Finger joint design and kerf


#1

Hi,

I’m new to laser cutting, and am trying to design a box with finger joints. I think I understand how to adjust the width of the tabs for kerf (essentially shrinking the tab width by the total kerf, so that half is adjusted for on each side of the tab on both pieces), but I seem to be having trouble with the tab depth. My understanding is that the tab depth should just be the same as the thickness of the material, with no need to adjust for kerf, because the kerf will essentially shave off the same amount from the top and bottom of the tabs. But what I’m finding is that the tabs don’t seem to be deep enough.

I measured the material (Proofgrade medium maple plywood) with digital calipers and it seems to be exactly 0.125 in thick, and in my AI/SVG files the tab depth appears to be the same, but they seem to actually come out around 0.1 in in depth. (I can’t measure that very precisely.)

Any thoughts? My SVG file is here: https://www.dropbox.com/s/rta5qwxwzif9rw4/tab-depth-issue.svg?dl=0

Thanks!


#2

I have not made any of these yet (still waiting) but have played with the programs a bit and they seem valid.

If your project is sort of a standard run of the mill box, this may help.

http://www.festi.info/boxes.py/


#3

Your digital calipers have a depth measure on them, right? It’s the thin thing that pokes out from the other end when you separate the jaws.

Two things occurs to me. One is that the laser doesn’t cut perfectly perpendicular to the surface because it makes an hourglass shape around the focal point. Another is that the two wood surfaces are a little bit “rough” which keeps them minutely farther apart than perfectly touching.


#4

Are your other dimensions cut accurately by your machine?


#5

Your understandings are all correct. I don’t know where you have gone wrong in AI as that is not a program I use.

Your fingers hight should be the thickness or a bit more if you want them to stand proud. The length of the fingers as posted is 0.094". This is why I am SUCH a big advocate for Fusion 360 for physical things like this.

One other minor observation. You want the width of any fingers to be at least as wide as your material thickness. You have a couple of finger on the second piece that will snap off way too easy when you are putting it together.


#6

@markevans36301 - Ah. The scale of the SVG is wrong for some reason. If I open it in AI it shows the correct original size, but in Inkscape and as printed it’s at 75% size. That should’ve been the first thing I checked :slight_smile: I’ll need to figure out how that happened when I exported the SVG.

And yeah, I need to redesign it to fix the thin finger - I was just seeing if I could get the basic joint to work.

Thanks.


#7

This sort of thing is usually an issue with dpi in art programs. save in inches or mm.


#8

OK, I think I know what happened, although it’s a bit mysterious why. When saving as SVG from AI, somehow the “Responsive” checkbox got checked, which makes AI omit the width/height attributes that map the pixel-size viewbox to physical units.

What’s strange is that even if I uncheck the “Responsive” setting, the width/height attributes are written in pixels. I could swear that the first time I did this just a few days ago, it properly wrote them in inches, but I can’t find the magic combination of settings to make it do that now.

I might just switch to PDF for now…


#9

Try these settings for saving your SVG:


#10

Thanks. Interestingly, it seems that just unchecking “Responsive” seems to fix the problem, even though it’s writing the width/height in pixels. Even though those pixel sizes are the same as the viewBox size in the SVG, somehow the size ends up being correct when I upload it to the GF app.

I think I must have been hallucinating about it writing the width/height in inches…I looked back in my Dropbox version history and none of the SVG files I’ve written since I got the Glowforge have had physical sizes in the root width/height - only pixel sizes - but they’ve all printed out at the proper size, as long as the width/height were actually set and not just the viewBox.


#11

Apparently, Glowforge has a detection algorithm that figures out the dimensions from user pixel space for Illustrator when the aspect ratio of the space is 3:5 (12:20).

It’s not recommended for other reasons (output errors), but there is a way to force AI to output proper dimensions.


#12

This is true if the only thing you’re concerned about is the depth of the tabs (“length” is prolly a better term for this measurement). You don’t need to kerf-compensate tab length.

Assuming you consider the “tab” to be the part that sticks out (as opposed to the “slot”), you will actually want to expand the width of the tabs.

It doesn’t matter if you’re cutting out tabs, slots, or kitty-cats. For all closed shapes you offset all lines by half the kerf width. For the outsides of closed shapes you offset to the outside, for the insides of closed shapes (AKA “holes”) you offset to the inside.

There is no need to do special kerf compensation based on what the shape you are cutting will be used for. Rectangles are rectangles and the laser doesn’t know or care that you’re calling one of them a “tab” and anothet one a “slot”. Make it easy on yourself and also make your design better by offsetting ALL of the outside perimeter outward by half the kerf width and ALL the inside perimeters inward by half the kerf width.

…Sorry, no such luck, @Hirudin of the past.


#13

The Glowforge workspace is set to 96ppi whereas most other software defaults to 72ppi. That is why you’re seeing a 25% reduction in size. Either remove the size designation in your files, or set it to 96ppi to get it to display properly in the GFUI.


#14

Yes, sorry, I meant that I shrank the cutouts, which is the same as expanding the tabs. I was thinking of the cutouts since I was using Boolean ops.

I got everything working and the end result came out great!


#15

Nicly done little video. Thanx