Vibration in "cantilever" rotating bar

[sempiternum]

Hi. I'm working in the design of an auotomatic bar feeder for a CNC lathe. I've established that the bars will not be longer than 1.2 m and will not have a diameter of more than 75 mm. Suppose they are steel bars, which would make them have a mass of about 80 kg

So, bars are tightly held in one end by the lathe chuck, as shown in this image

Now, on the other end there'll be the automatic feeder. When the chuck starts to rotate to, let's say 1000 RPM, the free end will not naturally stay centered, so, I need to calculate the magnitude of the force created by the vibration on the free end as to design the components that will hold it steady.

How can I approach this calculation?

Thanks.

 

[Mech_Engineer]

Vibration analysis is a somewhat complex topic, for a start you might take a look at some of MIT's Open Course information available on the topic: MIT OpenCourseWare - Mechanical Vibrations.

Generally speaking, I think you'll need to minimize the cantilever load to minimize the amount of vibration induced by both 1) asymmetries in the bar, and 2) tilt due to run-out in the lathe chuck and mechanical interface. How much of the bar is being processed, and how straight will they typically be?

 

[sempiternum]

Vibration analysis is a somewhat complex topic, for a start you might take a look at some of MIT's Open Course information available on the topic: MIT OpenCourseWare - Mechanical Vibrations.

Generally speaking, I think you'll need to minimize the cantilever load to minimize the amount of vibration induced by both 1) asymmetries in the bar, and 2) tilt due to run-out in the lathe chuck and mechanical interface. How much of the bar is being processed, and how straight will they typically be?

Yes, actually, the bar won't be in cantilever, it would be held in place through all of its length (Outside of the lathe) with 3 or four devices like shown in this picture

What I want to calculate is a magnitude of force "F", which should be big enough so that there's minimum eccentrity, without applying excesive pressure.

I presented it as a cantilever because I thought it would simplify analysis since it would be the most critical situation.

 

[Mech_Engineer]

The supports you've outlined will need to be almost perfectly coincident with the lathe chuck to work, and will only work if the bar is very straight. Also, I'm not sure a simple v-block like you've outlined will be a good choice for support of a rod rotating at thousands of rpm, friction may be a problem (and will be loud). You will probably want rollers/bearings instead.

This is a problem that has been solved many times, do a search for "long part on a short lathe" for some ideas. Take this thread for example: http://www.practicalmachinist.com/vb/general/long-parts-short-lathe-234926/

 

[sempiternum]

The supports you've outlined will need to be almost perfectly coincident with the lathe chuck to work, and will only work if the bar is very straight. Also, I'm not sure a simple v-block like you've outlined will be a good choice for support of a rod rotating at thousands of rpm, friction may be a problem (and will be loud). You will probably want rollers/bearings instead.

I've based my design on what I've seen in most bar feeders (see notes). Also, "V" channels will probably be constantly lightly lubricated, and the force from the plastic roller applied through a spring. Most machines actually use 2 "V" channels, but I'd still need some sort of notion of the force to be applied by the top "V" channel.

Notes from: http://www.productionmachining.com/...derations-for-high-speed-lights-out-machining

Guide Channel Sets
Most magazine bar feeders use some form of guide channel sets made of polyurethane that are sized to handle a specific range of material. Guide channel sets form the long channel inside a bar feeder that opens and closes to capture the pusher and barstock and help create a hydrostatic bearing when flooded with oil in automatic operation. A few years ago, polyurethane use emerged in items such as spindle liners, bearing blocks, rollers and guide channel sets because of its inherent lubricity, durability and ability to absorb vibration and noise.

Roller Steady Rests
Another development that occurred during the last few years and was embraced by some bar feeder manufacturers has been the use of roller steady rest systems featuring adjustable polyurethane rollers. Some bar feeders place this mechanism internal to the bar feeder; others place it between the bar feeder end plate and the lathe or provide both internal and external roller systems. The bottom line is that these devices help support the rotating bar and promote higher spindle speeds.​