Question about part chatter and spindle bearing wear

[MadMax]

I'm machining some acetal (plastic) parts which are fairly long. During one of the roughing cycles, the bar resonates a fair bit due to flex/chatter of the part. I don't really care about the noise and the part surface quality ends up being acceptable, but I'm concerned that the bar resonance could be adding a high load on my spindle bearings.

The bar is pretty much a wet noodle compared to more rigid steel stock so I'm not sure that much of a load ends up being transmitted to the spindle bearings. Should I be concerned that I'm going to be wearing my spindle bearings machining flexible materials?

[g-codeguy]

I'm not a maintenance man, but I'm pretty sure you are safe. We use to run a lot of glass filled Teflon...wide open with the first Hardinge 42 Conquest Super Precision lathe we bought. It is still running at a sister company. Never had a spindle repair of any kind in the approximate 12 years we had it. Of course, I am not aware of its maintenance record since the lathe got moved to the sister company about 8 years ago.

However, we have 3 Daewoo Lynx lathes that have all had their spindle bearings replaced. They are around 8 years old. My personal opinion is that it wasn't due to any chatter from machining, but by having guides with too much clearance. Three-four feet of bar whipping can do some damage! We have extensions on the spindles so we can run 4 foot bars.

Our one Daewoo barfeed with chuck (rest have collets) has had a service man in twice to line up the spindle. (No bearings replaced.) Again because of bar whip in the spindle. It has Live Tooling and the spindle doesn't have to be very far off before it won't run.

Don't step on a $20 bill to pick up a penny. Use correct size guides!

[von007]

I have probably rebuilt close to 40+ spindles in my day and not once has chatter been an issue arising as a cause of spindle failure. Running yur turret into a 2000 rpm chuck at 1000 ipm on the other hand......means i eat for another day

[MadMax]

Thanks Von. My machine is my first production grade piece of equipment. I've got little experience in the long term wear issues. So far so good. I got a great deal on my machine, but that doesn't mean I get a commensurately low cost for maintenance so I'm pretty "motivated" to not overstress my gear.

Out of curiosity, how much does it roughly cost to rebuild a spindle? My machine is a Puma 200 MSA. Spindle bore is 1.77" on my machine. The service logs show that my machine's spindle was rebuilt a couple years ago. I hope I won't do a big crash anytime soon. I've been limiting my rapids in the parameters during code tryout. The rapid override is a good feature, but it's still not hard to accidentally leave it at 100% sometime during a tryout. So far I've only bumped the subspindle. Lots of crashy bits to look out for in a subspindle machine.

[Dr Dug]

MadMax

If you call Doosan or one of their distributors they can quote you on spindle rebuilds.

Alot of times the total bill is travel and expenses. If you talk to the service coordinator you can often times get service when other customers in your area get service. Then your travel and expenses are less... just a thought.

[Geof]

On a lathe chatter is caused by the tool vibrating, the workpiece vibrating or both; most often this chatter is a fairly high frequency. The spindle bearings are never going to "see" this chatter because between the part that is vibrating and the bearings is the big heavy mass of the chuck which cannot vibrate quickly.

[MadMax]

Initially I had thought that the chuck mass would have isolated the bearings from vibration, but I've noticed that I can feel the vibration in other parts of the lathe. It might be psychosomatic, but I think I even feel the floor vibrate a bit. I'm figuring that the vibration I felt in the other lathe components might be sonic resonance though which wouldn't be the same thing as vibration transmitted thru the bed.

[camminc]

Originally Posted by MadMax I'm machining some acetal (plastic) parts which are fairly long. During one of the roughing cycles, the bar resonates a fair bit due to flex/chatter of the part. I don't really care about the noise and the part surface quality ends up being acceptable, but I'm concerned that the bar resonance could be adding a high load on my spindle bearings. The bar is pretty much a wet noodle compared to more rigid steel stock so I'm not sure that much of a load ends up being transmitted to the spindle bearings. Should I be concerned that I'm going to be wearing my spindle bearings machining flexible materials?

Hi. My name is Randy. I have good knowledge of chatter in machining operations, been trained by PHD's in the concept. MFG-Labs.com

Anytime you have vibration due to chatter, unbalance or other forces during the operation, it will dampen or transfer forces into all other connecting components of the machine tool. Just as you hear it you will feel it at many other area's of the machine tool and connecting components. Thus, what you feel and hear is, yes, going thru the spindle bearings, etc.

Think of it this way, if your driving your car down the road and you feel vibration, then it is also resonating into the vehicle connected. Yes, it can do damage. You feel it and you know it will cause damage. Your concern is valid. To not validate this phenomenon is to ignore it.

A simple thing such as a car tire being out of balance either static or couple or a drive shaft or wheel bearing, even a u-joint, is certainly felt by the driver. And it certainly does damage.

Whether it is chatter of the cutting tool, part or unbalance of the rotational speed of the part, it is still a force that you feel or hear and should cause alarm. You can hear it in MHz, you can feel it in vibration of that megahertz of the rotational spindle speed or resonate frequency of the tool vibration at its close proximity MHz natural frequency. Either way, you can add a vibration sensor to monitor and control this problem and control the spindle speed by the controller or you can simply use your common sense of the magnitude of the vibration and operator adjust.

Cheap way out: You could try programming the roughing operation to change spindle speeds during that operation at intervals to change the frequency of the vibration spindle speed to another frequency, since the part might be flexing and caught into a chatter condition that it can't get out of. Just like an operator changing the spindle speed when they hear it chattering and lower the spindle speed or increase it. My guess would be it will possible climb out of that frequency and into another frequency until it again begins to flex and vibrate again and then the program will change it to another or the previous spindle speed to to eliminate. This will eliminate an operator doing it themselves. It takes a little time for the part to begin chattering at a frequency and once it starts it will not stop if it is kept at the frequency of spindle speed it is occurring at. By changing the spindle speed frequency it settles down until it digs in again and starts over again. By changing spindle speed frequency you take that initial chatter time to another threshold, etc.

Another approce is to use a smaller radius on the insert, less force. Or different insert, coating, etc. You have options.

If the part is long and sticking out of the machine tool and that is causing the vibration then that is unbalance for sure. Runout, not supported properly, chuck only clamping on the front and not the back, etc.

[DSIF]

Max,you are not going to hurt your spindle machining Delrin(acetal). Delrin was made to machine. Treat it like aluminum,rev the spindle up and GO! It's not going to melt like soft plastic. You don't even have to run coolant on it while turning or milling. I have 20 yrs experience cutting the stuff. The black Delrin puts off formaldehyde fumes that will irritate your eyes,nose,etc..Ugh!
Dan

[DSIF]

Max-let me clarify my post before Randy comes back to shoot me down. Harmonics ("fretting"),are not good for your spindle. However,occasional chatter is not going to kill it. I have rebuilt quite a few spindles in my career. The most common causes of failure are too much grease,(can't dissipate heat) and too much thrust on the bearings. Crashes don't do much good either.
Dan