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View Full Version : Black Sap marks on the back of baltic birch cut



Karin Voorhis
11-14-2006, 2:44 PM
I am wondering after looking at all the great cuts I am seeing from the photos. How to get less black sap like stuff a little yellow and burn is ok but seems like a lot of the back is bad on some of cuts.

Any ideas on this or a way to clean it up or what I can do to look bett. some are very bad on the backside.

Joe Pelonio
11-14-2006, 3:00 PM
Apply transfer tape to the back first. Then apply to the front after engraving but before cutting. Or you can apply to both sides and with just a little more power/less speed engrave thru it.

Shaddy Dedmore
11-14-2006, 3:37 PM
Get a handheld sander for unfinished birch. Works great. For a tip, if you're cutting out shapes or ornaments (like Buzz's stuff), you can use the main sheet that the things were cut out of to hold the ornaments in place while you sand.

Or use stain first, the smoke doesn't show up as well. But I like the unstained look for some things. So i use my ROS (random orbit sander) after I cut out. TOo hard to hold down with my hand while sanding, so I drop back down into the cutout hole and it's easier to keep in place.

Shaddy

Joe Pelonio
11-14-2006, 4:40 PM
Get a handheld sander for unfinished birch. Works great. For a tip, if you're cutting out shapes or ornaments (like Buzz's stuff), you can use the main sheet that the things were cut out of to hold the ornaments in place while you sand.

Too hard to hold down with my hand while sanding, so I drop back down into the cutout hole and it's easier to keep in place.

Shaddy
That's a great tip, I should have thought of it because that's what I do for engraving two sides, but never thought of sanding that way. Keeps you from sanding the skin off your hand.

Michael Kowalczyk
11-14-2006, 5:17 PM
Karin,
Can you post a picture of your problem and what kind of wood you are using. Is it solid or ply? what are you cutting on? Factory grid table or ...?

Michael

Karin Voorhis
11-15-2006, 8:51 AM
Wow the transfer tape sounds like a great idea that should help lots.

The grid I am using is a like grill grid on top of a wire rack. Seems to work pretty good just needs to be cleaned a lot. The baltic birch 3 ply I am using has a lot of sap or maybe it is the glue coming out of it?????

I can have pictures when I get back into my shop after work tonight.

Nancy Laird
11-15-2006, 8:59 AM
I am wondering after looking at all the great cuts I am seeing from the photos. How to get less black sap like stuff a little yellow and burn is ok but seems like a lot of the back is bad on some of cuts.

Any ideas on this or a way to clean it up or what I can do to look bett. some are very bad on the backside.

Karen, we were having the same problem, and solved it by using plain 2" masking tape on the back. The burn marks stay on the tape instead of the wood, and peel right off with the tape. If you have a downdraft vector table, it helps, A LOT!! We still get a little bit of burn, but not nearly as much as before.

Nancy

Shaddy Dedmore
11-15-2006, 9:51 AM
Karen, we were having the same problem, and solved it by using plain 2" masking tape on the back.

Nancy

Wow, you have patience. Try using some wider application tape, like from Signwarehouse (http://www.signwarehouse.com/shop/merchant.mvc?Screen=CTGY&Category_Code=A-EM). Or Laserbits for over 3 times their price :eek: I use 6" tape for most things.

For me though, sanding doesn't take long, no tape weeding, less time applying tape/squeezing out bubbles... But that's just for the baltic birch. The pre-finished strips need to be masked. Sometimes both side if it's something that will have its back seen.

Shaddy

Karin Voorhis
11-16-2006, 2:29 AM
Karin,
Can you post a picture of your problem and what kind of wood you are using. Is it solid or ply? what are you cutting on? Factory grid table or ...?

Michael

Here are 2 pics a cut an ornament from a file here so you would see on something you all might have cut. also here is my $20 grid for now. I raise it up with a petistal about 4" above table.

Nancy Laird
11-16-2006, 9:01 AM
Karin,

Those look like burn and flare marks to me, just like we were getting on the backs of our pieces. Masking the back of your workpiece will virtually eliminate that. Do you have a pretty hefty blower on your laser to draw smoke? One of the things you probably should do is make sure that your working material is finished on both sides, then mask the back. When you peel off the tape, any small burn and flare marks can be removed (on both sides) with a coating of liquid floor wax for wood floors. Stubborn ones may need to be sanded a little, but the extra minute or two for masking will eliminate 95% of your problem. What power are you cutting these on? Sometimes lowering the power will cut down on the burn and flare. Might try reducing that too.

Just my .02.

Nancy

Karin Voorhis
11-16-2006, 9:15 AM
Karin,

Those look like burn and flare marks to me, just like we were getting on the backs of our pieces. Masking the back of your workpiece will virtually eliminate that. Do you have a pretty hefty blower on your laser to draw smoke? One of the things you probably should do is make sure that your working material is finished on both sides, then mask the back. When you peel off the tape, any small burn and flare marks can be removed (on both sides) with a coating of liquid floor wax for wood floors. Stubborn ones may need to be sanded a little, but the extra minute or two for masking will eliminate 95% of your problem. What power are you cutting these on? Sometimes lowering the power will cut down on the burn and flare. Might try reducing that too.

Just my .02.

Nancy

Thank you Nancy,

I am using 100% power and 2.3 speed. if I go higher on the speed it does not always cut threw all of them....

Michael Kowalczyk
11-16-2006, 4:03 PM
Here are 2 pics a cut an ornament from a file here so you would see on something you all might have cut. also here is my $20 grid for now. I raise it up with a petistal about 4" above table.


Karin, Looking at your grid may be part of the problem also.
It appears to be a metal screen used for stucco. Some of the grids may be perpendicular to your laser and bouncing back.

I use the egg crate plastic grids you can get from Home Depot, Lowes or other sources for 8-12 dollars and I get multiple uses from them by rotating them 90 degrees. I cut them to size on my table saw first but be careful because they can break apart if not done slow and smooth. when they are too cut up from the laser we spin or toss them out and put a new one in. I am still looking for another source but for now this works for us and very rarely do I get burns like yours. Usually it means time to change the grid because it is melting.

hope this helps

Joe Pelonio
11-16-2006, 4:25 PM
Karin, Looking at your grid may be part of the problem also.
It appears to be a metal screen used for stucco. Some of the grids may be perpendicular to your laser and bouncing back.


That could be it, there are flat surfaces. You'd be better off using an old piece of 1/4" acrylic where most of it is cut out but enough of a skeleton
is left to support your work. On that metal grid it might help to regularly paint it black to reduce the bouncing. You could also try not only transfer tape but then mist it on both sides with water before placing in the laser, the transfer tape will absorb water part way and keep it cooler.

George M. Perzel
11-16-2006, 4:27 PM
Hi Karin;
I think the black marks are smoke flare ups caused by laser bouce back on the underside of your work. Your grid should present no more than a knife edge-minimal cross section, to your work. Mike's egg crate is the right way to go but a metal eggcrate, like that sold very reasonably by Mike Null, is even better.
No, I don't get a commission from Mike and have no business relationmship with him-his grid just works well. You can cut down on the power and ppi which will help, but backing off too far will result in partial cuts-a real pain.
Like Shaddy, I sand-no big deal. A great tool for sanding ornaments, especially those with delicate cut outs, is a vertical oscillating sander - Grizzly has a great small worktop model for about $125. You can also use a large pneumatic drum in a drill press but the sander works great and fast-less than 5 seconds per ornament.
George