Looks like you have a nitrogen purity issue.  Try with another pack of nitrogen.  If it's the same, maybe oxygen or air is leaking into the nitrogen.  Close your oxygen supply, purge it out and try again.
If possible, do the same with the air supply.

That could be a case because fist i was cutting SS with N2 then i switch to cutting CRS with O2 and after that all started when I switch back to cutting SS I could not cut it through. When I lowered feed rate I was getting same golden cut color and dross. When I came back to set up feed rate same color more dross.
Thank You for the tip.

it's possible on some gas systems for one gas to bleed back into another gas. We had some bad valves let O2 into a huge N2 bulk tank. Had to dump the whole tank........

Chris Krug http://krugtech.com/
Maximum Up-time, Minimum BS

Yes problem was with N2 tank (I have 16pack interconnected) when I switched to a different tank problem was gone. I'm not sure if that was because of bleeding or not. How can I check if O2 leeks into N2 tank?

you need to look at your gas handling skills and your gas setup. Are you using the same regulator for O2 and N2? If you have multiple 16 packs behind the same regulator then all you have to do is open the O2 before you shut the N2.

Chris Krug http://krugtech.com/
Maximum Up-time, Minimum BS

To get oxygen to bleed into your nitrogen bottles, your oxygen pressure would need to be higher than the nitrogen pressure.  With a 16-pack, I doubt that will happen as your nitrogen pressure will be very high (unless you are almost empty).  It would be a different story if you used liquid nitrogen.
A fairly easy way to check if you do have oxygen leaking out through the nitrogen line would be to disconnect your nitrogen line.  Then run an oxygen test.  While running the test, either check by the disconnected nitrogen line if you "feel" gas coming out.  If it's very low pressure and volume, it may be hard to "feel".  In that case, you could connect the gas line to a bucket of water and check for bubbles.   

Are there separate regulators for the O2 and N2? If you have the gasses on a manifold behind the same regulator, all he has to do is use a little N2 to drop the pressure in the N2 16 pack, then open up a fresh O2 16 pack whilst forgetting to close off the partially depleted N2 16 pack and the O2 will fill the N2 until the pressure equalizes.

The gas company near me is a little crazy, they sell a kit to put handlebars, fins, a seat and nosecone on a high pressure gas bottle. Liftoff is via hammer to valve....... Where's my crash helmet?

Chris Krug http://krugtech.com/
Maximum Up-time, Minimum BS

OK this is my situation I'm using 16 pack for N2 and a single cylinder for O2. Gas pressure in O2 when fresh shows 2500 and fresh 16pack of N2 just 1/8 of of that. I have separate regulators for high pressure N2 and O2.  

What is the output pressure set on each regulator?

N2 is set to 7 O2 to 1 kg/cm
 

What is the output pressure set on each regulator?
 Did you mean from cylinders?
Pressure in in the output lines is correct according to AMADA settings.

Sorry guys probably I have to tell you that I very new to laser technology. Company I work for just bought a 1989 1212aIII and gave me bunch of AMADA pdf files to read and figure out how to operate that machine. So I'm running that beast since January of this year and making programs on CNC/CAD Metalix.   

Since you have separated lines for N2 and O2, and since the N2 line pressure is higher than O2, seems like you received a bad 16-pack of N2 from your gas supplier.
That being said, if you still want to check that the machine gas system is OK (basically to check that the non-return valves are doing their job), you can do the bubble test mentioned earlier.

I thought about that but in his OP he stated he N2 cut OK before he went to O2, and there was no mention of a tank change at that point. The bubble test is a good idea. I see this a lot on older lasers. Like you said, bad check valves mostly.

 

Chris Krug http://krugtech.com/
Maximum Up-time, Minimum BS

Just did bubbles test. No leaks. Changed N2 tanks everything back to normal.  

i am new here,and from sweden,so my language is not so good.
i think the problem with the golden colour is the proportional valve on your machine, some leakage between oxygen and nitrogen inside the valve,even a small leakage from oxygen will kontaminate the stainless .