Bouncing Motor and Mount on 12" Craftsman

[The_General]

So, I recently acquired a 1940's Craftsman 12" 54-inch bed lathe. Great shape and I've been cleaning it up and making sure everything is in working order. I replaced both belts with link belts (so I didn't have to pull too much apart).

One problem that I thought the new belts would fix was a Bouncing Motor (and mount). Basically when running in direct drive on the lowest speed settings (both belts on slowest settings) the motor and mount will bounce at about 1-ish Hz rate. Of course, everything is bouncing at the same rate (bed, chuck, etc) so it's not causing any cutting issues, but it makes it dang difficult to smoothly move the hand wheels and get steady feed rates.

I'm attaching a few videos that show the bouncing ('Bouncing Lever' and 'Motor and Frame Bouncing'...

Funny thing too - If I go to higher speeds by moving the belt to position 3 or 4, the bouncing all but disappears ('Smooth Lever' video below)...

Anyone have any ideas what might be causing this? Of course, I've checked for excessive play in the bearings, etc and can't really notice anything..

Thanks in advance!

-Mike

 

[markba633csi]

I think the fundamental issue is your bench is not rigid enough and there also may be some runout in one or more of the drive pulleys
With the motor hanging straight out like that it acts like a lever and magnifies any flex in the mounting structure.
Adding a support strut under the motor to the lathe bed or bench may cure it or significantly reduce it
-Mark

 

[Bi11Hudson]

Check your motor mounts and frame. There is a "tensioning" screw in the web of the motor base that ostensibly holds belt tension. When I acquired my C'man 101.27440, I changed the inside belt to a sectional just so I wouldn't need to dismantle the spindle. The first level (motor to countershaft) belt was left intact. I did have to adjust the tensioning screw when I first set it up. It has never vibrated since. That tension screw is important, the weight of the motor is not stable enough for belt tension.

.

 

[mickri]

From what little we can see of the lathe stand and bench top it does not appear to be stout enough. The MOLO recommends a bench top of at least 2" thick of clear hardwood. The top should be either cleated or have three to four 3/8" threaded rods pulling the boards together. Two layers of 3/4 plywood glued and screwed together would also work. The stand needs to be well braced and cross braced with 4x4 legs if made out of wood or 2x2 steel construction. You want the stand and top to be so stout that it doesn't move or flex.

Having an adequate stand and bench top may solve your problem. If not then you should try adjusting all of the drive pulleys, belt tension, etc. like Bill mentioned. The MOLO has instructions on this. If you still have the bounce then you will have to take everything apart looking for worn/broken parts.

 

[The_General]

Check your motor mounts and frame. There is a "tensioning" screw in the web of the motor base that ostensibly holds belt tension. When I acquired my C'man 101.27440, I changed the inside belt to a sectional just so I wouldn't need to dismantle the spindle. The first level (motor to countershaft) belt was left intact. I did have to adjust the tensioning screw when I first set it up. It has never vibrated since. That tension screw is important, the weight of the motor is not stable enough for belt tension.

.

Tell me a little more about how you adjusted the tension. As you know with the 101.27440, the motor hangs off the back and its weight sets the tension to the inner belt. But the first-level belt (motor to pulleys) tension is set by that adjustable screw. I discovered changing that tension affects vibration and noise. But it's all eye-balling it. How did you find 'the happy spot'?

thx

-Mike

 

[The_General]

I think the fundamental issue is your bench is not rigid enough and there also may be some runout in one or more of the drive pulleys
With the motor hanging straight out like that it acts like a lever and magnifies any flex in the mounting structure.
Adding a support strut under the motor to the lathe bed or bench may cure it or significantly reduce it
-Mark

Tell me a little more about pulley runout. What exactly is that? I 'think' that the right-most (smallest) pulley (the one that yields the worst bouncing, seems to be out of true (oblong). If I touch a screwdriver blade to the rim of the pulley while its turning, it will rub-miss-rub (repeat) indicating that its not perfect. Is this runout? And of course, how do I correct it short of totally replacing it. Which, in my opinion, would not be a guarantee that the replacement cast part would be any better! So your insight would be greatly appreciated.

Thx

-Mike

 

[The_General]

I think the fundamental issue is your bench is not rigid enough and there also may be some runout in one or more of the drive pulleys
With the motor hanging straight out like that it acts like a lever and magnifies any flex in the mounting structure.
Adding a support strut under the motor to the lathe bed or bench may cure it or significantly reduce it
-Mark

The lathe itself is NOT firmly bolted to the bench... Its an original cast-iron legs with original wooden top. Howwever, when I tried to lag screw the bed to the wooden top, I discovered that the wood seems 'soft' and that the bolts were not biting very well. I installed a through-bolt where I could, but only one (the bed through holes don't line up with any holes in the leg).

I will try re-bolting to the table top, and if the wood just isn't hacking it, replace it.

Thanks!

-Mike

 

[The_General]

By the way, I should have also mentioned that if I disconnect the belt between the pulleys and the head stock, there is NO bouncing... So I know its not the motor and its belt/pully.

the Head Stock has Timken bearing (original) and they seem to still be very tight. And it smooths out the faster it goes! (So i shouldn't be an unbalance issue)

 

[markba633csi]

Yes the out-of-round /oblong is what we call runout. You can fix that with a lathe, unless it's part of the lathe!
Most machines have some but your particular design makes it more apparent.
Do some work on the bench/ rigidity issues and it should help
-M

 

[The_General]

OK - Update on applying some of the above suggestions...

I re-screwed the base into the board. Got it much tighter. No change in bounce.

I did some up-close eyes-on and noticed that the headstock 4-position pulley seems to be 'untrue', or at least it seems to be 'untrue' while rotating. I stopped the motor and carefully tried to move the headstock against its bearings: No motion. However, the 4-position pulley does wiggle... Is this normal? I checked the screw and its tight. So I kind of wonder - what is that screw doing if the pulley stays a little loose... I'll watch a YouTube video on disassembling the headstock to get some insight...

Also, if I apply a small amount of pressure to the countershaft tension rod (with slight downward pressure) it stops the bouncing. This would seem to imply that its a 'small' issue rather than a heavy-duty one...

Thanks again in advance to everyone for their thoughts and advice.

-Mike