Box Joint Issue

Hi,
I have a lowrider2 that I have been using for about a year. I have cut furniture, toys, signs, etc. I can’t cut box finger joints! I am using fusion 360 and the joints come out horrible. I have tried different speeds, feeds, woods, joint size and nothing works. I have attached a photo of the sloppy joints. I 3d printed the box parts and they came out fine. The lowrider can male the smallest circle for a toy wheel or a 42" round intricate sign with excellent accuracy.
Any help is greatly appreciated. Thanks in advance

I came across this article looking for voronoi stuff, it mentions a box joint generator for Inkscape. Looks like a possibility for the CAD anyway.

I did some simple box joints for my Zen table, for the LCD holder:

I did the CAM in EstlCAM. I don’t know what the procedure is in Fusion, but that doesn’t look right to me. In EstlCAM there’s an option to “overcut” the corners, which moves the tool slightly out at an angle to ensure that the whole inside corner is cut out, but it only does it right at the end., so there’s a tiny gap right in the corners of the tabs, but it’s at most 41% of the width of the bit, which is pretty small when you’re dealing with a 1/8" bit. (Assuming a right angle)

I assume that Fusion’s CAM has a similar option somewhere, or else there should be a way to let it ignore the inside corners, and you would have to complete the cuts yourself, or round the tabs by sanding them.

Your corners look like they were overcut the whole way around, or like they were done following the line, rather than cutting the part out. What Estlcam would call a carve, rather than as a part.

That kind of hurts seeing something like that happen to a nice piece of oak. I’d definitely want to get it working with something cheaper and easier to work with before trying it with a hardwood like that again.

What are the intended dimensions of the tabs, and the real ones? Is the difference equal to the diameter of your bit? 2X the diameter?

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I don’t use fusion for CAM. So I am not able to help. Maybe post some details about your cam and some of the fusion folks will come around and figure it out.

It definitely looks like it wasn’t cutting inside or outside the line.

Here’s how I set up to do corners in fusion, I make a circle the diameter of the endmill that is placed tangent to the inside corner (I’ve made a line that is the radius of the endmill placed at 45° from one of the sides so it’s evenly placed) so you don’t end up with rounded inside corners. I also have started using the feed optimization and finish and spring passes to help with accuracy.

For reference, I’ve seen these described as “dog bones” in other articles.

Another option, although mechanically more complex, is to mount/clamp the sides vertically, then the bit can cut straight through and leave a square end to the joint. This requires a pass-through in the table for any but the shortest box sides.

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Funny coincidence that you are posting this topic - I’ve been meddling with the same issues myself. I don’t have such a severe gap as your photo show, but I’ve struggled with getting a snug fit. I’m using estlcam. My working hypothesis is that the mill has a runout, and I have to adjust the mill size in the CAM, to get a snug fit. I’m updating my thread when I’ve tried it out: Fingerjointed boxes (I don’t have too much time to tinker these days, since most of our spare time goes into renovating a holiday house)

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The very first time I tried this, I got a perfect fit. Easily press fit with pressure from the fingers, a little dab of glue, no clamping required.

Repeating that has been difficult.

I made tabbed connectors for my desk supports, and those were overtight, I had to expand the slots with the tablesaw in order to get them to fit. Next adjustment for something similar, and the slots were slightly oversized.

The last ones were good, but “sand to fit”

I’ve been fudging with the bit size in Estlcam to get dimensions exactly where I want them, and checking stuff with the calipers. The steps/mm seems to be right, in that the difference between something that should be 50mm and something that should be 100mm is correct, but getting the actual 50mm and 100mm seems a bit trickier.

I have found that climb milling with a finishing pass seems to work better, and gets me more consistent results, fwiw.

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This seems to be a long-running issue for us all: Parts Too Big and Holes Too Small
I was looking this up this morning because a gap between parts I cut yesterday that should have been 15mm was 13mm and I had to “sand to fit” as well. 2mm is a really big difference when I can run an engraving to an accuracy so small I can’t measure an error. I’m using Estlcam for the CAM.
Had this problem drilling dog holes too. Holes were too small.
There are a lot of variables at play…Actual bit size, deflection, etc. There may be something in the finishing pass treatment, too.
Things to investigate but no silver bullet it looks like.

Hi, Thank you for the response. The difference in the diameter is not 2x the 1/8" bit. Not sure you can see it from the Photo but the left have vertical is curved and the horizontal is curved but the second vertical is straight and dead on.

Thank you. I haven’t even gotten to dog bones! Not sure you can see it from the Photo but the left have vertical is curved and the horizontal is curved but the second vertical is straight and dead on.

I think that could possibly be from deflection, how many passes are you doing? I was thinking that feed optimization setting might help with that, it would slow the feedrate down in those sections to make the corners more accurate.

So it turns out to be a fusion problem. When I save the fusion 360 sketch as DXF and load it into estlcam there is no issue. Tedious process but it works. I can’t figure out what setting in Fusion would cause this issue.
Thanks for all the suggestions.

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