Table saw arbour nut unscrewed

[Gremmy]

Happy Easter all!

Bit of a weird one I’ve never encountered before and could have potentially been very dangerous. I was wondering if anyone could shed any light and advise me on what has happened and how to ensure it doesn’t happen again.

I’ve a single phased table saw - motor plate attached



I had been repeat cross cutting spruce lengths to 90, straightening up the rough sawn ends. After every cut I stopped the saw using the dome headed estop button below.




I then cleared away each sliver of waste before starting the next cut.

On the occasion of the incident - I pressed the e stop and the saw blade slowed down and a tooth caught a piece of off cut while stopping, I heard a metallic spinning noise but the saw blade didn’t stop, it kept rotating at a slower speed making a humming noise. I immediately unplugged the machine at the wall to kill power but it still proceeded to hum and rotate for about 15 secs. It did stop and I was obviously slightly concerned that the motor somehow still ran even when disconnected... if that blade had come off or collided with the machine?!? :shock:

Upon inspection the arbour nut had unwound itself about 4 thread turns and the blade had clearly been rotating loosely as can be seen by the red markings on the flange. It is a left handed thread.





Has anyone ever encountered something like this description? My thoughts are a problem with the motor - Faulty capacitors? Saw blade too loose?

 

[9fingers]

I cannot think of a motor fault that would cause this.
Agressive braking combined with a very heavy blade might cause it to loosen but I suspect you have neither a brake or a heavy blade in this case.

Firstly take the blade off and refit the flange and nut, tightened as normal. measure the gap between the two flanges. This MUST be less than the body plate of the saw blade.
it is essential that the blade is gripped by the edges of the flanges not the centre of the arbour.
If it does not grip properly then have someone with a metal lathe undercut the centre of the outer flange. 2 minute job.

A smear of grease on the nut should facilitate adequate tightening in the future. Check this now and then and if it works loose again then consider a second nut fitted as a lock nut although being LHD thread, not easy to find.
Bob

 

[sunnybob]

Wot 'ee said.
But also....... check just how loose the nut is on the thread. Thats an old machine, the nut has been tightened hundreds of times and nuts DO wear out. :shock:
If the nut is floppy, then replace. If the new nut is floppy, then a new arbour seems the cure.
Of course it could just be operator error from the last blade change :eusa-whistle: :eusa-whistle:

 

[sunnybob]

Also, its not best practice to use an emergency stop every time. They are not rated for hundreds of thousands of switchings and could soon wear out with continuous use.
But thats nothing to do with your wobbly nut problem :shock: :eusa-dance:

 

[Cabinetman]

I can’t think of why it should undo as it is a left-hand thread, and it looks from your photo as if it’s on the left hand side as you face the front of the saw so should tighten up naturally.
Except in one instance, and you’re probably not going to want to hear this. If perhaps the last time you did that nut up you maybe didn’t quite do it tight enough, then when the saw first contacts a piece of wood it could jiggle the blade fractionally backwards and slightly loosen the nut? I’m clutching at straws here and it does sound unlikely I will say.
Just realised the fault in my argument, does your blade have a locating hole in it which locates on a little stub to stop it spinning? If it does that’s my theory shot to pieces. But you did say t that the blade span so I suspect you don’t have a little stub. Ian

 

[Trevanion]

Left-hand threaded nuts tend to self tighten as the saw cuts so it’s a bit unusual for it to come undone unless the saw is braked.

Do you use bore bushes at all?

 

[Gremmy]

Thanks for the replies gents.

9fingers - I now think that the blade worked itself loose upon the saw blade tooth contacting with the off cut at exactly the same time that I pressed stop, creating a massive deceleration force that loosened the nut. The funny-metallic noise I heard was the saw blade free spinning on the arbour and would explain why I perhaps thought it was the motor still turning and why it continued for 15 secs after the power was shut off, due to the speed that the blade was travelling. As you can clearly see, my motor knowledge is very limited, I know capacitors can hold charge, and assumed one was still somehow feeding the motor.. :lol:

I’ve checked the distance between the flanges with no blade in place, there is no gap. The flanges and arbour are in good condition, I’ve recently changed my arbour and trunnion for a much later model that has a Riving knife assembly, it had not had any use really in its life according to the seller, so the thread and flanges almost like new, there is no distortion, I’ve checked on an engineering plate.

I’ll certainly grease the nut so that it will tighten fully.

Sunny Bob - I could very well have just not tightened it enough - I’ll be more vigilant. Regarding the estop dome switch - you are correct, it’s not rated for hundreds of thousands of operations, it’s actually rated for a million :lol:
https://cpc.farnell.com/hylec-apl/1de-s ... d4QAvD_BwE

Cabinetman - It doesn’t have locating pin holes in this blade- I think you could be right that perhaps I just didn’t tighten the nut enough :shock:
I’ve put a micrometer on the arbour and it’s less than a thou short of 5/8th, I wouldn’t have thought this would affect anything? Surely within tolerance?

Trevanion - yes I do use reducing bushes, and they are thinner than the body of the saw blade and they are turned not stamped from sheet metal. There isn’t enough room for the bush to work loose into the flange void, and I’ve superglued it in place. I’ve hoovered up your saw blades info and made sure that I’m up to speed with it. Have I preempted your thought process on the bushes?

I still don’t have a definitive answer, so will just ensure I’ve heeded all advice given, and be more careful changing my blades and tightening nuts!

 

[Trevanion]

Trevanion - yes I do use reducing bushes, and they are thinner than the body of the saw blade and they are turned not stamped from sheet metal. There isn’t enough room for the bush to work loose into the flange void, and I’ve superglued it in place. I’ve hoovered up your saw blades info and made sure that I’m up to speed with it. Have I preempted your thought process on the bushes?

I still don’t have a definitive answer, so will just ensure I’ve heeded all advice given, and be more careful changing my blades and tightening nuts!

Yes, you have

I'm not really sure what's caused the problem aside from not being tightened properly initially, even if you caught an offcut on wind-down it would still want to tighten the blade because of the left-hand thread.

You could always get a second nut to act as a locking nut, which isn't an uncommon practice.

 

[Gremmy]

Wasn’t meaning to steal your thunder there Trevanion, just showing you I’d done my homework
I’ve inspected the nut and it’s a really tight fit on the thread, no movement or slop..
I’d like to put another nut on now as I’m becoming paranoid and have visions of flying saw blades! :shock:
Seems to be a square threaded left handed nut. They are available but outrageously expensive!! Might have to do some saving.
As I’ve stated the fit of the nut is good, but for peace of mind would putting a few winds of PTFE be a good or bad idea? Pros vs. Cons?

 

[DaveL]

I have an AGS10 and have never had this problem. Mine has a 3HP single phase motor.
Does your arbour have the hole in the fixed flange to allow you to stop it rotating? I use a C spanner to hold the flange while doing up the nut.

 

[Lons]

Shouldn't happen as the nut will naturally tighten in use but a locknut is a good idea if you can find one and if the arbour is long enough to take it, another possible solution if it isn't is a serrated lock washer but as said all should be well if the flange is a good fit.

 

[Doug71]

I'm not really sure what's caused the problem aside from not being tightened properly initially, even if you caught an offcut on wind-down it would still want to tighten the blade because of the left-hand thread.

I would have thought if the blade hit an offcut on wind-down it would cause it to decelerate but the nut would want to keep spinning in the direction it was going (clockwise) so would want to unscrew?

I only suggest this because on our old Wadkin spindle moulder that had a hand operated brake sometimes if you braked it up quickly (and my father had tightened the top nut :eusa-doh the shaft would stop spinning but the top nut would keep going and unscrew itself

I do think the nut not being properly tightened on the saw was probably the main factor in this incidence.

 

[Trevanion]

I'm not really sure what's caused the problem aside from not being tightened properly initially, even if you caught an offcut on wind-down it would still want to tighten the blade because of the left-hand thread.

I would have thought if the blade hit an offcut on wind-down it would cause it to decelerate but the nut would want to keep spinning in the direction it was going (clockwise) so would want to unscrew?

I only suggest this because on our old Wadkin spindle moulder that had a hand operated brake sometimes if you braked it up quickly (and my father had tightened the top nut :eusa-doh the shaft would stop spinning but the top nut would keep going and unscrew itself

I do think the nut not being properly tightened on the saw was probably the main factor in this incidence.

That happened on the spindle because you were applying braking force to the shaft, not the the blocks on the shaft as would happen if an offcut came into contact with it.

The weight of the cutter blocks or blade causes inertia, when you apply a braking force to the spindle the intertia of the blocks wants to keep spinning on whereas the shaft is trying to come to a stop, since the nut is left-hand thread the spinning force of the cutter blocks is enough to undo it.

 

[Doug71]

I'm not really sure what's caused the problem aside from not being tightened properly initially, even if you caught an offcut on wind-down it would still want to tighten the blade because of the left-hand thread.

I would have thought if the blade hit an offcut on wind-down it would cause it to decelerate but the nut would want to keep spinning in the direction it was going (clockwise) so would want to unscrew?

I only suggest this because on our old Wadkin spindle moulder that had a hand operated brake sometimes if you braked it up quickly (and my father had tightened the top nut :eusa-doh the shaft would stop spinning but the top nut would keep going and unscrew itself

I do think the nut not being properly tightened on the saw was probably the main factor in this incidence.

That happened on the spindle because you were applying braking force to the shaft, not the the blocks on the shaft as would happen if an offcut came into contact with it.

The weight of the cutter blocks or blade causes inertia, when you apply a braking force to the spindle the intertia of the blocks wants to keep spinning on whereas the shaft is trying to come to a stop, since the nut is left-hand thread the spinning force of the cutter blocks is enough to undo it.

Thanks Trevanion, that makes sense.

Quick thread drift but kind of related. On spindle moulders that can run backwards do they have a special nut/bolt to lock down the block? In the past I have thought about running mine backwards to make certain moulds easier, it's not meant to run backwards but it's three phase so I figured I could just swap a wire round. One of the things that stopped me was the fact that the block would be running the wrong way for the thread on the nut, it wouldn't be tightening when it was running, would you see this causing a problem? At least the nut shouldn't spin off when I brake hard :lol: :eusa-think:

 

[Trevanion]

Thanks Trevanion, that makes sense.

Quick thread drift but kind of related. On spindle moulders that can run backwards do they have a special nut/bolt to lock down the block? In the past I have thought about running mine backwards to make certain moulds easier, it's not meant to run backwards but it's three phase so I figured I could just swap a wire round. One of the things that stopped me was the fact that the block would be running the wrong way for the thread on the nut, it wouldn't be tightening when it was running, would you see this causing a problem? At least the nut shouldn't spin off when I brake hard :lol: :eusa-think:

Typically on a reversing moulder the top of the spindle will either have a slot across the top that the “nut” (more of a countersunk top hat in this case) slots into and a countersunk screw is used to hold down the nut, or with a threaded shaft there will be a form of key way slot down the threads that a loose part of the nut locks into and by some black magic this holds it in place during reverse cutting.

With a plain right-hand threaded machine you can fit a secondary nut (if you have a rather tall nut that eats up all the thread you will need two shorter ones) on top of the existing nut which will stop it from coming loose during reverse cutting.

You also need to make sure that the shaft won’t end up unwinding out of it’s socket either! :lol:

 

[Doug71]

You also need to make sure that the shaft won’t end up unwinding out of it’s socket either! :lol:

That could be quite an interesting experience :eusa-think: think I'll stick to running it forwards. Thank you.

 

[Trevanion]

That could be quite an interesting experience :eusa-think: think I'll stick to running it forwards. Thank you.

It all depends on the machine, I wouldn’t do it on a bog standard older model Sedgwick for example as the shaft is only held in place with a draw bolt through the bottom of the shaft, fine for anti-clockwise cutting but I would think it would actually unthread the shaft from the draw bolt in clockwise motion.

 

[Doug71]

It's an old Wadkin BER2, I'm not sure how the shaft is connected so don't think I'll risk it :eusa-naughty:

 

[Vann]

...
The weight of the cutter blocks or blade causes inertia, when you apply a braking force to the spindle the intertia of the blocks wants to keep spinning on whereas the shaft is trying to come to a stop, since the nut is left-hand thread the spinning force of the cutter blocks is enough to undo it.

What he says.

The same forces that want to tighten the nut on startup, want to undo the nut during rapid deacceleration. And I'm assuming you have an electric brake on this saw. I'd guess catching the offcut must have caused some additional harmonic - just enough to overcome the friction holding the nut.

I know nothing about electric brakes. Is it possible to slightly reduce the braking (increase the rundown time) to reduce the "undo" forces?

Cheers, Vann.

 

[RogerS]

...
The weight of the cutter blocks or blade causes inertia, when you apply a braking force to the spindle the intertia of the blocks wants to keep spinning on whereas the shaft is trying to come to a stop, since the nut is left-hand thread the spinning force of the cutter blocks is enough to undo it.

What he says.

The same forces that want to tighten the nut on startup, want to undo the nut during rapid deacceleration. And I'm assuming you have an electric brake on this saw. I'd guess catching the offcut must have caused some additional harmonic - just enough to overcome the friction holding the nut.

I know nothing about electric brakes. Is it possible to slightly reduce the braking (increase the rundown time) to reduce the "undo" forces?

Cheers, Vann.

I don't think electric brakes generate that amount of braking force to influence the nut tbh. It is certainly not instantaneous....nothing like.

 

[Trevanion]

I don't think electric brakes generate that amount of braking force to influence the nut tbh. It is certainly not instantaneous....nothing like.

It really all depends on how harsh the braking force is, with DC injection you can stop a motor in a split second and generate enough stopping force to shear the motor shaft clean off given there's enough inertia on it such as a large and heavy pulley.

I've certainly worked with a Wadkin BSW rip saw that had a 30" blade that came to a complete stop in 3 seconds, if you let it run down without the brake this would a few minutes to come to a stop. I wouldn't want to imagine the forces that were at play when the brake was applied.

 

[RogerS]

I was only going by my weedy Hammer !