Pause between passes, not easy

I have been experimenting with cutting thick acrylic sheets, 1/2 inch thick or bigger. My machine is not powerful enough to do it in one pass (I think), so I started trying multiple passes. Yes, I tried moving very slowly, but I get unwanted melting at the edges of the cuts. The workaround is to wait a minute or so for the plastic to cool down (in the existing cut) before cutting deeper.

Long story short, the only way I’ve found to do this is to add a non-functional “cut” of a reference line as a second layer, with power at zero – a time waster. I then create as a third layer a duplicate of the first one, finishing off the cut.

Seems like a kluge; is there a better way? I’ve found it difficult to work with overlapping duplicates of the same design, particularly if I want to make changes.

Any/all help is welcomed, thanks.

You would expect an 80w machine to cut 1/2" acrylic. What is the focal length of your lens? I’d try a longer lens. Can you increase the air flow to cool it down?

Hi Dean and thanks for the reply. I’ve been intentionally running the machine at less-than-full power to lengthen the life of the tube. I can get about 80% of the way through the 1/2" acrylic at 6mm/sec, 45% power. I could bump it up as far as 55% and still be safe, I’ll try that.

Will get back to you…


Further testing…

I was able (almost) to get through 1/2" acrylic by raising the bed a bit, bumping power to 24ma (about 50%) and going 5 mm/sec.

I think your comment about a longer lens is probably the best idea, will order one. It’s presently with the factory lens, 50mm or thereabouts.

Later, and again, thanks.

If possible, it’s usually helpful to give us the mA reading if you know it.

Comfortable ‘high end’ to me is about 18mA with a placard tube maximum current of 21mA on my ‘China Blue’ 50 watt, which is really 44… so… 18/21 = 85% power if you are setup correctly.

There is a tradeoff of good performance/longest tube life. Generally they claim it to be up around the 80% ranges. The gas breaks down over time also…

You have to pick what’s ‘comfortable’ for you to run, power wise.

I run mine comfortably at 18mA quite a bit, cutting. I cut 1/4 inch quite easily, I would think you should be able to cut twice what I cut.

I’d say you are wasting it… A max of 55% is not really using it. You should be able to run 80% of placard current and be in the safe range. A mA meter is almost a necessity. You can make do with a digital one, but you have to be careful about when and how it’s reading.

This sounds like you need to set you focus point more in the center of the material, not the surface. This is also what’s behind a longer lens… I usually use my 4" for cutting…

Do you have a low pressure air assist?

This can be of great benefit for many materials.

Good luck


Agree with Jack’s 80%. But make sure you have a good chiller. Poor cooling will kill a tube far faster than amps at the tubes rating. The downside of a longer lens is that they require an outside airline. Unless you get a lens tube assembly designed for that lens.
Acrylics are hard to cut due to that meltback tendency. Not sure that’s a word but it should be. Seems like high power and a tuned speed is the way to go.

Thanks for the suggestions.

On my machine 55% in Lightburn causes 26ma, as measured on the ma meter I installed. This is the tube’s max safe current.

Turns out I can cut the 1/2 inch acrylic with two passes with my 2 inch lens, but this brings me back to my original question… how to inject a pause between passes. It’s needed to allow cooling of the plastic.


I’m pretty sure Lightburn has a feature that allows either multiple passes to all take place on the same shape or a single pass across the entire pattern (layer) followed by the second pass. This might help it cool down. Or try to increase your air flow. Wonder if a fine water mist would work.

Trotec site has some suggestions on their site for getting the best results with acrylic. From their site

When cutting acrylic, you should definitely use the nozzle with the large diameter and – if controllable – lessen the Air Assist to a max. of 0.2 bar. That gives the material enough time to cool off - and glass-clear edges result. In contrast, if a nozzle with the small diameter is used or the air pressure is too high, the result is a dull, milky cutting edge because the material cools off too quickly.

They have a lot of information, might give it a browse if you haven’t.

I think what you might want to do is see if you can change the order so it moves further away for the next part and/or returns to finish it or one nearby. This allows it to cool down.

I saw this somewhere, but can’t find it again.

Good luck


Thanks guys. Dean – I’ve been looking for that exact function, where I can do one pass of a layer, do the next layer (which is an intentional time-waster), then come back and do the second pass of the first layer. Haven’t found it, but that would be ideal. If someone knows how to do this, please chime in!

Neil, it would be in the cut optimization section if it existed but apparently it does not, sorry. It might be too difficult for the program to cycle thru layers multiple times, each with a potentially different pass count.

The simplest solution is to set pass count to one and run the job twice.

I think I saw a feature request to do allow for # passes at the job level. So passes at layer level would still be configured but then allowing for a separate designation for overall job. I haven’t heard anything beyond that but worth upvoting if it matters to you.

berainlb, I was hoping you would jump in because you know this software very well. I’ll bet its too complicated, however adding a pause for a set time between passes should be pretty easy. Not sure how often it would be used though.

Yeah… there’s always that balance between effort and benefit. I don’t think it would be particularly hard to implement but the design considerations are potentially larger.

For this particular issue if the only requirement is that there be a pause after the first pass then you might be able to use “End pause time” found in the Advanced tab for a Line operation. Enable this by setting “Cut Through” with a Power of 0%. It’s specified in ms and only goes up to 10000 ms (10 s). Don’t know if this is sufficient of a pause. You could further hack it by using “Start pause time” to add another 10 s. So a total of 20 s between cutting operations but with 10 s wasted at the start.

This only works for Line operations. No good if you need a Fill.

I suspect the pauses would be inserted only before the first pass and after the last one, so getting it to do what you want would mean duplicating the shapes and using multiple cut settings, each with a delay.

The cut through delays are intended to give the machine time to pierce thick material before continuing on, so having it delay with each pass would be a bit odd. On the other hand, I wouldn’t expect anyone using cut-through to use multiple passes as well, so I could go either way on this.

We could probably add a “delay between passes”, but the moment I do that we’ll be inundated with requests to make that happen after the whole layer runs, instead of per-shape, which would overly complicate things in the cut planner / builder.

In that case, this would be an even more impractical hack than what @neilr is already doing. Sorry Neil. Was hoping this might be an alternative.

I tried a variety of permutations – pass count, layer cut order, fiddled with the optimizations. No joy. It always did all of the passes of any given layer before going on to the next layer.

Of course I don’t know the internal workings of LB. My hope is that it’s possible (someday) to either insert a pause between passes, or have it do a pass of layer one, a pass of layer two, rinse, repeat. Looks unlikely at this time.

This may be a strange way of doing things, but I really do need to let the plastic cool for 30 seconds or so before doing another cut pass, otherwise I get unwanted pooling.

I agree with the earlier comment, just run the job twice. Seems counter-intuitive, but does get me there.


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