Speed vs Power

[Ross Moshinsky]

Engraving cutting boils down to speed : power. So if you break things down to actual units of measure watts:ips, you'll find that you can ballpark settings pretty accurately regardless of laser. I've never believed in "my laser is unique so my settings are different". The same thing has been said forever in the car world and it's simply not the truth. There is a shred of truth to it, but for the most part 10 of the same laser are going to be more alike than not.

With all that said, I can't speak specifically to how Chinese lasers control their power and speed. As stated in this thread, it seems they are a bit unpredictable and lack a linear result that you see from Western/DC type lasers.

[Rich Harman]

...but given a fixed focus can speed/power affect whether the cut gets a slope?

Yes. Think of it this way - if you cut just a bit too fast it will not quite cut all the way through but it may be enough to free the pieces. The top of the kerf will be wide but the bottom will be zero, so you have a slope. As you slow the cutting speed the bottom of the kerf opens up. You should be able to find a speed so that the top and bottom of the kerf are the same. I haven't tested this but I believe this process is easier if you are focused on the surface. I always cut full power (except for really thin stuff), always single pass and focus on the surface and can get perpendicular cuts on up to 3/4" material. Actually 1" if you count foam, that's the thickest material I have cut so far - and in that one case I focused half way into the material, the edge was vertical and very slightly convex.

[Mike Lysov]

I've seen a lot of mentions of edge quality but nothing yet on the characteristics it refers to. Do you mean the smoothness of the cut face? Or the edge between the faces? I haven't run into any problems with the edge yet, other than a tendency of the laminate to peel if the cut isn't thorough on the bottom. I've also been keeping an eye on how perpendicular the cut is, and I'm finding some variation within the piece. I know focus affects this, but given a fixed focus can speed/power affect whether the cut gets a slope? It's very slight, but I'm doing a lot of tab and slots so I'd like to keep them as straight as possible.

Hi Ian,

I meant charred sides by edge quality but it is not about level of charring. For example I can cut through 16mm thick MDF with the maximum speed of 800mm/min, however even though it is completely cut through edges are not perfectly smooth. It has some imperfections on an edge close to the back side. If I do the same cut with 600mm/min it makes a smooth edge. Straight edge depends on how thick is your material and what lens you are using. I have found some website where depth of field has been calculated for each lens. As far as I can remember 4" lens can give you maximum of 7mm depth of field.
I have no problems doing 18mm thick MDF with 3" lens but I set focus for wood from 12mm to 18mm deeper into the wood. It is not completely straight but I would not say it is really noticeable.

[Mike Lysov]

Does charring depend on the wood? I've only been cutting 1/4" melamine MDF so far, but there isn't a setting that will cut through that doesn't leave a black edge. Would that be more on the density of the wood, or is it going to be impossible to cut above a particular thickness without putting enough energy in to char it no matter what? Can you cut the energy in half and do multiple passes and avoid charring that way?

I do not think so. I have cut a lot of different types of wood. It always comes with charred edges. Multiple passes make it even worse because you get charring after the first pass and the second one makes this part of completed first pass even more charred.

You can get rid of residue at the back and at the front if you use a good air assist and you raise material above a cutting table. It works for me on my Laser Pro Spirit and all MDF cuts come very clean. However my other laser even with 65PSI of air assist sometimes leaves some residue at the top of cutouts. I believe it happens because my air compressor cannot keep recommended 70PSI all the time and pressure drops to 45-50PSI after compressor works 2-3 minutes. Anyway it leaves residue only at the piercing point so it is not really hard to clean.

The back of cutouts is kind of tricky on my 280W laser. I am using blades as a cutting table in this laser. I raise material about 2mm above the blades. But because the blades are dirty with residue when beam hits the blades it generates some flame at the back. That how I get some smoke stuck to the back of cut outs. Anyway it looks much better than cutting with material placed on the blades directly. If it is sitting on the blades I am getting reflections from the blades plus some residue stuck at the back.
I tried to cut with material hanging between blades but I was experiencing another problem. When cutout was complete and starting falling down the beam was still hitting it and was making some unexpected burns on them.

[Morris Mac]

There is a relation between both of them. When you cut a straight line there is no problem machine give you power and speed from the settings. Now when machine comes to a corner or do a small intricate cut, if machine keeps same power same speed you will see big burn on corners and because the machine have to slow down to make the turn is were software come in play to tell the laser to reduce power so you will not see those burns on corner. Machine can turn a corner or process geometry only as the mechanics allow, (is like driving your car around a corner with 100 miles an hour) sometime mechanical is not possible. Machine have to accelerate and de-accelarate and power of laser need to adjust as well.

[Allen Rawley]

In my experience, the following factors (in order of importance) will reduce charring when laser cutting quarter inch and thicker wood:
1. Reduce the nozzle orifice size the smallest acceptable diameter. Typical diameters are 2 mm and less.
2. Increase nozzle air pressure to maximum available (good settings start at 30 psi, and go up to 70 psi). By having a small diameter nozzle the velocity increases tremendously.
3. If more than one laser is available, then choose the one with the highest power as it will cut fastest and minimize the time to char.
4. Add a pierce or dwell time to penetrate the wood, then optimize speed for the cutting. In other words, it takes longer to first cut through the wood and if you maintain that power while cutting then you are simply adding unnecessary energy into the wood while cutting and increase charring. You Lasercut users have this feature accessible in your parameters setup--learn it and implement it. Older Lasercut users--the parameter for delay it is accessible in your syscfg.ini and found by using your text editor and searching delay and changing the value until you get it.
5. Industrial alcohol and charcoal lighter fluid are excellent to remove soot afterwards.
6. If nitrogen is available, then use it in place of compressed air (without oxygen there is less charring).

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