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.