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Yes, I know that this is quite a BORING question (get it ;) ), but I really DIG this kinda stuff (no really).

If you have watch an action children's show or a cartoon of some kind, we have all seen that moment when they use some large drill to tunnel under the earth or go dig under a wall to get somewhere of some do-thingy like that. In TV-shows etc they usually look like this:

A giant drill-like boring machine

But thing is, I remember that I was quite shocked to learn when growing up that THIS is what REAL bores looked like (and actually they do look quite BORING):

A flat-faced boring machine, used to dig Seattle's link light rail

Hey!? What's going on? Is there a physics or engineering (which I presume) reason why we use this big flat bore instead of the pointy one? I was going to use one in my story/short film but I was conflicted — obviously the pointy one is cooler, but it is kinda based on science soo....

In my quick little evaluation it doesn't seem like either is really worse off, except perhaps maybe the pointy one might become dull at the tip, but is that a really big issue, and would it not be prevalent in the flat one too?

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Is there a physics or engineering(which i presume) reason why we use this big flat bore instead of the pointy one?

A minimum area cutting surface is best in hard rock; so disk beats cone.

If memory serves, the flat face of the real tunnel-boring machines is pressed -- with enormous force -- against the rock face, as it rotates. The cutting wheels and teeth generate huge, localized pressures, to fracture away bits of the rock face. The flat, disc face has the minimum area and can be made strong and simply. The real one also has strong hydraulic jacks that grab the already-bored tunnel walls, to ensure that the cutting disc rotates, rather than the body of the machine spinning uselessly. (They also have an impressive system for conveying the ground-up rock, etc. out the back and away.)

A grooved, conical, cutting 'nose' as in the illustration might work in soft soil -- if that machine had something to prevent the body from counter-rotating and a means to move the pulverized soil out behind it.

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  • $\begingroup$ "minimum area cutting surface is best" but the pointy one has WAYY less surface area right?? $\endgroup$
    – John Hon
    Commented Jan 24, 2017 at 13:23
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    $\begingroup$ Nope, the cone has more surface area. Search for the formulae and you'll see. Intuitively: for every narrow pie slice of the disc (say 1 degree), the corresponding area on the cone is a triangle with the same base, but a much longer height, via Pythagorean Th. Then integrate around the disk (or cone) to get total area. $\endgroup$
    – Catalyst
    Commented Jan 24, 2017 at 13:28
  • $\begingroup$ i do understand ur point but i guess i was thinking about the penertration point (the tip). I guess my logic oculd be a little foul $\endgroup$
    – John Hon
    Commented Jan 24, 2017 at 13:30
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    $\begingroup$ @JohnHon The pointy one sure will have lower surface area when first getting started, but after you've drilled in at least the length of the cone, the entire cone will be in contact with the bore hole walls. $\endgroup$
    – Shufflepants
    Commented Jan 24, 2017 at 16:22
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    $\begingroup$ also note that most of the light blue parts on the real machine are replaceable (from inside the machine) and wear out quite regularly. and it runs at about 1/1000 th the speed of the cartoon drill $\endgroup$
    – simpleuser
    Commented Jan 24, 2017 at 19:24
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Is there a physics or engineering(which i presume) reason why we use this big flat bore instead of the pointy one?

Yes. A bore is essentially a really big drill bit. As such, the shape of the bit is determined by the material you are cutting.

The cartoon bore has a twist bit that has a high twist rate resulting in a large surface area for the swarf (the stuff being removed) to pass by. This will result in lots of adhesion and will cause galling. Large amounts of lubrication will be required to prevent this: water, oil, etc. anything to keep the rock from sticking to the drill bit. The real-world bore uses a conveyor belt to move the swarf rather than a spiraled groove (which is just an Archimedes Screw).

The cartoon bore also has very acute point angle. This is great for drilling through soft materials (clay as James points out), but is terribad at drilling through hard materials like rock. Take a trip to a hardware store and look at the difference between drill bits for wood vs. those for metal or concrete. The angle will determine the surface area faced by the cutting edges, and more surface area equals more friction. Friction creates heat, and heat causes more wear and tear. This will again require more lubrication to draw off that excess heat, but how would you pump all that lubricant to front of a pointy drill bit?

So while the cartoony bore might work, it would wear out quickly, require more lubrication to prevent galling, and ultimately be much more expensive to run (as Molot points out) because you would constantly be replacing a giant bit.

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    $\begingroup$ Which is exactly why bad guys use it! They need something that wears out quickly so their plots can be foiled by mechanical failures :D $\endgroup$
    – Doktor J
    Commented Jan 24, 2017 at 15:49
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    $\begingroup$ Probably want to explain "galling" $\endgroup$
    – R.M.
    Commented Jan 24, 2017 at 17:15
  • $\begingroup$ @DoktorJ - International Rescue are the good guys and their Mole is pointed. $\endgroup$
    – APC
    Commented Jan 26, 2017 at 13:46
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First of all in the cartoon one is based on a drill that you use on a soft material that is consistent. Like wood or plexi. The idea behind this is that you have a room for the shavings so they don't block your tunnel (because there is no tunnel) The real one is build to drill through materials of different density at the same time while the shaving can pass inside the drill to get moved on a belt transmission.
This design also help cooling the drill while the regular one don't have any cooling.

And to point out the obvious thing - the cartoon machine is bigger than the drill so it would be useless.

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    $\begingroup$ /based on a drill/ so viewers will recognize what it is. Plus it is pointed and sharp so it looks more wicked. $\endgroup$
    – Willk
    Commented Jan 24, 2017 at 14:49
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    $\begingroup$ @Will because they're bad guys! You can't have a bad guy with insidious instruments of crime that look dull and mundane! $\endgroup$
    – Doktor J
    Commented Jan 24, 2017 at 15:48
  • $\begingroup$ @DoktorJ You can't have a superhero with normal-looking toys either. Imagine the Batcave with a regular desktop PC, keyboard, mouse and three or even six (two rows of three each) monitors. Just wouldn't be the same, would it? $\endgroup$
    – user
    Commented Jan 25, 2017 at 13:09
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Lots of issues here that make flat better than pointy...most of which can actually be boiled down to a single idea... surface area.

  1. More surface area (a cone) generates more heat.

    • More heat requires more cooling equipment and more maintenance
    • More heat reduces the life of the cutting edges... Heat > Steel
    • More heat is bad for workers underground in a confined space

  2. Geologic stability

    • A cone is going to cause cave-ins more often that the flat guy. The machine (as mentioned elsewhere) is designed to support the weight of the earth it removes until supports can be placed. The cave ins are going to cause you to have to dig, and then re-dig as things fall from above.

The only situation where I can see a cone working better would be a situation where for some reason you want to dig a tunnel into clay...but...why would you want to do that?

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    $\begingroup$ Fairly certain tunneling engines intended for clay has the same flat front. $\endgroup$
    – Taemyr
    Commented Jan 24, 2017 at 18:56
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The cartoons leave out a very important fact: To make a hole, you have to remove a "hole-shaped" volume of material to some place else.

Wood is fibrous, which means it actually contains quite a lot of air spaces, so if you are inserting a fairly small wood screw, you are just squashing those spaces without actually "removing" any of the wood. The fact that the wood is then trying to spring back and fill up the hole improves the grip on the screw. It is much easier to drive a screw into a wood like pine which contains a lot of air space, compared with a something like oak which has less.

But to make a larger hole that will stay open on its own, you have to remove the material somehow. The cartoon boring machine doesn't have any way to do that, except by material travelling up the spiral grooves, which inefficient because of the amount of friction between the moving material, the unbored material, and the borer itself.

In the real tunneling machine, there are holes in the cutting face along the edges of the cutters, so the cut material can fall through to the inside of the machine, and then be removed with the minimum amount of effort so long as you can remove it as fast as you are cutting more material.

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I know this question is more on the physics, but this is the Worldbuilding stack so I want to help you with your writing problem.

I recommend looking at how "Avatar the Last Airbender" handled this Issue in the Episode "The Drill" . It is a childrens Cartoon, and their Drill is a mixture of the two.

It is almost flat at the front, but still has a little pointyness to it. It looks more menacing then the real one you posted, and is more realistic then the comic one you posted. enter image description here

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  • $\begingroup$ Avatar....the last good thing to come from Nickelodeon......and that is an epic drill/bore/machine....I want one $\endgroup$
    – NZKshatriya
    Commented Jan 25, 2017 at 23:45
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If you look at old hand auger bits (as used with a brace and bit), they had a tapered screw-like thread on the end and then a body with a helical rib. The helical rib was obviously to carry the cuttings away from the work area, and, to the casual observer, the screw-like end did all the cutting. Only on close inspection was it clear that some chisel-like surfaces at the junction between the tip and the spiral were what actually performed the cutting.

Even more confusing is a bung auger, where a short auger bit (of the above style) is combined with a conical wood plane.

It is easy to see how a person who did not actually work with such implements would come to believe that the pointy-ended device in the first image would make sense.

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In cartoons they are drilling through 2D ground. It's like piercing paper. You're squeezing the ground sideways rather than around and behind.

Therefore they don't have to worry about swarf or friction and the pointier the better as it'll move you forward faster.

Depending on the physics of the cartoon universe, the ground may backfill/unsqueeze after you've passed or not.

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