Hello everyone, long time lurker, but I am completely stumped and can't find information specific to my issues. I will try to give as much information as possible and a little backstory. Feel free to skip the first paragraph if you don't want the back story, but it might contain useful information. Note to mods: I'm sorry if this is in the wrong sub-forum.

I own a fabrication shop in California, and deal mostly with aluminum. I bought a cnc router (48"x48" working area) back a few months ago and have learned the ropes by making wood projects. The reason I bought it was for aluminum routing though. I'm currently using a liquid cooled spindle that appears to have a spindle RPM range from 1-24,000, I have been too worried to run the spindle under 6,000 rpm so far though. I have got to where I can make parts from aluminum with decent success, but engraving has been a real challenge for me.

I have .080(2.032mm) sheet aluminum I am trying to engrave for some information tags for my products. I started off with 5052 alloy but I don't think it machines very well. It seemed like it melted no matter what i tried with my feeds and speeds. I was able to get it to work, but it required too much sanding which was taking too much time. I then tried engraving some 6061 aluminum tubing (to test the other alloy) and it worked beautifully. I bought a sheet of .080 6062, but am now having issues again. I will attach pictures below.

I am currently using an Onsrud .020 SC SE 30 DEG engraving bit. The completed plate I made was made with a Onsrud .005 SC SE 30 DEG engraving bit. The tech at onsrud recommended a slow travel speed because of the size of the letters. The bits have a .0045 recommended chip load, but I have tried everything from .0038 - .005 with no success.
- The letters I am making are two different sizes. One is about 1/4" (6.35mm) tall, the other size is 1/8" (3.175mm) tall.
- I am trying to engrave in .080" (2.032mm) thick 6061 alloy aluminum at a depth of .010" (.254mm)
- I have been using 30 ipm for the travel speed because that was the maximum the tech recommended for travel speed.
- I have tried 8000, 7000, and 6000 RPM for the spindle speed. giving me chip loads of .0037 .0043 and .0050 all with about the same results.
- I do not have a mist attachment, but I hold an air hose on the work piece while it is routing. I have tried both dry cutting, and using soap. I did not get a mist attachment because I was worried that the spray would contaminate the aluminum making it difficult or not possible to weld.

Here are the issues I am having:
- I surfaced my spoil board, but the cut depths still vary. I am only trying to engrave 4 plates at a time. The stock size is 5.5" x 14" (14 x 36 cm). I am clamping the material to the spoil board with 2 clamps on each side and nothing on bottom and top. I do not have a vacuum table, so I am trying to keep the metal on the small size to minimize any up/down movement of the material while machining. I think the depth of cut issue is caused by not perfectly flat material, but I'm not positive. The engraving cuts look better when they are more shallow than when they go deeper, but if I go any more shallow on my depth, it won't engrave the low parts of the metal.
- I am not sure if the material is moving at all. It is moving any it is too slight to be able to tell. I have it clamped down very well on the 5.5" sides but don't really have a way to clamp it down on the 14" sides.
- I am not sure if the material is melting, or just not cutting chips all the way out. the bottom of the cut looks decent, but the top of the cuts is pretty rough and does not scrape off easily.
- The aluminum tubing of the same alloy as the sheet I am currently using engraved beautifully. My thoughts are it might be more rigid? It might just be that it is thicker? It might be more flat and i'm getting a more consistent depth of cut? It's only 1" tubing so there is almost no chance of up/down movement as opposed to the sheet I am using.
- Should I be using a different cooling method like mist or putting oil down?

-One more random question, I have been getting the bit close to the surface of the work, then measuring the distance between the bit and the work surface. I then use g commands to bring it down until it is .002 - .003 above the surface and zeroing the Z axis. Is there an easier way to deal with this when I need to be this precise with the depth of cut?

Please let me know if I left out any information. Thanks to anyone who took the time to read this. I attached 3 images. 2 of them are showing the roughness of the engraving I am trying to do currently with the 6061 alloy. The other imagine is a completed plate made from 5052. The finished plate looks good enough but I do not want to have to scrape the letters out, paint it, then sand the surface off if I can avoid all those extra steps, It takes far too much time and doesn't look as good as I would like it to.