Tablesaw arbor nut comes loose and I know why

Travis Porter

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292
Location
Wake Forest, NC
I have a PM 66 Tablesaw with a zero clearance insert. One of the ones you buy premade. Occassionally now, the arbor nut will come loose when I turn the saw off. The first couple of times I thought maybe I didn't tighten the nut properly, but just this past time it did it and I know I tightened it. Any thoughts or suggestions as to why this is happening?
 
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This one is tough... I was taught to NOT tighten the arbor nut, but just make it finger tight, and let the saw tighten it as it cuts.

European saws are required to have dynamic (electronic) braking to slow the saw blade faster (not the "instant" stop of SawStop), but those blades have three rather than one hole, so it doesn't loosen the nut.

Could there be some degree of braking provided by the motor, that may be set too aggressively for a "one hole" blade? I believe the European saw motors have an adjustment for the degree of braking.
 
Does the nut come loose using the standard insert? If not, maybe you have some friction with the zero clearance one during braking?

Are you sure your arbor shaft, face plates, washer are completely clean and free of sawdust when you put the blade on. Might not be getting a firm tightening of the nut due to sawdust causing the alighnment of the blade to not be straight.

Just some thoughts.
 
my thoughs are in line with charlie here....when the brake kicks in it`s probably causing the nut to back off.....tod
 
My Thoughts, the nut in reality is more or less joined to the blade by the washer on the nut side of the blade by friction. The Washer is joined to the blade my friction, To me the blade is slipping on the inside washer when the brake is applied, thus turning the nut and loosing it on the threads. So the Inside Washer thing must be turning on the shaft.
 
I don't believe the saw has a brake. I am going to try to clean the arbor, arbor washers up and rought the washers up a bit and see what happens. The point that the blade is slipping on the inside washer does makes some sense to me.
 
The inside and outside Washer Things are Stabilizers to give support to the blade and also to keep the Blade running true and also to give more area for friction driving the blade. It could be not tight enough fit to driving shaft, it could be vibration caused from V belt memory if its belt drive, or out of balance blade. Try some pipe dope on threads of arbor to help keep nut from unscrewing. Or a little nail polish as some location where nut and arbor threads meet after tighnening nut, let it dry before use. Force of unscrewing will easily break the nail polish, its kind of like an external lock tight.
Roughing up the washer will not do much just clean blade and washer well with a good solvent, such as denatured alchohol or paint thinner.

If its belt drive and vibration is the culprit just replace belt with a sectional V belt as it will not develope a memory, Most woodworking tools can be greatly improved by this method as when a V belt sets for any lenght of time it develops a memory where it goes around pullys which when running turns into vibration.
 
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If the culprit is the inside flange washer slipping on the arbor, just get a thin internal lock washer and slip on first. its a round washer smooth on the outside with little serrations on the inside.
 
Well I have pulled the motor cover off and blew out the motor real good with an air hose. Additionally, I pulled out the dial indicator and I have less than .001 of runout on the arbor. The threads on the arbor look good and both flanges of the washers or whatever you call them are clean, no play whatsoever. I got out the blade cleaner and cleaned the blade since I had it off as well. One thing I did notice is that it seems the belts have gotten a "memory" where they want to sit in one spot. I am going to contact Powermatic tech support to see what they say.
 
Unless yours is different from my older PM-66, there is no brake on the motor, so it has to be the memory in the belts. The set in the belts, (mine has three), can and will cause a vibration during slowdown, which can cause the blade to wobble enough to drag on the ZCI and that could possibly help loosen the nut. The belt memory problem is the reason that before I change my belts this spring, I am going to check with PM's Tech Dept about the possibility of changing out the arbor and motor pullys to use a flat belt like the new PM-2000 uses, (if it will all fit). I LIKE Flat Belts, (even better than the link belts, and they are MUCH quieter running than the links).
 
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...which can cause the blade to wobble enough to drag on the ZCI and that could possibly help loosen the nut. ...

Does the nut come loose using the standard insert? If not, maybe you have some friction with the zero clearance one during braking?
....

If the blade drags on the ZCI (for any reason) that would tighten the nut, not loosen it, just as sawing tightens the nut.
 
Well, the belts definitely have developed a memory in them. The saw is probably 15 years old now (I bought it new), and I have never changed the belts.

I do believe Norman's comments are on track. You can "hear" the memory in the belts as the saw slows down and it is probably dragging the ZCI. I have taken the ZCI out and have not had another incident of the nut coming loose, but to Charlie's comment, I don't understand why it is coming loose since you would think it would tighten the nut.

Powermatic has yet to respond, but I am going to go ahead and order two belts for the thing as I have never changed them and it is probably due.
 
Well, the belts definitely have developed a memory in them. The saw is probably 15 years old now (I bought it new), and I have never changed the belts.

I do believe Norman's comments are on track. You can "hear" the memory in the belts as the saw slows down and it is probably dragging the ZCI. I have taken the ZCI out and have not had another incident of the nut coming loose, but to Charlie's comment, I don't understand why it is coming loose since you would think it would tighten the nut.

Powermatic has yet to respond, but I am going to go ahead and order two belts for the thing as I have never changed them and it is probably due.

Please keep us updated as to the outcome of this, and any comments you get from PM, (you may have to call the Tech Dept to get a response). I agree with Charlie that in "Theory" dragging on the ZCI SHOULD tend to tighten the nut, but with the jerking effect of the arbor caused by the "Memory" in the belts, who knows for sure what effect the accelerate/decelerate of the arbor, will have on the blade and nut if the vibration causes the blade to intermittantly drag on the ZCI. It's kind of an unknown zone, and I have seen strange things happen with machinery before that were just as baffeling and seemed theoretically impossible.
 
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Heard back from WHM tool group yesterday. Got a phone call and an email with two different answers.

The guy on the phone was hooked on the arbor and the bearings. The email said that it could be the threads wearing out and that the arbor nut was the likely culprit.

I got pricing from them for the arbor assembly which includes the arbor, bearings, locking pins, and nut is $79. Chepaer than I would have thought. So, I am going to bite the bullet order the arbor assembly and two new belts. If it is the arbor/bearings it is not something I look forward to changing. It looks like I will have to take the top off the saw just to get to it good.

I did shift the belts around on the saw last night and put the ZCI back in, but I haven't used the saw enough to have an opinion whether or not it has done any good. Once I get the parts I can go through the process of elimination.
 
Boy, this is a weird one. I don’t understand how the ZCI would have anything to do with the problem though. I don’t have enough faith for “finger tight”, I always use the wrenches to snug up the nut on my Unisaw. The first thing I would do is clean everything with a solvent like acetone or lacquer thinner.
 
I shifted the belts as someone suggested to me, and the arbor nut hasn't come loose in almost a month. If that fixed it, which it appears it did, I still don't understand why it fixed it.:huh:

I did get the arbor, bearings, nut, and belts. Since I am going to make a new extension table, I may go ahead and replace them anyway.
 
Just a theory, but.... Perhaps bump caused by the set in the belts acted like a brake for a tiny portion of each revolution. And perhaps that bump was enough to gradually jar the nut loose. Since cutting normally tightens the nut, could you have been doing work with a larger percent of "running without cutting" or very light cutting?
 
I a new extension table for my tablesaw this week, and a buddy of mine came over and talked me into going ahead and doing the arbor change out on my saw. So, we take it all apart, fence rails, extension table, wings, top, motor, and go to town.

Once you get the pulley off, the arbor itself is a shaft, two bearings on the outside, a metal collar, and a piece of spring steel to keep it all snug with lock rings on the outside to keep it together. I get the assembly out, and low and behold, the arbor assembly itself has been slipping some. There is an allen screw in the top of the cast iron assembly that tightens on to the metal collar and it wasn't tight.

After taking the old arbor off and spinning the bearings, you could hear a little noise in them, so I went ahead and put the new one in with new belts. I turned it on, and I can say it is a whole lot smoother running.

Unfortunately, now I have to finish putting it back together and lining everything back up.
 
Well, Kiss my Grits,........... I totally forgot about that Allen screw. At least you FOUND the problem and fixed it without too much hassle, and it didn't turn out to be something serious or expensive. Now that you've got the bearings and belts changed and then get it realined to perfection, it should be good for another 15 or 20 years.:thumb:

Thanks for the report, Travis. (I had been wondering what the outcome was)
 
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