[SMallas]

Hi Laser Doc,
Could you please clarify your comment on the tightness of the RF beam being better, as the first thing that is recommended to improve an RF beam is to expand it?

Ps. You can't really tar all glass laser tubes with the same brush. There are some manufacturers producing some fantastic tubes, that I would put up against any RF tube.

Ps. It's the longer the focal length of the lens, the longer the depth of field ( cuts thicker ). Shortening the focal length will increase the beam intensity, but at a cost of depth.

[pontiacg5]

Hi Laser Doc,
Could you please clarify your comment on the tightness of the RF beam being better, as the first thing that is recommended to improve an RF beam is to expand it?

Ps. You can't really tar all glass laser tubes with the same brush. There are some manufacturers producing some fantastic tubes, that I would put up against any RF tube.

Ps. It's the longer the focal length of the lens, the longer the depth of field ( cuts thicker ). Shortening the focal length will increase the beam intensity, but at a cost of depth.

Expanding the beam reduces the amount of divergence, less divergence means less spot size change across the table when using flying mirror setups. You can always focus back to the small spot size at the head.

Divergence isn't as big a problem with glass tube lasers, only because the beam starts out huge to begin with.

I agree with you on the lenses. No idea what the laser doctor was thinking saying a shorter lens cuts deeper, it is always the other way. Longer focal length always means deeper cuts as the beam is thinner for a greater distance than a short length lens. A thinner beam means more power density which is always a better cut. The disadvantage is you won't be able to focus as small, so engravings won't be as nice.

Remember the free advice you get on the internet is worth what you paid for it!

[keebler303]

I agree with you on the lenses. No idea what the laser doctor was thinking saying a shorter lens cuts deeper, it is always the other way. Longer focal length always means deeper cuts as the beam is thinner for a greater distance than a short length lens. A thinner beam means more power density which is always a better cut. The disadvantage is you won't be able to focus as small, so engravings won't be as nice.

Remember the free advice you get on the internet is worth what you paid for it!

I think the devil is in the details here. A short focal length will give you a higher power density. A higher power density will cut deeper than a lower power density. A long focal length maintains a "good enough" power density over a wider Z range so it works better for cutting thicker material.

You're both right!

Matt

[pontiacg5]

I think the devil is in the details here. A short focal length will give you a higher power density. A higher power density will cut deeper than a lower power density. A long focal length maintains a "good enough" power density over a wider Z range so it works better for cutting thicker material.

You're both right!

Matt

A normal 2.5" lens will never cut as deep as a 4" or 5" lens. Even if it could "cut" it would leave such a horribly tapered finish that it would be entirely useless.

The fact that a longer lens doesn't focus as small is completely and totally overpowered by that greater power density over a longer range. The extra cut width due to that slightly bigger focal size leaves more room to clear material from the cut anyway, something that is a huge problem when cutting thick wood.

Yeah, you won't engrave as much detail with a short lens but that's the entire reason we have them. Trying to say that a shorter lens will cut as deep as a long one is crazy though...



"I always say more power, more speed. The higher the power the greater cutting depth and less time to perform the task. Cutting depth is dependent upon a few things. Consider beam diameter and the length of you focal cutting lens. The shorter the lens focal distance the smaller the spot size, thus the higher power density and the greater depth cut. Small laser beams produced by the RF laser will always be more efficient than a glass laser tube. This is why they make such great cutters and engravers. Research laser power density, formulas are available to tell you how many actually watts of cutting power will be produced, depending upon the laser type you use. I could give you the formula but that would be to easy.
Reply With Quote"


More power does mean more speed when cutting and engraving, but too much tube power can be a bad thing for engraving. Higher powered tubes have larger aperture sizes so it's harder to focus to as tight of a spot. This is why most will say an 80W tube is the max for engraving, and probably why the original question was asked of the Laser Doctor. Higher powered tubes can't scale power back as much either, so trying to engrave at 20W with a 150W CO2 tube would be nearly impossible. That might mean you burn up whatever you are engraving, loosing a whole bunch of detail.


Does the Laser Doctor have much experience with Co2 laser tubes?

[Rodney Gold]

The bigger the beam diameter , the smaller the spot..check these calculators
Laser Optics Calculators
Try the spot size (and energy desnity) calculator and increase the beam diameter...

[pontiacg5]

The bigger the beam diameter , the smaller the spot..check these calculators
Laser Optics Calculators
Try the spot size (and energy desnity) calculator and increase the beam diameter...

That calculator assumes your laser has a perfect Gaussian tem00 output mode, something a co2 tube laser never has.

So since the spot is never truly round, it can never be focused perfectly round. The spot is almost always made up of smaller spots of different levels of power (usually weaker towards the middle.)

Since the spot is made of little spots a shorter focal length will "funnel" them all together into a finer point. A larger focal length lens does not have this benefit, so the spot never gets as fine.

I don't know anything at all about RF co2 lasers, but if they do output in tem00 mode with a very low divergence that would explain the reason why a beam expander is common on those lasers.

[Rodney Gold]

I almost all cases , a beam expander is used in conjunction with a collimator , increases beam diameter for a smaller spot and corrects divergence. Works as well with RF as with the glass tubes I use.

In just about all laser applications , the initial thermal shock that aids vaporisation is based on energy density. If you use a longer focal length lens , you may NEVER achieve enough energy density to vaporise or start the thermal process and even tho you may , in theory , be able to cut thicker stuff , you may , in reality , not be able to do so well. Experienced this many time with my 30W RF lasers and even my 80w RECI glass lasers.
Some materials , like acrylic , are actually wave guides and it really is to some extent irrelevant what focal length lens you use , you can cut thru far thicker material than the focal length suggests , in that case , energy density is king and you DO get far better cuts using shorter focal lengths.

I can tell you that in the real world , out of the 12+ "western" lasers we have used or owned , with Synrad and Coherent tubes , I have never got anywhere close to 20k hours outa one , more like 3-5k hours at best. Often a LOT worse....

In terms of engraving and cutting quality , my chinese 60w and 80w RECI tubes do as good a job as my 30w RF ones in terms of engraving and they cut a LOT faster.

I think we are being ripped off big time in respect of the price one pays for a mainstream RF tube..I can buy 15x 60w Glass tubes for one 30w RF exchange/repair (especially since I am in South Africa and have to send the RF tube away vs a simple spare glass tube swap). 60W glass is $170 , RF repair is way in excess of $3k for me.

If both glass and RF tubes do the same job for your application, it makes no sense to use a RF tube when the replacement cost of a glass tube is 1/15th the RF.
In fact , I have no intention of even replacing a RF tube for any of my 6 GCC lasers when they die , Its more cost effective for me to import and run a bigger and more powerful chinese machine than repair the tube..

I have been running my 2 Shenui machines in a serious production environment , at least 30 hrs a week continuous and have been running em for around 18 months like that , my RECI 80w has done in excess of 2000 hrs and is as good as it was when new and I have replaced one 60W due to operator error (ice cold water added to a chiller .. which caused a glass crack). Not one other issue barring that with those machines. To me , thats very good going....

[Laser Doctor]

Boy, I didn't realize I opened pandoras box. The comments above are all correct. I was simply trying to state, shorter focal lengths produce smaller spot sizes. longer focal lengths produce larger spot sizes. Speed and power awill always determine the "depth' of the cut or mark.

[Laser Doctor]

..and give Rodney Gold a cudos for his comment on spot size cutting.

[mochouinard]

I was simply trying to state, shorter focal lengths produce smaller spot sizes. longer focal lengths produce larger spot sizes. Speed and power awill always determine the "depth' of the cut or mark.

Using a smaller focal will make smaller spot size but only in a thin focus range. This is why using a longer focal length is needed for depth cutting in addition to speed and power. Longer focal length allow to have a small beam but for a longer focusing distance, the best would be the thickness of your material.

I guess a multi-pass cutting with readjusting the focus lower for every layer would also do the trick too, but would take longer.

If you have a very thin material but very hard to cut, a shorter focal length would help to concentrate the energy to a smaller area.

[pontiacg5]

You'll never get a laser table true enough to see the difference in spot size between a 2.5 and 4" lens anyway. Nor will you ever hold a perfect enough focus to keep the top of the material in the sharpest part of the focus.

You will always cut deeper easier with a longer lens. All this about thermal shock is moot, you have to vaporize the entire thickness of the material. The energy density of the shorter lens will be nowhere near as dense as the longer lens at the bottom of the material you are cutting. You can see this when you cut wood with a short focal length. The lens vaporizes a bit, and the air blast blows it out of the way. The rest of the remaining material gets red hot and starts to smolder because the laser doesn't have enough power at that focal point to vaporize the wood. So it sits there, heated red hot with the air blast fanning away at it making it burn that much more.

I've seen this countless times with my laser. Try cutting 1/2" MDF with a 2.5" lens in one pass. After that try some 3/4" plywood. 80W RECI tube...

From 1/4" to 2" tool foam, 1/2" to 3/4" acrylic, 1/4" ply+, UHMW thicker than 1/4", all cut better with a longer lens. Less taper, less charring, faster speeds...

I guess a multi-pass cutting with readjusting the focus lower for every layer would also do the trick too, but would take longer.

That never works well in practice. If you move the focal point down after ever pass you waste most of the outside diameter of the CO2 beam on the top surface of your material still. The outside portion of the beam is the most concentrated, you really can't afford to loose it.

Multiple passes never seems to work well. With wood it chars the crap out of it. Acrylic is not as bad, but the finished edge is a lot cleaner with one pass.

[Rodney Gold]

IMO The smaller powered lasers are the wrong tool for cutting wood , unless of course you actually like and want char.....
As to multipass , I agree with Pontiac well it's generally a disaster for various reasons , firstly the dross or melt does not get entirely blown away , it's fiddly , the item can expand cos of heat affected zones and then the next pass is slightly off register due to the expansion and in general , one tends to overheat the item. I never use multipass...EVER!!!!

[OldEngraver]

Looks like the laser doctor has left us again!

[Laser Doctor]

I'm here. The Laser Doctor was off yesterday playing golf.

[Ant-Lion]

I have a question. I have a 40W K40 type Chinese laser engraving machine and lately I have found that the maximum current seems to be dropping. I have never driven the machine above 18ma but recently the maximum current read on the current meter was 15ma and as of today 14ma.

I am worried that the power supply is in the process of failing. I am woundering if you can give me some trouble shooting ideas to see if the issue could be caused by something other then the power supply.

[herby08]

Hi there
how old is your laser tube? they god down after about 1 year even if you did not drive it to hard.
also depending on the manufacturer of the tube.
if the power goes down how does the corona inside the tube looks if it looks whit or whitish the tube is on the way out. in 99% its not the power supply!
greetings
walt

I have a question. I have a 40W K40 type Chinese laser engraving machine and lately I have found that the maximum current seems to be dropping. I have never driven the machine above 18ma but recently the maximum current read on the current meter was 15ma and as of today 14ma.

I am worried that the power supply is in the process of failing. I am woundering if you can give me some trouble shooting ideas to see if the issue could be caused by something other then the power supply.

[wetcoast]

Do spare glass tubes degrade while sitting in storage? Should a person only have spares that they are going to use within a certain time period. If so, what would that time frame be?

Thanks Laser Doctor!

[photoguy]

I've got a Laserpro Mercury laser machine and I firmly believe it to be either possessed or schizophrenic.

It'll run like a dream one minute. But, if I turn it off and come back later, laser won't fire.

All of the moving parts work fine. It's just the laser that will fade out and become intermittent. Usually, it does this with raster engraving. But now even vector lines are hit or miss.

It did this before. But then it stopped firing altogether. And after a month of being idle it started working again. (I was out of town on business and no one else here knows how to use it) It was working without a problem for a few weeks and now it's back to not firing/intermittent firing.

I've checked for loose connections, made sure things are clean and without obstruction, and anything else i can think of. Do I need to replace/refill the tube? Or is there something else I can try?

[TechGraphix]

Things that can cause similar problems:


Contact - no contact - sometimes contact - half contact..
Especially heavy components and parts that get warm tend to do this.

Carefully take your HV-supply apart and check the solderside of the PCB..
Warranty seal stickers? Unless you want to return the supply, break them..
Mind the voltages left in capacitors.. Check them and shortcircuit them.. they can be lethal!

Kees

[Laser Doctor]

Sure. Your problem is the gas mix in the laser tube or a small leak that has developed. The mixture on these tubes is disassociating causing a reduction in current drive. Always suspect the power supply. But this sounds like what Ive heard over and over again.