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View Full Version : Focus lens tube length?



Mike Audleman
08-22-2014, 2:09 PM
Ok, if I understand the way this beast works...

The length of the vertical tube is not critical? I mean the distance from the down turn mirror at the top of the head to the top of the focus lens. The IR laser beam is columnated and doesn't spread, or at least not much. So if I increase its travel by an inch or two, that shouldn't affect it, right?


My tube (probably like many others) slips into the tube on the mirror mount and is held by a cinch nut. It can adjust maybe an inch or so for height. I was wanting to make it as long as possible. This would let me drop the table down lower and hopefully allow larger objects to extend out the front of the unit. I have to get the object's top focus low enough to get below the top I-beam support structure for the z-axis screw shafts.

And I was considering having a local shop make an extender tube if needed. Its just a hollow aluminum tube.

Yes, I am aware that this will decrease my maximum object size that I can work on and I am ok with that.

Is there any downside that I am not considering here?

Dan Hintz
08-22-2014, 2:51 PM
The final lens is doing the focusing... any beam before it hits that final objective should only be spreading by a tiny amount (milliradians).

Bill George
08-22-2014, 2:54 PM
Mike, see my posts on the same issue.

David Somers
08-22-2014, 2:55 PM
Dan,

(I feel like I am following you around today asking pesky questions <grin>)

If someone has a tube made from aluminum, should it be coated with any type of anti reflective coating inside? Or does that make a bit of difference since the beam is coherent until it reaches the lens?

Dan Hintz
08-22-2014, 3:01 PM
If someone has a tube made from aluminum, should it be coated with any type of anti reflective coating inside? Or does that make a bit of difference since the beam is coherent until it reaches the lens?

There shouldn't be any side-scatter of the beam for an AR coating to make a difference. That said, lenses aren't perfect, so there will be some reflection... but THAT said, there's so little power reflected (<1%, generally), if any fraction of THAT should make it back down to the substrate, it won't leave a mark.

Mike Audleman
08-22-2014, 3:30 PM
Mike, see my posts on the same issue.

Rofl,
Extendable nose cone (http://www.sawmillcreek.org/showthread.php?220934-Extendable-nose-cone)

I just poked into that thread moments ago and saw you were kinda talking about the same thing. The "nose cone" title didn't immediately register in my mind you were talking the same thing I was here.

That said, it seems like some here have different tube lengths.

Any downside to using longer ones?


The final lens is doing the focusing... any beam before it hits that final objective should only be spreading by a tiny amount (milliradians).

Ok, thats what I thought. I just needed so say it out loud and get a confirmation.

Now I just have to find someone to make the extender tube for me. Guess its time to go find a metal fabrication shop around here unless yall know of someone online who knows what they are doing and I can order one from them?

Bill George
08-22-2014, 5:22 PM
Mike see my posts on making one. I have been busy re-aligning my laser mirrors, well the last two anyway or I would be out looking though my shop metal pile right now. Next its out to mow the yard, yet again.

David Somers
08-22-2014, 5:25 PM
Thanks Dan!!! Appreciate it!

Dave

Rich Harman
08-22-2014, 8:13 PM
The problem I see with extending the tube, or any long tube in general, is that it is more likely to vibrate the further out you put the mass. More likely to get wiggles with sudden direction changes. Of course, the faster your laser, the more important this becomes.

Because the extendable tube on my laser is secured by just a thumbscrew (not very secure) I always run mine with the tube as far up as it will go.

Bill George
08-22-2014, 8:20 PM
When I get time to make my extension tube it will be to a lot tighter tolerance than the Chinese made one. I've done machining before, when it gets done there will be no wiggles. I am only going 1 inch longer.

Kev Williams
08-23-2014, 1:04 PM
The problem I see with extending the tube, or any long tube in general, is that it is more likely to vibrate the further out you put the mass. More likely to get wiggles with sudden direction changes. .
In a word: Nope. First, the tubes are light. My factory tube weighs 36 grams, my longest 8" tube weighs 64 grams. Not a whole lot of inertia to worry about! ;)

And Chinese machines don't make "sudden changes". No matter what speed I run my machine, there's no sudden shock whatsoever. The faster the machine is set to run, the farther the edge to edge overreach is to allow the machine to slowly stop for the turnaround. It's extremely smooth, and I assume most Chinese machines are pretty much the same. But even in a 150" per second machine, I doubt there would be any wiggle or movement of the tubes.

And I have no problem with a thumbscrew type clamp. Every engraving machine I own have spindles that are held in place basically the same way, just 2 set screws. The spindles weigh a ton and get more force exerted on them than one might imagine. As long as the screws are tight, the spindles don't budge.

Dan Hintz
08-23-2014, 1:30 PM
It's not the overall speed of the carriage that's a problem, it's the acceleration/deceleration... I think I calculated Trotecs at 5G(?) accel//decel, ULS was a more sedate 3-3.5G (but those are from memory). Even lightweight objects can be subject to a lot of flexure and movement when they're at the end of a long arm and subject to high Gs. Chinese machines are usually much lower G ratings, which helps in such a case... even if the Chinese machines could run at 150ips, they don't stock large enough motors for a high-G turnaround.

Rich Harman
08-23-2014, 3:27 PM
In a word: Nope. First, the tubes are light. My factory tube weighs 36 grams, my longest 8" tube weighs 64 grams. Not a whole lot of inertia to worry about! ;)

And Chinese machines don't make "sudden changes". No matter what speed I run my machine, there's no sudden shock whatsoever. The faster the machine is set to run, the farther the edge to edge overreach is to allow the machine to slowly stop for the turnaround. It's extremely smooth, and I assume most Chinese machines are pretty much the same. But even in a 150" per second machine, I doubt there would be any wiggle or movement of the tubes.

Nope? I think that is unnecessarily dismissive. Like I said, the faster the machine, the more important it is. Acceleration/deceleration values should be set for best performance. Add a couple ounces hanging down a couple inches lower and the laws of physics dictate that the situation will change. Maybe the acceleration settings are conservative enough on your Chinese machine that you won't notice.

Incidentally, the "wiggles" I spoke of will appear when vectoring, not rastering. For example, I have to lower the power to cut 1/8" MDF. If I cut at full power the speed is too fast and I get wiggles after each change of direction. So I slow it down to a speed where the wiggles don't appear then adjust the power accordingly. If I hung a 6" tube off the laser head with the lens at the end I can guarantee that I would have to alter my speed again because the wiggles would appear at a slower speed. That's just physics, the laws of nature.


And I have no problem with a thumbscrew type clamp. Every engraving machine I own have spindles that are held in place basically the same way, just 2 set screws. The spindles weigh a ton and get more force exerted on them than one might imagine. As long as the screws are tight, the spindles don't budge.

You would have a problem with my thumbscrew clamp. The fit of the tube in the laser head is loose. When the single thumbscrew is tightened there is still a little play in the tube. In the full up position, it is solid.

Bill George
08-23-2014, 4:42 PM
Well I have ordered my material, heavy wall 1 inch diameter (25.4 mm) aluminum tube. The net length gain will be 1.250 inches and I can weigh the extension when finished and you engineers can do your calculations. Trust me If I could have ordered a pre-built tube it would already be in the mail.

IF there is going to be a mass demand for this, I have a CNC job shop less than 10 miles from me.
My OEM tube holder has 2 thumb screws, is there going to be an advantage in going to 3 ?

I have an indexer for my milling machine that will be used to drill and tap the holes so 2 or 3 not a big deal.