Laser cutting issue

[Graham Facer]

How long do you have

The problem is the reflective index of the Mo mirrors, they are quite well below both copper and gold PVD coated so will get a lot hotter when running than a gold mirror does. They also corrode quite quickly and lose more of that index in a short period of time.(think 95% after a week or two) that 5% of the ERP adds up to a lot of power in a very small area so will raise the temperature of both the optics and the holder quite quickly.

Set the CW5000 to come on at 19C and go off at 17C, anything over 22C on a fatbody tube will cause problems even though the makers say 30C is safe (it is but it's unwise to say the least)

cheers

Dave

Hi Dave,

Good to know. I did read up on some discussion on mirrors but I wasn't sure what other mirrors would do OK at 150w so I just replaced the MO. I might order a set of the others to have. The mirrors would explain a some but I think the mirror heat was due to other issues - like the overall cooling (which I now have to 10C) and the fact I did not replace the head mirror (since done) and that I found a vibration on the #1 mirror near the tube from when I changed the plane.

I think another issue generally is that I had changed out the power supply once and forgot about it but did not check the new power output, so I have been running it in the red zone (31ma) if you will. Not above its max but above its happy long life levels.

So for reference I am now getting a speed of 25mm/s (power 50% = 18ma) on 3mm acrylic in the far corner during a test strip file lasting about 1.5 minutes on the 150w tube. Or this morning I was. I think 20mm might be more like it on a longer run.

I have some production parts to cut now so I'll see how that goes.

GF

[Graham Facer]

Well the production did not go super - better than usual for sure but not super.

The mirror holder on the head and mirror #2 seem to be OK - slightly warm but nothing major. The issue is still that cuts are degrading as at the far side (from the laser) as the run progresses. I was OK on the first run or 10 pcs 3mm of about 6 minutes at 20mm/s and 50% power (18ma) along the top of the table. I then moved in down a row on the Y axis and immediately started the program again and had issue on the 2nd have of the run. No power spikes, heat maxes out at 11.3C from a start of around 10.7-11. The chiller shows a drop in temp while cutting so it seems that the chiller is keeping up.

I also did see one point where the chiller went up when I was not cutting or had just finished and the chiller was still showing that it was on a cooling cycle. That would seem to be evidence that the tube was still releasing heat after the cut was done, otherwise I would only ever expect the chiller to show a drop when not cutting if in a cooling cycle. But water flow being what it is, its still pretty small evidence.

Anyhow after the problem run, I let it chill for a minute or so and then ran the same program again on the lower row (ie same spot). It still had some issues but we got further along (away from the laser) before the showed up. Also if I paused the program and let it chill a bit and restarted, then I would started cutting through again (for a little at least). All the while, the chiller was always good and +/- 0.4c typically.

I guess the cludge solution is to start away from the laser and work toward it - which is what I used to do. But still seems like odd behaviour, I'm beginning to wonder if their is a setting I can do as a test cut and then do a full sheet worth of things with. I have a potential customer that is looking at cuts utilizing the whole table so I'm really trying to get this consistent.

Thanks for all the suggestions - I am much further along than when I started but just not 100% there yet.

[Dave Sheldrake]

The mirrors would explain a some but I think the mirror heat was due to other issues - like the overall cooling

nope....physically impossible A laser beam has no intrinsic heat as it has no mass (outside of the lonnnngggg explanation of theoretical mass of lasers) they cause heat by molecular vibration in the target substance. For the purposes of generated heat in a mirror the actual tube cooling has no relationship.
Mo mirrors the strongest by a long way but the copper versions have a better reflective index and thermal stability. That said expect to pay $600 for a set of 3 copper mirrors (decent one's anyways) of 25mm diameter so well above what is normally acceptable for a budget laser system.

What model tube are you using? a 150 watt or a 130 with a peak of 150?

cheers

Dave

[Rich Harman]

My solution is to lower the speed as the parts get further from the laser source. In RDCam it is easy. You just select all the parts, rotate 45 degrees, then select parts and assign colors. After selecting de-rotate.

I rotate because making diagonal sections is not possible, or very tedious. This enables me to create three or four zones so that I can gradually decrease cutting speed across the table.

[Graham Facer]

Rich- I've done that in the past and it might work again now that I've tweaked other things. Before it would work for a while, then stop working. Right now I changed the origin to the farthest corner but then RDcam ignored it mostly and went in a totally strange order of cutting anyhow. Its good to know you need to do that too. I know you have more experience on these machines so if you need to do that too then I can live with it. Just was looking for a 'min setting' that would work table wide for as much as I can throw at it but that seems elusive.

Dave- I understand that lasers have no mass but I had thought that the same issue that degrades the cut at high cooling temps also would spread the beam and thus increase the absorption on the mirrors. If not, then why does the cut degrade with higher cooling temp? Oh and its a 150w with a rated peak of 162w as per the sticker.

Anyhow, trying a full sheet of stuff (ie full bed). Still issues but if I pause and restart it will cut through a line it had just failed to cut through. Still was able to use all the parts with a little deburring.

Graham

[Dave Sheldrake]

Dave- I understand that lasers have no mass but I had thought that the same issue that degrades the cut at high cooling temps also would spread the beam and thus increase the absorption on the mirrors.

Nope, the high temperature issue with lasers that affect the beam and scatter are to do with efficiency of the resonator and the gas it contains, gas that has been heated takes far longer to return to the base state thus longer to become available to lase again.

would spread the beam and thus increase the absorption on the mirrors

The emitted power hasn't changed, 150 watts is 150 watts whether it is spread over 1 sq MM or 1 sq Foot, in reality a beam that increased in diameter would result in a smaller cutting point and not bigger (hence the reason tube with a bigger incident beam produce smaller spot sizes) If you mean the larger surface area having a larger ability to absorb then in the face of it ,it may seem plausible but remember the bigger beam will also have a lower power density so the amount of heat forced in the mirror will be pretty much the same (if anything a little less allowing for intrinsic losses)

cheers

Dave

[Graham Facer]

Dave - cool - learn something new everyday. And it wasn't even the hard way today! Anyhow the mirror temp seem fine now so something worked for those.

Do you think it plausible that the cooling/water tubes in the laser tube itself are not sufficient to cool the gas/tube (enough) at certain settings after a certain length of time, regardless of water temp? I just can't see another root cause and I certainly don't think that its beyond Chinese engineering to under engineer that part (or engineer it once for 80w and then just copy it for larger wattages).

I feel its like a welder with a 50% duty cycle (not that I am a welder so this is anecdotal). Everything still works but a spot weld and a weld after the duty cycle limit is reached goes to pot. I might not notice it on the close side of the table but it appears on the far side where everything is a little weaker.

GF

[Bill George]

Do you have a way to actually measure the chilled water temperature? A temperature probe on the water leaving the tube or two in and out would let you know what is going on. Could be as simple as a defective thermostat on the chiller system.

[Graham Facer]

Bill - possibly but the cutting got so much better with the lower temp that I doubt it is defective. But it might not be as accurate as it seems too. Not sure I'm going to chase this issue that much further.

I also noticed that there is a lag in the cooling system so the temp was going up while not lasering was not evidence at all. It seems to go up still even after the cooling cycle starts and then drops and continues dropping after the cycle ends before reversing and starting again.

I think it is partly the combination of a large wattage, Chinese engineering (I'll bet the water intake/outflow tube one a 150w and an 80w are the same - I should check) and large table, which allows larger run times for cutting.

I think with a 100-130w and a 900x600 table I might not have noticed an issue or if I did it would have been easier to work around. Greg (who ended up with an 80w after all his replacement tubes broke in transit) does not have the issue on the same size table (or hasn't noticed it at least).

Our conclusion together is the 150w might be too high for the set up and basically not worth it.

I think I'd do 130 or 100w (and maybe dual tube if for higher production) if I had to redo the purchase.

[Dave Sheldrake]

Lowest powered tube I run is an EFR 180 watt (205 peak) and that runs from an HPC CW4000 chiller (it's actually not a CW chiller but the naming convention was used for clarity by HPC over here) the flow rate is far far better than I ever got from the CW5000 I used to have (the CW5000 also has a couple of design flaws that make me avoid them like the plague)

http://hpclaser.co.uk/index.php?main...products_id=10

Must admit it runs 10 to 14 hours a day and never goes above 19C and yet is cheaper than the Weike made 5000 (chiller set to a high of 19 and a low of 17)

I have a couple of friends over here who have experienced problems with the 5000 not cooling properly

On paper the CW5000 will cope with a 150 watt tube but in practice the internal pumps seem to leave a bit to be desired resulting in fluctuating power and beam scatter (mode failure), as yet I havent had time to pull a 5000 dud apart and compare pumps but from what I have been told by people I trust there isn't much difference between the 3000 pump and the 5000 (I threw away a 3000 that was almost new a while ago, useless junk to say the least)

Again on paper, a laser will run VERY well at extreme low temperatures (think -200c down) but in practical terms for what we are doing the issue seems to be stability of temperature (within safe margins) and not so much low temperature per-se.

I'd council to try and keep a stable of 17-19 and see how that pans out, also ensure there are no trapped air pockets around the mirror chamber in the tube, that can cause a multitude of problems from power fluctuation to dead tubes with cracked internal mirrors.

Overall 150 watt tubes are great for high volume cutting but you need to match all the train of components or problems crop up very quickly (and usually in an expensive way)

cheers

Dave

[Graham Facer]

Hi Dave,

Sorry to restate this, but you're likely the only one that might know. The question I had is, might there literally not be enough surface area on the cooling coil/glass in the tube to cool the gas fast enough to maintain the cutting quality at a given level? Flow would play a part of course too but given the low level and little fluctuation I see on the chiller, I just don't see it at fault (or not the primary issue at least).

At some point, if this premise is right, then the glass must warm up a little despite the cooling flow, as the capacity of the glass to transfer heat exceeds the heat being generated from the laser? Though from your 17-19 c advice I suspect that I'm thinking of this as too sensitive of a variable.

If that is the case though, it would point to test cuts being inherently inaccurate compared to production cutting and to set a production speed, a long test cut should be done first to establish what the cutting speed might be when everything is 'warmed up' to a similar state as the production run itself will generate. I imagine that many production shops that cut many of one pattern end up at that speed anyhow but as we often do one offs and small production runs, I only see this state when doing the production runs.

[Dave Sheldrake]

Hiya Graham,

I can't rule it out as it is *possible* but from experience I'd say very unlikely.Be careful with the temp monitor on the chillers, they don't update very quickly (the sampling rate is low) so using the digital readout won't produce very reliable data.
There could also be a build up of algae in the tube ends that restricts flow but on the fatbody tubes this is also quite rare compared to the metal ended tubes (the cheap tubes with aluminium end caps).
I'd be looking very much at the chiller and it's output as the 5000 has a history of sporadic faults

cheers

Dave

[Bill George]

Its very hard with a simple on-off control to hold temps to a 2 degree swing. Compressors do not like being cycled that much. Start capacitors and relays are required, everything wears faster, and fails faster.
When constant temperatures are required it gets expensive. Doubt if the Chinese are doing that on these units. The lab and good commercial grade units have a number of ways to hold temperature constant as EPR (evaporator pressure regulators) and hot gas bypass are two right off the top of my head. Compressor runs 100% and temperature is regulated as above.

[Graham Facer]

OK new lead on this issue.

First - I found the arc that I heard - not the emergency switch this time but under the board on the power supply. Not sure what would have caused that but clear scorching on the underside. I found it because the laser would not fire this morning. So I tried the emergency switch - which was the issue last time - then the power supply. The on board fuse on the power supply was well gone and then I found the arcing as I borrowed some screws for the other power supply I had (the original one to the machine). Not sure why it would have failed when not in use though I do leave the machine on (but not operating) for long periods.

Second - in inspecting the tube for cracks (before I started taking bits apart) I removed the external cap (my tube protrudes from the machine out the side) and noticed that the water hose is somewhat crimped where it connects to the glass and hard as a rock by this point of course. So I'll be replacing that shortly and see if it helps.

[Mike Lysov]

It could be that the machine is not level.

I do not think it is just leveling because Graham says that the first sheet is good, the 2nd and 3rd "may be great" and the fourth is not good. It looks like a problem with his tube overheating. If his table had not had been leveled properly he would not have been able to do the first sheet.