How to CNC a finger-jointed box?

Signals
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How to CNC a finger-jointed box?

Post by Signals » Mon Apr 28, 2014 2:30 pm

I'm having some difficulty figuring this one out.

I want to CNC boxes that are finger jointed together.

However with the CNC, you're going to have the inside corner's radiused since the CNC can't make a perfect square cut on on the inside corners of the fingers.

Therfore the finger joints on the other pieces will have to be radiused similarly to fit together. This is kind of tricky to CAD!

I was thinking of attempting to do it in Sketchup and then use MeshCAM to generate the tool paths.

Does anyone have any experience doing this, tips, or other joints that work similarly but more are more CNC friendly?

WillAdams
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Re: How to CNC a finger-jointed box?

Post by WillAdams » Mon Apr 28, 2014 2:36 pm

There's discussion of this in: http://www.flexiblestream.org/Digital-W ... ts-001.php

which is linked to from: http://www.shapeoko.com/wiki/index.php/ ... 6_Hardware

and there's further discussion here: http://www.shapeoko.com/wiki/index.php/ ... abrication

I've always thought finger joints were a rather ghastly compromise based on automation limitations to begin w/ (who wouldn't prefer dovetails?) --- we really should be able to work up better techniques w/ CNC gear. My next modification to my SO1 is enlarging a hole I added to my torsion box for drilling into the end of a board so that it allows me to present two boards at a right angle to each other, and 45 degrees to the bit.

If you presented the boards to the machine through a hole in the bottom of the machine you'd be able to mill fingerjoints w/o any issues though, but given how quick they are to do w/ a jig on a tablesaw, why bother?
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cvoinescu
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Re: How to CNC a finger-jointed box?

Post by cvoinescu » Mon Apr 28, 2014 3:29 pm

There is a neat trick to get a nice-looking finger joint on a 3-axis CNC mill without turning the boards on their side. I'll explain it in two steps: firstly, imagine you mill finger joints with dogbones, so that they fit (but look butt-ugly). The top of the sheet (the side you mill from) would be the inside of the box. Secondly, reduce the depth of you mill the fingers to, keeping a layer of material intact under them. Add a lip as wide as this remaining layer is thick to one of the parts. That will cover the joint, and you won't be able to see the fingers (and their ugly dogbones) at all. The downside is that the depth of the fingers is reduced by the material that hides them, so the joint is somewhat weaker. The difference does not need to be great: a thin layer is enough to cover the joint, so you can mill the fingers through most of the width of the material.

[Edit] The very last image at Will's first link (the Mixcubes) shows exactly what I tried to describe. Search for Davetone Mixcube, you'll find more images. The only difference from what I said is that Dave didn't add the lip I described, which would have completed the joints to nice, sharp 90 degree edges.
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Signals
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Re: How to CNC a finger-jointed box?

Post by Signals » Mon Apr 28, 2014 4:03 pm

Well, I have to mill thousands of these things so ideally I don't want to be doing any of it by hand.

Trying to streamline as much as possible.

I think I get what you're saying cvoinescu, although it's hard to wrap my brain around without actually playing around with the idea in 3D... so I will!

I'll show everyone what I end up doing.

I'm thinking the ideal solution if I can manage it would be some kind of radiused dovetail or the like.

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Re: How to CNC a finger-jointed box?

Post by WillAdams » Mon Apr 28, 2014 4:18 pm

You could use a 90 degree V-bit and just cut positive-negative Vs out of a pair of boards to join them at a 90 degree angle --- this would give you a decent amount of gluing surface, and would be a full-blind joint and wouldn't show on the inside either.

Rather than start w/ how things have always been done, let's start again:

- How strong do these joints need to be?
- how thick is the material? What are you cutting it out of?
- Why not just use a dovetailing bit and cut full or half-blind dovetails?
- Or a straight bit and cut a locking rabbet?
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Signals
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Re: How to CNC a finger-jointed box?

Post by Signals » Mon Apr 28, 2014 4:30 pm

WillAdams wrote:You could use a 90 degree V-bit and just cut positive-negative Vs out of a pair of boards to join them at a 90 degree angle --- this would give you a decent amount of gluing surface, and would be a full-blind joint and wouldn't show on the inside either.

Rather than start w/ how things have always been done, let's start again:

- How strong do these joints need to be?
- how thick is the material? What are you cutting it out of?
- Why not just use a dovetailing bit and cut full or half-blind dovetails?
- Or a straight bit and cut a locking rabbet?
Haha, I think what might not be coming across is my ignorance in wood working. I am not from a woodworking background, I am more from a 3D background - things that might be obvious to a woodworker completely escape me.

The joints don't have to be particularly strong. It's for a speaker.

Will be cut out of red oak or pine. Or some other raw wood.

I don't use dovetailing bits because I'm not familiar with them, haha.

Locking rabbet sounds like it might be exactly what I'm looking for. Looks like the inside corner radiusing could be avoided with "dog bones", haha. Unfortunately can't easily be cut flat with a CNC.

I guess I could do simple "dado" joints I believe they're called?

Correct me if I'm wrong, but I believe that this joint could be milled entirely with a flat end mill on a flat piece of stock? I think this is a winner as far as simplicity is concerned. It would also be good for ensuring an air tight seal, which is essential to the speakers performance.

http://www.onlinedesignteacher.com/furn ... 0large.png

It would also avoid tear out from rounding over the edges on a router table after construction, which is nice if I choose to do that.

I just was thinking about it, and there is a design problem with that joint as well.

The speakers need to be flush mounted from the front, which involves a large counter sunk hole to be drilled. That means the piece of wood would have to be flipped over to do both that and the joint. More work than I want to do for mass production... trying to make it so all finished necessary parts and joints are from a single flat piece of stock - hence the finger joints, except they can't be milled through either due to the inside corner radiusing issue.

I think I have a solution utilizing a different orientation of that dado joint, but I'll have to give it a shot in 3D to see if I run into any problems... works in my head. Basically just take the front panel where the speaker would be mounted, and carve a border around the perimeter about half the depth of the material. Put slits in the side panels, which the front panel would dado into.

I'll cut the top/bottom/side dados like this (look at lower left corner), but then I'll add slits to dado the front into the box as well (so the front would be locked in, the top and sides would have to be constructed around the front (and back the same way most likely).

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Re: How to CNC a finger-jointed box?

Post by WillAdams » Mon Apr 28, 2014 4:56 pm

Yes, that's one form of a locking rabbet joint.

My inclination would be to just drill the holes when making the joints, then use a pair of drills, one w/ a countersinking drill bit, the other w/ a driver bit and do the finishing as part of assembly.
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cvoinescu
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Re: How to CNC a finger-jointed box?

Post by cvoinescu » Mon Apr 28, 2014 5:32 pm

You can mill everything from one side with a single, straight tool and get a joint with no visible fingers or voids -- the only imperfection would be a seam a few millimeters from the edge. Check out the "50 digital wood joints" page Will gave the link for. There's a .zip to download with all 50 joints; the ones that would work best for a speaker enclosure are in file

Digital_Wood_Joints/CONTENT/ENGLISH/D_V_E/BRE_E/B_E_E.PDF

at pages 3 and 6; they are called "Secret Finger Tenons" and "Secret Fingertip Tenons" (file names F_006 and F_007).

There are CAD files too in the archive, for each of the 50 joints.
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twforeman
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Re: How to CNC a finger-jointed box?

Post by twforeman » Mon Apr 28, 2014 6:11 pm

As a woodworker, I would like to point out that if you are making speaker boxes there really is no need for a CNC (except for maybe milling out the holes for the speakers.)

If you have a table saw, a standard miter joint, with a spline if you feel you need the strength, is more than sufficient. Or even use a dado stack and make some rabbet joints.

There is no need for box joints, dovetails, etc, unless you are doing it for looks.

A table saw is going to be much faster than using a CNC, and if you are doing this as a production run, that would be my first choice.

My favorite axiom goes here: Use the correct tool for the job.
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Re: How to CNC a finger-jointed box?

Post by Woodworker » Mon Apr 28, 2014 8:14 pm

I agree, if you are going to do "thousands of them"' invest in a table saw and a router. I can't seem to find an earlier post i did but the quickest way is to cut your top, bottom and sides square, rabbet the front and back edges of all 4 pieces, then pocket screw them into a box shape. The front and back panels can be done quicker with a router. It could be done on the CNC machine if you wanted to do something else while it was working. Drop the front and back panels into the rabbets with glue and an you are done.

I plane to use my SO2 in my woodworking business but can't imagine running more than a dozen of anything, in a week.
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