Knives Hewn from Brake Discs Past

Knives are tools that rely heavily on material quality to do their job right. A knife made of cheap steel won’t hold an edge well, and blunt knives are more likely to cause injury, or at the least, be more difficult to use. The trick to making a good knife is to start with good material. Disc brakes just so happen to be a great source of cast iron, and are readily available, so [Diesineveryfilm Customs] has machined a knife out of a brake disc.

The first step is to roughly cut out the knife’s form from the disc. It’s easy enough to cut out with an angle grinder, following up with a belt sander to finish up the grip. After sharpening, the sharp blade is taped off for safety while a wooden grip is added. Holes are drilled in the brake rotor, allowing the wooden parts to be pinned and glued together before a trip to the belt sander for shaping. A string and dye are added to the handle as finishing touches.

It’s a great use of high-quality scrap material to produce a useful tool. An earlier disc brake knife video shares some useful techniques of its own – we liked the shortcut of measuring the disc thickness, then using a matching drillbit to mark the centerline for sharpening.

Perhaps your own knives aren’t sharp enough – check out this home-built adjustable sharpening rig.

53 thoughts on “Knives Hewn from Brake Discs Past

  1. Is it good or bad that these were likely coated with asbestos? I guess you could clean them up first but not doing so and then hitting it with a grinder and belt sander might not be the best idea?

    Not as bad as trying to make knives out of brake pads directly (not that such a thing would work) but it’s still worth mentioning some of the hazards here. Also might want to consider the temper of the brake discs as well.

        1. I was about to rubbish this post (as a garage owner I see a lot of brake pads lol) as the only time I have ever seen an asbestos clutch or brake lining (interestingly never a brake pad as disk brakes pretty much came about when asbestos was being phased out) is when stripping an old / classic car. However a quick google shows companies in china advertising asbestos brake pads for modern cars…. I hope they never find there way to my work.

          FWIW asbestos friction materials, pads, shoes, clutches etc are usually visually failrly obvious as often they have a light color to them from light grey to beige to yellowy brown, but different enough that anyone who has done a few sets of brakes will notice the oddity straight away.

          1. A certain car brand in Australia was forced to recall a heap of utes as they had asbestos brakes. The asbestos had been coloured black so it was not at all obvious. As they are cheap in Asia I would not be surprised it a large number of grey market brakes contained asbestos.

          2. Unfortunately you have a lot of less scrupulous competitors who don’t fear skimping on quality to earn an extra buck or 2. Canada still allows importing Asbestos containing materials (for example: http://canada.autonews.com/article/20160926/CANADA/160929879/asbestos-kills-but-its-still-allowed-in-canadian-brake-parts) and I wouldn’t be surprised if many of them find their way into the US and Europe as well.

            Most “car guys” won’t have a problem because they/we pay attention to what parts get put on the car and will opt for good quality from a reputable brand. Many less car savvy people will just accept whatever their mechanic puts on their car, which is often the equivalent of fancy cardboard for the price of a decent set of pads from any other supplier.

      1. Yeah, I survived, lead, asbestos, acid rain, nuke testing and chernobyl fallout, some 60 apocalypses, got a graduate degree before wikipedia and google were a thing, I must be freaking badass :-D

          1. Well I’m an first half gen Xer and will have to survive the both of them…. I’m hoping it will work out in ways like selling a single family home at peak demand, and buying a retirement place for peanuts.

          1. You tried to wash your cat in case it was covered in nuclear fallout? Where do you live, Belarus?

            I remember in Britain, we had to throw a lot of milk out, but cows graze a lot of acres between them, so that concentrates anything that might be absorbed into their bodies.

            Wouldn’t be a problem now they feed farm animals on asbestos, antibiotics, and ground-up sewage.

          2. @greenaum All over Europe and even way over in in Israel people were warned to not go out in the rain and if they were for some reason caught in the rain to wash their clothes and take a shower.
            And the EU said decades later that statistically people in the EU were still dying from Chernobyl. And probably still are.
            And you still can’t eat wild boar in Germany I hear because they eat truffles and those soak up radioactive element.

            Also: the UK government also said BSE was safe until the public forced them to admit it was not. Don’t trust the UK government for health advise.

      2. Yeah cute, not everybody survives, and if you had ever seen someone who got asbestos related cancer you’d be singing a different tune.

        To say you are invulnerable because you didn’t die (yet) is just the summit of silliness.

    1. You mean the temper they get when they heat up at the one or other emergency stop? :-)
      And why should they have any coating? They wear off steadily while in use. And if you really have that old discs which were used with asbestos containing pads and have the fear that some single fibers could hide somewhere, then wash it.

    2. This is a motorcycle disk. I have never seen what we used to call “break dust” on a motorcycle breaking system like you see it on car rims but wash it anyway, can’t hurt.

      Motor cycle break disk have a higher temper (harder) because they (at least the front) have to work far more efficiently than a car break. This is a good thing for making knives anyway.

      Car disks are a lot softer, I think the criteria there is that the disk must wear *slightly slower* than the pads.

        1. Car breaking systems are extremely poor because there is no point to make them any better (for standard vehicles) as standard drivers wouldn’t know how to use better breaks. In a car people just hit the breaks hard and lock up the wheels or rely on the ALB. There is no concept of controlled breaking.

          Locking up the wheels on a motor cycle is no fun! so motor cycle riders have to learn how to control breaking to maintain traction so there is an advantage to better breaks. A motor cycle has two independent breaking systems that are separately controlled. One for each wheel.

          Stronger is a difficult word to use here. The function of a breaking system is to convert momentum to heat. The strength required is determined by the inverse square of the time (or loosely distance) the break has to do this and efficiency it has as well as other variables such as momentum and initial velocity. Breaks also loose efficiency as they heat up so initial temperature, cooling and ambient temperature are factors as well. On top of all this is traction so the quality of tyres also plays an important role.

          So to *NOT* answer your question – it’s simply *not* that simple.

          1. Well, to simplify it: I’ve seen car brake disc video where they glow red hot, but never saw a video of a bike disc doing that.
            Mind you those were not street-standard cars I think, probably juiced up ones like supercars and shit. Still though, do racing bike discs glow like that? Never saw it.

          2. Low end racing cars can have glowing disks as they use a less efficient system. Formula 1 use a different system (forced air flow cooling) and different compounds in both the rotor and pads and the disks don’t have enough thermal mass to glow. Disks glowing red hot is normally an indication of break system failure as breaking efficiency reduces with disk temperature but on a racing car it is more of an indication that the breaks are getting towards their limits.

            I have never seen glowing disks on a racing bike but they do get hot at times, they also cool very rapidly too.

  2. “The trick to making a good knife is to start with good material. Disc brakes just so happen to be a great source of cast iron”

    Cast iron is an awful material for knives. Modern motorcycle disc brakes like he used are usually made from stainless.

  3. Heat treating stainless is a real pain in the ass. You should leave that to pro’s. Better to use 1095 steel if you want to heat treat yourself. Or, if you feel a little more DIY, a farriers rasp. Find it on the YouTubez.

      1. Check out the “Man at Arms” team on the “AWE me” channel on you-tube. I seem to recall they’ve fabricated swords from leaf springs (possibly rail-road ones) and all manner of other random bits of scrap as a way to start with specific metal/carbon levels along with making a few swords all the way from ore.

        Don’t think they’ve made any nunchucks but they’ve done a lot more than just swords.

      1. Hmm. No, not *far* more range. :D The range of each is the distance that you can walk before collapsing, plus the range of the item itself. And unless you can walk only a few meters before collapsing, the range of the item is very small compared to the range of your walk and can be disregarded. So, the range is about the same, unless you are very unhealthy. ;) ;) ;)

        1. This only is true assuming that you are in battle on a clear, flat, open field. In that case you are correct – but from a defensible position, such as a tower or castle, the distance an attacker can walk while injured is irrelevant.

        2. That’s only true if the people you’re “fighting” are completely defenseless, unarmed, and asleep. If your opponent can fight back, it’s much safer to kill him from a distance, than hand to hand. At any range beyond a few metres, crossbow beats sword!

    1. My grandfather made a crossbow with a leaf spring and a gun stock. I remember how awesome I thought it was when growing up but my Dad said it was incredibly difficult to use, so it hung around on the wall for the longest time. Not sure what happened to it actually as I haven’t seen it in at least a couple of decades.

        1. Which is why I recommend them as a mounted weapon in case of zombie emergency, on top or in back of a vehicle, drawn with a 12V ATV winch, and crew served with belt feed sharpened shovels as projectile.

  4. Using steel disks (NOT CAST IRON), leafsprings, etc… for knives? Not my choice. These Steels are designed for a completely different purpose, have moderate carbon Content (say 0.5 to 0.6%), often Si etc.
    This might work for a General purpose outdoor knife. but you will never get a fine and Lasting edge on this.

    Good tool Steels are affordable and much better suited. I would spend the Money and be sure that I put my work into a a high Quality knive.

    Personally I think old files are an excelent source, but you should not grind them with a machine after hardening.
    File: 1.4%C, some grain refinement (do not use any case hardenend material)
    Ball Bearing, often 1,2%C, a bit of chromium, not as fine as files, but excelent for Allrounders
    I personally prefer Tungsten alloy stells 1.2516, 1.2419, 1.2562, it’s a big step up from 1.2008 (files).

    There is better suited scrap material than brake disks!

    Greetings
    GvT

    1. If this project were “How do I make a good knife?”, you’d be right. But it seems like it’s more “What can I do with these old brake discs?”.

      The only reason an ordinary person need to carry a combat knife would be in some sort of zombie-related emergency, so you’d have to make do with what you have. That’s when making knives out of bits of steel lying around would be important. Otherwise it’s just for fun and doesn’t really matter.

      1. What you need is a shield rather than a knife, in case some nutjob pulls a knife (or axe) or the cops come at you a shield is handy. I wonder if a brake disc can stop a small caliber bullet, or at least a rubber bullet.

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