PPI question

[Pete Thomas]

Hello all, I have a question dealing with the PPI setting. I have a ULS and was vector curring some 1/4" acrylic at 100p and 4.0s with 500PPI.

Here's the question how come when I up the PPI to 800 I could not cut through, you could see that the cut only made it 3/4 through. When I drop the PPI back to 500 when through with plenty of power to spare. I was under the assumption that pulsing the laser more would make a deeper cut. If this is not the case then what would be the optium PPI setting for max cutting power.

Still always learning something new......

Thanks, Pete

[Robin Lake]

Good evening --

My name is Robin. Can I take a shot at answering this?

I do not "yet" have a laser machine, so this is just speculation.

I think the reason 500 ppi (pulses per inch?) will cut the material and 800 ppi will not relates to the duration of the pulse. More pulses must be of shorter duration, so less time to cut.

Ok. Tell me I'm wrong and show me the door. :)

Robin

ps - I have been reading this forum for a while, going back to some of the earliest posts. It has been unbelievably helpful reading as I sort through the various choises of which laser to acquire. Thank you all for your generous contributions.

R

[Joe Pelonio]

On my Epilog 45 watt I just cut 12 sheets of 1/4" acrylic into letters yesterday. My settings were 6 speed, 100% power, ppi frequency 5000.
The edges were nice and smooth, like they had been polished.

The higher frequency provides a smoother cut, meaning that if I'm correct your cutting at a low frequency will have rougher edges. What does your manual call for?

[Lee DeRaud]

What Joe said. Can't comment on your other settings since you didn't say what model/power ULS you've got, but the ULS settings "book" says 1000PPI for acrylic vector cutting.

[Jerry Allen]

Increasing the PPI will increase the power density in a given area at the same power and speed. That would translate to a smoother cut in acrylic and more charing on wood. You may be getting melt back. Try adjusting the speed to 5 or 6 at 800PPI and see if it cuts deeper. Also try reducing the power to 70-80% at 800PPI and check the result.

[Mike Mackenzie]

Pete,

Believe it or not robin is correct the higher the PPI the faster the duration the beam turns on and off. So it makes sense that it would not go through at 800ppi verses 500. If you think about it this way maybe you could understand it better Less Pulses mean more intense pulses more pulses are less intense but heat up the affected area more.

If you want a clean smooth edge we recommend using 1000ppi and then slow the speed down.

The difference in time is minimal between say 5% speed and 3% speed maybe 2 seconds difference.

The ppi setting does make a huge difference when cutting woods lower ppi settings like 100 - 300 will cut woods with much less burn. With acrylics most people want polished edges thus higher ppi settings.

[Pete Thomas]

Wow, thanks for all the words of wisdom. BTY I have a ULS X-660 60W system.

So in summary, when you increase the PPI you should lower the Speed. Higher PPI gives smoother cut as there is consideriable more beam overlap, but the price you is a lower laser output power which is compensated by using a lower Speed.

I'm going to give the 800PPI at lower speed a try tonight, it seems to make sense now.

Thanks, to all
Pete

[J Porter]

hmmmmmm.... According to what is being said here suggests to me that the driver for the laser needs some re-design.

In my driver you set the ppi and then set the parameters for engraves and cuts. So for some things, particularly photos on wood and plexi, I should put my cuts in a separate layer and run them separately from the engraves as opposed to just using the colors and running it all at once.

My system is a Pinnacle 25. Do other systems allow a ppi to be set individually for each power/speed parameter?

~Joe

[Lee DeRaud]

My system is a Pinnacle 25. Do other systems allow a ppi to be set individually for each power/speed parameter?ULS VL200 does, at least with the "advanced" driver.

[Rodne Gold]

Varying the PPI should not affect cutting depth , the pulses the laser fires should not be longer neither should they vary in power as PPI changes , the tube is capable of firing X amount of pulses per second at Y power (which the user sets) and this will set the upper limits of pulses per inch based on how many inches the laser can travel per second.
PPI applies only to cutting , it has NO effect when rastering , the laser actually treppans a cut , its a series of holes. If one has raised PPI it means your holes that have enough power to pierce the material are closer together. If the holes overlap you run the risk of putting too much heat into the material and thus increasing the heat affected zone (HAZ), hence the fact that wood chars less with lower PPI. If you rely on the heat of these overlaps to to cut , your power settings are wrong , the beam should vaporise the material with the minimum heat transfered to adjacent areas and not rely on putting temperature in the material to do the cut.
In the case of acrylic and some other plastics , this undesireable side effect of increasing PPI is a benefit as it promotes a melt of the material to give a polished edge or smoother cut cos of melt.
Ideally PPI should be set to just enough so that the spacing of the pulses is close enough to give a continuous cut with the minimal HAZ.
There is another side to this too and that is air assist , in the case of acrylic , using it is vital to obtaining the best cut , ideally it should be strong enough to eject the melt thru the hole that the pulse produces , but not too strong as to promote rapid cooling and thus a frosted edge.
Air assist will generally do 2 things , cool the material and reduce HAZ enabling cleaner engraving and cutting and blow away or remove bye products of the lasering so that they do not impact on the area where the next pulse hits.

[Jerry Allen]

As usual, another great answer from a pro. I'd like to meet this guy.

I was just getting ready to apologize for my earlier response after Mike's message. And I may well still be wrong, and if so, my apologies.

Then I read Rodney's response. Now I'm confused.

All I know is that with a 25W laser, I use 300DPI and Auto PPI, which yields 300PPI. I get nice smooth edges on 1/4" acrylic material. 1.2% speed, 100% power. (GCC Merc)

It is possible that different machines handle DPI and PPI differently due to the driver? I would be interested to know specifics even if relative to different machines.

From my understanding of the the crappy manuals is that DPI is vertical and PPI is horizontal and that more PPI equals more power density per line.

I have no problem cutting a nice clean line as stated herein. But in trying to create cuts and grooves, i.e., dados, I have found that as I increase the power, the dado has a tendency to fill in due to the melt. I try to compensate but am always disappointed with the mediocre results and apparent excessive power required.

[Rodne Gold]

Jerry, Dpi is not relevant in terms of PPI , it is only related to raster engraving where PPI does not feature. PPI is only related to continuous cutting or vector engraving and it wont feature on horizontal scanning.
In essence however , there is a sort of realtionship as increasing DPI will once again "space the dots" closer on the y axis and once again , you might put too much heat into the material.
Decreasing DPI can save huge amounts of time , for example we had to do 15 000 tags , 4"x1" with a raster border and 3 lines of text , at 500 dpi the best we could do was 58 secs a tag including the cut (rowmark flexi) , by taking it down to 250 dpi and one or 2 other changes , we reduced cycle time to 21 secs with almost no loss in quality whatsoever, I had charged based on 1 minute and the customer was happy with that price so I made almost triple what I was expecting on the job. More importantly we made the deadline with about 2 hours to spare.
Often reducing DPI gives better results than higher dpi cos of stuff like the effects of heat , it's sometimes actually crisper than high dpi prints. Apart from anything else , it stops the warpage or "print" thru you often get on thin materials.

[Jerry Allen]

Thanks RG.
That makes it somewhat clearer than the superstitions I have accumulated.
So, from what I am getting is that DPI is more related to rastering and PPI to vector cutting.
It was also my understanding that increasing DPI/PPI without changing speed or power increases the power to a given area, not lessens it. Even though the single pulse should cut at equal depth the cumulative effect heats the surrounding area, and particularly in plastics, melts the adjacent areas.
So adding more power or less speed is adding insult to injury. Or, to draw and analogy , using an axe when a knife would do.
So, in this case, do you increase speed, lowering the power, or decrease PPI? Or is it something you just have to play with depending on the machine and material? It seems like melt is a double edged sword. I have increased power or reduced speed when it seems like it should be the other way around. In that case, I never get what I consider to be a good cut in acrylic when I do this. It may be smooth but the kerf is wide and the slant excessive.

[John Bailey]

Robin,

Welcome to the "Creek." Don't be afraid to jump in with a comment. All are freindly on this "civilest" of sites.

John

[Robin Lake]

Welcome to the "Creek."

Thanks, John. I appreciate being able to take part. Lots of information here.

Robin

[Rodne Gold]

I'll start last , the reason your kerf is wide and slanted is most likely an alignment one. One of the most important things one can do is align the laser , the beam has to hit the final optic dead centre and must be perpendicular to it. A good alignment can often up power 25% or more at the table and maintain it all over the table , effectives often giving a sort of cumulative increase in that figure too, Alignement is fiddly but not rocket science and most operators should be capable of performing it. Think of what a laser with 25-30% more power would have cost you buy and then you realise time spent checking or doing it is time well spent.

PPI is not at all related to raster engraving , most drivers will disable it as in raster engraving the laser stops and starts many many times in a single X pass and to apply it is almost impossible and somewhat pointless. Increasing power is not the same as increasing ppi or dpi . Generally increasing power will result in a cleaner cut or better ablation (removal of material) , compare it to a lighter or a pencil type gas solder buring a hole thru paper , the likelyhood og the lighter hole being far more ill defined and charred are high. In the case of cutting , you still need X amount of power to "drill thru" , all the PPI will do is space those holes closer. You will actually add more power to a set area if you increase ppi or dpi , but the extra power may or may not be desireable. For example wood doesnt actually darken well with high clean power , it zaps the wood without charring it so "correctly" engraving wood for contrast is not a great idea , one might up the DPI to get more heat affected areas for not only a deeper , but a darker engraving , bear in mind engraving does not pierce the item , so more dots overlapping will tend to engrave on existing engraving.
Experimentation is the key here.

[Jerry Allen]

Thanks for the reply.
Perhaps I was not clear about the kerf and slant. I was referring to when I increase power to overcome melt. Nomally I get a nice clean cut and negligible slant. When trying to cut a dado however, the power required is not linear relative to a through cut. As I increase the power to achieve the depth I want, the walls of the cut become slanted and distorted. I have not tried adjusting the PPI yet which would probably help. There is a big difference between a single cut line and cut lines that are touching. I used a line pattern so that adjacent parallel cut lines were not cut immediatey after the previous one which helped some.
I understand that when rastering, the PPI is not a factor. Is there is a constant PPI when rastering? Or is it equal to the DPI in that case?
Regardless of the fact that I am ok with my normal cut and slant, I will attempt an alignment just to be sure and to familiarize myself with the process. I notice that when I place the red laser on a point and then focus, that the point shifts as the table rises. Doesn't seem like that should be happening, but then the table does not move vertically when cutting or engraving.

[Rodne Gold]

Try the slot/dado as a raster engraving and run a vector around it when finished Take the table up 1/2 the depth of the slot after focussing on the material surface. I have never found the laser good for this type application and normally use a rotary for it.