I must be doing something wrong (focus/alignment)

Yesterday I decided to cut a small box for my wife to use in our bathroom for some cosmetics. This is the first time that I have cut plywood for a box. I was using 1/8in Baltic Birch plywood from Ocooch Hardwoods. Using the stock mirrors and lens, I set my distance from lens to material top to approximately (using a jig) 49.3mm. My mirrors were cleaned and I thought my alignment was good. But, it might not be after all. I also think that I need to work on settings. I tried to cut the plywood at 4mA (20% on my machine) at 15mm/sec with good air assist. Any slower and it really scorched. Any less power and it doesn’t really do much cutting. I ended up having to do enough passes that I got distracted and lost count of how many it took…

Issue: the cut is not at 90º from the surface. There is a slight bevel. I noticed this in particular in areas where a hole was cut for wood fingers to fit into. I used a chisel to square up the fingers and a rasp to square up the holes. I tried to take a photo but the angle is so small I cannot catch it with my camera. My initial guess was focal distance is wrong but I measured it from lens to material. Perhaps I should measure with a ruler and not my jig.

Any ideas?

3-4ma is basically about as low as you can go and have the tube laze. Cutting wise I only go that low for things like wood veneers or paper. I cut 1/8" ply in one pass, I also get from Ocooch hardwoods, at about 7-8mA at 10mm/s.

The only way to avoid scorching is to use masking. I use this stuff in 8.5" wide rolls.

The bevel may indicate that you don’t have the focus set right. For a nominal 50.8mm focus lens the most efficient cutting height should be 49.3mm, as you have it, to put the focus half way into the work piece. Ideally in that case your cut would have a slight hour glass shape to it. You may need to do a ramp test to verify your lens focus is really 50.8mm from the lens. It’s also possible your laser head may not be parallel to the surface.

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4ma is at the threshold for ionization for these machines. Doubt you get a stable cut at this level.

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@donkjr and @NedMan Thank you for the info! I have a couple of rolls of transfer tape. It doesn’t seem to adhere well in the very cold temps of my garage (48ºF right now). I found that it lifts off the surface and then smoke/particulate gets trapped and it adheres. That being said, I will try it with a higher power and see.

I have not done a ramp test yet, I will do that in the next day or two. I will also check the laser head to see if it is perpendicular.

@NedMan our have cut a lot more than I. Shouldnt you expect some bevel on the cut due to the beams power profile and can’t you minimize this by focussing the beam at the center of the material?

Yes, you will always have some bevel. If you have the focus 1/2 way into the piece you end up with half an hour glass shape as that’s the beam profile. So the top and bottom would be the same while the middle would be narrower. For 1/8" material this is actually fairly minimal if everything is dialed in right.

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Transfer tape comes in different levels of tack. The one I link to above has very good tack, but I can’t attest to how it performs in colder conditions. That Transfer tape was recommended by a member (@ashley_M_Kirchner_No) from the G+ days that worked in a sign shop and tried a bunch of different ones.

I almost never have problems with the tape lifting off during use. If you are sanding the wood first, make sure to use a brush to remove all the dust as that can cause problems with adhesion. I also use an ink brayer roller to make sure the tape is adhered well.

I use this 4" soft rubber one with a firm rolling pressure. They come in different widths.

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I added that brayer to my amazon cart a while ago after reading about transfer tape in that post but never ordered it. I will order it now. I haven’t sanded wood first but I think that doing that would be far easier than sanding a bunch of tiny pieces by hand later.

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I typically sand with 150 grit on the orbital sander to level all the grain out and then sand with 220 if needed. Removing the transfer tape can pull some fibers up but that’s easily address with a sanding block as needed.

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I found some info on focus here

and more here

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I cut 3mm Birch Ply at 8mA and 8mm/s with very little scorching, cleans up with light sanding. Be sure to set your focal point 1.5mm (1/16") below the surface.

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