I was in a discussion last night with some folks on FB that were absolutely convinced that their generic 60 watt C02 lasers were capable of producing 1000 dpi engravings. Based on what I know (or at least what I think I know) about the laser physics and high voltage power supplies I told them that 1000 dpi was not possible. Most of the power supplies in this market have a response time of 1 millisecond (or greater) so if the laser head is moving at 100mm/sec it will cover about 4 inches in 1 second.
So it seems to be at the modest scan speed of 100mm/sec 250 to 300 dpi is about the best resolution you can hope for.
If the speed is slowed down any more the power level of the laser would need to be lower as well and what I have been taught about CO2 glass tube lasers tells me that you need to have at least 7% power to have a stable beam formed. So there is a real lower limit on the power level. I did not even get into the actual size of the dot being produced, that is whole nother can of worms.
The people I was chatting with were not impressed with my logic, they felt they were able to do 1000 dpi all day long and I was a heretic and a scoundrel for questioning them.
Am I wrong? If so what did I miss? Does some manufacturer of cheap generic power supplies make a super fast unit? Are there some special glass tube lasers that can run at really low power levels (one guy told me he routinely used less than 1 percent power).
I am sure someone in this group knows the answers.
You are correct in your assumptions/calculations, Russ Sadler has exhausted this notion on his YT channel and reached the same conclusion. I see no difference in processing pictures at anything higher than 300dpi, even as low as 150 on slate.
Thanks, just wanted to be sure I was not totally off base.
Both the dot size and the travel speed / psu response are the controllers (not sure if step size is a factor here). Most people do not have a true understanding of how small a .001" dot is.
…and even if they could, few materials would take an engraving at that resolution…
Some things that are really important are the original resolution of the image and material too. Very few people seem to grasp that concept. The phot’s resolution is critical to know first. Most of my projects are wood engraving and fine detail cutting. In alder, birch and aspen I can get clean engraves at 265 - 320 dpi (appropriate scan gap). I have been able to stretch hard rock maple to 400 dpi and I’ve seen a difference. The physics do dictate the reality. Considering the lateral move is the time/firing issue the vertical can be positioned at lower scan gap settings so it can go higher than the horizontal (it isn’t affected by the time, just power). All this is trumped by bad positioning of the focal point as well.
So the vertical (Y) step is a combo of spot size and step /travel distance, where the X step is controlled by the optic system. power supply/Laser tube response time, and software frequency.
Exactly. Where we do have control is in our lens selection. A 1.5" lens gets me the smallest dot size but sacrifices some power. So the lens is a critical point that needs to be included in the equation.
I did not think the shorter focal point reduced power, if anything it would increase power density since the same amount of power was focused into a smaller dot. The 1.5 inch focus lens does reduce the vertical margin where the beam is focused a mm up or down from the focal distance results in rapid loss of focus. A longer focal lens will keep the beam in focus over a longer distance (with a larger dot size). So a 4 inch focal length lens is better suited to cut thicker material since the beam will maintain a good focus through the entire depth of the material.
It does reduce power available to cut thicker stock. Limited focal range tolerance (very high divergence) is the real issue. The power falls off much quicker so that’s what you give up. Effectively, that’s what is happening. You can test it yourself. Universal Lasers use a High-Power Density-Focusing Optics™ (HPDFO) system that I would love to have. Here’s a link for lens choices and good explanations of power.
I think that is exactly what I just said, the power delivered to the focal point is higher (due to the smaller dot = higher power density), but power density falls as the beam loses focus (larger dot = lower power density). With a short focal point lens, the beam loses focus more quickly than a longer focus lens.
Just a note: Delivered power stays relatively constant even outside of the focal point, it is the power density that falls off.
Correct, Although my brain has a hard time accepting the short focal length - smaller dot, My approach is to get alignment, quality of mirrors & Lens, and focus as good as possible (to keep power density high), but use a 3 inch focal length lens to have a better range of depth. Gonna do some tests to find thickness, speed results.
I purchased a 3 inch focus lens and to be honest I cannot tell much difference with cutting depth between it and the 2 inch lens.
I have noticed a difference when trying to engrave on curved objects (like a softball) the area I can engrave is larger with the 3 inch lens than the 2 inch.
But when trying to cut 1/4 or 3/8 inch thick wood the two lens perform almost exactly the same.
This is true, but then some materials act something like a wave guide - you can focus on the surface and cut significantly deeper than the hourglass shaped depth of focus would otherwise indicate.
I use a 2.5" lens almost exclusively, but that is only to allow for variation in focal distance due to materials being less than perfectly flat. If I had a smaller machine and cut/engraved materials that were very flat, I would use a shorter lens.