Accurate Butterflies (Bowties?) for reinforcing cracks

[smorgasbord]

Watched some videos of people doing it, and a couple with people explaining it, and decided that my chiseling skills aren't at the level I'd need to achieve the results I want. Re-visiting some v-carve inlay videos, I decided to adapt that strategy. This could be further adapted to the Shaper Origin, too, which would actually be a bit easier, since with that you wouldn't need the mortising template - you could just directly SO rout the mortise into the slab.

Here's the Fusion360 layout for the two pieces: the butterfly itself and the template for cutting mortises:

The key here is to use a shallow angled tapered router bit to both cut a butterfly with angled sides and to cut the matching mortise. Since the mortise and butterfly walls are both tapered to the same angle, insertion/gluing/fit is all but guaranteed.

In case you don't know about V-carve Inlays, here's an image showing a cross-section:

You then plane/sand the inlay to be flush with the top. With the CNC, you typically cut both the mortises and the "plug" on the CNC, then fit and glue, then plane/sand to flush.

Now, these use rather shallow angles, but I'm not sure why. Any inaccuracies in height/depths will change the apparent width/length - and the shallower the angle the more the distortion. So, I started this with a 2.4º bit, and just received a 1º bit that I'll try out tomorrow.

Unfortunately, I built the butterfly in Fusion before creating components, and for some reason once I've done that, I can't seem to put what's built into a component and then create another component in the same file, so I created two files. Any Fusion experts that want to help me out - I'd be much appreciative.

Here are the parameters for the Fusion Files. First the Butterfly itself:


The idea is I built the shape I want to see on the tabletop surface, parameterized by overall length, overall width, and the angle at the outside corners. That gets extruded down into the top as thick as I want, and also up above the table in case the mortise ends up slightly bigger and the butterfly sinks in a bit deeper. The "TaperAngle" needs to be the angle of the final pass bit - easiest to use the same bit in the CNC as in the router cutting the mortise. Since the butterfly is milled upside down on the CNC, it all matches.

And then for the Template:


There's a measurement/calculation that needs to happen plus a bit of math:
1) Depending on the depth of the mortise you want, the tapered bit's diameter at the table top's surface ("BitDiaAtTemplateTop") will vary, but this is just Pythagorean triangle stuff.
2) Depending on the outside diameter of the bushing (What, Smorgasbord is using a router bushing after posting they're dead!?!) you want to use, you need to calculate the offset ("GuideOffset") of how much larger the template has to be to get the mortise size you want.

If everything is cut perfectly, the butterfly will stick up "ProtrusionAboveTop" distance above the tabletop. Note that in this example, the two "Thickness" parameters are not the same, in this case the mortise will be cut 18mm deep but the butterfly will have only 16mm below the tabletop. If the mortise is slightly big, then this 2mm gives some room for the butterfly to sit more deeply without bottoming out. Thinking about this, 2mm is maybe too much, but I'm still doing trials and will cut them open afterwards to see what's going on.

To do this:
1) Put butterfly stock good face down on the CNC bed and cut out the butterfly plug.
2) Cut a straight-sided template.
3) First pass on the mortise uses a ¼" upcut bit with a 7/16" guide bushing with a hand-held router to hog out center waste.
4) Second pass on the mortise uses the tapered bit with a ⅜" guide bushing.
5) Chisel the corners of the mortise by hand.
6) Apply glue to mortise only (don't want plug to swell), insert, and hammer/clamp.

I've done one so far with some scrap and it turned out OK. I'll use the 1º bit next and see how that goes.

The taper bits are ¼" shank, with a 1/8" diameter bull nose and long enough to get the taper angle right. I actually did order a 0.1º tapered bit but that might be too little taper.

One side question: Why is it that "Vee-Bits" have their angle as the angle between the two cutting edges while the "Taper Bit" have their angles as the angle of the cutting edge from vertical? These are really very similar profiles, so I would have expect the angles to have been measured the same. Took me a long minute to confirm this. IOW, a 90º Vee Bit cuts 45º angled walls while a 2.4º taper bit cuts 2.4º angled walls. Weird.

I'll follow up in the next few days with photos.

 

[luvmytoolz]

Doing butterflies is tricky at the best of times. I do a lot of them and have sort of got it down to a process, but there's still lots of manual finessing required at times. I cut mine out on the CNC in the shape of a butterfly instead of bowties, and make a router template for machining out the pockets. I use a Makita trimmer with a 1/4" single flute cutter, and the standard 10mm template guide. The pockets are around 0.1mm-0.25mm oversize depending on the timbers used, as there is always a minute bit of variance in the machined butterflies.

With the difference in Vee and tapered cutters, from my understanding it's because vee cutters are used full width up to full depth, whereas tapered cutters tend to run along a path machining one side of the material. If that makes sense?

 

[smorgasbord]

With the difference in Vee and tapered cutters, from my understanding it's because vee cutters are used full width up to full depth, whereas tapered cutters tend to run along a path machining one side of the material. If that makes sense?

Yeah, I guess so. Did waste almost an hour measuring and cutting and measuring to be sure, though. Now I know.

BTW, one thing I didn't mention is that, typically, the V-carve inlay process is used with the "shallow" Vee-Bit angles so that you can control the width of the inlay lines by varying the depth of the Vee bits. For this butterfly application, though, we don't want to vary any lines, we just want an uber-tight fit. So, the steeper angles/taper bits being used.

I cut mine out on the CNC in the shape of a butterfly instead of bowties

Pics? Are they actual insect-looking butterflies?

So far, the two inlays I've done haven't needed any manual finessing, just the paring of the mortise corners to be sharp.

 

[luvmytoolz]

Here's some pics of some I've done. They're a pretty firm fit, and I make around 5-6 different sizes in many different timbers as I really like the decorative effect, like in the pics of the small table top with around 9 or so of them.

The shape is as close to a butterfly as was practical, given they are all the same shape scaled up or down, and have to allow for the minimum possible radius of the template guide and the cutter.

 

[Cheese]

Just a note of caution & something to think about...there are bowties/barbells used for functional purposes such as to prevent further splitting of the wood and there are bowties/barbells used for decorative purposes. The angle/V-Carve inlay method would only be good for decorative purposes.

 

[smorgasbord]

With the difference in Vee and tapered cutters, from my understanding it's because vee cutters are used full width up to full depth, whereas tapered cutters tend to run along a path machining one side of the material. If that makes sense?

Thinking about this some more, it's not making that much sense. I think the Vee Bit are used for the inlays as well as lettering, and the tapered bits are also used for delicate lettering. So, in some applications you're using creating a V-channel for both bits. At any rate, I do think how the bit's angles are measured should have been consistent, but that ship has obviously sailed.

Just a note of caution & something to think about...there are bowties/barbells used for functional purposes such as to prevent further splitting of the wood and there are bowties/barbells used for decorative purposes. The angle/V-Carve inlay method would only be good for decorative purposes.

I thought about this, and I'm not sure I agree:
• The wedge shape and side glueing should be strong
• If you design the depths close enough (and I think I can get there after a few trials), then using epoxy to glue them in should easily span a, say, 1mm or so gap at the bottom.
• One could also do what Nakashima did (and apparently Mira Nakashima still does), which is to screw the bowties in from the bottom.

 

[luvmytoolz]

Thinking about this some more, it's not making that much sense. I think the Vee Bit are used for the inlays as well as lettering, and the tapered bits are also used for delicate lettering. So, in some applications you're using creating a V-channel for both bits. At any rate, I do think how the bit's angles are measured should have been consistent, but that ship has obviously sailed.

The taper bits for the most part are generally only good for very light cuts as they have so little cutting edge, especially closer to the tip. I only use them for a finishing pass where I'm skimming 0.25mm-0.5mm max off the surface for a 2.5D carving, or to follow up after routing a channel, to give a taper to one side.

For example, on my silicone molds I machine a taper along only the outside edge in the master, so the molds have the outside walls wider at the base than at the top.

 

[smorgasbord]

The taper bits for the most part are generally only good for very light cuts as they have so little cutting edge, especially closer to the tip

The ones I've got are essentially ball-end mills with tapered sides. I expect they cut like any straight ball-ended mill.

https://www.amazon.com/dp/B086HM5Y3S

 

[luvmytoolz]

1/8th tip, that's quite a decent size, that cutter will definitely move a fair amount of material. The attached pic is an example of the ones I use for finish passes and tapering sides. The tips vary between 0.1mm-0.5mm radius.

 

[smorgasbord]

So, now my question is which angle is best:
• 2.4º
• 1.0º
• 0.1º

I'm thinking for the 0.1º I'd just cut the mortise flat and then taper the bowtie plug to 0.1 just for insertion ease. Might not even notice that.

But, I'm guessing for what I want, the 1.0º will give me the tightest joints with the least hassle.

 

[luvmytoolz]

I'd agree the 1 deg would give you the least hassle as 2.4 is just too much I reckon, but if you're cutting these out on the CNC I'd go the 0.1 deg, but I'm used to doing mine at 90 deg and finessing and you probably don't want that drama.

 

[Cheese]

As a starting point, why not measure the angle of a standard plug that you cut from wood using a plug cutter? Those give a nice tight fit...I'd guess it's probably 1º or less.

 

[rmhinden]

I haven't done this in a while, previous efforts were to cut the bowties on bandsaw, and then use a router to clean most of the mortices and clean it up with a chisel.

If doing it now, I would use my Shaper origin for both. I would make a design and first cut the bowties out with an outside cut. All the same size. Then use the same design using an inside cut and adjust the offset to get the fit I wanted. Adjusting the offset gives excellent control over the fit. I don't think an angled cut would be necessary.

Bob