# Kerf Offset problem

I had a post going elswhere but it told me the problem was solved and to start a new topic.
I’m totally confused with kerf offset.
I followed The Louisiana Hobby Guy’s Youtub video:

I did exactly what was in the video and ended up with a mess.

According to the video’s instructions I came up with a kerf of 0.0685 but that seems to be way too big.
I ran 3 other tests juggling those numbers but still my inlays do not fit right.
I wouldn’t be asking if I wasn’t lost but here I am.
Thanks,
Tom

Can you post your Lightburn here and your material thickness?

Every type of material will have a different kerf… depends on how the laser damages the material.

From you post, I can’t tell if the image is before or after you followed the video…

1. let us know what the material is… this would help us help you as most of us know the about size for kerf in different materials.
2. what is the actual question you’re trying to get answered…
3. optional: It would be nice to have the link to the previous discussion you mentioned.

Might check out @BillieRuben tool for deriving the kerf … pretty slick stuff

I can put my lightburn in here but I have been trying all different types os wood with different thicknesses.

One issue I’ve had (1.5xx) is when switching my object to inches from mm’s, the layer kerf setting seemed to stay in mm, but was interpreted as inches. Bit me several times as my cutout was smaller that expected until I did the mertic → imperial conversion and reentered the correct kerf offset

Thank you.
I will do the kerf test again and see if I can find either what I’ve done wrong (LOL) or if lightburn is messing me up.
Thanks again,
Tom

I never gave that a thought but will check it out.
Thanks,
Tom

If you used the Kerf Test Jack mentioned, it is possible to get those squares rotated 90 degrees. That would give you a smaller kerf value. Use the little lines to make sure the squares (actually rectangles) are properly orientated.

I had to lay a small piece of wood over the squares to keep them from popping up.

Steve/Hobo with Wood actually has a novel way (to me anyway) for calculating kerf. Just using boxes.py (actual site is www.festi.info/boxes.py/ ) and the burn test. Since lasers usually have the beam ( as far as this is relevant) more of a rectangle than square, rotate the design by 45°. X and Y axis will always slightly differ in kerf size, so this reallly is the best approximation of both. I won’t attempt to paraphrase him, but I do highly recommend his video on this and the reasoning why it works.

Just search Youtube for Hobo with Wood, it is not that old of a video. Forget exact name now, but it is relatively recent and mentions kerf in the title. Using the traditional way finding kerf, well you will find that depending on how you rotate and move pieces around to save material, some will fit great, some not so great. It really doesn’t have anything to do with varying thicknesses on the same piece of wood as some have led us to believe.

I have tried the 45° thing. It works OK if your designs are mostly right angles. Throw in odd angles or curves and it makes little/no difference.

I have not seen this video, but a tip I would add for those looking to use this technique is to rotate the workpiece 45° also. That makes layout and material conservation easier.

I assume this is in inches which would be 1.7399 mm. That’s a very large kerf value. There may be a procedural issue in how you’ve calculated this.

Can you articulate the steps that you took?

Also, does 0.0685 represent the full kerf value or is that the value you entered in as the kerf offset amount? Note that you’d want half the full kerf value for the kerf offset.

I did the Vernier test on my Sculpfun 10w, and got very similar results (can’t find the posting where I reported this) of less than .07mm. I did the test in 2.8mm Baltic birch because that is what I use the most. I also did it twice because I thought that was too good to be true.

Sculpfun laser modules are known to have a tight beam and long depth of field. One of the few times advertising claims seem to be true.

Is your position that you believe the number reported is actually in mm and not inches? We would need OP to clarify but I’ve inferred inches based on screenshots showing use of inches for all other functions as well as the Preview showing fairly extreme kerf offset appearance.

This is why I’m suggesting a procedural error of some kind. A nearly 2 mm kerf is likely too high a measure. Something is likely getting lost in process and I haven’t seen anyone raise that issue as yet.

I have to agree with @berainlb … From the documentation on the Sculpfun site:

``````Laser focus:   0.06x0.08mm
``````

If it’s 0.08mm size spot, it’s clear you’re not measuring in this direction…

If the lasers kerf is 0.06mm and your measured kerf is less than a hundredth of a mm larger than the beam … I question your numbers and procedure… again agreeing with @berainlb that somethings askew.

Knowing how the Chinese measure stuff, the kerf value of 0.06x0.08mm is probably the very best of the batches of lasers they could get… so I’d say it’s likely to be a bit optimistic. Much like output power of their co2 machines.

Maybe … but I doubt it… and the keywords are seem to be true

Unless Sculpfun has figured a way around physics, beams size and dof (depth of focus) are directly related… larger dof, larger spot size. They’re playing the same game, don’t let them fool you.

D is input beam diameter, f is focal length.

``````DOF = 2.5 * wavelength * ((f/D)²)
``````

The only way to change DOF is to change one or more of these three variables. If you change D, it being in the denomiator, then a larger beam gives you a smaller spot, along with a smaller dof.

This calculator works well - check it out… maybe I messed up…

Just MHO …

Valid point. I would have come to the same conclusion.

Yes, that is more like plasma torch numbers.

Not the first time. If you can tell me how I could possibly do it wrong, I will be happy to correct my mistake. I used the Vernier test you suggested. If you see numbers you do not like, that does not mean they are wrong.

I can run another test and mail the wood pieces to you if you want. That way you can verify if I used the correct procedure.

Don’t worry about me. I go by results, not claims.

I did…
Wavelength = 455 nm
Focal Length = 50mm
M^2 Factor = 1.5 (I used the default)
Beam Dia. = 0.08mm (I used major dimension)
Calc Results:
Spot size = 541.734 Microns (0.542mm)
Depth of Focus = 444,335.938 microns (444mm)

I know for sure I do not have a 1/2mm dot size. I also know for sure my DoF is not 17.5 inches.

I have been a hobbyist astronomer since pre-1970, so I understand the optics and math. What I dispute is applying that calculator to diode lasers. If I made a data entry error, show me so I will not make that mistake again.
There seems to be an assumption that the focal point cannot be less than the advertised beam diameter. It seems the concept of “focus” was lost somewhere. The purpose of the lens is to take the slightly divergent beam of coherent light and re-aim it at the smallest possible spot, smaller than the original beam diameter. If this is not true, why bother with the lens at all?

Agreed …

I think the issue is beam diameter… This is the diameter of the beam entering the lens, not the resultant focused beam… I’ve never seen this specified … anywhere.

The other question that keeps me up, is the M2 factor. How this applied to a rectangular emitter, I haven’t come to grips with yet… maybe it’s something simple like 1

It comes down to the same thing, you get a large dot with large dof or vice versa.

Love the night sky… moved to a dark city … actually see the milky way at nigh…

I screwed up and went the other way. Harrison Co., IN dark skies to now Tampa Bay, FL with a gazillion lights.

Yeah, I have no idea what that is.

Thanks for the link. I knew what they were talking about. I just did not know it had a name.

That is a parameter I doubt the cheap laser manufacturers would spend the money to determine. I am in NY, so I cannot see if my laser has it on the label.

I’d be surprised if there was much useful information for these… they target a plug and play environment… the less detail the less they have to explain.