Speed VS Power - Just a thought


(Peter) #1

I’m just starting with my diy 50W Laser cutter and there are a lot of discussions about that using less power is better for the laser tube.

Which I understand but when you’re using less power your cut is going to take more time and thus your tube is running longer.
(Asuming that you are using the least amount of power to cut through a given material at al times)

Let say i’m running my laser at 20mm/sec with 80% of power (material and wattage aren’t important just the calculation)
And i’m cutting a square of 100*100mm. It would take the laser 20 seconds to complete the cut.
Running it at 15mm/sec with 60% of power would take about 27 seconds which is about 35% longer.

So is it really that important to run at lower power with longer running time?


(Dean E Forss) #2

The CO2 tubes have a power sweet spot you can find by testing it on pulse and watching the power meter. For instance, my current tube measured at 171 watts when it was first installed. It slowly looses power over time as the gas dissipates. There’s a “J” curve on the power issue. My most efficient highest power setting is 71% for this tube. Each tube will be different. If I run over that power point I will use the tube up much faster. It’s all about finding the balance. Using a tube at 90-100% will kill it much faster.


(THX) #3

And because the tube power curve isn’t linear if you reduce the power by 25% you don’t lose 25% of the tube power, but much less.

Imgur

This is the curve I measured for my RECI tube (100W peak, 90W working), and at the recommended power setting (24mA, written on the tube) it’s at 75% of the peak output power but outputting a respectable 92W (almost exactly what the tube is rated for).

Notice how the power levels out at 100W – the advertised peak power. And the advertised working power of 90W is about exactly the recommended mA for this tube. When manufacturers use the peak power as the power of the tube they’re not ripping you off. You need that information to figure out the tube characteristics. The peak power is a constant from which all calculations are derived, and the working power is variable. So those advertising the peak power are technically more truthful than those advertising the working power because the peak power is an absolute whereas the working power will vary a few percent from tube to tube and over time. Those working with CO2 lasers know this and it’s not an issue for them.

Tubes are always outgassing and lose power whether you use them or not so it’s not worth fretting over it and just consider tubes an expendable item that needs to be replaced every few years when performance drops enough to annoy you.

Not to mention always cutting at the peak power and using just speed to adjust cutting depth not just makes it faster, but with less variables to consider, over time you’ll be more able to easily notice changes in the laser like if it gets out of alignment.


(Joe Alexander) #4

Quick question about your data: I see that the final temp in °C goes all the way up into the 40s, and starts roughly the same @ ~17°C.
-Isn’t this exceeding the recommendations of keeping your coolant around 15-20°C throughout the job?
-What kind of chiller are you running? And if the exposure time is only 41 seconds for each of the various data points on the chart, can your chiller mitigate that much thermal gain?

BTW great explanation on the tube power characteristics :slight_smile:


(THX) #5

That’s the temperature of the aluminum block being heated by the laser. It rises because higher power is used. It’s quenched in a bucket of room temperature water between runs to get the temperature back down to room temperature.

There’s a thermocouple in the aluminum block, and by measuring the temperature change before and after heating it with the laser you can figure out how much energy the laser is generating.

This run was done in spring, so room temperature (in an unheated garage) happened to be down around the same temperature I keep the cooling water. The cooling water stayed 16C during testing.

The chiller is the venerable CW5000. It has a compressor and heat exchanger (like a refrigerator) so it can get the water temperature down below room temperature. The popular CW3000 uses a fan and radiator and can get down to room temperature.

The CW5000 has no problem holding the temperature at 16C, even during days spent cutting at high power all day.


(Joe Alexander) #6

Ahh got it, thanks for explaining. And yes nearly all chillers bow to the CW5000 :slight_smile:


(Allen Massey) #7

Where did you find your aluminum block (black body). I have tried to buy a “dohicky” from Russ Sadler (RDWorks Learning Lab) but I have not received a response.


(Robert Tate) #8

I bought one from Russ about a month ago. I don’t remember the details, but I followed what he said to do in the YouTube video comments and he emailed me the info to buy it. My thoughts are that he is not doing this full time, so if he is busy or away on holiday, it may take a while. If it has been a week or so, try commenting again.


(THX) #9

Same. I think he makes it difficult because he doesn’t want to get into dohicky manufacturing.

However it arrived really fast.


(Allen Massey) #10

What email did you use to make the request?


(Mike) #11

This power meter is available at a decent price and gives you a digital readout:


(Robert Tate) #12

Yea, looks good, but Russ’s can be built and used for around $40! Way less!


(Dean E Forss) #13

Excellent description!