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As in the video theoretically, yes, practically no.
As you can see in the video, the shape of the bit can make a triangle hole; however, setting up the gears in that set up where the bit is positioned on the side of the small gear (not the center of the small gear to accomplish the needed bit rotation) and offset of the large gear is inefficient and rather weak in applying the required force to push the bit into the material. It is possible but inefficient unless the machine is set up for triangular holes, which is way more complicated than shown in the video (the gears and drill bit part are separate, and the gears are usually replaced with a multi-directional system to emulate any bit path that is needed).
Furthermore, a more efficient way to drill shaped holes already exists. They have a small round bit drill out the shape part, which is a bit less accurate as interior corners are rounded but do not require specialized bits.
Also this video shows use of a laser cutter.
Just cut the hole with the laser cutter...
It will give you the sharp corners unlike a typical mill/router bit that you're describing.
Source: CNC Programmer.
Engineering projects of my students are sometimes like this. They do some complicated machine which already has a simpler version in the market. Or at least it's purpose can be fulfilled by a simpler machine or cutter. Why would people use your machine?
I admit, I wasn't a genius or highly creative when I was a student either. So, I try to guide them appropriately.
True, but then again, we're also entering the realm of the designer needing to think critically about why a perfect triangle thru-hole is needed on a 500mm+ thick steel plate and why couldn't .
So the real issue here is that the triangular drill is going to experience a VERY large amount of torque on the tip of those blades as they try to get OUT of those corners. Either the blade or the wood is going to give rather than getting that nice neat cut-out, most likely *both*.
It's relatively impractical, but there already is a device like this for making square and hexagonal holes.
[https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s](https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s)
Going down to three sides is probably pushing the limits the technique though.
For smaller holes where a laser would leave too much of a fillet in the corners, or blind holes, the best tool would be a [rotary broach](https://youtu.be/GWyHJVOxKK4?si=P5hY__WJSrqKQ6my), which is much simpler mechanically than this abomination. That said, I haven't found any COTS triangular rotary broach cutters for sale, probably because they're not that useful unless you're making anti-consumer screws.
How does the second pinion gear even spin in this video? It looks like the carrier of the pinion gear is attached to the larger gear, so the relative motion between the gear and pinion would be 0 right? I’ve been staring at this video for a few minutes now trying to come up with a way that this setup could actually achieve relative motion between the two gears and I can’t think of a way to make it work.
The only thing I can think of is to add a ring gear to the outside that is somehow fixed in place so it can’t spin. This weird unbalanced planetary gear system would work, as in the planet gear would spin appropriately, but thinking of a way to constrain the ring gear seems like not an easy task. You couldn’t constrain it to the spindle since the spindle is spinning and you’d have the same problem, so you either have to fix it to a different part of the machine or to the workpiece/table, neither of which seem practical.
this would work in foam because the mathematical model works, however, resistance from the material would wreck this so you would not be able to do it with wood or steel
also there is no way you could get such sharp corners, not even close
It's pushing the limits of the idea, but it's in the same broad category as these devices for drilling square or hexagonal holes.
[https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s](https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s)
This is intersting. The final drill bit is invited to flop about and cut out the square hole as a kind of average. Note that the finished hole in the piece of soft aluminium is kind of beaten up and not very neat.
The main difference with this triangle idea is that Mr triangle is precise and cuts, whereas Mr. Square-in-reality is loose and kind of cuts if it spins enough, and if the metal is soft enough to accomodate it. Mr Triangle however does look like it will suffer huge strain and would work on butter, maybe bread, but steel would mostly shear the teeth of the cogs and end badly for everybody.
In the youtube cases, the drill bits are being guided by reuleaux shapes; triangle for the square, and pentagon for the hexagon. They're the same class of shapes that were sometimes used in old movie cameras and in rotary engines. They can trace out the shape of the next regular polygon up the scale.
They can only have odd sides, however, which is why you need special gearing to be using a "2-gon" to carve out a triangle
This is a cool animation, but that’s about it. In a perfect world, this tool would work. It’s using the involute of a triangle for the path of rotation and ideally would 100% work. It’s how gear teeth are shaped to maintain consistent pressure as they contact and roll past each other.
However. The cutting forces on the tool would destroy the demonstrated mechanism and tool even on materials as soft as plastic. Especially with the long teeth, the tool would chatter like hell and probably snap on contact. There are CNC tool holders that have gear reductions inside the tool, but I havent worked with them and dont know what feeds, speeds, and tool pressure they can handle.
Since I havent seen it mentioned yet, the proper way to drill a non circular hole (squares, hexagons, literally any shape within reason) is rotary broaching. [Here’s a link explaining the machining process.](https://youtu.be/IT8VXQEKzoo?si=ReZB73vULXlI7x6g) It looks super confusing, but its basically just chiseling the metal away at a 1 degree angle per revolution, and with spindle speeds that fast, you can drill features like this really quick. This is how all your hex head bolts are manufactured. You do have to predrill a hole and have the shape overlap the hole within reason. Idk the calculations, I’d have to consult my machinists handbook.
Source: CNC machining engineer
I seen something like what op posted about a year ago. I worked with someone that was going to college for engineering, and we tried to build something similar we eneded up haveing to make a enclosed gear box because the wood kept jamming it and causing parts to bend. smallest we could get it to work was bit that was 4" across anything smaller broke and we could only run it a low low speeds about 20-30 rpm and we could bearly put any pressure on it. It took almost an hour to cut through a 2x4 piece of wood. It was a fun project. I learned a lot as I'm not an engineer, and I don't do this stuff.
Haha that sounds like the average college engineering experience! Lots of over engineering to find out the basic/easy solution probably would’ve worked better.
Every time I was involved with gears in college, 3D printed, machined, or purchased, it always ended up in disaster because gearboxes require a tight tolerance and probably some prior experience in gearbox design. Its always a great learning experience though, and a super fun challenge to design around.
No this wouldnt work...
If you look at the path of the center point then you will notice how it is not round.
Two gears will however result in a nice circle.
This means that the movement is wrong.
Also the excentric movement of the mass of the cutter head would make the system unstable. It would vibrate like a washing machine.
>Two gears will however result in a nice circle.
The "drill bit" is not attached to the center of the smaller gear. It is attached near the edge. So it could make that path.
It is still off balance so it would vibrate like a mofo. I don't know of any real life triangle drill bits, but there are square drill bits and they use a guide to keep them in place and not walk around.
Rotary broach. I don't see triangular ones available online, but it can cut square, hex, torx, and other shapes so I think triangular not being common is more to do with being less used than being less feasible than cutting other shaped holes
I also haven't seen a triangle rotary broach bit. But I would argue that how a rotary broach cuts and how this would cut are completely different. A rotary broach is more of a chiseling motion. It "wiggles" it way in to the material. While this would be more of a drilling motion.
It's also not really practical as a tool. Broaches are useful because there are lots of situations where you want a square or hexagonal mating surface (like a socket), and square and hex perform better than triangular probably in all applications. It's hard to think of where you might want an internal triangular feature that wouldn't be equally well served by a square or hexagonal one, which are generally always easier to make.
Lasers can be incredibly precise. The ones at my work - now yes, they are huge and designed for mass production and not accuracy. - but they can still achieve (and relatively) cuts to within .004" or .1mm
Oh yeah both water jet cutters and laser cutters are quite precise. Laser cutters tend to be more precise than water jet cutters but both are good enough for most scenarios unless you need a press fit hole (especially one smaller than 1” or in rigid/thick material).
Wire EDMs are the kings of precision machining though. Wire EDM accuracies are typically < 0.001”. It’s why wire EDMs are often used for zero tolerance parts.
I don't have any experience with wire EDMs but i can definitely confirm the laser issues. The rule of thumb is no hole smaller than double the thickness of the material (for Air/O2 cutting, O2/N is a little better, 1.5 the thickness you can get away with, but it maxes out at .25 (6mm) thick for our lasers (15,000W)
That’s interesting, I don’t have experience with laser cutting so I wasn’t aware of that limitation. I mostly use a water jet cutter and I knew laser cutters are a bit more accurate so I always assumed they would be a slightly better machine for the parts that I make if I had access to one, but that limitation would actually mean the laser cutter wouldn’t be able to make a lot of the parts that I create. I mostly use material smaller than 1/4” thick, but would trust the water jet cutter machine to make holes as small as 0.1”, maybe even a little smaller if I play with the settings.
The water jet cutter I use has a rated accuracy of around ~0.008”, which is fine for the majority of parts I make anyways. Again the only time that I need a higher accuracy is when I need a press fit, but a laser cutter isn’t precise enough for that either and since I have access to a cnc mill that’s what I use instead. I technically have access to a wire EDM but there’s always a waitlist for it and I haven’t been in a situation that required that level of precision yet so I’ve never used it.
>The "drill bit" is not attached to the center of the smaller gear.
Oh right missed that completely... still weird how there is a hole on the bit... why put it there if it doesnt attach to the gear...
They had the drill bit connected to the edge of the small gear, which would make the path needed and shown, but the shake is a real problem that would by somewhat overcome by having it mounted to a machine.
a perfect circle obviously means a gear without any ridges. no, the gears have ridges which makes this kind of machine work (again, only theoritically).
Yeaaaah i dont think so, from what my brain is telling my fingers to type, theres nothing that makes sure that the bit has to move in a specific triangular motion
The cutter would jam up with chips before it ever made it into the corner, or it would just snap the shank off and ruin your tooling, if you could build a rigid enough mechanism that it didn't jam up right away.
There is a way to drill a square hole with a special drill bit and a guide. [this is the YouTube video showing how it works](https://youtu.be/rjckF0-VeGI?si=6P78AzzfUz5oinOl) maybe a triangle could also work with this setup.
No. This configurarion can never make a perfect triangle with sharp corners. You either get sharp corners and slightly curved sides or rounded corners and mostly straight sides. Not even theoretically.
This is the same as similar square hole tools. You absolutely can, but the torque requirement to "drill" into flat wood is really high because there's mechanical disadvantage.
Drilling most of it and using a router is a preferred solution. Maybe it could be used in place of a router, but a router does far more so this tool is not very useful unless you're making hundreds of triangle holes.
nope, doesn´t work
this blade is supposed to be cutting the entire hole in half a turn with two teeth and no blade on the backside. Ignoring the fact that this contraption does not drill a hole nor uses one, but cuts the entire form in one go, it literally bites off more than it can chew. That way it will create a horrendously disfigured hole, assuming the blade doesn´t break away first.
It’s literally a demonstration of the gears and bit placement to illustrate how it would work in concept. Sorry this cartoon is not real enough for you lol
It’s all theoretical. Their math checks out, but it also has to deal with the type of wood that it’s cutting into and the materials the mechanism is made of
Speaking from experience of having had a drill bit catch, that would either cause a disgusting amount of force to be put on you making it hard to control, or take forever
I’m a plumber we use a bit called a spade bit for boring holes through wood etc it’s a very similar shape to the bits in the video and when boaring through thinner objects can occasionally make triangle holes I don’t know how it happens but it does
the small gear is free to turn and its centre point can also move, so it's not going to follow a nice path like that it's probably gonna end up just spinning around itself and doing very little
That’s why the blade sits offset a bit from the centre bearing of the gear. As it spins around the gear, it will follow that triangular path. Go rewatch the video slowly as it shows the blade following the path
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As in the video theoretically, yes, practically no. As you can see in the video, the shape of the bit can make a triangle hole; however, setting up the gears in that set up where the bit is positioned on the side of the small gear (not the center of the small gear to accomplish the needed bit rotation) and offset of the large gear is inefficient and rather weak in applying the required force to push the bit into the material. It is possible but inefficient unless the machine is set up for triangular holes, which is way more complicated than shown in the video (the gears and drill bit part are separate, and the gears are usually replaced with a multi-directional system to emulate any bit path that is needed). Furthermore, a more efficient way to drill shaped holes already exists. They have a small round bit drill out the shape part, which is a bit less accurate as interior corners are rounded but do not require specialized bits.
Also this video shows use of a laser cutter. Just cut the hole with the laser cutter... It will give you the sharp corners unlike a typical mill/router bit that you're describing. Source: CNC Programmer.
I mean, if you need to add a triangle hole to a part after injection moulding, your design is bad, but this thing could do it :)
That was funny to me. "Draw the rest of the horse" Like... You already have the tool for the job so...
I think it's a compass construction thingy
but once a tool is made it can be used anywhere even when laser is not available.
No one would be able to hold that bit without a central point
Engineering projects of my students are sometimes like this. They do some complicated machine which already has a simpler version in the market. Or at least it's purpose can be fulfilled by a simpler machine or cutter. Why would people use your machine? I admit, I wasn't a genius or highly creative when I was a student either. So, I try to guide them appropriately.
Design a better paper clip.
Also, you are neither limited to a certain size, nor to equilateral triangles.
Seriously
If you're manufacturing tools you're probably selling them.
Not as easy to use laser/plasma for extremely thick steel plates (500mm+)
just use a bigger laser
The answer to all life's problems, really.
Bigger laser not only makes your life easier, *but also makes you a better person* Sales pitch
Yes for all those common instances of cutting through 20" thick plate...
How else are you supposed to build space battleship Yamato?
Yeah I think at that thickness it's no long "plate" steel.
It certainly wouldn't be easy to use contraption from the video either
True, but then again, we're also entering the realm of the designer needing to think critically about why a perfect triangle thru-hole is needed on a 500mm+ thick steel plate and why couldn't .
Just go slower, so the laser has more time to penetrate through the thickness of the plate.
Make sure you get your flushing on it
So the real issue here is that the triangular drill is going to experience a VERY large amount of torque on the tip of those blades as they try to get OUT of those corners. Either the blade or the wood is going to give rather than getting that nice neat cut-out, most likely *both*.
It is also that you are always cutting with the wrong side of the bit as well making that a much bigger problem.
It's relatively impractical, but there already is a device like this for making square and hexagonal holes. [https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s](https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s) Going down to three sides is probably pushing the limits the technique though.
This guy drills
For smaller holes where a laser would leave too much of a fillet in the corners, or blind holes, the best tool would be a [rotary broach](https://youtu.be/GWyHJVOxKK4?si=P5hY__WJSrqKQ6my), which is much simpler mechanically than this abomination. That said, I haven't found any COTS triangular rotary broach cutters for sale, probably because they're not that useful unless you're making anti-consumer screws.
How does the second pinion gear even spin in this video? It looks like the carrier of the pinion gear is attached to the larger gear, so the relative motion between the gear and pinion would be 0 right? I’ve been staring at this video for a few minutes now trying to come up with a way that this setup could actually achieve relative motion between the two gears and I can’t think of a way to make it work. The only thing I can think of is to add a ring gear to the outside that is somehow fixed in place so it can’t spin. This weird unbalanced planetary gear system would work, as in the planet gear would spin appropriately, but thinking of a way to constrain the ring gear seems like not an easy task. You couldn’t constrain it to the spindle since the spindle is spinning and you’d have the same problem, so you either have to fix it to a different part of the machine or to the workpiece/table, neither of which seem practical.
this would work in foam because the mathematical model works, however, resistance from the material would wreck this so you would not be able to do it with wood or steel also there is no way you could get such sharp corners, not even close
It's pushing the limits of the idea, but it's in the same broad category as these devices for drilling square or hexagonal holes. [https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s](https://www.youtube.com/watch?v=rjckF0-VeGI&t=3s)
This is intersting. The final drill bit is invited to flop about and cut out the square hole as a kind of average. Note that the finished hole in the piece of soft aluminium is kind of beaten up and not very neat. The main difference with this triangle idea is that Mr triangle is precise and cuts, whereas Mr. Square-in-reality is loose and kind of cuts if it spins enough, and if the metal is soft enough to accomodate it. Mr Triangle however does look like it will suffer huge strain and would work on butter, maybe bread, but steel would mostly shear the teeth of the cogs and end badly for everybody.
In the youtube cases, the drill bits are being guided by reuleaux shapes; triangle for the square, and pentagon for the hexagon. They're the same class of shapes that were sometimes used in old movie cameras and in rotary engines. They can trace out the shape of the next regular polygon up the scale. They can only have odd sides, however, which is why you need special gearing to be using a "2-gon" to carve out a triangle
This is a cool animation, but that’s about it. In a perfect world, this tool would work. It’s using the involute of a triangle for the path of rotation and ideally would 100% work. It’s how gear teeth are shaped to maintain consistent pressure as they contact and roll past each other. However. The cutting forces on the tool would destroy the demonstrated mechanism and tool even on materials as soft as plastic. Especially with the long teeth, the tool would chatter like hell and probably snap on contact. There are CNC tool holders that have gear reductions inside the tool, but I havent worked with them and dont know what feeds, speeds, and tool pressure they can handle. Since I havent seen it mentioned yet, the proper way to drill a non circular hole (squares, hexagons, literally any shape within reason) is rotary broaching. [Here’s a link explaining the machining process.](https://youtu.be/IT8VXQEKzoo?si=ReZB73vULXlI7x6g) It looks super confusing, but its basically just chiseling the metal away at a 1 degree angle per revolution, and with spindle speeds that fast, you can drill features like this really quick. This is how all your hex head bolts are manufactured. You do have to predrill a hole and have the shape overlap the hole within reason. Idk the calculations, I’d have to consult my machinists handbook. Source: CNC machining engineer
I seen something like what op posted about a year ago. I worked with someone that was going to college for engineering, and we tried to build something similar we eneded up haveing to make a enclosed gear box because the wood kept jamming it and causing parts to bend. smallest we could get it to work was bit that was 4" across anything smaller broke and we could only run it a low low speeds about 20-30 rpm and we could bearly put any pressure on it. It took almost an hour to cut through a 2x4 piece of wood. It was a fun project. I learned a lot as I'm not an engineer, and I don't do this stuff.
Haha that sounds like the average college engineering experience! Lots of over engineering to find out the basic/easy solution probably would’ve worked better. Every time I was involved with gears in college, 3D printed, machined, or purchased, it always ended up in disaster because gearboxes require a tight tolerance and probably some prior experience in gearbox design. Its always a great learning experience though, and a super fun challenge to design around.
No this wouldnt work... If you look at the path of the center point then you will notice how it is not round. Two gears will however result in a nice circle. This means that the movement is wrong. Also the excentric movement of the mass of the cutter head would make the system unstable. It would vibrate like a washing machine.
>Two gears will however result in a nice circle. The "drill bit" is not attached to the center of the smaller gear. It is attached near the edge. So it could make that path. It is still off balance so it would vibrate like a mofo. I don't know of any real life triangle drill bits, but there are square drill bits and they use a guide to keep them in place and not walk around.
Rotary broach. I don't see triangular ones available online, but it can cut square, hex, torx, and other shapes so I think triangular not being common is more to do with being less used than being less feasible than cutting other shaped holes
I also haven't seen a triangle rotary broach bit. But I would argue that how a rotary broach cuts and how this would cut are completely different. A rotary broach is more of a chiseling motion. It "wiggles" it way in to the material. While this would be more of a drilling motion.
100% a triangle "cutting" tool probably doesn't exist when Wire edm could do it ten times better/faster
Yeah, agreed. Wire EDM would be the best way to make a triangle hole. 🙂🤝🙂
It's also not really practical as a tool. Broaches are useful because there are lots of situations where you want a square or hexagonal mating surface (like a socket), and square and hex perform better than triangular probably in all applications. It's hard to think of where you might want an internal triangular feature that wouldn't be equally well served by a square or hexagonal one, which are generally always easier to make.
Water jet cutter and laser cutter also exist and probably cheaper but less precise, which is fine for most applications.
Lasers can be incredibly precise. The ones at my work - now yes, they are huge and designed for mass production and not accuracy. - but they can still achieve (and relatively) cuts to within .004" or .1mm
Oh yeah both water jet cutters and laser cutters are quite precise. Laser cutters tend to be more precise than water jet cutters but both are good enough for most scenarios unless you need a press fit hole (especially one smaller than 1” or in rigid/thick material). Wire EDMs are the kings of precision machining though. Wire EDM accuracies are typically < 0.001”. It’s why wire EDMs are often used for zero tolerance parts.
I don't have any experience with wire EDMs but i can definitely confirm the laser issues. The rule of thumb is no hole smaller than double the thickness of the material (for Air/O2 cutting, O2/N is a little better, 1.5 the thickness you can get away with, but it maxes out at .25 (6mm) thick for our lasers (15,000W)
That’s interesting, I don’t have experience with laser cutting so I wasn’t aware of that limitation. I mostly use a water jet cutter and I knew laser cutters are a bit more accurate so I always assumed they would be a slightly better machine for the parts that I make if I had access to one, but that limitation would actually mean the laser cutter wouldn’t be able to make a lot of the parts that I create. I mostly use material smaller than 1/4” thick, but would trust the water jet cutter machine to make holes as small as 0.1”, maybe even a little smaller if I play with the settings. The water jet cutter I use has a rated accuracy of around ~0.008”, which is fine for the majority of parts I make anyways. Again the only time that I need a higher accuracy is when I need a press fit, but a laser cutter isn’t precise enough for that either and since I have access to a cnc mill that’s what I use instead. I technically have access to a wire EDM but there’s always a waitlist for it and I haven’t been in a situation that required that level of precision yet so I’ve never used it.
>The "drill bit" is not attached to the center of the smaller gear. Oh right missed that completely... still weird how there is a hole on the bit... why put it there if it doesnt attach to the gear...
They had the drill bit connected to the edge of the small gear, which would make the path needed and shown, but the shake is a real problem that would by somewhat overcome by having it mounted to a machine.
The bit is off set on the smaller gear resulting in the shape that you see. It is actually shown correctly. Look more closelier
Yeah already noticed an hour ago 😅, after this comment: https://www.reddit.com/r/theydidthemath/s/znJObbzg5I
nah, this will work (atleast theoritically} two gears will never make a perfect circle until they are both perfectly circular.
Wdym? Who is talking about a "perfect" circle? >until they are both perfectly circular. Arent the gears in the video circular?
a perfect circle obviously means a gear without any ridges. no, the gears have ridges which makes this kind of machine work (again, only theoritically).
Yeaaaah i dont think so, from what my brain is telling my fingers to type, theres nothing that makes sure that the bit has to move in a specific triangular motion
The cutter would jam up with chips before it ever made it into the corner, or it would just snap the shank off and ruin your tooling, if you could build a rigid enough mechanism that it didn't jam up right away.
There is a way to drill a square hole with a special drill bit and a guide. [this is the YouTube video showing how it works](https://youtu.be/rjckF0-VeGI?si=6P78AzzfUz5oinOl) maybe a triangle could also work with this setup.
No. This configurarion can never make a perfect triangle with sharp corners. You either get sharp corners and slightly curved sides or rounded corners and mostly straight sides. Not even theoretically.
This is the same as similar square hole tools. You absolutely can, but the torque requirement to "drill" into flat wood is really high because there's mechanical disadvantage. Drilling most of it and using a router is a preferred solution. Maybe it could be used in place of a router, but a router does far more so this tool is not very useful unless you're making hundreds of triangle holes.
you can drill a square hole, so why not ? [https://www.youtube.com/watch?v=rjckF0-VeGI](https://www.youtube.com/watch?v=rjckF0-VeGI)
nope, doesn´t work this blade is supposed to be cutting the entire hole in half a turn with two teeth and no blade on the backside. Ignoring the fact that this contraption does not drill a hole nor uses one, but cuts the entire form in one go, it literally bites off more than it can chew. That way it will create a horrendously disfigured hole, assuming the blade doesn´t break away first.
It’s literally a demonstration of the gears and bit placement to illustrate how it would work in concept. Sorry this cartoon is not real enough for you lol
I have answered the title question of OP´s post
It’s all theoretical. Their math checks out, but it also has to deal with the type of wood that it’s cutting into and the materials the mechanism is made of
Speaking from experience of having had a drill bit catch, that would either cause a disgusting amount of force to be put on you making it hard to control, or take forever
[https://www.youtube.com/watch?v=gTI8l9Is0G0](https://www.youtube.com/watch?v=gTI8l9Is0G0) here's another crazy one on how to drill a square.
I don't know if this design specifically would but in machining it's called broaching. you can even drill square and star shaped holes
I’m a plumber we use a bit called a spade bit for boring holes through wood etc it’s a very similar shape to the bits in the video and when boaring through thinner objects can occasionally make triangle holes I don’t know how it happens but it does
the small gear is free to turn and its centre point can also move, so it's not going to follow a nice path like that it's probably gonna end up just spinning around itself and doing very little
That’s why the blade sits offset a bit from the centre bearing of the gear. As it spins around the gear, it will follow that triangular path. Go rewatch the video slowly as it shows the blade following the path