Author Topic: Adjustable focus rather than a moving Z-Axis?  (Read 3052 times)

mrehmus

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Adjustable focus rather than a moving Z-Axis?
« on: March 28, 2017, 08:20:34 PM »
Since we now adjust the lens focus to accommodate differing thicknesses of material, would it not be simpler and easier to use a very small stepper motor through a reasonable gear train to adjust lens focus for object thickness before a run and for adjusting focus during the run when using multiple passes?
 
I have very small stepper motors (cost about $2 each) that could be run from a third axis control board replacing what we no have or, perhaps, operated via a separate control setup.

Certainly a small stepper and gear train would be easier to set up than a Z-Axis motion system and much lighter. Probably an entire setup would be no more than a few ounces.
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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #1 on: March 28, 2017, 08:28:57 PM »
>and for adjusting focus during the run when using multiple passes?

FYI -- BCL can generate g-code to support a Z Axis if you have one installed on your laser machine.
« Last Edit: March 28, 2017, 08:30:00 PM by Administrator »
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mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #2 on: March 28, 2017, 08:59:58 PM »
Yes, I noticed that. Good thinking there.
mrehmus
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ggallant571

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #3 on: March 28, 2017, 09:18:24 PM »
Welcome to the Society of Z-Axis Builders. There have been a few solutions presented, each with its own set of problems. Some issues:

1. Laser beam alignment
2. Home switches
3. Weight
4. Z-Axis motor drive
5. Speed

1. I built a simple mechanism about 6 months ago and it fails miserably on beam alignment. If the beam is not perfectly vertical you will expend energy burning the sides of the cut trough. Looks poor and the cut is extra wide. Really shows up bad when cutting plastic as the walls melt and fill the cut path. A nice test pattern is a large square (50mm x 50mm) where the Z-Axis is lowered 1mm for each 5mm of travel.

2. Home switches are a necessity!!! You will want to focus the laser at a known height and lock it in place.

3. Adding weight mean more mass. My factory supplied stepper motors could not handle the weight of a NEMA17.

4. Extra motor means extra stepper driver and associated firmware. I wrote my own embedded firmware and the ISR for servicing 3 steppers uses about 60% of the CPU cycles on a 16MHz 328 chip.

5. To lower the weight I went with a small motor with internal gear box. Slow. Takes about 30 seconds to move 1mm. Might not sound like much today but when cutting it is a pain. Also, when finished it needs to return to the home position. BTW - make sure you turn off the laser when doing Z motions.

I was designing a "better" mechanism but my interests have changed from cutting to etching. I primarily use the z-axis for focusing and test burns. To etch 3mm slate I focus the laser beam on cheap neon cardstock and do a low power burn. Once OK with the test burn I raise the laser appropriately.

For my next laser (parts on workbench) I am planning on raising/lowering the work surface. It has a few design issues, 4 stepper motors and wiggle, to solve.

Please post your ideas and pointers to purchasable solutions. If we get enough response we could start a z-axis thread (assuming Ralph hasn't already).

« Last Edit: March 28, 2017, 09:19:34 PM by ggallant571 »
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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #4 on: March 28, 2017, 10:06:17 PM »
>If we get enough response we could start a z-axis thread (assuming Ralph hasn't already).

We already have one: http://benboxlaser.us/index.php/board,70.0.html
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mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #5 on: March 31, 2017, 01:53:17 PM »
I've ordered miniature steppers and a handful of plastic gears to start experimenting. I'm going to drive the Arduino with a rotary encoder.
First attempt will be to just motorize the existing lens setup and see if it will or wont move the beam around (misalign the laser) when adjusting focus.
Iif that does not work I will mount another lens in a brass sleeve and use an external and coarse acme thread to adjust the lens more like a regular camera lens focus mech.
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Koskee

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #6 on: March 31, 2017, 05:13:24 PM »
Well I'm having trouble finding it, but I'm pretty sure I saw some sort of electronic controlled version of this:
http://www.banggood.com/44-inch-Lab-Lift-Lifting-Platforms-Stand-Rack-Scissor-Lab-Lifting-Aluminum-Oxide-p-1022996.html?rmmds=search

I thought I saw it on Banggood, but I can't seem to locate it atm. Even if you attached the hand crank in the picture to a stepper motor instead, or mounted the platform to a linear actuator/threaded rod type of actuation might do the trick. These ^ seem cheap enough, but I think the other one was either 50 or 100 bucks each if I remember right.

mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #7 on: March 31, 2017, 09:12:26 PM »
Lab jacks are small platforms that don't match the operating envelope of the laser.
mrehmus
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Koskee

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #8 on: April 02, 2017, 09:38:27 PM »
Lab jacks are small platforms that don't match the operating envelope of the laser.

Thewha? lol I couldn't even begin to guess what an operating envelope is, and I've come across the term envelope in a few different (sometimes technical) meanings. No idea here..

mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #9 on: April 02, 2017, 10:39:27 PM »
operating envelope is the physical space in which a machine tool can work on, well, work. For a laser, the space is only two-dimensional whereas in a milling machine, it is three-dimensional, a cube.
mrehmus
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Koskee

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #10 on: April 04, 2017, 08:32:59 PM »
Thank you. So you're saying it's essentially too tall or at least not really going to fit or be appropriate for these machines?  Or did I misinterpret that entirely?

What I had in mind was more of a raised laser/cnc frame and/or recessed jack type setup, where the jack is just used for the Z axis actuation either 1 centered, 4 corners or 5 in an x pattern depending in the size of the vertical platform. And using some sort of electronic control on those jack(s) to raise and lower in unison and with precision..

Simplest method would prob be to have a square hole cut out of the surface of the table and a platform(shelf) of some sort to support the jack on the bottom such that the top lines up with your laser x-y, drop it into the hole, add electric actuation, and voila. you have a vertical axis

Obviously would be a lot of custom design and construction involved, but I didn't think that was part of the question/most upgrades to these lasers are homebrew anyway

ggallant571

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #11 on: April 04, 2017, 09:17:18 PM »
I think it is essential that there be minimal slop in the X & Y dimensions. The Z is less critical. Suspect something like 4 motors for lifting along with a couple very good vertical alignment guides are needed. Please post ideas and plans.
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mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #12 on: April 05, 2017, 04:53:56 PM »
Good minds think alike.
I thought about the problem from the viewpoint of bothering the existing gantry structure as little as possible because it is a marginal design (but good enough). It's main problem is that if you pick up one corner, the system distorts a small amount.

Jacking the 4 corners is relatively easy I think. That sort of thing is done all the time with other machinery such as saws, planer and wide-bed sanders.
4 jackscrews driven together might be the best solution except the mechanism has to avoid the gantry carriages. Still, if one started off with the premise that frame will have to be higher, then a common drive is not impossible.

I'm going away for a few days so I'll think about how best to achieve common movement at all 4 corners.

Another thought would be to place the laser as-is on a platform that raises up and is not a part of the laser frame. Might be the best way to go.

This is certainly a case where it would be necessary to focus the laser on the baseboard and then raise the frame to accommodate the thickness of the material to be cut then drop it as the cut progresses.
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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #13 on: April 05, 2017, 05:06:23 PM »
I'm still inclined to think the best Z axis laser frame is the i3 Prusa 3D Printer frame. It's Z axis is built like a tank and uses two steppers. I don't think there is that much work to do to convert one over to a laser machine.

It runs Marlin firmware and I have already modified BCL to work with Marlin -- however -- I prefer to use Grbl, so if I were to build one, I'd just remove the RAMPS 1.4/Mega2560 unit (this one is becoming my less and less favorite choice) and replace it with an L8 or Mana SE and flash Grbl 1.1e. Then connect all the wires which should fit?

All that is left to do is remove the Extruder and mount a laser 3D printed mount and fan.

I really think this would be the best way to go. With all the posts on this forum about trying to add a Z axis to the existing 20x20 laser frame design, there are nothing but problems, problems and more problems.
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mrehmus

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Re: Adjustable focus rather than a moving Z-Axis?
« Reply #14 on: April 05, 2017, 06:42:16 PM »
You would need a Mana CNC board, would you not for a 3-axis setup?

I think placing the laser on a movable platform is quite doable and much less expensive than a 3D printer chassis which does not have that big a work platform does it?
mrehmus
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Elekslaser A3 2.5 watt via Banggood
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