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I recently bought a new Dewalt DWE7491RS table saw (pictured below). And on two separate occasions the arbor nut has come loose on me. Once with a dado stack that threw blades across the room! (Very SCARY!)

The second time it was when I was taking a standard cross-cut blade off and noticed the tension on the nut was very loose, although the blade hadn't slipped out of place yet.

I'm almost certain that I am tightening the nut down pretty well; probably more than I should. And I do put the washer on the nut between the blade and the nut. This is also with all stock parts that are less than a month old.

Any ideas what might be causing this to happen?

Table Saw

JohnFx
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    This just happened to me with the same saw and a dado stack. I was cutting a rabbet, so thankfully the fence held the blades in, but it did take a nice chunk out of my fence. The arbor nut came flying out and is still in my yard somewhere. As you said, very scary. It’s a brand new saw, but I also thought I had tightened the arbor nut enough. Since reading this post I have re-cleaned the spindle with some mineral spirits and ordered a new arbor nut. I’m very hopeful that I don’t experience anything like this a second time. – James Craig Jun 27 '20 at 23:04
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    Wow. Mine was brand new too. I wonder if there is a manufacturing defect. – JohnFx Jun 28 '20 at 00:42
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    Had this issue also with this same model and a brand new Diablo dado stack. I did some testing and it doesn’t seem to matter how hard/soft the nut is tightened; it was coming loose during saw start. Not during a cut, not on shutdown. It’s a very noticeable sound difference on start when the nut comes loose. – Michael Thompson Jun 28 '20 at 20:02
  • Interesting that you experienced it on start.

    I cannot be sure, but I thought it might have loosened on shut-off, when the brake kicked in. The weight of the dado stack could have torqued the nut opposite the braking and therefore loosened. The first thing that I noticed that indicated something was wrong is that the blades were still spinning at nearly full speed after I cut the power. I'm pretty sure this happened well before the blades walked over into the fence.

     – James Craig Jun 29 '20 at 20:33
  • @JamesCraig interesting data point, and if there is a brake on the arbour then your hypothesis makes sense. Dado stacks would also have to be torqued a little more, as there is going to be some flex in the pack to overcome. –  Jul 01 '20 at 22:59
  • To top things off, I've been able to replicate this issue on saw start with a normal blade. Hand-tightening the nut and then starting and instead of locking down or getting tighter it'll just spin off and the blade comes loose. I bought a new nut and compared, I think something happened to mine at some point. So maybe try buying a new nut? Think mine had the blade on it when I bought it so maybe during shipping it wobbles or gets stressed. Either way, we're returning ours. – Michael Thompson Jul 13 '20 at 14:22
  • After replacing my arbor nut and throughly cleaning the arbor, I tried again with various dado configurations and conventional blades. Thankfully I have not been able to reproduce the problem since the first time. I'm keeping my saw because I'm otherwise happy with it and with DeWalt customer service. – James Craig Jul 13 '20 at 22:21
  • My saw is doing the same thing! ‍♂️ I changed my blade to a 60 tooth and doubled the blades up so I get a thicker cut. And for the life of me I can’t keep the arbor nut tight after I run the saw once. It’s frustrating since it’s a brand new saw. I’ll order a new nut and go from there. I’m calling DeWalt Monday morning. – Joshua Oct 18 '20 at 02:12
  • Same saw same problem. I first noticed it when I shut the saw off and the blade continued to spin because the nut was away from the washer about a quarter of an inch. You would think that if the thread of the arbor is opposite the blade spin it could only get tighter. 50 years of standing behind saws and never had this problem – Mike Oct 21 '20 at 00:20
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    There seem to be a lot of "me too" comments, so I did a search to see if this was common across the board. This is the only result I found for this particular model. However I came across this forum post where the OP fixed his problem (on a different model DeWalt) with tightening the arbor nut by flipping it over - he'd put it on the wrong way out. It seems to be a reasonable thing to check for those who are having this issue. Not saying it's the problem here, but it's worth knowing - I wouldn't have thought of it – FreeMan Oct 22 '20 at 11:20
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    I feel compelled to chime in on this one. I have this same table saw and I was ripping a long board and when i turned the saw off the blade kept spinning forever. I thought the braking system on it failed or something. I slowed it down with a scrap piece of wood and opened it up to find out the arbor nut was loose, hence the spinning blade. Only happened to me once though, and not with a dado stack. – Nick H Jul 27 '21 at 17:26
  • I've been having similar problems. The arbor nut has come loose multiple times, the most recent being after just 4 cuts or so since I last tightened the arbor nut. Scary! Dewalt customer support has been miserable. They've sent me 3 different replacement nuts, none of which fit my saw and one of which was a nylon lock nut for a completely different product. – Tim Herzog Aug 05 '22 at 02:03

2 Answers2

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Once the arbour nut is tight, the rotation of the spindle will tend to tighten it. So you just have to make sure it is torqued down appropriately.

Well, it is the rotation into the material that tends to tighten the arbor nut. Once powered down and free-spinning the mass of the blade is now free to loosen the arbor nut, which is why dado sets and saws with brakes may tend to loosen the arbor. The model described here doesn't seem to have a brake, but I suppose a valid check is to make sure the blade or blade stack isn't exceeding their recommendations. (Though, they usually discuss blade size in terms of the the riving knife installed rather than mass.)

This is how most saws like to be treated:

  • Make sure there is no oil or grease on the nut or the arbour. Make sure the entire spindle and all the spacers and washers are free of sawdust and other debris. If this is a new saw, I'd carefully degrease the entire spindle and blade hardware because it might still be covered in a factory anti-rust coating.
  • Stack up your spacers or blades correctly; there is usually an identical cupped washer on both sides of the blade, for example. What we don't want is mismatched torque on either side of the blade or blade stack.
  • Thread the nut on finger-tight, making sure all the slack is taken up as much as possible.
  • Either lock the spindle or use two wrenches to tighten the arbour a 1/8 to 1/4 turn. The idea is to snug things down fully without stretching the threads too much -- we want to be within the natural elasticity of the spindle metal.

That should be it. If it is working loose after torquing it down then there is a problem with the arbour nut, washers or spindle.

  • Make sure the blade is torqued down evenly. Spacers or washers must be the same mass and dimensions.
  • See if the spindle (or arbour nut) threads are obviously crossed or stripped out. Maybe they are stripped toward the end and you aren't getting a proper torque.
  • If you have over-torqued the arbour nut in the past, you may have stretched the threads on the spindle. There are gauges to check for this, but the idea is that the threads should be the same distance across the spindle. Look closely at the place where the nut tightens for really shiny spots or obvious metal movement.
  • Check for excessive run-out or vibration. Make sure the bearings aren't failing. There are ways to check for run-out without an indicator which are easy to find on-line. Vibration should be obvious while the saw is running at speed under no load. The unpowered spindle should turn easily with your fingers with no "grumble".
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    Stumpy Nubs' latest VLOG? :-) – Graphus Jun 29 '21 at 16:24
  • Eagle eyed! Yes, when he mentioned that, I was "oh, of course!" I never really though of the spinning mass of the blades, but it makes sense given that every report for this saw has been losing the blade when powering it off. –  Jun 29 '21 at 18:30
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I would say that there should be no good reasons for this to happen. All that should be required is that you tighten the nut correctly, and then as others have said, the spin direction of the spindle combined with the thread direction of the nut should work to tighten it.

Given that it's happened twice to you and that there seem to be several other people in the comments saying they've had the same problem with this model, I suspect something wrong with the manufacturing - though heavens knows what - perhaps an improperly machined thread on the nut or the spindle? Maybe they have a worn or misaligned tool somewhere in their process?

I've worked in professional woodworking shops for a total of ~16 years, with many different kinds of saw and many different makes and models running for probably tens of thousands of hours in total while I've been there, and I have never heard of a saw arbor nut coming loose like this - it just shouldn't happen.

For a critical safety defect of this nature, I would strongly recommend returning the saw for a different make/model, if you're able - it's just too dangerous to risk it.

WhatEvil
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  • This may, in fact, be the correct answer for this specific question. –  Oct 21 '20 at 18:00
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    It happened a third time just today. The blade didn't come off, but as it was spinning down it started to wobble and made a terrible grinding noise. I opened it up and sure enough the arbor nut had unscrewed itself a full 1/4 inch and wasn't touching the blade. I contacted Dewalt this time. This has got to be a defect. – JohnFx Jan 25 '21 at 04:03
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    Make a real stink about it. DeWalt are a reputable manufacturer and if you insist from the safety angle that you want the unit or at least the faulty parts replaced then they may just do it. – WhatEvil Jan 25 '21 at 04:13
  • Would it help to obtain a second arbor nut and thread it on to lock down the first one? – gnicko Jul 01 '21 at 02:35