 and X27Y0, it was slightly to the right of center.

I was basing the alignment on instructions I found for an old speedy 100. (Adjust mirror 1 to center of mirror 2 in Y0 position, move to Y17 and make adjustments to beam to match, repeat until they fire in the same spot, slide mirror 1 if not centered left to right, etc). It was my very first time aligning a laser and I learned a lot about how each mirror affects the next. I've done it so many times now the past three days and I still can't figure out any way to fix the deviance through mirror adjustment. I may have to start looking at the right side rail and see if there's something out of place. It's almost as if there is a slight twist in the rail at the Y17 end that is causing the mirror to shoot high. It's dead straight until 10-11 inches, then it starts to raise the beam. 1/16" might not even be out of spec. I "think" the margin is something like 1/6 the beam diameter, but I can't say for sure where I read that.

One thing I did find that I didn't correct was that the laser itself is firing a bit high on the mirror 1 entrance hole. It's dead center left and right, but a bit high. I figured out how to tilt the whole apparatus (laser, mirror 1, beam combiner, etc) to slightly lower the tube so it wasn't shooting high from mirror 1 to mirror 2, but I couldn't figure out how to only move the laser down a smidge while leaving the mirror 1 in place to center the beam at the entrance hole of mirror 1 (or slightly raise mirror 1 to compensate). I wasn't sure of the function of the black hex screw between the laser tube and the aluminum housing supporting the mirror 1 assembly, so I decided to leave it alone.

I figured as long as it hit the mirror 2 hole dead center I'd be fine.

[Scott Shepherd]

Once mirror 1 was adjusted, I went on to adjust mirror 2. I'd burn a mark at X27/Y0, then move the head to X0/Y0 and burn another mark. Make the adjustment while at X0/Y0, new tape, and repeat until satisfied. Once both marks were the exact same for adjusting mirror 2, I'd put it at X0/Y0 and very carefully slide mirror 1 to center the mark. Sliding mirror 1 was a frusterating experience due to the slop in the milled slot that the mirror 1 housing slides in. Every time I touched the mirror 1 slide screws, it would change the alignment in all directions (up, down, left, right, as well as sliding back and forth). I found that I had to go back to the slide adjustment a few times because the "sliding" action would sometimes pivot the mirror a bit when I locked down the screws. I ended up removing the mirror housing and cleaned the nail polish off the housing, screws, and washers so it wouldn't affect the tightening of the screws. It's helped a bit, but it was still a bit tricky dealing with the side to side slop. I'd rather not have to touch the mirror 1 slide, but when the burn dot was the same at X0Y0 and X27Y0, it was slightly to the right of center.

I think that's not correct. Once you get mirror 1 set, you shouldn't be sliding it anywhere. The only time to slide mirror 1 is when aligning mirror 1. If you're moving mirror 1 to get some result on mirror 2, I can understand how frustrating that could be.



One thing I did find that I didn't correct was that the laser itself is firing a bit high on the mirror 1 entrance hole. It's dead center left and right, but a bit high. I figured out how to tilt the whole apparatus (laser, mirror 1, beam combiner, etc) to slightly lower the tube so it wasn't shooting high from mirror 1 to mirror 2, but I couldn't figure out how to only move the laser down a smidge while leaving the mirror 1 in place to center the beam at the entrance hole of mirror 1 (or slightly raise mirror 1 to compensate). I wasn't sure of the function of the black hex screw between the laser tube and the aluminum housing supporting the mirror 1 assembly, so I decided to leave it alone.

I figured as long as it hit the mirror 2 hole dead center I'd be fine.

Also, I don't think anything in twisted, and I disagree with the adjustment you made that rotated the mirror assembly at mirror 1. I think you could have caused issues there. If it was aligned properly and you were adjusting the wrong mirror (1) to move an adjustment on mirror 2, then you just changed all of that and you can be chasing your tail for some time now.

The procedure (as I understand it) should be :

1) Align mirror #1 at Y0 and Y17. If it's in the center of the opening in both those places, then mirror 1 is pretty much a done deal.

2) Align mirror #2 at X29,Y0 and X0,Y0. Make all adjustments at X0, Y0.

3) Align mirror #2 at X29,Y0 and X0, Y17. Make all adjustments at X0,Y17

4) Align mirror #2 at X29,Y0 and X29, Y17. Shouldn't need any adjustments, it's just a check.

That's pretty much it.

My beam was below center coming out of Mirror #1 by probably 1/8". I couldn't adjust it in, it just wouldn't come up any more. I loosened the tube and pushed it up, and tightened the tube. That put the beam in the center of the hole. I assume it settled in shipping.

PM me your email address and I'll email you the service manual I have.

[Ronald Erickson]

PM'd. Thanks Scott.

The only reason I had to slide mirror 1 when adjusting mirror 2 (to hit mirror 3) is because when mirror 2 was perfectly adjusted to hit mirror 3 at X0Y0 and X27Y0, it was off-center. I agree that once mirror 1 is adjusted you shouldn't have to adjust it again, but if I center mirror 1 to the entrance hole for mirror 2, it would be off center on mirror 3 no matter what I do. It was in the sliding of mirror 1 that things became more difficult because of the loose fit of the mirror 1 base in the slot used for sliding mirror 1.

I'll take a look at the service manual and take pictures of the process. Maybe I can put together a pdf photo log of each of the mirrors and what each hex screw adjusts. I have an idea about making some targets that will fit into each of the mirror holes like a plug. It would make it much easier to eyeball with a target grid to hit.

How important is it to hit the mirror 1 entrance hole dead center? This is the hole before the beam strikes mirror 1. It's shooting a bit high and I don't see a way to adjust it without actually shimming the mirror 1 base.

[Martin Boekers]

Listen up laser manufactures.......if you come up with a way of auto alignment you may sell a few more machines.
I have two machines that both could use it right now, it's hard to be shut down right now for a few hours though,
so just muddling through until Thanksgiving when I can spend a day on much needed maintainance! :)

[Scott Shepherd]

I have to say, the Universal Laser's alignment process is about the easiest in the business. You can align the machine in about 15 minutes.

I don't think hitting the center of the hole in Mirror #1 is important.

[Ronald Erickson]

I'm currently in the process of redoing the alignment and have taken some photos to explain what I am experiencing.

What I have done is adjusted mirror 1 to shoot dead center of the entrance hole to mirror 2. Then I reattached mirror 2 and set it to hit the exact same mark at the X27Y0 and X0Y0 positions. Here are the results:

245812

As you can see, the burn mark is off center. I cannot adjust mirror 2 left/right to center the burn mark at X0 without affecting the location of the burn at X27. The only adjustment I can make at this point is to adjust the mirror 1 slide to center the beam the mirror 3 entrance hole. While this makes the beam off center at the mirror 2 entrance hole, the important mirror centering is at mirror 3.

When I loosen the screws on the mirror 1 slide and attempt to slide mirror 1, rather than just sliding in back and forth, the mirror 1 holder also pivots in place changing the angle. Changing the angle at mirror 1 is bad at this point because it changes the 90 degree angle I have accomplished in the mirror 1 setup to mirror 2 and mirror 2 to mirror 3. Here is the amount of deviation that occurs at mirror 3 in the X0 position JUST by pivoting the mirror 1 holder.

245813

If I manage to pivot the mirror 1 holder even slightly while I'm sliding the mirror 1 holder, I have to restart the alignment.

Ultimately, adjusting mirrors won't fix the issue I have with Y11+ :(

[Scott Shepherd]

The first photo is perfectly acceptable. I don't consider that off center by any means, in the world of aligning lasers.

The 2nd photo should have nothing to do with mirror # 1, unless the rotation you did earlier has messed that up. That should be an easy adjustment. Top right adjustment screw of mirror #2. It's very sensitive. Move it a fraction and try it again. No reason you can't get that lined up. It's very close.

[Ronald Erickson]

Here's my fix for adjusting the mirror 1 slide to correct side to side alignment at mirror 3:

245816

Not a perfect fix, but it was a simple solution to the pivoting issue I was having when adjusting the slide. Now the mirror 3 entrance shoots dead center at X0 and X27 in the Y0 to Y11 range.

[Scott Shepherd]

Your 80 watt is setup different than my 75. My "area" is different than that. Glad you got it fixed.

I don't think anyone checks any of this stuff from any manufacturer. We've had our Universal for about 4 years now, I guess. I replaced some belts and bearings a few weeks ago and decided I'd check the table to make sure it was level. It was out by .030". So I shimmed that all up, got that really close, and then I decided to put a precision square on the table and check to see if the beam assembly was perpendicular with the table. My jaw dropped. The beam is out 1/16" over 2" No visible adjustment for making that square. So my table is level with the rails that move the beam around the table, but the beam that the Mirror Assembly travels on is out of square a crazy amount. It's run for 4 years like that. I can't imagine they ever checked it.

Haven't figured out how to fix that one yet, since there's not enough adjustment in anything to resolve it, plus I don't want to go off on a weekend of working on the machine.

[Rich Harman]

Haven't figured out how to fix that one yet, since there's not enough adjustment in anything to resolve it, plus I don't want to go off on a weekend of working on the machine.

Don't know how the Trotecs are set up but on my machine the Y axis motor is in the center of the laser driving a torque tube. I can loosen the connector for one side of the torque tube and adjust just one side of the Y axis to bring the X axis into perpendicularity.

[Scott Shepherd]

Don't know how the Trotecs are set up but on my machine the Y axis motor is in the center of the laser driving a torque tube. I can loosen the connector for one side of the torque tube and adjust just one side of the Y axis to bring the X axis into perpendicularity.

Rich, it's not the Trotec that's out, it's our Universal. Yes, it has a torque tube as well, but there isn't enough adjustment in that to rotate it as far as it needs to rotate to make it square.

[Rich Harman]

Rich, it's not the Trotec that's out, it's our Universal. Yes, it has a torque tube as well, but there isn't enough adjustment in that to rotate it as far as it needs to rotate to make it square.

How about loosening the belt and shifting it by a tooth or two, then adjust the torque tube?

[Ronald Erickson]

Your 80 watt is setup different than my 75. My "area" is different than that. Glad you got it fixed.

Actually, I just got it aligned so that it isn't making slanted cuts. It's still having an issue with the beam moving up in mirror 3 when the head moves past 11 inches on the Y-axis. I'll probably have to call Trotec to figure out what's causing the issue. I'm suspecting some type of issue with one of the Y-axis rails. If I don't fix it, any cuts done in the lower left corner of the 17x27 bed will taper to the left on the left and right sides of the piece I am cutting.
=======================================

To all: (a summary)

The issue I was having with slanted cuts on thicker materials (like 1/4 inch acrylic) was caused by laser alignment. The alignment was up and right on mirror 3 and needed to be adjusted.

245827


Here's a diagram I made of how the beam alignment will affect the cut.

245826

If you're vector cutting and not getting verticle edges, you may have an alignment issue. I'll post an update after I've talked with Trotec about the remaining issue.

[Mike Null]

Ron

Eliminating slanted cuts is not possible. The very shape of the laser beam cuts a kerf that is slightly angled. Wider at the top than the bottom. On thicker material you can minimize it with multiple passes. On thinner materials it rarely shows.

I find it odd that you are having so much of an adjustment issue and that you have apparently decided to attack your perceived problem before any contact with tech support.

[Ronald Erickson]

Ron

Eliminating slanted cuts is not possible. The very shape of the laser beam cuts a kerf that is slightly angled. Wider at the top than the bottom. On thicker material you can minimize it with multiple passes. On thinner materials it rarely shows.

I find it odd that you are having so much of an adjustment issue and that you have apparently decided to attack your perceived problem before any contact with tech support.

Mike,

I suppose you can say I've been conditioned to providing tech support to the tech support from my professional work experience, so I like to wrap my head around an issue and understand a problem prior to contacting tech support. For me, it's much easier to talk to tech support if you understand the nature of the issue at hand.

The parallelogram cuts (2 sides parallel and slanted in the same direction) weren't just a "perceived" problem, but an actual issue. I'm not trying to totally eliminate slanted cuts, but I was able to turn a .015 parallelogram cut into a .005 hourglass cut that's even on all four sides of the cut. Even the Trotec Speedy 300 service manual specifically states "If the laserbeam is out of the middle of the lens, you will never reach a right angled cutting line on all four sides of a working peace." (grammar error not mine, that was a cut and paste from the manual :o) The left and right sides were noticeably slanted and was not an acceptable cut that I would sell to my customers. After the alignment, the vertical cuts actually look vertical and are now acceptable.

The adjustment issue stemmed from the rotational slop in the mirror 1 base. Ideally, the base shouldn't rotate in the slot at all, it should only slide back and forth. I can go into more detail if you'd like, but the fix was simple and turned an aggravating experience into a much more manageable one. (If your listening Trotec, tighten up your tolerances on the mirror 1 base, or better yet, add a hex screw adjuster on a sliding base that allows you to slide the mirror assembly without changing the angle of the base. :) )

The alignment / slanted cutting issue I have solved. With the other issue I am having, I haven't figured out the cause. The laser tracks perfect in the center of mirror 3 (burned on tape) from Y0 to Y11. At Y11, it immediately starts to rise until it's at its highest point at Y17. Here is a diagram of the tracking (Y0 is on the right, Y17 is on the left):

245850
It would seem to me that I have an issue with one of the Y-Axis rails. Do you have any input?

[Rich Harman]

Ron

Eliminating slanted cuts is not possible. The very shape of the laser beam cuts a kerf that is slightly angled. Wider at the top than the bottom. On thicker material you can minimize it with multiple passes. On thinner materials it rarely shows.

Don't you mean wider at the bottom? If you focus on the top of the material then the beam is smallest at the top and widest at the bottom.

If you cut at a speed that just barely cuts through then your kerf will be wider at the top. If you slow down you can widen the bottom of the kerf. I cut stuff all the time where you can stand it on edge and not perceive that it leans one way or the other. I routinely cut 1/8" up to 3/8" acrylic and sometimes 1/2" and never make more than one pass - and my edges are vertical. Probably not perfect, but you can't see any slant.

[Rich Harman]

The alignment / slanted cutting issue I have solved. With the other issue I am having, I haven't figured out the cause. The laser tracks perfect in the center of mirror 3 (burned on tape) from Y0 to Y11. At Y11, it immediately starts to rise until it's at its highest point at Y17. Here is a diagram of the tracking (Y0 is on the right, Y17 is on the left):

245850
It would seem to me that I have an issue with one of the Y-Axis rails. Do you have any input?

I had a similar issue with my machine. The Y rails did not lie on the same plane. The solution was to shim up the top portion (near 0,0) of the Y axis rail.

[Mike Null]

Rich

I meant wider at the top. The hourglass shape of the beam would seem to support your theory but in my experience the energy is absorbed at the point of contact and dissipates deeper into the material. Properly adjusted settings--just enough power to get through the material--will produce a smaller kerf at the bottom. For this reason I always cut from the reverse side when I'm doing inlay work.

[Scott Shepherd]

How about loosening the belt and shifting it by a tooth or two, then adjust the torque tube?

It's all pinned together with roll pins, so I guess they didn't want it moved!


Ron, what did you find out from Trotec today? What did they say about it dropping off like that at Y10?

[Rich Harman]

It's all pinned together with roll pins, so I guess they didn't want it moved!

Perhaps there is an adjustment where the belt attaches to the X axis? It would be bad engineering to have no way of fine tuning, there's got to be a way.

[Ronald Erickson]

Ron, what did you find out from Trotec today? What did they say about it dropping off like that at Y10?

I called Trotec and spoke with tech support. Their suggestion is to try to alter the mirror alignment to minimize the issue, specifically to adjust mirror 1 slightly low in the far field position (Y17) then compensate for the lowness by shooting mirror 2 slightly higher.

I'll try putting the laser out of alignment to compensate for the problem as suggested, but I believe I'm on my own finding the real cause of the problem and a solution. I still believe there's something with the Y-axis that's causing the issue and after the holiday I'll put more time into troubleshooting. I go with Rich's suggestion and check to make sure the Y-axis rails are on the same plane.

[Ronald Erickson]

Just a quick update. Trotec just called me (Amie McGee, Bret with Tech Support, and Chris my sales rep) to help diagnose the issue. It was a very unexpected call to receive, but a call that was very much appreciated. (Thank you Trotec!)

We formulated a plan of attack to first see how the lower left deviation affects the quality of the cuts (test cuts on all four corners and the middle of the table, measure the differences, and email tech support the results) and go from there. It may be that the deviation does not significantly impact the quality of the cut and it's a test that I had not done.

Amie also had some recommendations on the start/stop dimple issue that I'll test tonight.

[Dan Hintz]

Test with the shortest FL lens you plan to use (even if you don't currently own one). What may engrave fine with a 2"FL lens may get too dodgy using a 1.5"FL lens.

[Ronald Erickson]

Summary update:

Last night I spent 5 hours burning test pieces and taking pictures to send to Trotec. I cut 1/4 thick acrylic test pieces in all four corners and at the center of the table. Inspecting the test pieces showed a slight deviation in the lower left piece, but it was not significantly different from the other test pieces. Although the 1.3mm rise in the beam exists in the lower left corner, it should not presently impact the work that I am doing.

With regards to correcting the dimple, Amie had suggested I use a much higher PPi/Hz (5000-9500) to even out the initial pulse. It did not seem to make a difference in the dimpling. Two ways suggested to combat the start/stop dimpling are to start the cut where it will be hidden (a corner of a straight cut doesn't dimple), or add a lead in/lead out segment. I'll experiment this weekend.

[Scott Shepherd]

The only way to not get a dimple is to do a lead in/lead out. In order for a laser to work, it has to pierce the material. It has to hit it with high power and dwell long enough to get through the material and then move. That piercing of the material is never going to be dead straight. It's a very very common problem that's been around since lasers have been cutting. Don't want a dimple in your part, make sure the pierce is not on your part. Also keep in mind that every single person cutting with a laser is getting the same thing, so it''ll always be as good as anyone else can do, unless they are drawing lead in/lead outs.

[Mike Null]

Steve

I will acknowledge that a dimple sometimes exists but most of the time it is not visible. With my work on name tags the dimple is often visible but disappears with cleaning. On clear acrylics I cannot spot a dimple even though I know one should be there.

My theory is that the dimple is more apparent when too much power is used for the material. I have never had a complaint and as I said it usually isn't visible.

[Scott Shepherd]

I agree Mike, on nametags and thinner materials, you won't see it. It becomes very clear in thicker materials, like 1/4", 3/8", and 1/2" acrylic. The longer the dwell needs to be to puncture the material, the more noticeable it'll be. You can certainly minimize it. I'm only stating it as a point of reference so someone doesn't think you can put in a piece of 1/2" thick acrylic and cut flawless parts all day long, without having that mark, unless you do some work on the file, and that it's an issue that's been around ALL cutting lasers for a long time, not just C02 lasers.