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I have a grinder with a Dewalt diamond cup wheel I have been using to remove cosmetic issues on the floor of my basement before staining the floor. I am grinding the whole floor. I am about 1/2 done and need to get the diamond cup off the grinder but I am unable to do so. It is a reverse screw, yes I know I am turning it the right way, I've taken it off twice before.

I have also tried spraying some WD-40 in the threads and tried holding the lock with one hand and striking the wheel with a mini-sledge to loosen the diamond cup. Its still not coming off. I have a dust shroud on the grinder so I can't get a wrench on the back of the diamond cup. I can't take the dust shroud off without 1st taking off the cup. Any ideas how to get this thing off?

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Put the grinder on a concrete surface like you're going to grind it, hold the lock button to prevent the shaft from spinning, push down into the concrete and turn the grinder counter clockwise to unscrew the diamond cup.

If that doesn't work, do the same thing with the diamond cup on concrete but stand with your left foot above the diamond cup on the backside of the grinder, and kick the other end (power cord side) forward with your right foot. You need to press the shaft lock at the same time, but sometimes you can get it to stay locked or use a broom handle or something long to press it to keep the shaft locked when you kick.

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  • That or use a vice, I usually step on them because if it is stuck bad enough to need the vice I usually bend the lock pin, happens with rock & cement dust, I usually coat the threads with copper antisize as I have broke several grinders before coating the threads never since adding a touch of the copper antisize to the threads.
    – Ed Beal
    Jan 23, 2018 at 15:19
  • @EdBeal Great idea with the copper anti-seize, I'll start doing that too. Thanks!
    – Dotes
    Jan 23, 2018 at 15:50
  • I'll give those a try. Where do I get the copper the copper anti-seize? Is it the same thing you use on a car?
    – Kenny
    Jan 24, 2018 at 15:18
  • @Kenny Yes, most auto parts stores will have it.
    – Dotes
    Jan 24, 2018 at 16:24
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    I can confirm that the second approach worked for me. I applied penetrating oil a few hours prior, but I'm not sure if that mattered. Using my body weight to push the cup into the floor, and using my other foot to kick the grinder's handle, was definitely required. Using only my hands while leaning over the grinder was not sufficient.
    – Christian Convey
    Jun 5, 2019 at 16:33
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Well, its been a pretty long time, but I was browsing my Stack Overflow account, saw this and thought I should post my fix.

Unfortunately, none of the ideas I was given worked. I ended up having to use my drill press to drill through the steel portion of the diamond cup, around the threads, until it broke off completely leaving the threaded portion of the diamond cup still on the grinder. Then I used a pair of channel locks and I was able to remove the threaded portion of the diamond cup and then I had a working grinder again. The whole thing took me an hour, and I went through multiple drill bits and a whole bottle of 3-in-1 oil.

Needless to say, I have used copper anti-seize from the auto parts store every time I have used the grinder ever since. I've never had an issue with it coming back off, just need a dab of it on the threads.

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I welded a nut on and then used a socket wrench.

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