Stopping A Bench Grinder Quickly

In every workshop ever, there’s a power tool that goes unnoticed. It’s the bench grinder. It’s useful when you need it, and completely invisible when you don’t. We take the bench grinder for granted, in part because we keep it over there with that box of oily rags, and partly because it’s so unassuming.

But you can really mess your hands up on a bench grinder. Words like ‘degloving’ are thrown around, and that doesn’t involve actual gloves. For his Hackaday Prize entry, [Scott] is adding safety to the ubiquitous bench grinder. It’s called the Grinder Minder, and it aims to make the humble bench grinder a lot safer.

There are a few goals to the Grinder Minder, most importantly is DC injection braking. This stops the grinder from spinning, and if you’ve ever turned off a bench grinder and waited for it to spin down, you know there’s either a lot of energy in a grinder wheel. Grinder Minder also adds accidental restart protection and an actual ANSI-compliant emergency stop. All of this is designed so that’s it’s a direct drop-in electronics package for a standard off-the-shelf grinder.

The early prototypes for the Grinder Minder have the requisite MOSFETs and gigantic wire-wound resistors , but the team has recently hit an impasse. The current market research tells them the best way forward is designing a product for bigger, more powerful tools that use three-phase power. The team is currently researching what this means for their project, and we’re looking forward to seeing where that research lands them.

54 thoughts on “Stopping A Bench Grinder Quickly

  1. What a fantastic idea!
    I hope there will still be a model for the (single phase) home shop. Would be very welcome on many other common tools: circular saws, chop saws, smaller tablesaws, benchtop/wood lathes, etc.

    1. Don’t a lot of shop tools short the motor leads after disconnecting the power? Electro braking.

      But trying to stop a grinder wheel (instantly, not sure if that is what is being discussed) is how you make grenades.

      1. DC injection braking available from most Variable Frequency Drives is a pulsed PWM signal. The Teco FM50 on my surface grinder is adjustable in how many seconds it will take to spin down the motor. Neat timesaver when you’re constantly swapping/adjusting/measuring parts.

        Standalone DC braking without a VFD or three phase motor is a neat idea.

      2. It’s possibly becoming common on newer tools. Of the ones I own, possibly only the Skil contractor saw (1-2 years old) may do that–I’ll have to check. All of the other secondhand tools and my HF chop saw (>5y old) lack braking, so a retrofit would be a great way to increase safety.

      3. Problem with shorting the leads of a motor is you generate a lot of current in the motor windings and therefore a lot of heat. If it takes several seconds to get the grinding wheel up to full speed, that’s all of the energy over that timeframe dumped into the motor in a fraction of a second. You need some device that can absorb that energy. A super capacitor comes to mind for this application coupled with a traditional break pad.

        Better yet, if one could decouple the flywheel from the grinding surface with differential break, the grinding surface could be made lighter and be stopped safely, while the flywheel can take it’s time to slow down.

      4. shorting the leads on an induction motor will not slow it down much faster, the residual magnetic field is very weak and thus generates only weak EMF.
        DC injection creates a very strong magnetic field and the motor dissipates energy as heat in the rotor from eddy current losses.

        As for the heat being a problem – no. Do a bit of physics math.
        Even if the kinetic energy would be several kJ (which is unlikely, I’d guess a few 100J at most), the rotor would heat up only a few degrees (C or F, doesn’t matter), because it’s fairly heavy chunk of metal.

        Personally I’d be more worried about the mechanical integrity of the stone if you try to stop it too quickly.

        1. Where grinding wheels are concerned there is another problem. Any attempt to put a brake on then must be moderate OR, there needs to be a nut locking system on the shaft. Ifuot then the kinetic energy (especially in a big wheel) will cause the unit to spin offshaft. If not then the kinetic energy (especially in a big wheel) will cause the unit to spin off the shaft.
          D0 we really want several kilos of stone wheel carrering across our workshop? (potentially at several hundred meters /second.

    1. It requires modification of grinder hardware. That’s not always possible. For example in grinder that I have there is no way to add any kind of brake without modifying the chassis, which is one solid piece of metal…

      1. That’s certainly an issue, but there are cutting wheels.

        I think the main difficulty would be the risk of unbalancing the grinder at high speed, potentially causing mayhem.

        Er, also, the whole “vast quantities of iron filings produced by the grinding” thing might not play nicely with electromagnets and neodymium magnets for a braking mechanism.

        Maybe the solution is to lower a 30 gallon barrel over the grinder while it spins down.

    1. Now here’s me thinking of some kind of armature /shaft or even the other wheel as a disc brake affair then along comes Cliff with his ‘if you stop the shaft the nuts will be unscrewed by the Inertia of the wheels and they will inevitability come off, and all that that implies.
      which is probably why bench grinders have remained unchanged since before the beginning of time.

    1. Other than the fact that having a lathe practically necessitates that you also both own and use a grinder, what about the lathe do you think is more likely to kill you?

      1. At school in shop, I saw a friend have their jumper ripped off their back and were thrown over the lathe. They survived, with no permanent injures. This was long ago when safety was far far less than it is today. It is a machine that demands respect.

      1. Spinning brittle disk, which was properly secured is safer than a lathe where a spinning, very hard and massive possibly unbalanced piece of metal is PROBABLY properly secured, this time.

    2. When I was a kid I was lucky enough to have a 6 x 20 metal lathe that dad acquired after a lot of pestering from me :)
      He rigged up a safety cut-out before I was allowed to use it though. This consisted of a metal bar across the front of the lathe cabinet just under the lower edge of the drip tray. The bar had offset ends by 3 or 4 cm which pivoted. At one end there was a microswitch going to a box that had a seal-in (self-latching) relay controlling the motor.
      When standing at the lathe, the drip tray prevented it being triggered accidentally. But by lifting the knee to hit the bar, it would drop out. I was always pretty careful using that lathe (and any other) but I must say I did trip it a few times especially when thread cutting and the cross slide was about to hit the chuck! Thanks Dad, it was a useful thing.

    3. I love the discussion of which tools are the most dangerous! But a lathe, with super-fast spinning parts that have square almost-hooks protruding from its face, is pretty much tops on my list. Maybe second to my home-made, misaligned table saw. Or…

      The most dangerous tool in the shop is definitely linked to what tools are in the shop. In a lot of places, it’s the drill press, which can also do quite a pull-you-in-and-mangle-you number on long sleeves, but which is also so common that you forget about it.

      A woodworker friend of mine claims that in his shop, it’s the chisel.

      1. Wood working powertools that involve sharp teeth usually win this category, because they run FAST and with a lot of inertia, humans are softer then wood, so it chews through flesh with great speed.

        Metal working tools are much beefier, but also slower. Won’t make much of a difference if it actually manages to grab you, but they’re slower ;-)

      2. ” my home-made, misaligned table saw.”

        Years ago I bought a “table saw” at auction ($10).
        When I got it home and started checking it out, I found out that someone had replaced the original motor/arbor with a 7 1/4 inch circular saw!

        I took it apart and still use the circular saw from time to time (it is complete).

  2. In industrial applications I can certainly see the potential for such a safety implantation, but on a consumer level I think it’s not going to be that feasible. I think most consumer bench grinders are single phase brush-less induction motors. When the power is turned off there is no more field being generated and no back signal going into the line. In order to make it work your going to need to monitor the rpm of the shaft and the rotation and apply the reverse synchronized power to just the motor and keep that power out of the mains circuit until the shaft stops spinning. But its going to generate a lot of heat in the winding’s. your $100.00 bench grinder just became $500.00 In my non professional opinion…lol

  3. I had an uncle who made a foot powered bench grinder. He use a long spring (about 1 meter/3 feet) anchored to the roof, the split chain from a bicycle and the sprocketed freewheel from the bicycle. So your foot would pull the chain down against the spring and the freewheel would engage and spin the grinder wheel (and a heavy inline flywheel), Then when you would release your foot the spring would pull the chain back up into position and the freewheel would allow free spin back into position. You would pump the pedal several times to store some energy in the flywheel and do a bit of grinding, rinse and repeat. With 100% of the energy coming from a human, the potential injuries are a lot less if something ever goes wrong.

    1. Any chance for body parts to get caught in the chain, spring, sprocket or flywheel?

      Generally speaking, I’m more afraid of angle grinders than bench ones, especially angle grinders with cutoff wheels.

      1. The spring and chain were behind the machine near the wall. I’m not sure about the flywheel, I don’t remember it being an issue, but then again I last saw the machine about 40+ years ago.

    2. That sounds challenging when it comes to retouching drill bits.
      Inertia is most of the problem when it comes to spinning things, how it became a spinning thing is largely irrelevant if you don’t respect it.

  4. just as a side note – I finished reading this article walked across the kitchen and saw the latest Aldi catalog. this week they are flogging modeling tools including a mini bench grinder.

    The picture shows some numpty taking the edge off a chisel while wearing what appears to be leather welding gloves.

    Either we REALLY need a product like this or we need to remove ALL the safety features from machines and let nature take its course…

    1. These small grinders are low power and low inertia. They can be stopped by hand if you wear a glove. There is no danger to a grown mans hand when wearing a sturdy glove on these.

  5. This also needs a foot pedal to operate, because the brake would be hard to activate with the stumps where your hands used to be.

    Or, any danger with a bench grinder is when you are working with it and your hands are close to the wheel. Not when you’ve turned it off and are walking away from it.

    1. “Or, any danger with a bench grinder is when you are working with it and your hands are close to the wheel. Not when you’ve turned it off and are walking away from it.”

      You might be walking away from it, but I’ve seen grinders take minutes to spin down, and they are almost silent when they do so.

  6. I think something that acts practically instantly like the blade stop safety thingo on table saws is needed.

    Relying on human reaction time to hit the button means your appendages are long gone by the time you’ve even realised you need to hit the button.

    1. The Blade Stop device you mentioned is for emergency deployment only. It stops the blade within one or two milliseconds. It works by firing an explosive bolt through the spindle which destructively stops all rotation. Once fired, the entire spindle assembly must be replaced.

      It’s expensive, but it’s a lot cheaper and easier than surgically replacing a finger.

      It can’t be used for normal on/off activity.

      1. And grinding wheels are usually made from many tiny pieces of abrasive bonded together, where the table saw blade is usually one piece of steel (with carbide teeth added).
        Suddenly, stopping a grinding wheel could cause it to disintegrate, flinging many sharp chunks in at high speed and in many directions.

  7. Either too many either or missing a point. Either Give up the missing point.
    Easily the most non assuming dangerous tool in shop. Looks so simple. Have the same HF 8″ and will attest to its ability to drive a 14mm x20 mm bolt into 1/2″ plywood. Thats with the flimsy see thru cover plates which this one is missing on both sides. Not fond of factory shrouds which have the ability to accelerate and direct projectiles. Suggest building own and beefing up cover plates. And. A vent fan hood.
    Its a sub HP unit and easily stopped mechanically comparatively to gear drive industrial 3/4 HP plus. I think a foot pedal on/off switch would reduce incidents. (14$ same place).
    Not sure that proposed project would provide better safety.

  8. It doesn’t matter how much technology you apply to safety the ever inventive user will always find new ways to remove limbs that are not covered by the technology.

  9. Cliff Claven (above) is correct in commenting about braking too fast. There is a lot of mass in a grinding wheel. They’re only held on with a coarse thread handed nut (right hand thread for the right side, left hand for the left). The amount of inertia remaining in the wheel when braked is sufficient to reduce the bite of the nut and loosen the wheel. A castle nut and split pin could be a solution however that means every grinder would need that modification.

  10. A diamond dresser tool is used to flatten or round the edge of the grinding stone. It works great as a slow brake. If you get in the habit of using the dresser when you turn off the grinder, it will keep your stone fresh while not wasting your time worrying about walking away from a high RPM wheel.

    I made a simple contraption to lightly hold the dresser against the wheel after I turned the wheel off. It was normally pulled back an inch, but I could flip it forward when desired. I no longer use it because I’ve found that a slightly rounded edge is better for sharpening my chisels.

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