CNC Project #3: UJK Parf Guide II box (finger joints)

**Bernmc** · 30th August 2021, 08:09 PM

parf guide stylised.jpg

I'm trying to pick initial CNC projects that will teach me some essentials.

There's a lot more to get your head around with these machines than I expected! The software side is a particularly steep learning curve. I thought I was doing reasonably well with Fusion 360, but it's a whole new level of pain when you venture into the Manufacturing environment.

For Project #3, I wanted to set up a parametric Fusion 360 project for simple finger-jointed boxes. I wanted to be able to plug in length/width/height and material thickness, and have a complete box with appropriately sized finger joints generated, along with the required toolpaths for the CNC. That way, I can have anything from a storage box to drawers ready to be cut within minutes.

Fortunately, I stumbled across a youtuber who has clearly spent an unhealthy amount of time figuring out finger joints - he's using a laser cutter for very skinny plywood, and although there are issues with the way he generates some of his models (he uses rectangular patterns rather than e.g. mirrors, which causes problems if you add internal features like dados for drawer bottoms), his formulae and their implementation are well thought-out.

I need to do a bit more work on the F360 model, but it's essentially functional for different dimensions. Everything is automatic:

finger joint parameters.jpg

F360 box joints.jpg

Thicker panels:

F360 box joint thicker.jpg

Long & skinny:

long thin finger joint box.jpg

Of course, CNC routers use round spinny things to cut, so you have to add dog bones to the corners. There's a nifty dogbone extension for F360 that does this automatically. Oddly enough, it's called 'Nifty Dogbone Extension'. Saves a whole lot of time.

F360 box joint dog bone.jpg

F360 now has a 'manufacturing model' feature in the manufacturing environment that allows you to tweak your design for the CNC without destroying the original design. For example, you might have rounded-over edges which are quicker to do with a hand-held router. You can remove these in the manufacturing model so the CNC doesn't try to do the edging (or get it's toolpaths fuzzled). It will also flatten the model and arrange it in predefined stock - again, without affecting the original model.

F360 box joint layout.jpg

So, for my test box, I thought I'd see if I could do something to store my Parf Guide II bits. They've lived in the original cardboard tubing and a padded box up to now.
As usual, I got a bit carried away and ended up modelling a foam insert too:

parf guide box F360 design copy.jpg

It's very cool when something that was once a little thought bubble in your head turns into something physical courtesy of a robot:

CNC parf box cut pattern.jpg CNC parf cut out.jpg

Needed a bit of brute force and ignorance to assemble - one of the issues with the way F360 cuts and joins models is that there's no clearance in the joints (you need to add it manually), so the fingers were very tight. I also hadn't quite got my cutting heights (depth) quite right, so there was a bit of cleanup to do.

parf box assembly.jpg

parf box bottom.jpg

I didn't have foam-specific cutters, so thought I'd give it a whirl with a standard downcut spiral. It actually worked ok in a piece of insulation foam, which is fairly rigid:

insulation foam.jpg parf in insulation foam.jpg

So I did a test in some Kaizen foam:

kaizen test cut.jpg

Not bad, if a little furry around the edges. However, it didn't go so well when I tried to do the full insert. On the finer cuts, like the slots for the rulers, the cutter kept grabbing the foam and rolling it up around the cutter.

kaizen insert.jpg

But, I wasn't doing fine furniture, so in it went anyway!

parf box inside.jpg parf box 3qtr view.jpg

And of course, CNC's can do writing (if not reading or rithmatic), so it'd be rude not to...

parf box side.jpg parf box complete.jpg

The lid is made from a skinny piece of marine ply that I had lying around, and slides into a couple of CNC'd slots. I also cut a keyhole in the base, so it hangs up on the wall.

All-in-all, a pretty good early project. Lots of new F360 learned, lots of mistakes and swearing at the AvidCNC, and a useful result in the end.

**pippin88** · 30th August 2021, 10:00 PM

Nice work!

I've done a few drawers with my CNC, but all modelled and programmed individually.

A parametric / easily adjustable model is a great solution.

There are a few ways to achieve a bit of clearance:
1. Model it in
2. Allowance in CAM. I'm not very familiar with F360, but all CAM should have some option in the toolpath to offset the geometry. Often used to leave some material for future operations, but usually a negative value can be input
3. Lie about the cutter size. Tell CAM you are using a 5.9mm bit, when you are actually using a 6mm (for example).

PS: you probably don't need the Parf Guide now anyway...

**Bernmc** · 31st August 2021, 10:14 AM

Originally Posted by pippin88

2. Allowance in CAM. I'm not very familiar with F360, but all CAM should have some option in the toolpath to offset the geometry. Often used to leave some material for future operations, but usually a negative value can be input

PS: you probably don't need the Parf Guide now anyway...

I think this is probably the way to go - I was thinking about it during one of those 2am semi-conscious states. -ve 'stock to leave'... I have a few more drawers to make so will give it a go.

And yes, the Parf Guide will be featuring in the Marketplace soon.

**Sam** · 31st August 2021, 12:06 PM

I’ve no idea about CNC so excuse me if this is way off but do you thickness/size the material with the CNC too ? Like the how many finger joints fit across the width of the box, is the width done by the CNC or you trim it afterwards?

**Bernmc** · 1st September 2021, 11:08 AM

Thicknessing can be done on the CNC - big surfacing bits like this Amanatool beastie. However, it's a lot quicker to run stuff through a dedicated jointer/thicknesser/planer (I have a hammer A3) if the planer is big enough.

I bought a piece of 16mm MDF for my final workbench section from the Big Green Shed. Got home, cut to size, and then discovered that it was 18mm - buggers had put the wrong thickness on the shelf. I'd have been stuffed without the CNC. But a quick buzz with the surfacing bit, and I had it a matching 16mm.

MDF too thick.jpg CNC thicknessing.jpg worktop thicknessing.jpg

Of course, you can do some other cool things with the CNC...

workkbench story.jpg, which is a nice little finishing touch. final workbench nit.jpg.

The parf guide/finger joint box setup is designed for sheet goods, so no need to thickness. I just plug in the plywood thickness and it sets up all the cuts. As it's eating ply sheets, it cuts all four panels to size with joints out of a single piece. I'll make a derived design that will just cut the fingers into stock that's already cut to the correct size.

Had some thoughts around using offsets in the toolpaths to create joint clearance, and this is probably not going to work - it'll cut the offset into all surfaces along the joint line, so joint surfaces and exposed surfaces. Edges won't be flush. So I think it will need to be modelled as an offset face in F360, and repeat the pattern along the joint faces.

**harro1968** · 1st September 2021, 01:31 PM

Hi,

Perhaps I can help, particularly as I'm an experiences Fusion 360 user.

All you really need to do for your CNC is to create a tool in the machine library that details a cutter like this one - 1-5/8inch Finger Joint Reversible Router Cutter Bearing Drill Bit Box CNC Tool | eBay Just get one that doesn't have the earing on the bottom. Many of these bits are reversible to you can make the corresponding reverse stack cut to you faces line up.

All you need to do is define the tool width in the library which is the vertical flat face that will end up being the edge of your work. The fact that fingers will be inside the tool path isn't relevant.

This way you just cut your parts out as normal and they will have the finger joints in them.

David

**pippin88** · 1st September 2021, 07:14 PM

An option is to cut the joints separate to the overall shape.

Pocket out the joint (mortice in effect)
Then profile cut the whole outside rectangle.

Might have to draw what I mean later

**Bernmc** · 1st September 2021, 09:12 PM

I get what you mean. I'm not sure if I can get fusion to recognise all of the mortises minus the outside - if they have to be selected individually it'll be painful. Will experiment

**Bernmc** · 1st September 2021, 09:16 PM

Originally Posted by harro1968

Hi,

Perhaps I can help, particularly as I'm an experiences Fusion 360 user.

All you really need to do for your CNC is to create a tool in the machine library that details a cutter like this one - 1-5/8inch Finger Joint Reversible Router Cutter Bearing Drill Bit Box CNC Tool | eBay Just get one that doesn't have the earing on the bottom. Many of these bits are reversible to you can make the corresponding reverse stack cut to you faces line up.

All you need to do is define the tool width in the library which is the vertical flat face that will end up being the edge of your work. The fact that fingers will be inside the tool path isn't relevant.

This way you just cut your parts out as normal and they will have the finger joints in them.

David

David, I can't see how that cutter will do joints like those in the pictures above? each mortise is several centimeters wide.

**harro1968** · 2nd September 2021, 10:10 AM

Sorry, I misread your ask....

If you are just after glue clearance in your joints, just use offset face on your parts on the faces you want to add that clearance.

Best done of course before you mirror parts to create the multiple sides etc.

I do this quite a lot. Using offset geometry in your tool paths affects all your faces of course.

I have the odd joint in my current project, about 2000!

**harro1968** · 2nd September 2021, 10:25 AM

I tend to leave some interference in the corner as the wood with "smash together".

That way even with your glue clearance on the joint faces, you still have good positioning when assembled.

- - - Updated - - -

I tend to leave some interference in the corner as the wood with "smash together".<br><br>That way even with your glue clearance on the joint faces, you still have good positioning when assembled.<br>
<br>

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CNC Project #3: UJK Parf Guide II box (finger joints)

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