Bad results after using ply

[Karen Thompson]

Hi everyone.
I’ve been cutting and engraving some “laser grade” ply (3mm and 6mm)and noticed that after a while the results weren’t as good as they were to start with and it's not just on the ply. So I guessed it was time to check and clean the mirrors and lense.

The lense was filthy and the mirrors had a slight dust on them so I cleaned them all and checked alignment etc. But things haven’t really improved. OK an example is (and bear in mind I’m using an LS6040 Rabbit), before using the ply I could engrave on 3mm acrylic using Speed 400 and Power 21. Now I have to increase the Power to 50 to get anything decent.

I don’t know what I’m missing here, I haven’t changed any machine settings in Lasercut and I’m doing everything the same as before. I’m feeling a little confused and wondered if anyone can give me an idea as to where I’m going wrong.

Thanks
Karen

[Roy Nicholson]

It may be time to check the focus with the gauge that came with the laser.


Regards


Roy N.

[Karen Thompson]

It may be time to check the focus with the gauge that came with the laser.


Regards


Roy N.

Hi Roy,
The only focusing "tool" I was given was a 41mm square to manually focus the machine to the material - is this the "gauge" you mean?

Thanks for answering:)

Karen

[Dan Hintz]

And don't forget to check beam alignment...

[Alexa Ristow]

Hi Karen,

I have also cut plenty of 3mm ply on a chinese 6090. With the amount of smoke produced, as well as the oily nature of the residue, we had to clean all three mirrors as well as the lens with alcohol afterwards. In terms of power loss, this has always been caused by alignment issues. whilst it is fairly easy to check alignment at the head, an alignment error on the secondary mirror can go unnoticed. Run your machine for a few minutes, switch off and feel the secondary mirror mount. if it is hot to the touch then you have a beam edge touching the metal either on the way in or after reflection.

A thorough alignment, starting at the tube and ending at the lens a few weeks ago allowed me to significantly increase my cutting spead again.

Cheers,

Alexa

[Roy Nicholson]

Karen

I received a 2 inch plastic cylinder as a gauge to set mine.

I checked mine fo the first time in two years recently and was delighted with the results.

Regards

Roy N.

[Karen Thompson]

Hi everyone.

Thanks for your replies - I'll definately re-check the alignment and see how it goes from there.

Fingers crossed:rolleyes:

Regards
Karen

[william kaminsky]

Hi everyone.
I’ve been cutting and engraving some “laser grade” ply (3mm and 6mm)and noticed that after a while the results weren’t as good as they were to start with and it's not just on the ply. So I guessed it was time to check and clean the mirrors and lense.

The lense was filthy and the mirrors had a slight dust on them so I cleaned them all and checked alignment etc. But things haven’t really improved. OK an example is (and bear in mind I’m using an LS6040 Rabbit), before using the ply I could engrave on 3mm acrylic using Speed 400 and Power 21. Now I have to increase the Power to 50 to get anything decent.

I don’t know what I’m missing here, I haven’t changed any machine settings in Lasercut and I’m doing everything the same as before. I’m feeling a little confused and wondered if anyone can give me an idea as to where I’m going wrong.

Thanks
Karen


Your results sounds about right to me. Have done this dozens of times on a friends machine. and back a good ten years ago got the same old results that you are experienceing now.

Number one, 50 watts is sort of marginal for good cutting of 1/4 inch plywood. That is OK for the 1/8 inch stuff, but you got to run very, very slow to use that sort of power on 1/4 inch. I figure to cut one time all the way through you need to be down in the range of less than 3 inches per second on the thick plywood.

Number two. the lens is fogging due to the heat of the laser melting, then boiling away the glue in the plywood laminations. We have a vaccum point on ours near to the lens cover so that when cutting plywood, the vapors then get sucked into the bag, instead of going on the hot lens. You need a hood that will suck up the vapors and get rid of them, which is a nother noisey item you need to modify for or add on. We use a common Craftsman garage vacuum cleaner for this.

Number three. Been finding that when cutting thick plywood, not only expect a burnt edge, but expect the kerf to be very wide. Although the laser is reported to have a beam of .004" wide, why then (to get through the wood) does it create a kerf of about .018" wide (as measured)? The machine is being taxed to it's full abilities. It may cut it, but is not doing a very good job then. Once they get the kerf to narrow down at any setting, the manufacturers will then have something to boast about. But .018" is a little wide for me. Was told at one demo, to adjust my lines over a good .009" , yeah, like I need to do that for every thing I cut. As Tim the Tool Man Taylor once said.... More power is better.

After you get the Mods done, or adjusted for, then you begin to get the wood to catch fire. We use a common atomizer shooting water to pre-treat the wood, plus Nitrogen air supply, and... Still ruin quite a bit of that fancy imported wood.



William

[Dan Hintz]

Number three. Been finding that when cutting thick plywood, not only expect a burnt edge, but expect the kerf to be very wide. Although the laser is reported to have a beam of .004" wide, why then (to get through the wood) does it create a kerf of about .018" wide (as measured)? The machine is being taxed to it's full abilities. It may cut it, but is not doing a very good job then. Once they get the kerf to narrow down at any setting, the manufacturers will then have something to boast about. But .018" is a little wide for me. Was told at one demo, to adjust my lines over a good .009" , yeah, like I need to do that for every thing I cut. As Tim the Tool Man Taylor once said.... More power is better.
More power is not a solution to this "problem", as one has nothing to do with the other. The 4mil beam width is only at the apex of the beam's cone and will be wider both above and below the apex. At 1/4", the beam has increased in radius about 16mil. Focusing into the ply halfway (1/8"), your kerf will be roughly 16-18mils at the surface, exactly what you see.

You can narrow the kerf at the surface by using a longer focal length lens, such as a 4", as long as you understand there is a point of diminishing returns... as the focal length gets longer, the more difficult it is to keep a small apex, so while a 2" FL lens gives a 4mil apex, a 4" FL lens may give 1.5-2 times that.

[Rodne Gold]

The glass tubes used in the chinese machines dont have really good beam quality and have finite lives , like 800 hours or so , most likely the tube has lost some power and is not perfectly aligned.
The price quoted to me for a 50w tube was $125 , I would get a new one and try with that.

[william kaminsky]

More power is not a solution to this "problem", as one has nothing to do with the other. The 4mil beam width is only at the apex of the beam's cone and will be wider both above and below the apex. At 1/4", the beam has increased in radius about 16mil. Focusing into the ply halfway (1/8"), your kerf will be roughly 16-18mils at the surface, exactly what you see.


Am confused here....

How do you go about focusing about 1/2 way through the material then as you mentioned?

Would this have to be done on 5mm stock, 3mm stock, and 2.5 mm stock then?

Would you then have to offset the beam about 1/2 width to one side in order that you get a reasonable and accuracte cut then?

Wm.

[Rodne Gold]

William , the wide kerf is cos of the edges of the cut burning .. you need more vooma (power density) to cleanly vaporise the wood without gross heat affected zones.

[Dan Hintz]

Yeah, don't let me confuse the issue too much... as Rodney says, too little power and you spend a lot of time hovering in the same area, roasting the wood along the edges. If your speed is fast enough, however, the kerf will be little more than what the laser itself is creating. That said, you don't need to focus into most materials if they are cutting fine for you, but in some circumstances it can pay to focus deeper than the surface. My calculation of a 16-18mil kerf was based upon a 4mm beam width at the focus lens (which should be fairly close to reality with most lower-power lasers).

If you want the desired material to be as exact in size as possible, then yes, you'll have to include kerf offset into your design (this would be a nice feature to have on the laser itself, but then you run into the issue of the laser determining what is the desired material side and what is scrap).

On a side note, focusing into the material does not have to be strictly halfway... for thicker acrylic pieces (1"), focusing about 1/4 to 1/3rd of the way in works much better than 1/2 way in. In that case, you're trading a bit of kerf width (compared to focusing on the surface) to get through a thicker material.


Am confused here....

How do you go about focusing about 1/2 way through the material then as you mentioned?

Would this have to be done on 5mm stock, 3mm stock, and 2.5 mm stock then?

Would you then have to offset the beam about 1/2 width to one side in order that you get a reasonable and accuracte cut then?

Wm.