Confused about PPI

[John Noell]

I am unclear about the impact of changing the PPI value on my Epilog Mini. I can see that higher (5000) is better for acrylic edges but I am having trouble seeing just what difference it makes on some materials (e.g., coconut shell). In general, what is the effect of varying the PPI value on common materials (esp. woods)?

[Sarah Jones]

what is nice about engraving woods, is the fact that any pixel out of place is usually masked by the grain of the wood, so you can techniqually engrave a lower resolution image on wood and it still come out okay. I would not suggest the same for glass metal and acrlic engravings becuase Jpeg artifacting on those stick out like a sore thumb..

[Frank Corker]

John the 5000 that you see on the Epilog refers to the pulsing of the laser during vectoring. The laser is pulsing at 5000 is good for continuous cutting of materials like acrylic/corian - when cutting materials like wood, laserable plastic, leather, mat board it is reduced to 500. With a reduction like this there will be less charring.

Try cutting wood with the frequency at 5000 and then again at 500, I can't understand why you cannot see the difference. Same goes with leather, cuts much neater and smooth at 500 even though it can be cut with 5000.

[Sarah Jones]

Oh opps my bad! I was talking about raster engraving, the frequency is rather important when vectoring

[John Noell]

I have tried cutting coconut shell, Fiji mahogany (fairly hard), and kavula (fairly soft) at 500 PPI and 5000 PPI but I cannot see any significant difference. The cut surface appears to be equally blackened. I varied the power & speed from where I am just barely cutting through (i.e., pieces come out but mucst be pushed out) to where the pieces fall through. Maybe these materials are dofferent than most? (We have so many exotic hardwoods here and our own flavor of pine that it is not easy to get samples of typical woods available in places like the USA.)

[Frank Corker]

John for you and your exotic coconuts (whey hey!) experimentation is your only course of action. Try taking the frequency right down 50, 100. 150 etc until you find a desireable one. I've vector cut acrylic at 50 frequency and got a rather surprising result, dependent on the thickness, sometimes the finished edge can be smoother.

[Marc Myer]

Wow! I've got to try that!

[Gary Hair]

Marc,
I have used a very low PPI setting to perforate paper. Try something very low on paper and you can really see the results.

Gary

[John Noell]

Yes, I can see the difference with very low PP values on paper but not on the woods I have tried. Not enough time to experiment right now as we are still cutting and engraving coconut shells for a big resort. The biggest problem is the irregular shape and thickness which means every single piece must be hand arranged and the focus is a compromise. If I had the time I would have gotten a longer focal length lens but the resort is in a major hurry. Even DHL took 12 days to deliver a box from the USA. (And the box sent the same day via regular air mail only took 14 days.)

[Peck Sidara]

John,

PPI and DPI are essentially the same. It refers to the printing resolution. DPI is for raster engraving mode.

Frequency is adjustable from 100-5000 and applies to vector cutting only. As others have stated, wood is typically vector cut at 500 Frequency and acrylic at 5000. It's the pulse rate of the laser tube in vector mode.

Setting your frequency from 100-200 will result in preforation of your material. Where the laser beam is pulsed with spacing to allow for punching out the part later.

Reasons why you may not see a difference in the woods edge quality when adjusting the frequency from 500 to 5000 is possibly because:

1. You're using too much power for the particular wood which is causing the browning/charring of the edges.

2. You don't have enough power so the speed setting you're using for vector cutting results in too much dwell time causing the browning/charring regardless of your frequency setting.

HTH,

[Mike Null]

PPI and DPI are essentially the same. It refers to the printing resolution. DPI is for raster engraving mode.

You may be correct for Epilog but they are not the same for Universal and Trotec.

PPI refers to pulses per inch and differs from DPI. DPI refers to dots per inch and relates to resolution. I believe one of your manuals has an excellent explanation of dpi.

One thing i disagree with is the blanket statement that higher resolution produces better image quality. That is dependant on materials and, in fact. lower resolution may produce a better image with some substrates.

[Rodne Gold]

Peck , on other systems , PPI =Frequency.

[Peck Sidara]

One thing i disagree with is the blanket statement that higher resolution produces better image quality. That is dependant on materials and, in fact. lower resolution may produce a better image with some substrates.

Agreed Mike. I'm not sure who made this blanket statement but it wasn't me.

Mike & Rodne,

Thanks for the clarification on PPI vs. DPI. On your systems, are you able to adjust both PPI and resolution in raster mode? If so, please elaborate on how they affect raster engraving quality.

[Marc Myer]

...and it gets worse.
In the world of Graphics Dorks (me) working in Photoshop, ppi means Pixels Per Inch.

...and among fantasy writers, of course, it's Pixies Per Inch.

...and...

[John Noell]

Reasons why you may not see a difference in the woods edge quality when adjusting the frequency from 500 to 5000 is possibly because:

1. You're using too much power for the particular wood which is causing the browning/charring of the edges.

2. You don't have enough power so the speed setting you're using for vector cutting results in too much dwell time causing the browning/charring regardless of your frequency setting.

HTH,

Well, it is not the former as I am barely cutting through (and in many cases not all the way). But 45 watts may not be enough for some of the coconut shells. They vary tremendously in both thickness and hardness. The ones that are both very thick and very hard are extremely difficult to cut. Adding the curvature in, and thus uneven focus, makes it even trickier. (But this justified a higher quote so I am still making decent money for this part of the world - where skilled workers may earn less than $3 USD/hour.)