Constructing Janus, by Dirk

Post your build logs here

Re: Constructing Janus, by Dirk

Postby educa » Tue Oct 02, 2012 10:22 pm

So thats 40 watts. How thick plywood and how thick acrylic can you cut with GOOD cut ? I mean the quality of cut, not the speed, but it must be single pass.
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: Constructing Janus, by Dirk

Postby steppenshoe » Wed Oct 03, 2012 11:47 pm

Lovehappy is definitely still on ebay, I buy VFD's and spindles from him all the time.

http://www.ebay.com/sch/m.html?_nkw=las ... pyshopping
steppenshoe
 
Posts: 83
Joined: Sun Oct 16, 2011 3:59 am

Re: Constructing Janus, by Dirk

Postby dirktheeng » Thu Oct 04, 2012 12:38 am

educa wrote:So thats 40 watts. How thick plywood and how thick acrylic can you cut with GOOD cut ? I mean the quality of cut, not the speed, but it must be single pass.


I can comfortably cut 1/4 inch ply wood in a single pass as long as I don't hit knotts. I can definately cut 1/4 inch solid wood. I can do 1/4 inch plastics too.

The thing is that I have come to realize that a lazer cut for accurate parts isn't really that valuable for thick materials because of the shape of the kerf. It will always cut a "V" shaped kerf no matter what you do. For thick materials, the edge squareness is often important especially if the pieces have to fit together. In some respects, I wish I had build a cnc router first instead. However, for thin materials, a lazer can't be beat. So I am perfectly happy with my 40 watt lazer as my thicker materials will be run in a cnc router.

That's just my two cents.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: Constructing Janus, by Dirk

Postby dirktheeng » Thu Oct 04, 2012 12:45 am

@educa:

You should really carefully check out that smoothie cnc controller. I've been talking with the developers and the software architecture is really sweet. My only concern at this point is if the current chip they are using (32 bit 120Mhz single core) will have enough horse power to run a 1000DPI engraving at 4-6 inches per second. That said, they have some people working on going to either an xmos or FPGA for more parrallel power.

They have coded up a very basic engraver. I read through the code on github and it does look promising. The best part is that it is all open and active. The will be releasing the first version of the board which comes with 4 axis stepper drivers on it as well in a few weeks. I am really tempted to just buy one and play. It's only $140. It looks super easy to set up modules to do useful stuff too. However, the cautios part of me is saying just wait and see what kind of resolution they can get at what speed. At this point, unless it does good engraving, it doesn't buy me anything. However, if I were to start again, I would buy this as it would have saved some cash and provided a great educational experience.

Needless to say, i will be following the development of smoothie closely.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: Constructing Janus, by Dirk

Postby educa » Thu Oct 04, 2012 6:09 am

At 120Mhz they should surely have power enough.

With such controllers the biggest problem is always the same... The want everything parametered to be able to run every where on machine with all kinds of acceleration ramps (mm/s^²), top speeds (mm/s) and different stepper setups (mm/step)

I code for 1 specific machine. My acceleration for normal cutting is 6000mm/s^² with topspeeds between 500 and 700mm/s (where I will probably choose 500) and on X axis for engraving I can shoot 1000mm/s with 30000mm/s^² acceleration.
My machine does a nice 0.02mm / step

Once I write my softwarehardcoded on these parameters, it runs very smooth.





Now I have a another question since this whole "what laser should I buy" bothers me a lot.
I initially wanted to buy a 80 Watt RECI tube from cncoletech on ebay (good seller, allready bought a lot there)

But... since these are quite expensive, I did some research.
These tubes allready produce a 6.9mm diameter beam at the aperture and have 2.4mrad divergence.

My laser has a big size and the biggest distance the beam would have to travel is 2.3 meters. At that point, the beam is calculated to be 12mm diameter.


If I plug all these data into the DOF calculator, then a 12mm beam with 50.8 would have a 0.475mm DOF.

That sounds extremely little distance. I am aware of the V shape (or the X if you set focal point IN the material), but it sounds strange to only have such a small DOF


If I understand right, then this could be a good point for engraving, but a bad point for cutting (which I'll do most)


If I then look at tubes like Lovehappyshopping or coletech 40 Watt, then I see beam diameters of 1.95mm when leaving the tube. I didn't find divergence values for these tubes anywhere so I cannot calculate the spot size at 2.3 meters distance, but I guess that the beam will probably be smaller with these tubes and therefore give me more DOF?


Thats why I am trying to get any technical data about these 40W beams to see if they will be better solution for me.


Another option would be to go for the 80W anyway and use a 100mm focal lens for more DOF (my up down table has 300mm to go up-down)


Or I could abuse the fact that my laser table is so big to selectively put my workpieces where the application is best.
In my upper left corner on the table the beam is smallest (between 8 and 8.5mm) which will give me there a DOF between 4.1 and 3.6mm (I guess this'll shoot veyr nicely on 3mm ply) and in my lower right corner the beam gets upto 12mm giving me 1.8mm DOF there....


Its all calculations and its so hard to find good numbers.

If I could know at least the average divergence on such 40 watt lasers or the spot size of them at a certainl distance, then this comparing would be a lot easier. Thats why I tried to ask on the forum and in other places allready and nobody seems to have these data.


Kind regards,

Bart
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: Constructing Janus, by Dirk

Postby dirktheeng » Fri Oct 05, 2012 1:01 pm

Bart,

I think you are getting worried over a phenomena that won't really effect the quality of your cuts in the end. The reason I say this is because there is physics that occurs when the beam is cutting the material that actually shapes the beam and causes it to stay narrowly focused. The assumption that the beam will form an X shaped curf within the material when the focus is in the middle of the work I think is false. Will the beam form a focalpoint, sure, but it won't spread out when it is past the focal point when cutting in a material. This phenomena is known as "self focusing". There is a guy at work named Steve Woodruff who has spent his whole life working with lasers and built several very high power systems for specialized tasks. He showed me a wooden 2x4 he cut once as a test and the kerf was not X shaped it was a continuous V shape (the sides weren't straight but it never spread out to a wider kerf). This also matches my own observations about cutting deeply into the acrylic when I tested the PPI. With the acception of the tests I did where I put a lot of heat in the the material and basically melted and flowed the material around with the air jet, the kerf was always V shaped... widest on the top and narrowest on the bottom. If I think of it, I will ask him for some technical papers or something that will explain the physics. The long and short of it is that the material and the gases being vaporized act to keep the beam narrow even past the normal focal point. In fact, he explained that if you have enough power in an unfocused beam, it will hit the surface and self focus as if you had a lense.

Therefore, I think the only thing you have to worry about is getting the beam at or near focus at the top surface of your cut. The DOF doesn't really matter when you cut because of the self focusing interaction with the material being cut.

The other thing is that because of the same physics, the matearials can steer the beam.. that is if you are cutting thick stuff it may not go in a straight line... the longer the cut depth the more the material can interact with and move the beam off of a linear path. This was clearly evident with the deep cuts in the PPI I worked with in acrylic. When it cut the beam wasn't straight all the time.. it wabbled from side to side when it got deap and bent towards the cut when I was moving along.

Basically, a laser isn't really meant for cutting thick materials well. There are good reasons why most industrial applications are for generally thin stock. (I know somebody is going to post an example to try to prove this statement wrong... however, I think those examples are in the far minority). I think what an 80W laser will buy you is the ability to move faster... it may be able to cut deeper, but I think no matter what you do the cut quality will not be nearly as good as if you physically cut the material with some kind of blade.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: Constructing Janus, by Dirk

Postby educa » Fri Oct 05, 2012 1:25 pm

Dirk thank you for this nice explanation.

My main stuff to cut is 3 and 4 mm plywood and MDF.

The reason I wanted 80 watt was indeed to possibly increase speed a bit. The 80 watt is reasonably expensive, but... it also lasts a lot longer.

I think that I will always cut with PPI and therefore I thought that a 80Watt would have an easier job shooting pulses.


Correct me if wrong, but I guess the whole purpose of PPI is to shoot (if possible) the holes in 1 single shot and then kind of microperforate the material?


I will order my laser tube from cncoletech + power supply, chiller , GOGGLES!!! and I was also considering buying an extra 100mm focal lens (they offer me the matching air nozzle for free)

Only things I still need to buy then is the fume extraction and a airbrush compressor for the air assist.



so I guess what you try to say is that 100mm focal lens might be interesting , but maybe not needed at all ?



By the way I'm very happy that you are back online, since I think I'll need to ask you some separate stuff later on about PPI and maybe engraving. Those question will be mainly about timings since I nowhere find how long you should keep a laser tube OFF before shooting again.

With PPI and 3ms shots (which will probably be a little less for me with 80W) you can theoretically have 333 times 3ms shot in 3 second, but I wonder how much time you should leave the laser OFF between 2 pulses.

Kind regards,

Bart
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: Constructing Janus, by Dirk

Postby dirktheeng » Fri Oct 05, 2012 2:30 pm

Thanks for the kind words.

Re the 100mm lense... I think it will probably be worse for cutting. Because you can't get as tight of a focal point. I don't know for sure as I haven't tried it yet. However it may help engravings look a little softer though you could always defocus with a shorter focal length lense to achieve the same result. If it isn't very expensive you could buy and post about the differences.

PPI really just helps controll the heat going into the material so you get less melting or charing. I and others have found that it really helps when you cut thin materials. Theoretically you should also be able to cut thicker stuff too because of the dynamics of the laser turning on, bu I think that is tougher to achieve.

Re off time... That really depends on what you want to achieve... If you want to cut thick stuff you want to let the plasma to collapse in the tube and let the capacitor in the power supply fully recharge so it hits the on again with a lot of current. However if you want to do grayscale engraving, you probably want to leave the power on but at a very low level such that the plasma doesn't collapse in the tube but doesn't produce enough power to mark the material... That is called trickling the laser current.

Regarding the timing for PPI you could set up a test to cut into the edge of acrylic like I did before and set up a series of dots to cut at various feed rates. Set it up so that the dots are close but not overlapping. The feed rate will determine the off time. If the off time is too short the laser won't punch as deep into the acrylic. You want to find the fastest feed rate you can have while keeping the max depth. That should give you the off time to use to get max cut depth with PPI. I suspect that each system will vary somewhat.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

Re: Constructing Janus, by Dirk

Postby educa » Sat Oct 06, 2012 8:56 am

Thanks dirk, so 2 thing learned here

1) for engraving I have the impression that you say I should let the laser powered, but play with the PWM value that controls the power? I will have a RECI tube and there they state that at around 4ma the tube is in pre-ionisation state, so maybe I have to sweep the laser head and leave the laser on FIRE, but set the PWM to 4ma and for example 10ma for non-shoot and shoot (and then that10ma can be changed based on how dark I want to engrave, 10mA most probably allready being too much current)

2) I like your idea of testing the off time with a piece of plexi. This would indeed give a visual idea of how to shoot with a specific laser tube. And I guess with 80W I should be able to go deeper then with your 40W. Anyway, my most intended stuff to cut is 3 and 4mm ply so I guess you still call that THIN ?


Kind regards,

Bart
educa
 
Posts: 239
Joined: Thu Dec 22, 2011 9:13 pm

Re: Constructing Janus, by Dirk

Postby dirktheeng » Sun Oct 07, 2012 6:29 pm

Bart,

1) yes that is the idea that some people are promoting... keep in mind that I currently don't have an engraver set up so I don't know from practial experience what works.

2) yes 3-4 mm stuff should be fine... i've had some difficulty with 5 ply 1/4 inch stuff as knotts in the middle plies don't like to cut, but maybe an 80W laser will be able to power through that??

if you get too much thicker than that I think you will be unhappy with the squareness of the sides... I can't cut stuff and get gap free joints cuz of the shape of the kerf... sometimes I can get better results by flipping one of the cut pieces over so that the relative slope of the sides match up.
dirktheeng
 
Posts: 616
Joined: Thu Sep 09, 2010 4:49 pm

PreviousNext

Return to Build Logs

Who is online

Users browsing this forum: No registered users and 6 guests

cron