Help needed on setting up a new to me K40

I just bought this K40 locally including all that you see here. Most of the upgrades have not been installed yet. The previous owner installed the Cohesion board and display.
My questions:

  1. Replace the existing outlets with either a GFCI outlet or attach a power strip/surg protector?
  2. Get a bucket and use the existing pump or buy a CW3000 or CW5000 chiller?
    3.What was the plan for the mini radiator, 120mm fans, little clear tank?
  3. Should I go ahead and install the new blue head along with the 3 new 20mm mirrors?
  4. Use the aquarium pump for air assist or install a blower/fan?

I also notice this K40 didn’t include the “big red button” should I get one of those?

I got a pretty good deal on everything so I don’t mind adding some more money if it will make laser cutting easier for me.
Before this I had a 2watt Eleksmaker and have already been using Light Burn

Any help would be greatly appreciated!!!

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So far I’ve removed the stocked bed/fixtures. Remove the stock rear duct. 3D printing a replacement duct. Working on a shopping list.

  1. Analog mA meter
  2. rolling cart to hold the laser and either a chiller or a bucket and pump

If you just don’t use them and run everything off a switched outlet strip it might meet your needs?

Bucket or CW5000 or lightobject Q600; CW3000 will run the water temperature eventually a little above ambient. The mini-radiator won’t do you any good. It was probably one of those “seemed like a good idea at the time” things to the seller. Cooling the K40 laser tube has more advice and details.

Aquarium pump for air assist. Fan / blower only for exhaust, not for air assist. Fluid dynamics will not be your friend if you use a fan for air assist.

Some way to turn off all the power easily that is immediately at hand is good. e-stop buttons are a cheap way to do that; a power strip that is easily accessible (not behind the K40) could also work. Keep in mind that when you want to hit it the unit might be on fire and you are trying to kill power before hitting it with a CO2 fire extinguisher.

Up to you but if I were in your shoes I’d do changes in small increments and replace the head and mirrors at the end of the process, on the “change one thing at a time to know when something goes wrong” theory.

Very good call. :relaxed:


I have found the power switch that came with the unit to be sufficient to control the units main power.

I added grounded sockets for AC distribution that are controlled by a relay board with associated switches. All these sockets are wired to the mains.


This is great!! Thank you guys for all the great tips and opinions. For right now I think I’ll go the power strip route. Eventually I’ll make a new control panel. I did find a awesome cart for the laser today now I can roll it around my shop. I’m planning to put the chiller and aquarium pump inside the cart and mount an exhaust fan on the rear of the laser.


Here’s an update.

So far I have…

  1. 3D printed a new control panel
  2. 3D printed a new air assist nozzle and red laser cross hairs adapter
  3. wired in the mA meter
  4. Added the drag chain (I think it’s the wrong size, too big)

Next I’m

  1. wiring the test fire button
  2. Wiring the laser on button
  3. Wiring the potentiometer

Then I think I’ll be ready to power it up. Once I see that it’s working I start on the next phase.

  1. make a new control panel to accommodate more buttons and the laser water temp display that connects to the water flow sight glass.
  2. Mount the water flow sight glass
  3. Wiring the “big red button”

I’m still a bit foggy on what I control with the Cohesion3D GLCD. From what I gather I dial the laser max output with the potentiometer by reading the mA meter. I still need to look up what the max output should be dialed to? Then lock the potentiometer. After that Lightburn creates the gcode. I can either send that file over USB or save the gcode to an SD card which I insert into the GLCD. Oh… then scroll through the GLCD find the file then burn that file.


Our recommended maximum on a K40 is 15-18mA unless the tube has markings indicating a higher max current. 15mA for longer life; 18mA if you are willing to sacrifice a little tube lifetime for more power, I think.

However, you don’t necessary lock the potentiometer at that rating.

For example, if you are planning to raster an image, you might set the potentiometer lower so that it’s the darkest you want on the particular material you are using for that particular job. That gives you maximum contrast range and smoothest gradation of contrast on your raster engrave. You just don’t want to adjust it above that 15-18mA range no matter what. Similarly, if you are vector cutting, you are likely to want to set it to a constant power setting appropriate for cutting; if you are cutting 2mm acrylic sheet you might want a lower power setting on the potentiometer.

Does that help?

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What about Interlocks?


I was thinking that LB along with the Cohesion3d (mini in my case) would adjust the cutting power to suit the job? I did see that on my current setup I don’t have anything plugged into the PWM port. I assume that I will need to wire that up for the Cohesion3d to be able to adjust on the fly?

That is adjusting PWM, where it pulses the laser on and off rapidly. The potentiometer sets the range to which the PWM applies.

If you tell LB to use a range from 0-5% because you are burning something light, it will have fewer distinct shades of gray available from PWM. If you tell LB to use 100% but adjust the max with the potentiometer, then you get more distinct shades of gray from PWM.

Yes Don I will be installing a interlok to the main door. I’m planning to only run the one. The rear tube and controls area I’ll put the screws back in them. off the top of my head I’m wandering if you can connect the endstop switch to the cohesion and in the firmware add a “pause when opened” command. That way I can continue the cut/burn apron closing the lid. Is that even possible??

The normal setup:
The PWM is connected to the L on the power supply from an Open Drain output on the smoothie. I use P2.4. Nothing else other than ground is connected from the C3D to the LPS
The Pots wiper is connected to the “IN” signal on the LPS. The other legs of the pot are connected to the 5V and ground respectively.

I like to think of the pot as the intensity control where the L signal from the C3D is the dynamic power within that range.

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off the top of my head I’m wandering if you can connect the endstop switch…
[Do you mean interlock v endstop?]
…to the cohesion and in the firmware add a “pause when opened” command.

[I assume it is theoretically possible but I doubt its a simple change due to the queing of the input vs active print stream. I have never cracked the smoothie code. You should ask this in a separate post over in the smoothie category. ]

That way I can continue the cut/burn apron closing the lid. Is that even possible??

From what you’re saying it’s best to set the mA to a conservative 15mA and let LB adjust for the job. But if I see that the results need more of less power I can tune it with the potentiometer.

The GLDC lets you operate the machine locally.
Since LIghtburn I never use it but there is a menu for its functions.

The potentiometer sets the maximum power.

Like everything else, the details get more complicated…

  • For cutting, don’t exceed 15mA is a good rule
  • For rastering, if you want more “shades of gray” in a smaller range, you can set it to lower than 15mA
  • For rastering, you can reasonably set it as high as 18mA as long as that’s only used for small parts that fire only momentarily.

Basically, this means treat 15mA as the max constant power, and 18mA as the max momentary (peak) power.

If you want darker/deeper than you get from 18mA for rastering, or 15mA for cutting, use multiple passes.

Does that make more sense?

The pot sets the DF of the LPS internal PWM control.
The L signal is a controller PWM that turns the LPS PWM ON and OFF
Therefore the actual Duty cycle of the laser is the product of these two PWM’s.

The reason the pot is important is that lasers’ power diminishes over time.
What you set in a program as the power in a job at the beginning of the tube’s life will not be the same as the tube is used up.

I recommend setting the POT * Job setting such that the combination does not equal the tube max power. This allows some ability to compensate for lower total power by increasing the pots setting and not adjusting the job setting.

The exception is cutting I set the JOBS power for cutting to 100%.

I now have the POT installed and wired in. Now I need to install the “Laser on” and “laser test fire” buttons.



3D printed head could catch fire, you can seriously reduce your risk by glue/wrap some foil around it.

Worth considerating .

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Thanks StephaneB but the head is still the stock aluminum one. The 3D printed part is just an adapter that holds the air assist and a laser.