Can't understand something about Corels mirroring

[Joseph Tovar]

I'm not sure why, but I'm not comprehending something for some reason. It seams like this should make sense, but it's not.

Here's the scenerio...

I have a piece of 12"x24" acrylic that I need to cut 2 shapes out, but I need to raster something on the top. I don't want to burn the tops while cutting the acrylic, so I mirror the shapes that need cutting. After cutting, I flip the piece of acrylic around the same way I mirrored it, put the cut out pieces back in like a puzzle, and then raster the other image on top. This SHOULD align correctly in my eyes, but it doesn't. I don't understand why.

Ex:
Take to items, lets say 2 rectangles(outline red for cutting). then in the top right of each rectangle, put a smiley face(colored black for rastering). Now mirror vertically the 2 rectangles so you can cut from the back. Once cut, flip the whole peice of 12x24 acrylic over vertically and put the pieces back. Now raster the little smiley faces. In my mind they should be rastered in the top right like we placed them in the graphic, but the don't line up correctly.

At first I thought it was because when I mirrored the shapes, I didn't include a 12x24 box around them to simulate the acrylic. I'm asumming that when I mirror the shapes alone, they were be mirrored on the center of the verticle axis so it wasn't accurate. I tested this out by making one big item close to the top edge and 1 small onefar from the bottom edge. Highlighting them both and mirrorroring them should me that they mirror on their axis, so that was one reason I was off.
Then I thought to include the 12"x24" box so it mirrors in the center at 6", that way when I flip the piece of acrylic over, everthing should match. It still didn't work.

I hope this makes sense because my head hurts now. It seams so simple, yet I can't get it to come out like I thought it would.

Any suggestions?

[Joseph Tovar]

Attached is a Corel file that I used as a sample and it still doesn't work correctly. See how the star is close to the top and the polygon is further from the edge on the bottom? If you highlight them both and mirror them they stay in the same spot, but swap places. This was my first problem so I made the 12x24 box. Now select the star, polygon, and box, then mirror them, now this simulates what should really happen when I flip the acrylic over and cut. DON'T select the black dots, only the 3 shapes.

Once cut, select all three items again and mirror them back. See how the black dots in them stay in the same place from which they first started. Now if I select the dots and raster them, they aren't rastering where I thought they would(just like you see in the Corel file). They are actually off, and I'm not sure why.

[Joseph Tovar]

Now that I think about it, I'll just ask someone who's probably done this already...

How do I cut the shape from the back(to prevent burning on the front), flip the acrylic back over, put the pieces back in, and line up the rest of the items for rastering?

Seams straight forward, but I can't get it to work. I though mirroring the shape would work, but it didn't.

[Rodne Gold]

I engrave and cut in one go - what you need to do is to determine why you are burning the acrylic in the first place - It should not be neccesary for you to jump thru those hoops to do what you want.
At any rate , you can just leave the whole piece in place , cut your shapes , turn em over and place em in the same holes (if possible - they got to be bilateral to do this) and then engrave em.
You are possibly having problems with turning the sheet over and then misregistering the shapes cos the sheet doesnt see the exact same start point registration when turned over or some other issue.
I cant open your drawing , save it as an earlier version of Corel

[Dan Hintz]

I'm having difficulty understanding what is happening.. maybe pictures would help solidify it. One thing to be careful of is the objects selected before flipping. For example, if you missed selecting the box surrounding the entire piece, your centerpoint will be different and the flip will not line up.

[Niklas Bjornestal]

I would guess that the main reason you have trouble when mirroring the whole piece of acrylic is that it isnt exactly 12"x24"

[Dan Hintz]

Niklas' suggestion has the same effect... if the piece is not the same size as your bounding box, the centerpoint will be off and the flip will fail to line up.

[Joseph Tovar]

Rodne,
The reason for the burning is because cutting the acrylic is causing a small flame that is being pulled to the rear of the machine because of the blower. This is melting parts of the acrylic even with the protective paper overlay. As for the misalignment, I thought it was because I wasn't lining things up correctly, but it's not. I'm putting the piece tightly in the upper left corner everytime.

Niklas/Dan,
I think you may be right about the mirror problem. I have the bounding box measured as 12x24, but when I measured the acrylic piece, it's actually 1/8" smaller all the way around. I believe this is why it is off just a bit.

In theory, if everything is measured correctly, what I'm trying to do should work, right?

Here's another way I thought about doing this since every sheet of acrylic may not be exactly 12x24. Do the first one and make sure when I flip it over, the image being rastered is lined up. Once I do that, I'll just use that same piece as a template for all the rest. Basically, I'll cut out the pieces from each sheet, then put them back into the first sheet which should make them all line up correctly.

Attached is the first sample I made. Now I just want to make a bunch by cutting the shapes out from the back, then flipping them back to make the little ear piece.

[Mike Null]

Joseph

It won't matter how you position an irregular shape to your 0-0 position if it's not the dimension you think you're working with.

You should be able to do this job without turning the acrylic if you have the settings right and are using air assist. Don't be afraid to do the cut line twice using lower power to reduce the flaming. Use a paper mask. Reduce your PPI or Hertz setting.

If you have too much exhaust flow cut it back by using a gate.

[Dan Hintz]

Doing it in one pass without flipping is the best solution. However, if you feel you must flip, use the first pass to cut the main sheet to a slightly smaller size.

For example, let's say the sheet was 17.5" tall. Set your bounding box to 17.3" tall. During the first run, the machine will engrave your design and cut the sheet down to the size of the bounding box. Remove the scrap piece and flip the sheet. This will ensure your flipping it does not change the center point. The cut line to trim the sheet is far enough away from the actual design so you don't care if it burns a little, and you're guaranteed a match in sheet size regardless of what you start with (assuming the sheet is at least 17.3" tall to begin with).

[Steve Clarkson]

I completely understand where Joseph is coming from. I have the same issue.

My laser bed is 12"x24".......occasionally, I'll get someone that wants something that is 24"x24". So I stick a piece of wood/acrylic/Rowmark in that is 24"x24" (but in reality it maybe 23 15/16" x 24 1/64") and cut and/or engrave what I need on the top.....then I rotate it (in reality and in Corel) 180 degrees......no matter what.....it's usually off by a hair and I spend an hour trying to line it up correctly.

I have not found a solution to this yet...........

[Rodne Gold]

All you need to do is use Air assist - it is vital to use air assist properly when cutting acrylic - you are making it real difficult for yourself if you dont.

[Dee Gallo]

Okay Steve, I know you're good, but HOW do you get a 24 x 24 piece into a 12 x 24 laser?

Oh, and my procedure is to raster first, cut second both from the same side - you should not get any top burning if your settings are right... what are you using with what laser wattage? Just make sure you use tape and air! The only soot/smoke I ever get is on the bottom anyway. If you don't have air assist, you should check out a recent post where someone puts a fan in front of the laser and blows air that way.

However, there have been times when I need to position something precisely on a scrap and what I do is position the acrylic and check where the bottom horizontal and left vertical sides of the part I want to raster are on the bed ruler. Then, I make guidelines on the file which are the same. Then place the graphic exactly there and voila, perfect every time.

This is also a handy procedure for adding something to an existing piece, so I can pre-make something and then personalize later.

:) dee

[Dan Hintz]

Dee, one word... "doors".

[Niklas Bjornestal]

Dee, one word... "doors".
isnt that cheating :rolleyes:

[Joseph Tovar]

Thanks for all the suggestions...I'll give it another go.

Mike, I usually use leave the gate all way the open. What happens if I partially close it? I know it limits the air flow, but I thought we want air flow? What does limiting the air flow do during the cutting?

I also see comments about using tape also. Do you mean to put an additional layer of tape on top of the protective paper layer? The plastic I'm using is 1/8" thick and from Delvies.

Any setting suggestions for a 50W ULS? I have it at 100p 4.0s
The burning is not horrible, but the backside stays so much more glossy, while the top has a few "glazed" spots. You can really notice it when you hold it at an angle that reflects the light.

[Steve Clarkson]

Dan hit the nail on the head Dee.

[Dee Gallo]

Dan hit the nail on the head Dee.

Okay, I don't have a door on mine... or do I & I don't know it?

[Steve Clarkson]

Okay, I don't have a door on mine... or do I & I don't know it?

Mine is right in front......the door pulls open and down.......so you can see/clean under the table. I usually open this door whenever my auto plunger gets jammed in my cutting grid so I can pull those black rubber cords to manually lower the table..........

[Ernie DeMartino]

Joseph, you may want to try going up to 10% on the speed. I had the same issue with the small flame and it worked for me.

[Mike Null]

Joseph

Too much air flow will "fan the flames" so to speak. You're probably noticing more flame with front to back cuts than with side to side.

If you have air assist be sure it's on, lower your ppi setting and increase your speed a little. If that doesn't work try closing the gate a little to reduce air exhaust but not so much that you can smell smoke and fumes.

I'm one who would rather make two cut passes than risk flame damage to the material, though with 1/8" material that shouldn't be necesssary.