Need help with a problem with a Powermatic 66 table saw

[Charles R Johnson]

I've got a 1987 vintage 66 that has served me very well. I'm still getting excellent cuts but get a strange vibration / blade wobble when the saw comes to a stop. When shutting down the saw and the blade at about 1/2 the full RPM it suddenly - thumps and wobbles the blade. If there is wood around the blade it gets scared. It seems this coincides with the capacitive start function of the motor dropping out. But I'm not a motor expert.

The belts are tight and the arbor bearings seem good.

Any ideas of where to head on this?

[Mike Hollingsworth]

Mine did that too. Someone here will know. Looking forward to hearing the reason.

[Ken Fitzgerald]

I suspect it's caused by natural resonant mechanical frequency of the blade/arbor combination.

It's been decades....but as I remember it.....nothing is absolutely perfectly balanced....as in saw blades.....arbor spindles etc. As such when you spin them the combination of the imperfections will have a frequency.....a narrow RPM range at which their natural "oscillations" will be aided by the spin frequency of the motor....both as they spin up in RPM and spin down. This causes a vibration. The vibration isn't as noticeable while spinning up because you pass through this narrow band of RPM quickly while trying to achieve the running RPM of the motor. But when power is removed, the motor is allowed to coast down and as such spends more time in the natural resonant RPM range as it tries to coast to a stop and thus the vibration is noticeable.

Again....it's been a long time but that's how I remember it.....I'm sure others will correct if I am wrong.

[Rich Riddle]

I suspect it's caused by natural resonant mechanical frequency of the blade/arbor combination.

It's been decades....but as I remember it.....nothing is absolutely perfectly balanced....as in saw blades.....arbor spindles etc. As such when you spin them the combination of the imperfections will have a frequency.....a narrow RPM range at which their natural "oscillations" will be aided by the spin frequency of the motor....both as they spin up in RPM and spin down. This causes a vibration. The vibration isn't as noticeable while spinning up because you pass through this narrow band of RPM quickly while trying to achieve the running RPM of the motor. But when power is removed, the motor is allowed to coast down and as such spends more time in the natural resonant RPM range as it tries to coast to a stop and thus the vibration is noticeable.

Again....it's been a long time but that's how I remember it.....I'm sure others will correct if I am wrong.

You are correct Ken. The same thing happens to cars with a tire slightly out of balance. It will shake you mercilessly in about a 5 MPH range but outside that range it runs fine. Many think they need a new front end on a car because of it. With table saws, many think it's a much more serious problem than simply the balancing of all the moving components. Remove the blade, turn on the saw and see if it has the same issue without the blade. Chances are it will not have the issues.

[John McClanahan]

When mine gets at about 1/3-1/2 speed, the start winding switch clicks and the motor acts as if it has an electric brake slowing it down for a second or two.

I do remember having some vibration that seemed to come from the belts. I don't run my saw hard, so I removed one of the belts and it runs a lot smoother.


John

[David Utterback]

I have this problem with my jointer and someone suggested that the pulleys may not be co-planer or the pulley set screw may be loose. Have not checked them yet, though.

[Frank Drew]

Charles,

Have you noticed the vibration with other blades mounted on the saw?

[Harold Weaver]

You may want to have a machinist true up your arbor flange. I had it done on my 1972 model 66 when I restored it. Smooth as silk. Probably $20 or $30 well spent. In addition, I would replace the arbor bearings on a 1987 saw. Life expectancy on those bearings is about 20 years as a rule of thumb. If you have never replaced them, then assume they are the originals. You are well past that 20 year mark. Call Accurate Bearing and ask for Lynne. She'll hook you up with good pricing and good shipping. In addition, make sure the set screws on the arbor housing that holds the arbor in place are tight. If loose, the will allow vibration. Check the set screws on both the arbor pulley as well as the motor pulley for the same issue. Belts? How old are the belts? It could very well be time for those to be replaced as well.

Harold

[Judson Green]

You are correct Ken. The same thing happens to cars with a tire slightly out of balance. It will shake you mercilessly in about a 5 MPH range but outside that range it runs fine. Many think they need a new front end on a car because of it. With table saws, many think it's a much more serious problem than simply the balancing of all the moving components. Remove the blade, turn on the saw and see if it has the same issue without the blade. Chances are it will not have the issues.

Yeah its probably something like that, but I'd check the tension on the belts. Make sure there all the same and if possible replace with link belts. I had jointer with 3 belt drive and one of the belts wasn't tensioned the same as the others, being the lazy guy I am I I simply removed the offending belt, no more vibration.

Also make sure the belt seems are good and smooth.

[Mike Schuch]

My Powermmatic 65 uuused do that... a thump when it started and a thump when it stopped. A previous owner changed the 3 phase motor to a single phase motor with a smaller shaft and used a spacer to make up the difference in shaft size. The thumps on my saw were due to the pulley on the motor shifting on startup and stopping. After messing around with tightening the pulley several times I replaced the motor and arbor pullies with two groove pullies with pinch collet mounts. The pitch on the original 3 grove pullies were impossible to find. The thumps have been gone for four years now.

The spacer on the original pulley made my thump worse because it allowed more rotational play. You might just have a loose pulley allowing rotational play.

[Charles R Johnson]

Thanks for all the inputs!!! I'm just back in town after a week long trip away. I'll check into the suggestions and let you know what I find. Thanks again, CJ