modal analysis of pocket seat in milling

[freiFEM]

hi all,

can anybody give me an idea how can i conduct modal analysis of a pocket seat in milling in MSC NASTRAN for windows...thanks in advance..

 

[GregLocock]

I'd guess a pocket seat has a first resonance of interest of the order of thousands of Hertz.

You need to specify what you are trying to do.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[freiFEM]

Hi GregLocock,

Thanks for the reply. Actually i am performing the optimization task. I have finished it and conducted all priliminary tests on the machine and compared the both original and optimized milling cutters..everything was good... Now i would like to check the stability of both the cutters. For that we decided to conduct harmonic analysis to compare the frequency response function. When i have conducted normal mode analysis i got the frequency values as, for original one around 22000 Hz and for optimized one around 13000 Hz..but this is clear for me..since f=sqrt(K/m)..as i have optimized by removing the material..means ..mass..K value went also less..which gave 13000 Hz..but..for stability checking..i am not getting the idea..so i thought..frequency response function will help..however..i would like to know..how i can solve this problem..and how can i compare them....your suggestions will be helpful..

thanks in advance..
tobias

 

[GregLocock]

Well, I'd model it with the cutter in contact, and without the cutter in contact, and compare the frequencies of the cutter and the work part in those two conditions. That'll give you some idea as to whether you are getting significant mass (or stiffness) loading of one part by the other. I must admit I'm completely out of my depth on this one.



Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[freiFEM]

thanks GreLocock,

some how i have performed the normal mode analysis..but the results are a bit confusing for me..as i gave in my previous reply..there is difference..in frequencies..its k..but when i found out the machine frequency..its just 50 Hz..here in my case..as i mentioned earlier..in optimized case..the first mode value..or i would say..the least frequency value is around 3300 Hz..so..if i am thinking correctly..there is no chance at all for the resonance occurance...since..50 ..nd 3300 ...large difference...in this case..there is no need to perform dynamic analysis..please give me ur openion here..and your idea also..

thanks in advance..
tobias

 

[GregLocock]

No, the primary excitation will be the number of teeth on the cutter times the shaft speed, and all integer multiples of it (well, for practicial reasons it is more likely to be the odd integers rather than even). So what is the shaft speed of the cutter? How many teeth does it have?

By the way, I see you are from Germany. Your English is far better than my German, but it would give a much better impression if you used normal sentences and capital letters.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[freiFEM]

Hi,
Thanks for the reply. As suggested I will try to write in a promising way. Coming to the shaft speed, do you mean the maximum RPM of the milling machine? If so, it is around 20000 RPM. And coming to the number of teeth, the cutter i am using consists 8 number of teeth. Actually today i have tried to run "Frequency Response Analysis" in MSC NASTRAN for windows. First i wrote 2 functions, 1 is with Load Vs Frequency and 2 nd is with Critical Damping Vs Frequency.Here in 1 st function, i took "Equation" option and i gave x value from 0 to 50(This is because i thought this is exicitation frequency since machine frequency is 50 Hz) and Y value as 1(which is load i.e Amplitude). And then i have related this function (in modal->load->nodal)by using function dependence.And for the 2 nd function again i took "Equation" option and here x is again from 0 to 50 and Y is o.5 (critical damping factor for metals). Then in load->dynamic analysis i typed the number modes as 50, and modal damping is the 2 nd function i have written, and frequency response as 1 st function.Then i tried to run the analysis. Everything went fine,but when i tried to check the results,as usual..i can`t c anything.So i am requesting you that, please let me know, whether the procedure i have followed is correct, if not please give me your idea.

Thanks a lot,
Tobias.

 

[GregLocock]

Thanks for writing in sentences. I must admit I don't remember having to type any equations into MSCN4W, for a dynamic analysis, but that's beacause I only do normal modes, not transient analysis.

So, your fundamental excitation frequency is going to be 20000*8/60, 2666 Hz, and the harmonics will be 2,3,4, etc times that. As I said, Harmonics 1 3 5 etc are likely to be more important than 2 4 6 etc.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[freiFEM]

Hi,
Thanks for the reply.Here i would like to know some more information.If i would like to compare the deformation at some area(for example pocket seat in milling cutter), between 2 parts(in my case a standard original one and new optimized one), what are the possible ways? Please let me know.

Thanks in advance,
Tobias.