I need advice, what is going on?

[Chris Ghent]

I own but am an infrequent user of a 100w Redtube laser.
Today I did a test on plywood I make myself. It has a layer of ebony and then 2mm ply followed by a blackwood veneer, ending up 4mm thick. I established it cuts well with 3 passes at 35 power, 15mm per second, in fact it made a beautiful job on the test piece (one hole). When I put the full job (about 70 holes ) in at the same settings the top became charred and the wood was not pierced.

I am wondering whether, 1) lasers lose power when running a long job, 2) do I need to focus down between passes?

Cheers

Chris

[sebastien laforet]

many possible source of problem..

plywood is not homogeneous, so it can be harder in some place (inside knots).

your table may not be flat. so, your laser get out of focus of some parts of the job. thus, the charring. or the wood itself does not lay flat on the table.

your laser mirrors may be badly set, so, at the place you test, the power is ok, but on other places on the table, the laser does not hit the center of the 3rd mirror and you lose a lot of power.

in normal conditions, the laser power is consistent across time, provided it has correct cooling. with a 100W laser, you must have active cooling (like a CW5000), especially in summer!

combine all together : suppose you test in the corner closest to the laser : you have most power because the mirrors are ok for this position, the focus is also ok at this place. you launch your full job, and in the opposite corner, the laser beam is not centered and the table is 3mm below...

whatever, i am surprised you need 3 passes for a 4mm plywood. i can cut decently that at 10-12mm/s on a 90W tube in one pass.

[Chris Ghent]

Sebastian, thanks for taking the time to have a look and contributing your ideas.

While there is the possibility I have not performed these things perfectly, the mirrors were aligned and cleaned recently. The spot seems well aligned on the last mirror. I have a CW 5000 and the temp does not go over 20° C. The test and the larger job were done in the same place.

The reason I went for three passes is when I used one pass at higher power the top of the plywood would get charred. Out or interest, when cutting 4mm plywood you say 10-12mm/s but at what power..? And would you expect any charring of the top surface..?

I have heard that ebony is difficult to cut and the epoxy glue might not be helping either. The thing that really puzzles me is the test cuts were perfect.

[Jeffrey Aley]

I would try further experiments on a more homogenous material such as MDF or acrylic. Does your plywood have any voids? Is the epoxy evenly distributed? Even small variations can have a big impact on lasering.

I would also apply a masking to the surface of the plywood. Even ordinary masking tape can work well. Then you won't have to worry about surface charring.

Also make sure you have plenty of air-assist.

[Gene Uselman]

What is the max safe power of your tube? I would be using just below that in order to cut thru and blow the debris out the bottom- if you don't cut through the debris will be blowing up and causing a lot of problems. With 100w you should be able to cut in one pass unless that ply is almost uncuttable. Another thought is the burnthrough setting [laser through check box] and having a slight delay to begin the cut, it is easier to maintain the cut once it is through the material. In three years I cannot remember having to use 2 passes only a couple times.

[Chris Ghent]

Thanks for the advice. I am not sure of the safe maximum so have not exceeded 65%. I will look into the laser through box, it is ticked but I don't know what it does.
cheers

Chris

[Gene Uselman]

It is important to realize that the Max safe current for the tube does not correlate to the power scale of the tube/power supply combo. On my 130w tube the safe level is 45% power. If you don't have a mA meter you are only guessing where the safe power is. Some of the 644x Ruida controllers can communicate with the HV power supply and display the current on the machine screen. This feature has to be turned on as I recall, but don't recall how it is done.

[Chris Ghent]

If I have a mA meter connected how will I know when it the tube is being driven at maximum power? Is it a specific mA value, and if so, what is it..?

I have read up on laser through and open delay. This is not the solution to my specific job related issue. I am wondering if the auto focus might be focusing a little low, meaning the beam is wider and less strong at the surface of the ply.

[sebastien laforet]

If I have a mA meter connected how will I know when it the tube is being driven at maximum power? Is it a specific mA value, and if so, what is it..?

the max usable current is given by the manufacturer of the tube. obviously, if you have a "branded" tube with decent doc it's better. there's a "max" power (which will consume your tube very quickly), a somewhat "max usable power" which will provide power at the expanse of your tube's life, and there is a "safe" power, well below the max, that ensures that your tube will not burn itself.

on my reci tube, for example, the max is around 30mA, it is advised to stay always under 27-28mA, and the safe value is around 24mA.

I have read up on laser through and open delay. This is not the solution to my specific job related issue. I am wondering if the auto focus might be focusing a little low, meaning the beam is wider and less strong at the surface of the ply.

actually, you probably want to focus a bit low, so the focal point is "inside" the material instead of being on top. the "entry point" of the beam is a bit larger, but the exit point will be thinner and overall you get better results.

[Chris Ghent]

Thanks for your time...

It is a Reci tube, says,
Min 600mW
Max 102.3W
Average 96.29W
Std Dev 16.96
Overrange 0

In another box it says,
Peak power 102W
Agreement power 90W

Does any of these point to a way of detecting a usable power maximum?