View Full Version : Dark burnt cuts...

David Sacks
05-13-2009, 12:07 AM
I assume there is nothing I can do with cutting 1/4" wood - 100% power and 5% - getting burnt edges, leave your fingers black..

I'm now using the Masking Material for the face - and it works great, now I want to know if there is way to solve the full cuts.


Scott Challoner
05-13-2009, 12:33 AM
What kind of wood is it? A lot of it will depend on the type of wood you are cutting. I cut a lot of 1/4" poplar and it just darkens a bit. You can try lowering the ppi or increasing the speed.

David Sacks
05-13-2009, 12:37 AM
Alder - prefinished from Laserbits - if I raise the speed, it doesn't cut all the way through.... I'm running at 600dpi. Are you saying run the raster first. Then run the Vector at a lower dpi?

Chris J Drew
05-13-2009, 5:17 AM
For a less charred vector cut when you're lasering wood, just drop the frequency right down.
Frequency for the vector is independant of dpi... lowering it will mean less heat build-up in the cut & so less charring.
Still use full power for the vector so you can run the speed as fast as possible which also helps keep the cut as cool as possible.
You'll be surprised how low you can go with the frequency & still get a cut through wood.

Frank Corker
05-13-2009, 6:47 AM
Chris is right - when you cut you will see that there is a frequency setting at the bottom of the vector area on Epilog driver interface. Wood is supposed to be at 500 - the default setting is 5000 which if you haven't altered it will char the life out of it.

Chris J Drew
05-13-2009, 7:40 AM
500 is Epilog's suggestion - but all the values on their settings page should be viewed as a starting-point for your experimentation.
For example their suggestion for anodised ali can be way off depending on the thickness & quality of the anodising.
Wood in particular can be very variable.
I've cut wood with frequency below 100, this was 5mm cherry... certain plastics such as PETG which like to weld back together can also benefit from a really low frequency vector setting.

For your cut to be as clean as possible it's worth checking that your air-assist pipe is correctly aligned so it blows the vapourised material out of the bottom of the cut... over time it can "wander" a little by being occasionally knocked off-target when loading material etc.
Just aim it at your red-dot on some focussed material, there's a small grub-screw that holds it tight in the head (same size key as your lens-holder).

Albert Nix
05-13-2009, 7:51 AM
Will PPI have the same effect as changing the frequency, I do not think I can change the frequency settings on my MII??

Mike Null
05-13-2009, 8:49 AM
I make two passes and get a smooth medium brown cut. I cannot use charred wood as I must radius the edges and then lacquer them.

It is better to make two passes (it's really not much extra time) and get a good result than to have to do things over. We can all make the cut in one pass--who cares if it's not acceptable.

Chris J Drew
05-13-2009, 9:01 AM
I'm not familiar with machines which lack frequency control for vectoring, and have not used lasers with a PPI setting, but I'd guess they do a similar thing.

I think it may go like this: ( my guess )
Pulse-per-inch is related to the frequency but depends on the speed.
For example frequency of 500Hz @ 1 inch-per-second = PPI of 500.
Slowing to 1/2 that speed effectively doubles the PPI.
If you know the frequency you could work out the PPI, if you know the PPI you could work out the frequency.

Perhaps this is just different manufacturers doing things differently but achieving a very similar outcome.
Hopefully someone with more knowledge of this will pipe in & let us know if this is true.

Try lowering your PPI & see what happens... as with most things laser, experimentation is your friend!:)

Scott Challoner
05-13-2009, 10:56 AM

DPI only relates to rastering. It won't affect vector cuts. PPI is the same as frequency so the the others are right, lower the frequency. Alder should cut like butter.

Mike Null
05-13-2009, 11:08 AM

I believe that PPI is constant regardless of speed. It is "pulses per inch" and not related to time.

Albert Nix
05-13-2009, 12:52 PM
On mine PPI is varible when vectoring but not for raster. PPI range on mine is 30-1524 then it goes to X which is a steady burn.

Rodne Gold
05-13-2009, 5:30 PM
PPI is also a time based function as it assumes frequency , IE 1000 pulses per second at a speed of 2" per second = 500 pulses per inch.
so with that in mind , something I have never investigated is that : if PPI is always constant the machine must be constantly varying Freq to get this constant PPI cos true linear speed of the head vectoring is never constant due to accelerations , turning , arcs etc.
Or is that assuming oversophistication for the sort of lasers we all have?

Mike Null
05-13-2009, 5:51 PM

I have not seen anyplace where PPI is identified as being in seconds. Granted my experiece is limited to just a few machines.

David Sacks
05-14-2009, 1:59 AM
So I'm at 60 Freq - and run the raster (at 1200 dpi for nice dark images) first, then vector as a second pass...

But I have also solved the dirty faces too...

I first tried Pledge but found Murphy's Oil Soap better. A nice coat on both sides, and cover with light paper. Raser, Vector - and a quick wash in hot water - dry with a chammy.

This is working better than the lo-tack mask paper. which takes too much work to clean the small pieces inside of "O"s and the like.

Anyway - they look great..
Thanks everyone...

Rodne Gold
05-14-2009, 2:34 AM
Mike , Yeh , its never espressed in seconds , as its pulses per inch.
It has to be derived from frequency however - as it has to be dependent on speed of travel.

What Im wondering is .. we all realise that headspeed is never really constant ..do the machines that quote PPI actually vary freq continually to compensate for this?

In the case of a machine that allows you to set freq alone , the pulses per inch using that freq would be related to head speed. So a user would have to know head speed to set the correct PPI to avoid or minimize charring.

At the end of it all , getting back to the original question , the obvious answer is that the low powered laser is the wrong tool for cutting thicker wood if you want clean edges.

Mike Null
05-14-2009, 6:41 AM

Here's a view of how my Trotec handles the PPI/hz settings.

Dan Hintz
05-14-2009, 2:54 PM

I'm with you that it should be specified, and I've wondered the same myself. If a beam is specified in PPI, then speed of the carriage shouldn't matter... but what's the length of each pulse (i.e., the duty cycle)? Does the duty cycle vary with the PPI to remain constant in pulse length, or does it remain fixed leading to more power in the substrate with increasing PPI (max PPI means 100% power into the substrate, min PPI leads to near zero power... and if that's the case, then selecting power settings would have no effect)? Either one has very valid reasons for being chosen.

Andrea Weissenseel
05-15-2009, 1:54 PM
I have the same problem with the charred edges when I cut felt, I'll try out the PPI seeting maybe it helps there too.

Mike, when you cut in two passes, do you adjust the focus after the first pass ?

Mike Null
05-15-2009, 2:46 PM

I usually do not adjust the focus unless I'm working on 1/4",(6mm) or more.

Most of my work is at 3mm or less.

Chris J Drew
05-18-2009, 7:59 AM

I believe that PPI is constant regardless of speed. It is "pulses per inch" and not related to time.

I understand that PPi supposed to be a constant along the length of the cut , & frequency is linked to time.
Like the above posters, I' curious if this mean that a machine with PPi control maintains the PPi by adjusting the frequency according to the rate of movement of the machine?
- ie, if you set the PPI at a given value & then run the job slower it reduces the rate at which the tube fires?
Would speeding the job up result in the machine increasing the frequency in order to maintain the PPi?
How can PPi remain constant when the rate of movement of the machine cannot be so? ( for example when acceleration / deceleration takes place, or when cutting tight radiuses etc )
Maybe PPi is actually an "average" figure & is in reality an adjustment of the frequency but without the actual frequency value being revealed to the operator.

In your opinion which is better ( Frequency vs PPi ) and for what reasons?
Or, perhaps more fairly, what are the advantages & disadvantages of each system? ( if any! )

I guess it's irrelevant when you consider that the operator doesn't need to actually know the values of frequency or PPi, just that "less" is a colder cut (less charred wood, less melted & cleaner cut PETG etc) & "more" is a hotter cut ( smoother edge on acrylic cuts etc )

Rodne Gold
05-18-2009, 9:07 AM
Ultimately the laser treppans when it cuts , Ie it cuts by drilling lots of holes. Ideally one would want the holes to be spaced as far apart as possible to achieve a smooth cut and have the least heat affected zone.
However the ideal is not always ideal.
For example acrylic wants enough "drilling" overlap to just melt the edges of the cut giving a polished look , thus in most cases , the ideal PPI setting isn't...
The laser works by raising local temp to a state so that the material is vaporised , often you need the overlaps so the material actually gets to a vaporising temp - IE you need more heat affected zone for the laser to actually work. You might not , for example , be able to use a 25w laser to cut 1/4" wood without "overheating" where it cuts , so there might be no solution involving PPi to avoid char.
i dont believe that any of our lasers would continually vary freq to get a totally constant PPI. I think they just use the speed set and adjust freq to a single figure to average this out..I don't attribute that much sophistication to our type of machines.
IMHO , PPI is the better figure to use as it requires no computation and its derived from Freq where freq is pulses over time

If the Freq figure used by some mnfgrs is indeed the pulses per second , you have to know the speed in inches per sec to get PPI. However it might be that some mnfgrs just interchange the terms Freq and PPI , perhaps avoiding confusion with DPI or PPI (Pixels per inch).
Anyway , in 99.9% of cases when we cut , we just use the factory default......