Table Saw Blade Breaking Loose

[Michael Edward Hanley]

I have been working with a Dewalt 7491RS for over a year now and I started having a problem with the arbor nut breaking loose. It has happened once with a dado blade (without clamp washer) and once with a regular Freud blade (with clamp washer). Every time before I turn on the saw I check and make sure the blade is tightened down, but I am wondering if anyone has experienced this issue. If anyone has a remedy, I'm open for suggestions.

I can say this it is quiet terrifying to turn off your your table saw and see the blade still turning at 3400+ RPMs then start to wobble like it is going to jump out. I figured letting the blade slow down by itself is the best bet rather than try to stop it with a piece of scrap.

[jamil mehdi]

The arbor should have a left hand thread, so, if anything, it should get slightly tighter from the centrifugal force. Does this saw have a brake? If so, the braking action can cause the arbor nut to loosen if it wasn't tightened enough. Scary situation.

I had roughly the same situation occur with an angle grinder. It got loose after shutting down, every time. I threw away the grinder. Have you tried throwing away the saw?

Seriously though, try replacing the arbor nut and hope that fixes it. You can also try roughing up the threads with a wire brush. If it keeps happening, it's a mechanical problem with the arbor nut not holding because the threads on the arbor are slightly mismatched. I would definitely contact dewalt. This is a potential manufacturing problem for which they'd be liable and my guess is they'll bend over backwards to fix it.

If nothing else, put a lock washer on it, but don't keep using it with a freewheeling barely attached blade.

[Jamie Buxton]

Does your blade have that crazy Teflon coating? Freud puts it on their consumer blades. It allows the arbor nut to slip against the blade, and bad things happen. A fix is to sand the coating off the blade where the arbor nut is trying to snug up against the blade.

[Michael Edward Hanley]

I am going to try scuffing up the blade, arbor nut, and clean up the spindle assembly. This is the current situation: http://imgur.com/a/aymktx2

[Jamie Buxton]

Do you have an arbor washer? You're not mentioning it. This is a cupped washer which goes between the nut and the blade. If you don't have it, you may well be tightening the nut on the threads rather than putting pressure on the blade.

[Michael Edward Hanley]

Yep, I forgot to add that into the pics. When using the dado there isn't enough room to add the washer.

[Alan Schwabacher]

If your arbor bearings have failed, the increased friction can cause this.

[Andy D Jones]

Does that saw happen to have a brake that automatically stops the saw quickly when you shut the saw off?

If that brake is set too aggressively, the momentum of the rotating blade will unscrew the nut, and both nut and blade can hop off the end of the spindle.

-- Andy - Arlington TX

[Peter White]

DeWalt have a special nut to use with a dado blade doesn't come as standard.

[Kevin Jenness]

I had a similar problem with an open frame motor on a Unisaw. The centrifugal switch got sticky and created enough vibration when dropping out of the circuit that the arbor nut loosened up. I wound up replacing the arbor as well as the motor. Not fun.

[Ronald Blue]

Those are clearly right hand threads. It should want tighten as you use it. Unless something is causing it to decelerate quickly it shouldn't be coming loose. Is everything smooth when it's running? No unusual sounds or vibration? It should take a few seconds to spin down unless something else is going on. Is there any chance the nut is wearing out and slipping on the arbor? Is the nut brass? You might explore procuring a new nut. That looks like Acme thread which is pretty much the standard for a saw arbor. Just my 2 cents. That and 2 bucks will get you a gas station cup of coffee.

[ROBERT SILVERSTEIN]

I am going to try scuffing up the blade, arbor nut, and clean up the spindle assembly. This is the current situation: http://imgur.com/a/aymktx2

Hey Michael -- did you have any luck with scuffing up the blade? I am running into same issue as you. Almost new 7491 (5 months), and I have a Freud Diablo blade in it. Yesterday, for the first time ever, the blade "self-loosened". Fortunately, the nut was *barely* loose, but this is ridiculous, right?!

I hope to get in touch with DeWalt support and discuss warranty, but want to have as much info as possible before I talk to them. I've heard at least a half a dozen stories from others about this occurring either rarely or often with the 7491 (and barely any stories of it happening with other saws).

Thanks,
Bob

[Lee Schierer]

Your arbor has an acme thread, but your nut appears to be a standard thread. Is the arbor nut the one that came from the factory? Did you purchase this saw new or used? Looking at your arbor, you should always use the cupped blade washer (called a clamp washer for your saw (https://www.ereplacementparts.com/clamp-washer-p-91121.html)) that came with the saw. The nut will bottom out on the threads before it tightens the blade without the blade washer. You should always use a wrench to tighten the nut. I hold the blade between my fingers and tighten the nut until my fingers just start to slip on the blade.

Here is what your arbor nut should look like. Note the flat areas on the threads inside the nut. Also not the shoulder area on one side of the nut. That side should always be against the blade.
483665483666
Here is the cupped washer that should be used in combination with the arbor nut for all single thickness blades.
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I run Freud blades, including the ones with the coatings, on my Craftsman saw and never ever has a blade or dado set loosened in the 30 years I have owned this saw.

[Brian Tymchak]

I run Freud blades, including the ones with the coatings, on my Craftsman saw and never ever has a blade or dado set loosened in the 30 years I have owned this saw.

Same. Though for only about half as long as Lee. :)

[ROBERT SILVERSTEIN]

Here is what your arbor nut should look like.

Hi Lee -- thanks for replying -- for some reason I don't have permission to view those jpegs, although my account was "activated" yesterday. I get an error. Thanks!

[Lee Schierer]

Hi Lee -- thanks for replying -- for some reason I don't have permission to view those jpegs, although my account was "activated" yesterday. I get an error. Thanks!

You need to be a contributor to see photos posted on SMC. It costs a minimum of $6 per year. Cheaper than a cup of latte at Starbucks.

Bob, I just noted that this entire thread other than the recent comments is over 12 years old and has been resurrected several times. I suggest if you have any more questions regarding problems with your saw, that you start a new thread....

[Sam Force]

12 years!!! You might need a new calculator :eek::D

[Lee Schierer]

12 years!!! You might need a new calculator :eek::D

I definitely got my wires crossed somewhere.:eek:

[ROBERT SILVERSTEIN]

You need to be a contributor to see photos posted on SMC. It costs a minimum of $6 per year. Cheaper than a cup of latte at Starbucks.

Bob, I just noted that this entire thread other than the recent comments is over 12 years old and has been resurrected several times. I suggest if you have any more questions regarding problems with your saw, that you start a new thread....

Thanks, Lee -- I will become a contributor. Also, thank you for the suggestion about starting a new thread. Some forums prefer not to start new threads for "same topic" so I wasn't sure :)

[Tom Bender]

There should be no lubricant on the nut, spindle or washer. A toothbrush a rag and the right solvent will fix it. Dish soap should do it but since it is also a lubricant it will require sufficient rinsing.

[Lee Schierer]

After some additional thought and looking at the images supplied by Michael Hanley ( http://imgur.com/a/aymktx2), it would appear that the shoulder next to the flange nut is a bit wide. From where I sit, putting a thin kerf blade on that saw without the dished clamping washer between the nut and the blade would not allow the nut to properly apply pressure to the blade. Even with the dished washer in place, it appears that the shoulder may be too long. The nut may bottom out on the threads giving the indication that it is tight, without actually putting pressure on the blade.

As for Dado blades, it is common to not use the dished washer. However, if you use shims between the blades and chippers to get exact widths of cut, make sure your shims are not compressible. On a stacked dado it is a good idea to tighten the dado set and then bump the saw on and off quickly before retightening. Sometimes the chippers can drop down into the threads of the arbor shaft and bind when they are first assembled.

[Dan Raessler]

Just bought a used Dewalt 7491 job site saw, not knowing about this problem. Arbor nut came loose several times. Dewalt sent a replacement nut but it came loose, too (while ripping a 1/2" thick piece of pine!). Has anyone who has had this problem found a solution? FWIW, DeWalt says that as a second owner there is no warranty. Is there a way to find out how widespread this problem is? Thanks.

[Lee Schierer]

How are your tightening the arbor nut when you change blades? Are you using the flange washer that comes with the saw?

[Dan Raessler]

I put the wider side of the dome-shaped washer against the blade. The nut appears to be the same on both sides. I tighten by hand, then "snug" it a little tighter. I tightened the nut more than that once and it still came loose. Only has happened while cutting wood--not on shutdown, at least not so far. Thanks.

[Lee Schierer]

The shaft of the motor should be turning in the opposite direction from how you turn the nut to tighten. The sawing action should tend to tighten the nut not loosen it. I tighten by hand then hold the blade with my left hand and use the saw wrench to tighten the nut to a point just before it would slip my left handed grip on the blade. I've never had a blade loosen doing this for the past 40+ years

If your saw uses two wrenches then apply more torque to the nut.

[Dan Raessler]

Thank you for the suggestion. I will try that. I very lightly tightened the blade on my previous saw and it never worked its way loose--that is why this was such a shock. FWIW, I was using a thin kerf blade with a stiffener next to it the first time this happened. After removing the stiffener it happened several more times. I probably will try cutting some 1/2" plywood later this week (standing out of the path of the blade, just in case) and hope for the best. Again, thank you.

[Bryan Hall]

I had the same issue develop with that saw as well. I assumed I, or one of my crew members, had been overtightening it and caused damage to the threads. After 3-4 months of trying to fool with it and get a solution I gave up and just replaced it. I could have replaced the arbor I think, but since it's a work tool I just let it go.

[Lee Schierer]

FWIW, I was using a thin kerf blade with a stiffener next to it the first time this happened. After removing the stiffener it happened several more times. I probably will try cutting some 1/2" plywood later this week (standing out of the path of the blade, just in case) and hope for the best. Again, thank you.

Sometimes the arbor threads are cut too close to the fixed flange on the arbor and thin kerf blades or chippers in dado sets will drop down into the threads and bind when you are tightening the nut. As soon as the blade encounters resistance the blade will move and pop out of the threads, leaving the blade(s) loose on the arbor. I have a Craftsman table saw with a 1 Hp motor and I find that a good sharp full kerf blade works just as well and gives a better finish than the thin kerf blades. I no longer use any thin kerf blades. All my blades are Freud Industraial 60 tooth blades except for my Freud Glue Line ripping blade. Your 2+ Hp saw should easily handle full kerf blades.

[Dan Raessler]

I hate the thought of giving up on the saw. I will try a blade with a standard-width kerf. I think that that is what the previous owner had. I was disappointed at how much effort it took to remove his worn-out blade, though. Makes me assume he had the same problem and dealt with it by over-tightening the arbor nut. I also plan to call a number of DeWalt service centers to find out whether they have dealt with this problem. If I learn something from one of them I will post that information. Again, thank you for your thoughts about this and suggestions. Dan