Weird laser cutting issue (not cutting properly)

[Phil Horne]

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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
2013-02-04 17.28.22.jpg

Larger backside of template
2013-02-04 17.28.13.jpg

This is the position of the template while lasering
2013-02-04 17.40.32.jpg

[Dee Gallo]

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]

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]

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]

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]

. . . 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]

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]

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]

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]

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]

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]

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]

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]

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]

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