How I design kerfed finger joints (using Affinity Designer)

kerf
#1

My method actually focuses on creating the holes, not the fingers. It requires that you kind of think of the process inside-out, but it’s the only way I could create finger joints that actually work.

This is the sequence of steps for creating finger joints along one corner of a small box.

Sometimes it can be a little difficult to visualize how three rows of finger joints will fit together at a corner. This is what Amazon boxes are for – cut out some trial parts and put them together to make sure you haven’t made any mistakes, before you cut your actual material!

Note: for each of the steps below, click the triangle to see full details.

1. Create your box (or whatever) design using rectangles.

For this example, I’m just creating a small open-top box, 2" x 3" x 1".

2. Decide how wide (approximately) you want your fingers to be, and make a rectangle with **width = desired width** and **height = thickness of your material**.

I’m using 1/8 in Baltic birch, so my height is 0.118 in. I like thinner “fingers,” so I’m selecting 0.2 in as my width.

3. Make a row of these rectangles to check for fit.
  • Select this rectangle and use CTRL J (or command J) to make a copy.
  • Move the copy over by the width of the rectangle by adding its width to the X-coordinate (see step 9 for screenshots).
  • Give the copy a fill, to make it look different from the original rectangle.
  • Now select both of them and duplicate, then move by adding double the width of one rectangle to the x-coordinate.
  • Continue hitting CTRL J until you have a line of rectangles a bit longer than the longest dimension of your design object.
  • Now you can move this strip against each of the edges of your parts to see where the fingers and holes will end up on your piece. If desired, stretch or shrink the row of rectangles to make any changes you want.
  • When you’re satisfied with the fit, delete all but the original rectangle.
  • Make a note of the new rectangle width – this will be your new spacing to use in step 9.
4. Set this rectangle's line width to the width of the kerf for your material.

I’ve found that 0.008" works pretty well for me, for 1/8" BB. Note that AD lets you input your measurement in whatever units you prefer, and converts it to points for you:
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5. Check your settings for line ends (square) and join type (mitered), or things will get wonky in the next step!

You want to use square caps, miter joins, and align stroke to center.
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6. Select your finger rectangle and expand its stroke.

This creates two paths, one on each side of the previous stroke. Here it is zoomed in, so you can see before and after:

Before:

After:

7. Divide the resulting shape.

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Now it will look like this:

I like to remove the fill so I can continue working just with outlines:
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Now it will look like this:

8. Select the outer rectangle and remove it, leaving only the smaller one.

9. Duplicate this rectangle, spacing the new ones at intervals equal to its original width, until you have enough to go across the longest dimension of your design object. You now have a row of finger 'holes' with kerf already built in.

Select your rectangle and do CTRL J (Command J on Macs) to make a duplicate. Place the duplicate by adding the finger width you chose in step 3 to the current X-coordinate.
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Then continue to place new copies at the same interval by repeating CTRL J until you have as many as you need. (I like to have a few extras, just to be safe.)

10. Select the row of rectangles and group them. Place them in an out of the way spot in your design. You'll make a copy of this group for creating each row of finger joints.

11. Center copies of the finger strip against two facing edges of your design.

To align, select the finger strip first, hold down the SHIFT key, and select the object you want to align it to. Then use the alignment tool to choose where you want the strip to go in relation to the object. In this case, I’m centering the strip along the bottom edge of one of my box pieces.

And here are the finger strips placed along two facing edges:

12. Look at where the fingers/holes line up at each end of the object. If the spacing is awkward, ungroup your finger strip, delete one rectangle, regroup it, and recenter.

Here I’ve ungrouped one of the finger strips, deleted the excess, and deleted every other rectangle to get a feel for how they would fall along the edge.

This spacing is awkward, since it leaves me with half a finger at one end and half a “hole” at the other, so I’m going to try again by deleting one rectangle from my original finger strip and re-centering it.

I like this better. If the strip of holes and fingers doesn’t end up exactly at an edge of your piece, you can make small symmetrical adjustments at each end. In this case, the last “hole” or “finger” at each end will be just a fraction larger than the rest, but it will look fine as long as it’s symmetrical.

Before doing anything else, copy this new finger strip to the opposing piece, so you can work on them at the same time.

13. Now ungroup the strips and delete every other 'hole' along one piece, and the opposite ones on the other piece, so they will fit together.

14. When you're happy with the placement, stretch the end rectangles to make sure they overlap the ends of your piece, then stretch the entire strip to overlap the edge they lie along, to ensure you won't have any wonky little nodes left hanging out in space after you merge them.

End “holes” stretched:

Stretching the whole strip over the edge:

Ready to merge:

15. Subtract the row of 'holes' from each of the two pieces.

Select the finger strip and move it to the front. This ensures they will be the items to be subtracted:

Hold down the SHIFT key and select the object below them, then click the Subtract icon in the top toolbar:

Repeat for the other side:

Now just repeat this process for each of the other sides of your box. Repeat the same sequence, rotating the “finger strip” by 90 degrees to do the vertical sides.

Remember to keep that original “finger strip” intact while you work, so you don’t have to go back and rebuild it!

EDIT: Added a step (#3) for checking fit and making adjustments to finger width if desired, before adding in the kerf.

23 Likes
#2

Yeah this is what you do when you don’t use a CAD program designed to do this very task. It’s not that you can’t solve the problem, it’s just harder. These kinds of tasks are really hard to manage in a drawing program (because it’s not designed for parametric design). The problem with using a drawing package is that when you want to then change the underlying geometry all those steps above need to be redone, if you use a parametric CAD package it will auto-fix the joints. Also the CAD program will catch a lot of your stupid mistakes (The number of times I had bogus geometry which threw an error for a joint is huge, and it’s saved me a lot of dumb waste designs)

Not saying this isn’t good work, but just that there are easier tools out there.

9 Likes
#3

One of these days I’ll learn a new CAD program. I used to be pretty good at it; back during a short stint working in a sheet metal fabrication shop, but I can’t remember the name of the program we used, and it probably doesn’t exist any more, anyway. sigh

Same story with 3D modeling. Technology keeps outpacing me and taking away the tools I know how to use!

7 Likes
#4

Cool write-up. Good to see that if I’m ever stuck with just the iPad I’m not dead in the water.

More importantly though, how did you pull off that cool forum format thing?

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#5

It’s under the little gear icon – “hide details.” :slight_smile:

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#6

Oh!

Like this?

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#7

You can learn all sorts of things when you’re post-surgical and bored out of your mind. :wink:

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#8

Ruth, this is fantastic. I’m going to read it more carefully this evening and maybe set up a test run following what you did.

The finger parts are sort of a no-brainer, so I also worked with just the holes/slots. For instance with 1/4" acrylic, using the entire kerf .016", I just make one of the holes as a box that is .250 x .234 (.250 - .016), then duplicate the box for as many as I need for one side, after that, duplicating and rotating until I have enough of them, then when lined up, combine them with the rectangle (box side).

Sounds sort of similar to your method, but I’ll need to read yours again this evening.

Thanks so much for taking the time to do this.

2 Likes
#9

Excellent! Fills in a blank spot in the Matrix! (We didn’t have a writeup for AD.)

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#10

Wonderful job of explaining, and I really like the detail hiding thingy—it does help. I think it’s useful to know how to design something from scratch, even if there’s a tool that works. It’s kinda like making a loaf of bread from scratch even though you can buy a loaf much cheaper (but not necessarily better).

And does this mean you have had your surgery (again) and are in recovery mode? If so, best wishes and feel better soon!

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#11

Not yet! It was supposed to be May 20, but they pushed it out to June 3 so they can have 2 surgeons there…apparently my other ankle was too much work for just one! :stuck_out_tongue:

I’m working on setting up a small crafting space right next to my bed so I don’t go quite as bonkers this time. I can’t fit the GF there, so I got a Cricut to keep me busy until I can be upright long enough to 'forge. And the GF will be moving upstairs before I go under the knife, too – the stairs were a huge obstacle last time, because even when I got to the point of being able to navigate them, it still took so much energy that I didn’t have any left for 'forging!

Anyway, I was designing a printer stand to make space for paper storage underneath the printer, and figured I’d throw together the tutorial while it was fresh in my mind. :slight_smile:

5 Likes
#12

You need to rig up a Wallace and Grommit style bed. :laughing:

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#13

This is very useful! Thank you for sparing me a lot of trial and error.

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#14

Thanks for the great write-up! Kerf is something I’ve mostly been ignoring, but I’m tackling a project where it’s important. This will definitely help!

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#15

would not boxes.py do this?

#16

Sure, if you’re happy with how they come out and don’t want to do any fancy stuff. :slight_smile: I started doing my own when I designed my Mastermind game box.

2 Likes
#17

Well, I tried it. Took me forever. I took away some key points, which I really appreciate. Thanks again.

1 Like
#18

Nice write-up!
I keep a set of long fingers saved as a file I can drop into any drawing and cut to size. That keeps all the scrap with interchangeable fingers; that scrap from different projects fit with each other, These I glue at right angles that have many uses.

By making the fingers a bit long and using a guide for depth the other end overhangs and I get a clean cut. It does not take many times cleaning up that line connecting the end of all those fingers to not want to make it a habit, By playing with the line thickness I try to adjust for kerf; but I use copies of the side pieces to subtract from the bottom piece so I am sure they will fit. I can also copy and mirror a piece and use intersection to do the other side. That way you only do the hard part once.

3 Likes
#19

You could use one of these:

Image result for bed table tilt
I was given one years ago and I happily used it to do my beading in bed when I was too tired to sit up. Unfortunately I lent it to someone in temporary need and it broke.

3 Likes
#20

I’ve updated the steps to add a “check the fit” option (step 3) before finalizing kerf adjustments. It was a sudden impulse and I have to get ready for work, so there aren’t any screen shots for that step, sorry!

1 Like