Focal length of Chinese K40 Engraver

[Bruce Golling]

I wonder if anyone knows the focal length of the ubiquitous K40 engraver. I have seen a million tutorials about how to align the mirrors but there is no mention (that I can find ) of adjusting the height of the z table. I would think it matters because of differences of items being cut, or am I missing some obvious fact?
In other words how accurate does the focal length have to be to ensure a perpendicular cut?

[Dave Sheldrake]

Focal length doesn't control angularity, it controls depth of field. To get the machine to work properly you have a depth of field on a 50.8mm (2 inch) lens of 2.8mm (roughly) where the beam will be at it's thinnest point.

So the top of the material has to be 50.8mm (2 inches) from the back (upper) surface of the lens (give or take a MM as lens focal lengths aren't always accurate)

You could always do a ramp test to get the exact focal position.

[Bruce Golling]

Please pardon my ignorance, but what is a ramp test?

[Bruce Golling]

I just googled ramp test, so don't go to the trouble of writing it out. That ramp test seems like a good idea. btw I don't have one of these machines yet. It is still enroute from Asia. But my question is this: shouldn't these things have a moveable Z axis table or something. Because of the wide variance of things to be engraved.
Also, say for instance I am cutting a 3/4 inch item. Shouldn't the table be adjusted for additional passes?

[Kev Williams]

Take a piece of engraving stock- rowmark, anodized aluminum, etc, on the table, with one end propped up a bit higher, so it's now a 'ramp'. 1 x 12" works fine...

See my lame drawing ;) ...
343334

setup a ramp like this, position the lens head over the center of the piece, and set it so it's approximately 2" from where the lens sits in it's housing--

Now, you just need to engrave a straight line across the length of the plate. Say, 300mm/sec and low power...

The top view, note the line-- what you're trying to get is a line that gets bright and in focus in the middle and fades to nothing on the ends.

If it's bright for most of the length, then lower the power and/or pick up the speed. The shortest bright spot left is what you're after.

--The center of the bright spot is you're lens perfect focus...

Move the center of the laser head to that point.... Now what you need to do is make yourself 'focus shim', something that perfectly fits between the bottom of the lens cone and the material.

That will be your focus tool... make some spares! ;)

If you ever get different length lenses, repeat... :)

>edit< (you were posting while I was writing) -- your machine SHOULD come with some sort of plastic shim to do your focusing with. The ramp test will see how close it is. The moveable table is how to adjust focus.

And while is seems you should, you DON'T adjust focus for half-cuts in most cases. This results in the beam being out of focus when it enters the original kerf and that makes for power loss.

[Dave Sheldrake]

Also, say for instance I am cutting a 3/4 inch item

A K40 with a 30 watt tube isn't going to be cutting 3/4 inch of anything to be honest.It will cope with 3mm ply but even 6mm will be all kinds of slow and terrible quality.

Some K40's have a fixed table, some are adjustable but pretty much all of them arrive bent and of such low quality materials that they are close to impossible to align.

Sorry it's not better news but I have to be honest, if you can cancel the order please do so.

[David Somers]

Bruce, I heartily agree with Dave Sheldrake!! Cancel the order if you can and start researching something better than a K40. They are generally very poor quality and generally have zip for support. Would you mind letting us know who you ordered from?

Dave

[Bruce Golling]

Guangzhou Amonstar Trading Co.,Limited.

[Dave Sheldrake]

That company is a reseller, they manufacture LED strips as a main business so the K40 will have come from one of 4 of the main manufacturers who make them. The original K40 was a Shenhui machine, it was bad...but not really dire (you *could* make stuff with it) but many of the other copies of that design use seriously low rent rubbish metals that strip threads for fun and arrive bent out of shape making aligning precision optics impossible.

The last one I helped a mate fix was 6 degrees off of square and both bearing rails were bent when it arrived

[Bruce Golling]

I understand everyone's concern about the quality or lack of thereof, but I am grateful for these Chinese tools and for the likes of Harbor Freight. Otherwise I would only have catalog pictures to drool over. I like to think back to when I was in Viet Nam ( a verrrry long time ago) and see what the Vietnamese people could do with so little. I often think that if the VC would have had Harbor Freight, I might not be alive to complain about it. :)
All in all I think I can get along by having to tweak this machine when it gets here. Who knows, I might even learn something. I'll probably be back asking a lot of basic questions because you guys are the only ones I know who might have some answers.
Also, my main idea for this is to engrave turned pens. So I think if I can come up with some way of holding and turning the pen so that it can be engraved in 360 degs. Any other stuff will be for learning (that's fun in its own rite)
Thanks
Bruce

[David Somers]

Hi Bruce!

Apologies. None of us were disparaging Chinese machines. I bought one direct from China and am very pleased with it and the company I bought it from. And I buy a modest amount from Harbor Freight and Grizzly. But I know what I am getting and I know they will take something back if it is garbage. What we were disparaging is the K40 style machines. At least in the US, and also in the UK to read Dave Sheldrake's post, they are largely garbage and cause the buyer a lot of headaches with little recourse in terms of tech support or returning the product. In your case, I hope this works out well and a little bit of tweaking is all it takes to get it up and running for you. Some of the machines are reasonable and serviceable. Many are not.

I wanted to add one thing to the focal length discussion above. If your lens is a 2" (or 50.8mm) then as Dave Sheldrake said, you are measuring from the top of the lens surface to the object your want to cut or engrave. Remember though that you have a lens tube that the lens is mounted on. So really, your spacer or focusing gauge is only measuring the distance between the bottom of that lens tube (a cone shape) and the object your are working on. So when you do your ramp test and the distance comes up much less than 2" that is why. I helped a few years here in Seattle who had panicked when they did a ramp test and that number was way less than the 2" focal length they expected. They thought they had been given the wrong lens. Just FYI.

Good luck with the new machine.

[Dave Sheldrake]

Thank you for your service Bruce and if I can help when it arrives please feel free to email me direct.

[Bruce Golling]

Just wanted to drop a line about my k40 machine. I just got it set up and tuned in (pretty much) and figured out how to send a basic engraving to the machine. I am very impressed so far. It was a difficult enough chore just to get all the software installed let alone making it work.
If I can learn this software my next goal will be to incorporate some sort of axis that will turn in conjunction with laser engraving. Sort of a laser lathe if you will.
I'll be back for help you can be sure. Thanks everyone!
Bruce

[Matt McCoy]

Bruce: You will want to look into a rotary attachment that will fit your machine. They typically plug into the Y axis and rotate your stock.

[Dave Sheldrake]

There is a small diameter pen turning / engraving mod that can be done to a k40.....lemme see if I can find the details, it works by fixing to the gantry and using wheels turning on the bed to achieve the result. No need to disconnect the axis or anything like that...it's just a few parts that clip on from memory.....

[Bruce Golling]

I think I know what you mean. I saw a guy on youtube engraving rolling pins. He couples the rolling pin to the gantry and as the gantry moves up and down, the rolling pin turns.