Gantry Laser Build Question

[liaoh75]

Hi All,

I'm building a 4x8 gantry type laser cutter. I'm using a 150 watt Chinese glass tube co2 laser. I'll be using the 2012 DSP of course. I have a few questions if you guys don't mind me asking.

1. If my tube which is 1850mm long and 90mm in diameter, will it crack if the gantry is accelerating/decelerating too fast? I'm running 750 watt servos on each side of my gantry. My servos are driving lead screws with a pitch of 25mm/revolution with 2:1 belt reduction. My gantry is in the neighborhood of 70kgs and 2.7 meters long. My servo motor vendor promises me a speed of 600mm/second and acceleration from stop to full speed in one second and deceleration from 600mm/second to full stop in zero point one (0.1) seconds.

2. Due to the gantry design, cables will be through a cable chain. Now, does the laser power run in the chain as well or should it ride in the gantry behind the tube itself? Will it cause interference with my Y axis servo if I place the laser power supply in the gantry? Can the power supply take the acceleration/deceleration?

The finished laser cutter will be about 850-900 Kg so I don't see vibration as being an issue but correct me if I am wrong. For those that might be scratching your head as to why my gantry is so long, my gantry is running landscape orientation to accommodate to the long 1.85m tube.

Thanks to all that take interest in my project and kind enough to offer me your input.

David

[Tech_Marco]

Do you have a drawing for how you mount the tube on the gantry. FYI, 1 150W tube may weight about 8~10lbs (I'll weight it for you) but after water filled up, it may increase to 15lb. If you lay the tube in horizontal I don't think there is an issue. But if you do it vertical, then you may be having a trouble when speed of the gantry is going fast. But for cutting, you shouldn't go over 20mm/s, right?

I will suggest you to put the power supply on the gantry as the length of the HV cable may cause a problem. The servo is so powerful so it shouldn't be a problem (for cutting). Remember that the laser power is a high voltage device (32~42KV with 30mA = 1000Watt) so that the electromagnetic field created by the HV cable shouldn't be underestimated. Plus, the longer the cable, the more power lost expected. HV cable is not very flexible compare to other cable. You need to take it into account when it bended. If the HV cable broke during operation, you're expecting a blow on the PS and the DSP card! That will cost you big time.

I'm not an expert of CNC or mechanic. It's just some though that came to my mind after seeing your post.

Marco

[liaoh75]

Thank You Marco for the reply. I'll have pictures up soon as my machine drawings are with my brother inlaw. I think I'll put the laser power supply on the gantry. What you said makes sense.

What about the length of cable sending PWM signal going from the DSP I bought from you to the laser power supply. Does this length matter? I'm fairly sure that length will be at least 4-5 meters if not a little more. Speaking of that. Can I make the cable going from the display module to the board itself longer? Due to the size of my machine, it would be better to put your card in the control box and the display panel where the operator will stand.

The tube will be mounted horizontal on the long gantry. If I'm cutting cardboard or cloth with 150watts I think I should be going quite a bit faster than 20mm/sec. I think 120mm/second is more likely. Please correct me if I'm wrong.

Thank you so much.

David

[geotek]

I'm curious, what is the advantage of mounting the laser on the gantry as opposed to a flying optics system?
It seems that you have to deal with a tremendous amount of moving mass as well as electrical and water connections. In a flying optics system, the laser is stationary; the only connection needed at the cutting head is an air assist line.
If you were doing a lot of heavy cutting at slow speeds, the laser on the gantry would make sense, but for light high speed cutting and engraving, it would be a poor choice. The internals of a 150W tube are very fragile. No amount of external support is going to protect the capillary tube and its water jacket (the outer tube is only a gas reserve).

[twehr]

I'm curious, what is the advantage of mounting the laser on the gantry as opposed to a flying optics system?
It seems that you have to deal with a tremendous amount of moving mass as well as electrical and water connections. In a flying optics system, the laser is stationary; the only connection needed at the cutting head is an air assist line.
If you were doing a lot of heavy cutting at slow speeds, the laser on the gantry would make sense, but for light high speed cutting and engraving, it would be a poor choice. The internals of a 150W tube are very fragile. No amount of external support is going to protect the capillary tube and its water jacket (the outer tube is only a gas reserve).

The primary advantage of a gantry-mounted laser is the elimination of one moving mirror. The difficulty of keeping alignment goes up greatly with size. On the 4x8 table, keeping proper alignment on a single moving optic (mirror) will be a lot easier. The extra mass and connections are a real consideration. But if you are doing primarily engraving, then all the action takes place in one axis, so you don't need a lot of fast movement for the gantry itself. If cutting, you are typically going at much slower speeds anyway, so again, the mass may not hurt you much.

[geotek]

I see. You are not reducing the number of mirrors, because the beam must be folded, but the first two mirrors would be fixed (with reference to the laser). I remember seeing gantry machines where the laser was mounted vertically and no mirrors were used. I would think these machines were strictly for cutting.
I'm a retired design engineer so I know the idea of running signal lines, water lines, and 240VAC supply lines through the same run is not a good idea. UL, OSHA, and CE would not approve. The 240VAC for the laser power supply must be separate. You could place the signal lines (shielded) in the same chase as the water lines. On the other side of the machine put the 240VAC supply lines in their own dedicated chase.

[twehr]

I see. You are not reducing the number of mirrors, because the beam must be folded, but the first two mirrors would be fixed (with reference to the laser). I remember seeing gantry machines where the laser was mounted vertically and no mirrors were used. I would think these machines were strictly for cutting.
I'm a retired design engineer so I know the idea of running signal lines, water lines, and 240VAC supply lines through the same run is not a good idea. UL, OSHA, and CE would not approve. The 240VAC for the laser power supply must be separate. You could place the signal lines (shielded) in the same chase as the water lines. On the other side of the machine put the 240VAC supply lines in their own dedicated chase.

I was not saying you eliminate a mirror, I said one moving mirror. Sorry if
I did not make it more clear.