Optimizing regenerative chatter in milling.

[krishi1978]

Hi all,
as a part of my work i need to work on some task which is related to "Optimizing regenerative chatter in milling". For this purpose i would like to perform "Harmonic analysis" on the differential cutters with ANSYS. However,i would like to know whether the chatter in the milling cutter is minimum if the insert spacing is even or differential one? And is there any other easy way apart from my way of approaching(i.e Harmonic analysis). Please send me your replies and valuable suggestions.

thanks in advance,
regards,
Krishna.

 

[bklauba]

Most of my experience has been with the machining of large mild or stainless steel fabrications, so my comments are based on such. Chatter of milling cutters has numerous causes, amongst them the resonance of the in-fixture, as-clamped, workpiece, and whether the impacting of the cutter excites this resonance.

Another factor has to do with the condition of the workpiece. A "live" part can undergo what can be described as stress removal (as to be distinguished from stress relief), during rough machining. Some of the stressed material ends up being chips. The workpiece now suffers an unbalanced preload, and changes shape, often in a non-uniform manner, affected by the stress pattern, machining path, and clamping geometry. This is often mis-diagnosed as the workpiece having variations in hardness, with tool chatter and variations in surface finish being the result, while the actual problem is often due to variations in depth of cut, caused by the while-in-fixture distortion.

Ignoring these factors (especially on the types of parts mentioned), and only doing a harmonic analysis of the tool, might not resolve your problem.

Can you describe the types of parts you are dealing with?

BK

 

[krishi1978]

Hello bklauba,

Thanx for ur reply. My task is dealing with the "tool and work piece interaction". Meant to say, when both are in interaction, at some spindle speed(high spindle speeds) the vibrations are induced in to the tool(milling cutter) due to which the surface of the work piece will be non-uniform. So as my task demands, i need to simulate this process and need to optimize the pitch so that, less chatter will be the goal. I hope now it is clear to you. If still you need more details, i will try to provide. Thanx for your replies.

Regards,
Krishi

 

[krishi1978]

Helo,

here i would like to know some more information regarding machine vibrations also. If i would like perform the harmonic analysis to calculate the vibrations induces in to the machining, how do i need to model the task and force directions. Please give me your valuable suggestions.

thanks,
krishi

 

[vzeos]

Here's a pretty good article on chatter:

http://www.moldmakingtechnology.com/articles/090402.html

 

[BillPSU]

The problem as I see it in your approach is you are assuming total rigidity of the machine tool and the tool mounting and total rigidity of the fixture and fixture to machine interface. An example is the use of machine tool made from steel weldments with roller pack slide bearings versus a machine tool with cast iron construction and Turcite slide material. The cast machine will absorb some of the vibrations induced by a cutter and Turcite will also absorb some vibrations while a steel construction will be more apt to resonate with the cutting action. I have actually moved a job from an older machine tool (cast construction) to a new machine tool (steel weldment) and found I had to reduce cutting forces due to the differences in machine construction. There was a definite increase in the sound levels produced and loss of surface finish.

I feel your approach is overly simplistic. You can probably change the holding fixture or the tool mounting and reduce the chatter without changing the tool to workpiece interface.

I once read a study which was performed in a precision lathe application where the main problem with the finish produce by a lathe was movement between the cutting tool and the part. A higher stiffness spindle was required and a rock solid machine base was required. The machine which was ordered was equipt with a heavy duty air spindle for the part and jig mill with a rotary table for tool motion.

 

[krishi1978]

Hi BillPSU, and RGasEng

Thanx for your article on chatter. If you any other related articles or other ideas, please suggest to me.

and BillPSU, your explanation is quite near to reality. Itz true that i am assuming machine as total rigid body in my modelling. But when it comes to the spindle i would like to give more stiffness. However, your idea is better than my idea. And i will try to work on both. Could you please suggest me, how do i need to model(type of elements for base, machine body, spindle,tool and work piece)for Harmonic analysis. And how do i need to connect them elements. Just give me the idea, how you would this. So that, i can work more efficiently by mixing both the ideas.

thanks in advance,
krishi

 

[jbel]

This University of Florida web site has excellent information.

http://highspeedmachining.mae.ufl.edu/

 

[BillPSU]

Modeling needs some connection with the real world with empirical data. Start from some base condition and modify different factors to determine the affect so you can observe changes in behavior. After you have investigated various single changes combine changes and see if you can predict the result. Finally try to come up with some standardized formula for the different conditions and then change you base assumptions and see if the formula still works.

 

[krishi1978]

hi jbel, BillPSU

thanx for ur msg and link..but unfortunately i am unable to access anything from there int he link....could plz provide me fresh link...plz take a look...

thanks,
krishi

 

[jbel]

The provided link works fine. Here are a few more that may help

http://highspeedmachining.mae.ufl.edu/htmlsite/html_site.html

http://highspeedmachining.mae.ufl.edu/simulations.html

http://highspeedmachining.mae.ufl.edu/htmlsite/faq.html

The last link provides a history of chatter research and an excellent derivation of receptance coupling substructure analysis. I believe it is just what you are looking for.