"micro" air bubbles in laser tube

[Dave Vigness]

A big chunk of my rewire was the noise of the exhaust and air pump combined with a small work area. The blower was on a power strip almost from day one. The air pump was part of the 'mod. The heater and water pump were also in the mod, but as I said, part of an unheated enclosure. I set my air pump on a piece of that super soft foam it was packaged in instead of hanging it from the handle like Russ did. Any decent but not hard piece of foam would do the same.

I would stay away from any sort of inline flow meter / switch unless you can measure your output at the return and know you have at least 2 L/min with it in place. A simple pressure switch on a T takes any flow reduction out of the equation. You're not needing to measure any flow, just the fact that you have pressure from the pump being operational. I differ from Russ in one other point. He recommends having the return line below the surface of the water. I have it above for two reasons.

1. I can hear the pump running, but I can also hear the little splash as the water falls that last inch back into the container. I know by the sound if something is off.

2. And more importantly, if I have a power failure of some sort, even though we have a whole house backup generator now, with the open gap, the water in the lines and tube will siphon back. It's won't be a complete siphon just because of the nature of the tube and tubing. But it will be enough that it will not be a solid water fill and will lessen the chance of breakage if it gets down near freezing.

On your beam. Running a bit high on #1 and #2 is not that critical, you loose area horizontally from the 45 angle for the 90 turn. And if you notice the first several of Russ' videos, he had a line high on his head to note where the beam had to hit to get it down the center of the tube. The critical part is getting it parallel. to the gantry travel in X and Y. Only the lens is critical for centering and only in the tube, because unlike the mirrors, it's the only component in the chain that is not flat.

That red wire is that thickly insulated because of the voltage it is carrying. The output of your laser tube power supply is measured in KV. The tube uses it all and it becomes Ma at the other end. Be VERY respectful of that wire.


Oh yeah, and I have a bone to pick with these yaho's that think they can just change the plug to switch a machine from 220 to 110. You guys switch both hot legs and have a neutral and an earth. We have ONE hot leg, a neutral and a ground ( earth ). Drove me nuts trying to figure out why some of my electrical mods weren't working right until I remembered to check the main switch wiring. The neutral was still going through the switch as if it was a hot leg, and being disconnected when the switch was turned !!!

[Mark Malley]

Hi Dave

Noise can be a problem especially if you have one of those big tangential blowers that the larger machines usually come with. They seem to make a terrific noise like a jet taking off.

Air pumps can sometimes be noisy too. I have now found another use for the mountain of packing foam I have accumulated from this and other machines.
Put the air pump on it great idea ! I shall try it. The stuff is also useful for lining flightcases of which I have many but I digress.

My intended flowmeter is 3/4 BSP so it shouldn't cause an appreciable flow drop and I do intend to put it in the laser return line. putting it in the inlet line means that if a hose should fail, the meter might indicate normal but you are not cooling your laser merely trying to turn your laser cutter into a Barbie Doll sized swimming pool.


But pressure switches are very cheap so I might fit both. I have plenty of spare inputs of the Arduino and it's only a minor software mod to implement it.
What switch are you using so I can buy one similar. I imagine the pressure is fairly low perhaps a few hundred mBars at most.

I will go with the beam being at bit high as you (and Russ) have pointed out, in the first 2 mirrors you lose area horizontally but not vertically. So slight misalignment in the vertical plane is not so much of a problem. Its only in mirror 3 when it might become an issue as things are reversed and you lose area vertically. If it is a problem I will either shim the head a little or better still make it adjustable.

Yes the tube drops pretty much the entire HT voltage less probably a few hundred millivolts across the current sensing "resistor" in the Power Supply but the Red HT one carries around 18Kv (varies a bit) to earth at enough current to do you some serious harm if not kill you instantly. I keep WELL away from that one even with the Unit switched off, It's a switch-mode power supply and they can store high voltages in internal capacitors for quite a while.

In the U.K. our power supply is only single pole 240V 50 Hz about a grounded (theoretically) Neutral but I believe that in some parts of the states it's centre tapped about neutral giving a split voltage arrangement of 110V on each "live" leg and 220V between them
But where you are It is more like the U.K. system having only one live. or have I got that wrong.

Right, back to scorched masking tape with renewed vigour

Best Regards Mark

[Dave Vigness]

Pressure switch for water -https://www.lightobject.com/Water-Tubin ... protection

I put the T in up within a foot of the tube input, then an S curve of tube to the switch. I doesn't say if the switch is water or air, so I erred on the side of caution with the S curve of tubing. Only head scratcher was the installation. The switch has a 3mm fitting and comes with a short piece of 3mm tube and a 6mm T. Wound up slipping a short piece of the 3mm tube over the switch fitting, then the larger scrap of 6mm tube I had over it. A couple of zip ties just to be safe, and it's all in and working wonderfully. It was a toss up on putting it on the tube input or output, but the room for mounting and the fact that the factory inline switch was ahead of the tube made me decide on the input end of the line.

Sorry about the assumption on your cutting head. I've only glanced at the K40 in pictures. If you have the cylinder thing I see in the pics, that is definitely different. Then again you could probably get away with cutting some 'washers' out of card stock to shim it up if acrylic is too thick, or in combination to get your correct thickness.

[Dave Vigness]

Mark,

Something you said in an earlier post just hit me. Using the ten turn pot to control the supply.

Whether you use that or the control panel percentage, be aware of one iron clad fact of laser tubes...

THE OUTPUT IS NOT LINEAR !!!

No matter how you control it, it operates on a curve, and every tube has it's own curve. Mine has a very steep start up, so if i was doing a lot of detailed card cutting like Russ, I would be pulling my hair out trying to get decent cuts at low power. My top end is as flat as the front end is steep. I gain a whole three watts going from 55% to 62%

[Gene Uselman]

I have seen a couple of Bodors with Reci tubes and power supplies where the power supply was adjusted to set a upper current limit- could that be your flat spot?

[Dave Vigness]

Gene, Thanks for the thought. I was thinking of trying to figure a way to limit my tube so i can't over drive it. Gotta love the simple solution, just turn down the power supply.

[Mark Malley]

Hi All

Yes I agree, and Russ has proved it beyond doubt by rigorous testing, Laser tubes are decidedly NON LINEAR. In fact from what I see from Russ's tests they don't even appear to even have a mathematically predictable curve like a log relationship for example.

At the top end of the curve quite large variations in current have little effect on output power but a dramatic effect on tube life.

Being gas discharge tubes they are a form of thermionic device very loosely akin to an Audio (or Radio) Valve
Audio valves also have this rounding off effect at the top of their transfer curve although they are pretty linear throughout the rest.

This leads to an effect known as "soft clipping" which despite being a distortion hated by the HiFi folks is much loved by Electric guitarists and quite musical if not taken to extremes. Indeed Valve Guitar Amplifiers were (and still are) designed to actually exploit it.

Now I'm not suggesting you try to make an EL34 lase by sticking 18 KV across it or putting extreme voltage audio through your laser tube. but I thought it was interesting that other thermionic devices also exhibit strange nonlinearities.

My reason for thinking about the ten turn pot and voltmeter idea is two fold

1) Very precise control of power output with a good quality pot, exactly what you need in the lower ranges where the curve is very steep.

2) the ability to preset your power before actually firing the laser.

The voltage readings may be nonlinear and rather meaningless in themselves but once you know the relationship between voltage and laser output by plotting a calibration curve as Russ has done. you can set your laser accurately by simply setting the number (whatever it may be) that corresponds to the power you require.

I'm talking about a Stock K40 here which cannot be set by software.

With more expensive and sophisticated power supplies especially ones matched to a specific tube I think it is quite probable that they do indeed have a current limiting feature to protect the tube and the power supply itself. In fact I would be surprised if they didn't. Electronically it is not a very difficult thing to implement and most LV bench supplies do. Even some of the cheaper ones.

Best Regards Mark

[Dave Vigness]

You'll have to forgive my limited knowledge of the K40. Both my current very low end Top Wisdom, and my new to be installed Ruida, have good power and speed control. It may be a relative percentage, but it is at least consistent. I can set 7.5% or I can set 7.6% for my low end settings. 7.3% or lower just lets it go through the motions. I'd have to go back to my power curve from Russ to see what wattage that actually corresponds to, but I have my percentage / speed charts for various materials and work being done. If you do not have the ability to set power and speed at the control panel, an upgrade just got a whole lot higher on the list.

Hell, I'll give you a deal on my old Top Wisdom. I've only has the machine for a little over six month. I'd have to pull the meters, but I know I had less than 39 hours on the tube when it faded. The controller on time will be much higher, before I made my mods I had to leave the laser on for the pump to operate during freezing temps without swapping cords every time I shut down for the night.

PM me if you're interested.