PDA

View Full Version : Weird laser cutting issue (not cutting properly)



Phil Horne
02-04-2013, 9:57 PM
I have a template that I am trying to get cut. Everything cuts fine around the edges but in the middle of the 24" X 24" piece of MDF its not cutting all the way through. Take a look at the pics to see what I mean.. Any and all suggestions would be greatly appreciated.

Back side of the template
253531

Larger backside of template
253532

This is the position of the template while lasering
253530

Dee Gallo
02-04-2013, 11:46 PM
Don't know your machine, but have you checked that the table is level in all four corners? The focus could be uneven.

Phil Horne
02-05-2013, 12:02 AM
Thanks for replying Dee, I have checked for level and it appears to be.. Im going to get my hands on some machinists levels in the next couple of days and make sure the machine is properly tramed though...

Rodne Gold
02-05-2013, 12:46 AM
The material maybe warping as you cutting it ? Barring that , it could be alignment or the laser varying power , you need pretty constant cooling for power stability , dunno what chiller you are using , most likely the cw 5000..keep an eye on coolant temps. If temps start rising , the laser will be less effective.

walter hofmann
02-05-2013, 5:21 AM
Hi there
I see three major issues:
1 your beam is not dead on center every four corner
2.the focal distanc is not the same on all four corners dont worry to much that the table is level more importand is that the focal distanc is the same,
this smal machines tend to have a slop in the gantry.
3. the material looks like warped this is a problem with thin material on such a large area , I prevent this by making rather two pieces .
greetings waltfl

Richard Rumancik
02-05-2013, 12:07 PM
. . . 2.the focal distanc is not the same on all four corners dont worry to much that the table is level more importand is that the focal distanc is the same, this smal machines tend to have a slop in the gantry . . .

What do you mean Walter? Are you saying that there is substantial play in the vertical axis such that the carriage and lens is moving up and down? It's hard for me to imagine this . . .

Keeping a constant focal distance requires that the carriage have no vertical play AND the table is level. But I haven't yet heard of anyone saying that their carriage motion is not planar.

Edit: I realize that I said " AND the table is level" when of course I really meant to say "AND the table is parallel (to the lens plane)."

Richard Rumancik
02-05-2013, 12:23 PM
Phil, how close are you to getting through the material? Looks like for most of it you are 95% through. Perhaps you are expecting too much in terms of contant beam energy over this entire area. Due to differing lengths of beam travel and changes in alignment as you move around as well as other miscellaneous factors it is not uncommon to have a variation in beam energy over the table. You need to set your beam energy (speed/power) such that you have 110-120% of that required for cut-through in the worst place on the table. If you "calibrate" and test in the top left you can easily get this result. When you set the parameters for 120% of cut-through in the worst location, hopefully it will not have negative consequences in the "best" location on the table. If it does, then you may need other work-arounds. But the point is that your machine is probably not uniform over the whole cutting area.

If this happened to me when making a template (or other one-off) I don't get too worried trying to optimize. I would leave the sheet on the table, copy the pocket layout to a rework layer, change object color, look at what cut and what didn't, erase the objects that cut okay, and re-cut the "defective" pockets at high speed. For me this is the most expedient method rather than starting over.

For a production part, I would spend more time to find the best (minimum) cutting parameters.

Mike Lysov
02-06-2013, 4:05 AM
how do you focus? if you are using manual tool for focusing you can use it to check the focal distance at different points to see if there is any difference at the points where you can cut all the way through vs points where you cannot. It is important to check it with the focusing tool because wrong system leveling does not necessary mean that the laser head is not parallel to the laser bed.

If there is no noticeable difference it is mostly likely that you bean is not aligned to go the centre.

There may be one more reason but I have never seen it or heard of it. it may be possible that at some point you laser starts moving a bit faster than needed.

walter hofmann
02-06-2013, 6:00 AM
ok richard
the most importanmd part is the focal point distance from the laser lense to the material vene if the machine and worktable is level it could be and has been on some machines that the focal distance is different on some places of the worktable the best methode is :
put a strip of wood on the table top put on the right isde a piece of 1/2 inch thickness under to raise the strip then cut a line there and look where the line is the thinnest, measure the distance from the strip to the laser head add the thickness of the strip then make a template with this measurements and now go to all four corner of your table and see if the distance from the table to the laser head is the same everywhere and adjust where is needed. this way you have made sure your focal distance is right. if this does not solve the probleme you need to adjust your laser beam again on all four corner of the table and it needs to be dead center.
greetings
waltfl

Mike Null
02-06-2013, 7:53 AM
Phil

I have no expertise with Chinese machines but it seems that on long running jobs they tend to lose power--at least that's my take after reading many posts. Focus is likely the issue, whether that's a table or material problem. The reason I mention this is that you can re-draw your template and save at least 90% of the cut time.

Make vertical and horizontal cut lines spaced according to the dimension of your pieces. That way you will have 9 vertical lines and 20 plus horizontal lines (whatever the number of pieces plus one)

Mohammed Issa
02-06-2013, 9:20 AM
hey Phil,

i had the same problem before fixing my table.
the problem i had was the table was not level from the middle and from other weird spots.

the best thing to do is to get yourself a fresh piece of MDF maybe a bit thicker than what you have already used (to make sure its not warping) then manually check the focal length every 6 inches horizontally then move down 6 inches vertically, then repeat.

these chinese tables are not very reliable. so dont count on them at all. checkout a post i made about 2 weeks ago about fixing the chinese table (i pretty much made a new one).

also, a normal 'carpenters' level is no indication at all if the table is level or not. you need a professional machinists' level, that will give you an accurate indication if the table is level or not.

to find out if the table is flat or not, you will need to use a flat piece of anything you can find around as long as you know its 100% flat and doesnt warp.

just place it on the table and see if there is any play.

you might not need all this for just the jig your making, but its nice to know that you have a good 'flat' table on your machine!

hope that helps!

good luck!!

Phil Horne
02-06-2013, 10:18 AM
Thanks everyone for all the great info.. I used a laser level last night and made sure I am within 1/32 all the around on the machine and then also leveled the table. Ill check the focus accross the table today. I guess I need to learn how to optomize the cut paterns in lasercut (thanks Mike what you said makes perfect sense!!) Does anyone have any suggestions as to where I can get the info on how to set the cut path?? Right now Im just importing a DXF and lettting lasercut set the cut path.. Although I will have a new printer driver and will be able to print directly from Corel Draw later today!! Looking forward to being able to finally do that and glad that the software is finally released, although I do most of my work in illustrator and they wont have the update for the driver that supports illustrator for another 4 to 6 weeks :-(

Mohammed Issa
02-06-2013, 12:03 PM
Although I will have a new printer driver and will be able to print directly from Corel Draw later today!! Looking forward to being able to finally do that and glad that the software is finally released, although I do most of my work in illustrator and they wont have the update for the driver that supports illustrator for another 4 to 6 weeks :-(

will you be using laserworks? where did you get the printer drivers from???

thanks,

Richard Rumancik
02-06-2013, 12:04 PM
Well, I guess I look at the issue of the table being level a bit differently than other posters.

Personally, I don't really care if my table is level. What I do care about is that the table is parallel to the plane defined by a point on the carriage.

By leveling the table using a bubble level (or laser level) you are using the earth as a reference. Nothing wrong with this, as long as you then level the gantry assembly to the earth as well. How are you going to do that? Shim the side rails?

Why do you want to use the earth as an intermediate reference?

It really does not matter much if your table is at 0.000 degrees to the horizon. In fact, if you did level the table accurately, and moved the laser system a bit, then with the error on the concrete floor and the casters, you might just find out the the table is no longer level. What to do? Re-shim the side rails to match? This does not make sense to me.

All you want is to make sure the plane of the table is parallel to the plane swept out by the carriage (which contains the lens).

One way to check this would be:

1. Attach a probe, pointer, dial indicator or whatever to the carriage. This must not be able to flex. (For a GCC you could just used the focus probe as a tool.)
2. Place a precision shim, metal block, or similar object in far left corner of the main table (0,0) and elevate the table so the probe or indicator just touches the block. At this point the z-offset can be considered "zero".
3. Move the probe (carriage) to the near left corner of the table. Try to slide the block under the probe. If there is a gap, this implies that the table is low at that point. (You could use a feeler gage to measure the amount if you wanted.) If the block will not fit, the table is high at that corner (relative to 0,0).
4. If the table is supported in 4 points, test the table at the other two corners. If it is a 3-point table (like some GCC's) then do the measurements near the 3 leadscrews.
5.Then the z-offset can be measured in other locations on the table to determine is there is twist or warpage in the table.
6. What you do to remedy the error depends on the construction of the machine.

This test could be repeated with the honeycomb table in place, to see if the honeycomb surface is parallel to the main table.

Once the table and honeycomb are verified, the test can be repeated once again with the material on top of the honeycomb. (This test does not need accurate data, just a visual indication.)

You can see how the errors in the hardware stack up here (table + honeycomb + material.)

My machine is a GCC, so when I am setting up a job with the material in place, I focus on one point somewhere in the middle of the laserable area using the focus probe, (which just hangs from a hole beside the carriage.) Then I manually sweep over the material and watch both the focus probe and gaps (between the tip and material). This will visually tell me if the plywood is parallel enough to the focus plane to start the job. If I am unsatisfied with the deviations that I see, I will use shims or weights as required to resolve the problem. People with other lasers could probably make a little tool or probe that could be quickly clamped on to do something similar.

When doing this test, you intially assume that the gantry and rail hardware are all good. But in doing the measurements, you may find evidence that there is a hardware problem. For example, let's suppose Phil places a precision (straight) bar left-to-right on the table surface. If he zeros the z-axis on the left, and confirms it to be zero on the right, and the middle shows a .050" gap, then I would say that the gantry may be bowed. This would probably cause an out-of-focus condition at the middle of the table. (I'm not suggesting that this is the actual problem for Phil, just a hypothetical example.)

Phil Horne
02-06-2013, 12:25 PM
Mohammed, the driver works with Leetro boards i.e LaserCut boards so I dont think it will work with a laserworks based board.

Richard, I used the laser level to first level the gantry on both X and Y axis and then leveled the table.. Ill grab a dail micrometer and atache it to the head of the laser at some point and check to make sure the work surface doesnt vary from the gantry (just like tramming a mill I guess) I know one of the problems is the poor quality of the honeycomb table.. Now I just need to find a better solution to that and it should all be good.

Thanks again for all the help..
Phil

Dan Hintz
02-06-2013, 2:06 PM
I'm with you, Richard. This suggestion may be a holdover to other equipment such as lathes where leveling the entire system is done to prevent twist in the ways, etc. On larger machines this may still be a good idea, but on smaller systems it's really not that critical. Make sure the bed is level with the carriage on a grid of points on the table and be done with it.

Richard Rumancik
02-06-2013, 7:51 PM
. . . Richard, I used the laser level to first level the gantry on both X and Y axis and then leveled the table.. Ill grab a dail micrometer and atache it to the head of the laser at some point and check to make sure the work surface doesnt vary from the gantry (just like tramming a mill I guess) I know one of the problems is the poor quality of the honeycomb table.. Now I just need to find a better solution to that and it should all be good. . .

Phil: Well, if that's the case, then what you are doing is not wrong; I'm just surprised that there is a vertical adjustment on your gantry/rails. I have never felt it mattered if the gantry was running at a slight angle. Whatever the angle actually is, I just want my table to follow suit.

I suppose I am just accustomed to the "conventional" systems with rails secured tight to the frame. I would not really think of trying to adjust (shim) them on mine.

I will admit that a larger frame could twist as Dan suggests; in such a case I would probably shim the offending wheels (or use the screw levellers if supplied) and make it permanently immobile like a machine tool.

Rodne Gold
02-07-2013, 2:58 AM
If the table is out with respect to the head/lens plane , the easiest fix is to cut a nice flat sheet of 3 or 5mm anodised aluminium and shim it up atop the original table.
Honeycombs are a big problem in respect of maintaining exact focus , over time they sag and often the frames are out
When focus is critical , I would rather use the table and use blocks to elevate when and if cutting. The anodising of the ally (even when the table is true , we still use a thin sheet atop the table) actually sort of acts as a backstop to stop any flashback. If you use black , its easy to make templates , cos all you do is run the outline of the object and it comes out white..
Of course there are times when you HAVE to use the honeycomb ..but we try avoid it if we can..

Richard Rumancik
02-07-2013, 10:09 AM
Rodne, you are right about honeycomb; by its nature it has no torsional strength and a simple aluminum frame won't hold it from twisting. And it is easy for it to get a sag or bulge in the center.

I haven't actually used my factory honeycomb for some time - what I use is an aluminum grid that I picked up surplus - I think it was some kind of grille for HVAC (heating/ventillating). Although it is more rugged than honeycomb it will still twist. I used it on a wooden frame clamped to the tabletop. I lose z-clearance but I don't need much clearance for sheet. The wooden frame has rail supports for the grid along two edges with 4 jackscrews for quick leveling of a job with a screwdriver.

Phil Horne
02-07-2013, 11:32 PM
All is good!!! I cut a templet today with the same settings I was using the other day and it cut just fine!

Wes Moore
02-08-2013, 8:08 AM
I have had this happen several times. A couple of things that I identified
follow:

1 The material was near the maximum thickness that the laser could cut.
When starting first run the laser has enough power to punch through but
as it warms the output power decreases and is unable to cut all the away
through on a single pass.

2 Cleanliness is next to perfect work. If cutting material near the speced
maximum thickness and my lenses/mirrors have any leftover smoke from a
previous cut then this sort of thing will happen.

YMMV and this may not be your problem, but these two items have bitten
me a couple of time. Hope this helps.

Richard Rumancik
02-08-2013, 8:47 AM
All is good!!! I cut a templet today with the same settings I was using the other day and it cut just fine!

So what did you do differently, or what do you attribute the problem to?