Focus Lens Mounting

[trwalters001]

Hi All,
I'm using one of the Lightobject focus lens mounts. I'm not very happy with the way the lens is held in. I'm thinking I may be missing a piece.
When the lens is placed in the mount, it sits directly on bare aluminum. It's smaller in diameter than the mount, so it's difficult (or impossible) to keep it centered when tightening down the mounting ring. Also, the top of the lens (curved) hits the sharp edge of the mount. Tightening down the ring is out of the question, as it might scratch or break the lens.

Am I missing some sort of washer or spacer to 1) keep the lens centered and not rattle around and 2) allow a tight mount without scratching/breaking the lens?

What's everyone else doing?

Tim

[r691175002]

I just screwed it in as provided between the two aluminium pieces. My hope was that gravity/pressure from being screwed into place would center/level the lens a bit. I didn't worry about scratching since the edges of the lens don't matter anyways.

It isn't ideal but it does work. I bought a 20mm lens but the 18 should be okay as well.

[twehr]

... the top of the lens (curved) hits the sharp edge of the mount. ...

Tim

You are not missing anything.

Are you sure the curved sided of the lens goes up? I think I have mine down.

[trwalters001]

Hi Tim,

You get a much tighter focus with the curved side toward the laser. I was reading the theory behind this sometime in the past couple of days, but I can't find it again at the moment. When I do, I'll get it to you.

Tim

[artwood_decor]

There is some theory about the orientation here:

http://www.fullspectrumengineering.com/ ... ?f=3&t=257

[trwalters001]

That's what I was looking for (thanks artwood_decor)!

Here's the link referenced in that thread:
http://folk.uio.no/walmann/Publications/Master/node8.html

Tim

[bdring]

I have always been curve side towards laser (up). Every diagram I ever saw suggests that is the way. The Full Spectrum crowd seems to like convex down.

[twehr]

I'll be changing mine, if it is down, as I suspect it is. My original Chinese import came that way, so I am sure I continued to follow their lead without checking if it was right.

This morning, I ran a ramped focus test on the system as I have it currently set up. Tonight, I plan to flip the lens and run it again. There seems to be some thoughts that it will change the focus height, so will document any changes I find.

[twehr]

After work, I flipped the lens (it was curved side down). I reran my focus ramp and the optimal focus height shortened by 1.9 mm. Quality on the ramp test did not noticeably change.

So I tried some cuts of my craft plywood, since I have been working with it a lot lately and have a good baseline to compare with. Cuts do appear to be cleaner, with possibly a smaller kerf (though I have not tested for that yet). What about speed of cut, you ask. Well, I asked that, too.

Cutting speed appears to be up significantly. On my test pieces I have increased from my usual 4 mm/sec to 6 mm/sec. That may not all be due to the lens flip. I tested with small, very flat pieces of stock. Normally I work with larger stock that has some slight twists and bends. I need to do a full test of a normal project, but if I can get even 5 mm/sec, I will be happy. That's a 25% increase from the 4 mm/sec I have been used to. When I run the bigger test, I will let everyone know!

[r691175002]

Slightly unrelated but I've been sorting through some accuracy problems with my laser. It seemed like it was getting worse.

Anyways, the lens mount had gotten slightly loose. This was most likely caused by vibration from the air assist and was made worse because I couldn't see the lens (it is covered by the air assist) and the air assist would blow the lens around during cutting so all my lines were squiggly.

I decided to tighten the lens a little more and shattered the edge into four pieces.

On the plus side, I removed the pieces and the remaining 60% or so of the lens still works (with less power and off center). I'm accurate again which is very nice.

On the downside I need a new lens. I'll be using a drop of glue to keep the lens mount from unscrewing in the future.