Stop start lines on 10mm acrylic

[Keith Colson]

I have been cutting a lot of signage letters lately. One of my clients wants really clean 10mm clear acrylic letters. He does not like the little stop start line. e.g. when cutting an "O" I get a distinct line on the side of the character where the laser starts and stops.

Is there a simple method for getting rid of this? I am not keen to edit the files as to define my stops and starts with a lead in.

On a side note: When running a 1 hour+ job, I notice my power drops quite a bit too. I can cut 10mm acrylic with a cold machine at 100% power x 2% speed. When the machine is hot I need 100% power x 1.3% speed. I also noticed the cooling fans are running at full speed. It makes sense that the laser will lose power when it is hot as the laser will thermally/optically expand and electrical efficiencies drop too. I have readjusted my settings to 80% power x 1.3% speed and it seems to work good but I have not done a big job yet. The fans continue to modulate rather than hitting full speed. I also cleaned my filters but they were not too bad.

Cheers
Keith

[Scott Shepherd]

Keith, it shouldn't behave like that. It should be the same power/speed all the time. In the 6 years or so of running our Universal, I've never once seen power fluctuate like that under any circumstances. I think you should make a call to tech support, it sounds like something is wrong.

[Kev Williams]

The simple method for eliminating a start-stop line is called "tangential entry and exit"-- it looks like this:

http://www.engraver1.com/erase2/tangential.jpg

the actual start and end points cross each other, so that whatever's doing the cutting- like a plasma flame, vinyl knife, water jet, endmill, or laser beam- is never static at the actual start and end point. Works like a charm.

Note that these tangents are outside, which is what you want if you're keeping the characters. If you're keeping the "holes" and throwing out the characters, the tangents would be on the inside--

The trick is, you need a program that will create the tangents. What does mine is my trusty 1991 Dos version of Casmate, which was designed for vinyl cutting...

[Keith Colson]

Thanks for all the feedback so my next question is ,,,,

Is there a plug in or macro for Corhel draw x5 that can automatically drop the tangential lead ins/out into my artwork?

Cheers Keith

[Scott Shepherd]

There might be. One thing you can do is change the start point of the cut yourself. Make it a corner and the mismatch will all but go away. Of course that doesn't help on an "O", but it does help on every other letter.

[David Somers]

Keith,

Admittedly I am groping here. I don't have Corel available to me here and won't have access to it for a few days.

Without a plug in, the one way I can think of doing this in 5 would be to break a line in the letter and assign a 1 ray arrow to the ends. You want the ray to be going to the outside of the shape. That is a lot of work unless you set a macro to help you. I believe 5 has a number of arrow styles that has just 1 ray to them.

Sorry I cant be more help at this point.

Dave

[Dave Sheldrake]

Kevs on the money, industrial lasers do that for the exact same reason, the piercing action causes horrible spike lines on the job so a lead in lead out is the only way round it. Strangely enough the Chinese lasercut software can add them automatically!

cheers

Dave

[Mike Lysov]

What lens are you using Keith? 10mm is quite thick and if your lens is 2" or 2.5" your start and end points will never match because of short depth of field for 2"/2.5" lens. It should not produce a line though, it is more like a small tab at the back of your cut out. I have the same problem with material thicker than 9mm and a 3" lens. Up to 9mm it cuts without leaving a tab but starting from a 12mm thick material I am getting them all the time. To reduce their size on a thick material I usually move a laser head closer to the material than it should be. It moves depth of field deeper into it and reduces tabs size.

I am not sure about a macro for CorelDraw for lead in/out. I tried to find one once and I could not. I could find though standalone software called SheetCam where you can import your dxf file, add leads to your design and export it as a dxf.

regarding dropping power I am experiencing the same with my LaserPro after/during long job runs. I think it can be caused by bad tube cooling.

[Keith Colson]

I am using a 2.0" which cuts 10mm okay. I set the focus 2mm below the surface as this works best for my machine. I cut 100's of letters so manually editing the file would kill the job. I only need the really clean letters for clear acrylic as it can be optically visible when seeing light shining through. The customer is very happy with my white and black letters.

I am tempted to write some VBA within corhel draw. If I do go to the trouble I may as well get it packing the letters into the sheet.

Cheers
Keith

[Matt Turner (physics)]

Hi Keith,

Did you figure anything out? I typically make the lead-ins myself, but I just got a metal-cutting job (for a rocket skin!) that has a huge number of holes. I did find that the Gcodetools add-on for Inkscape will generate lead-in/-out paths.