I have a 4 1/2” diamond cup grinding wheel installed to my angle grinder.

I screwed it on all the way, but there is no room for the locking nut.

I called DeWalt and they said since it is a screw-on wheel, i do not need the locking device, but that kinda worries me.

Is this safe without a locking nut and how should I tighten it?

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2 Answers 2


The Dewalt person is correct.

Use the locking mechanism of the grinder to keep the shaft from turning while you tighten it down by hand. (Always unplug the grinder when replacing discs.)

The rotation of the shaft is such that it will cause the wheel to tighten, not loosen. There are flat spots on back side of the wheel that you can get a wrench on, but there is no need to torque it down very tight.

The locking nut/washer is used for discs that are not threaded, such a cut off disc.

  • 1
    I'd hazard a guess that the threads take up the whole shaft specifically so there can't be a locknut added. I'd hate to think what would happen if someone didn't screw on the cup completely, then added the locknut, only for the force of the grinding caused the cup to finish screwing itself on. Not only would that be a significant shock to the tool and grip, but it would also cause the locknut to be loose on a high RPM shaft. Sep 4, 2020 at 16:14
  • pro tip: keyring the otherwise lost locknut to the plug end of the grinder's power cord, where your wrench should already be to remind you of unplugging before use.
    – dlatikay
    Sep 4, 2020 at 17:43

Yeah, that's SOP for grinding wheels and wirebrush cups. The machine turns counterclockwise (from the business end), which will always tighten the screw thread. That's why the machine turns counterclockwise; if it turned the other way it would constantly be unscrewing the grinding wheels.

  • So in theory, if the grinder had an axle brake, or a mechanical failure that locked up the motor/gearbox while it was spinning, the wheel could unwind itself fast, drop, and make like Roadrunner ?
    – Criggie
    Sep 5, 2020 at 7:17
  • Yes. And it doesn't need all that much random chance to have it happen. If the wheel jumps off, DON'T KICK IT!
    – aghast
    Sep 5, 2020 at 10:30
  • 1
    @Criggie In theory, yeah. But in my experience those things torque themselves down pretty well just from use, and I'm using a 3" diameter cup brush so not a lot of transferred torque. A 6" wheel, certainly more. Sep 5, 2020 at 15:54

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