View Full Version : Rastering Circles - Any hints besides high DPI?

Sara Gould
12-20-2010, 2:53 PM
This might be a pointless question - but I thought I'd throw it out there as an answer would save me many many hours this week.

I have an engraving to do that has many small filled in circles. They're about 0.083" (Edited to add: RBG000 black vectors) in diameter and I'm engraving them onto coated aluminum. I did the first sample for this project at 300 dpi and wasn't happy with the circles, but that's the dpi I always use so didn't think about it until after I'd finished and moved it out of the laser. Once I remembered the setting existed I did some tests on scrap and realized that 600 dpi doesn't make much difference but 1200 does. Perfectionist me pointed this out to my client and now I'm stuck spending 4 times as long on each item as I anticipated when I quoted it. Hmmm.... any ideas?


40W Epilog Mini 24

Gary Hair
12-20-2010, 3:09 PM
I see a big difference between 300 and 600 but I agree that 1200 looks the best. You may find that slowing down will give you a better result. Even if you cut the speed in half, and reduced power as needed, you'll still be faster at 300 dpi than full speed at 1200. Also, my Pinnacle has a feature called "rastor tuning" that helped me to get rid of the "jaggies" that you see in your pictures. I'm not sure if the Epilog driver has anything similar but it might be worth a call to tech support to find out.


Ross Moshinsky
12-20-2010, 3:21 PM
I've done a lot of testing at different DPIs. My general opinion is that all engraving should be done at 500dpi. I notice basically no difference between 1000dpi and 500dpi. 400dpi is acceptable for trophy plates. Anything less than 400dpi I find to be unacceptable. I know some people have different opinions, but I also am of the opinion that laser engraved glass is unacceptable. I also think you can process a photo to run at 500dpi or 300dpi. It all depends on the way the photo is processed. So although there are exceptions, I'd say I run 95% of my engraving at 500dpi. One last statement about DPI. It is directly related to speed. If you run a plate at 500dpi and it takes 10 minutes and then run the same design at 400dpi it will take just about 8 minutes to run the job.

To solve your problem. I'd run it at 500dpi. I'd also run at 100% speed. Then I'd vary my power until the desired results were achieved. I try to run my laser at 100% speed whenever possible. If power doesn't solve the issue, like Gary said, try lowering the speed. Sometimes this makes quite a difference. I also wonder, how is your artwork? It doesn't look like vector art to me.

Martin Boekers
12-20-2010, 3:37 PM
I have remade plates that were done at a different shop because the customer felt they were out
of focus. I run at 600DPI and definatly nicer. I gain a few clients here and there because of this.

I hit most everything at 600 as it is a default with Epilog and then I stay consistant.

I do crystal at 1200 seems to work best for me.

The best way for others to figure out what works best is to test, test, and test!

Richard Rumancik
12-20-2010, 3:51 PM
. . . I have an engraving to do that has many small filled in circles. They're about 0.083" in diameter and I'm engraving them onto coated aluminum.

Sara, when you say "filled in circles" what exactly do you mean - a) are the circle outlines drawn and rastered along with the inside fill or b) is the outline "invisible" and you are only rastering the "fill"? If the answer is "a", then do you have the outline color and fill color identical?

Sounds like you are letting the laser do the vector to raster conversion. You could consider converting it to bitmap in Corel rather than letting the laser do it. If you zoom in on the dots you will see what you are getting. I think that with a small (.083) circle you would get flat spots on the top and sides at 300 dpi.

With your 300 dpi example I don't really see flat spots - so I'm wondering if there is some play in your x-axis motion. In fact at the 0 degree ("equator") of the circles I'm seeing a bit of overshoot on a row of circles. I would have expected a flat vertical section near the equator. Play (backlash, hysteresis) could cause an offset between each left-to-right pass and the subsequent right-to-left pass. Some lasers have a tuning function so this can be tweaked in the driver to minimize the offset between rows of engraving. I don't know if your laser does - check the manual. Also check that belt tension is good, sprockets are tight etc.

How well can your laser draw a vector circle? If it can make a clean circle at .083 then you could vector the outlines and fill them in with raster. (Actually, I don't think my laser could draw a good circle that small.) But if there are too many circles this might not be practicable as it would be time consuming. That trick could help on larger circles if there are not too many.

Maybe try to convert the pattern to a bitmap at 300 dpi and then plot it at 300 and see whether it is the same as what you are getting. (In this case Corel is doing the vector-to-raster conversion instead of the driver.) Sometime I have converted at higher dpi eg 1200 dpi but still plotted at lower dpi and found an improvement. Not especially logical to me and I cannot prove it, but I sense that it often looks better.

David Fairfield
12-20-2010, 6:55 PM
Odd, I almost never need 1200 dpi, even in my miniature work. Compared to the OP's example, I get better results t 600 dpi, but I usually step the speed down for extremely fine work. I find 30-50% speed, 600 dpi is my sweet spot for engraving, and 99% of my engraving is directly off the vector graphic.

But yeah, try different stuff. Richard's tip is really interesting. Also I wonder if taking the piece a click or two out of focus might help?


Sara Gould
12-20-2010, 8:34 PM
Thanks for all the suggestions. It's always trial and error - but having more things to try never hurts!

I converted the vector graphic to bmp in Corel with anti-alias ON and it's doing so much better on the little circles - even at 300 dpi, which seems to be the best setting for this paint/coating and my laser. I'm really pleased, and would never have thought to try that! Focus I play with all the time because the surface has some curvature, speed and power I've played with a lot, but I never thought about the fact that the driver is rendering vector graphics into bmp and maybe Corel would do a better job.

Kim Vellore
12-21-2010, 12:36 AM
The DPI setting is something that has to be tested to get it right. No one setting will cover all types of variables. The biggest variable is the substrate. If you raster wood you wont see much difference between 600DPI and 1200DPI but if you raster Delrin you will see a difference between 1000DPI and 1200DPI. I found this out while rastering a delrin rod with a rotary fixture. Kim

Martin Boekers
12-21-2010, 8:30 AM
thanks for the tip Richard, I to would not have thought of that.