Arbor nut keeps working itself loose on my table saw. What am I doing wong?

Question

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?

Answer 1

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".

Answer 2

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.