Dynamic stiffness of machine tool

[kingmeister]

I have just designed the fixturing and acquire the apparatus to perform a static stiffness test on our machine tool at work. I have not carried out the experiment however as I need to purchase a load cell which I am hesitant to do at the moment as I do not know whether I require an output from a load meter when I come to measure the dynamic stiffness of the machine.

I would appreciate any help you could offer in explaining how to calculate the dynamic stiffness of a lathe. I found a very informative website (below) however the steps to measure dynamic stiffness are missing.

http://www.engissol.com/Article2.htm

Specifically I don't understand how to acquire the vibration data if I use an unbalanced mass.

 

[Cockroach]

Vibration of under balanced masses is something common in rotating machinery applications. Obviously it depends on the RPM and the specific centrifugal force generated from the axis through the piece.

I think searching rotating machinery or advanced books in vibration analysis would help you. I vaguely recall doing this in Vib classes, but that would be twenty (20) years ago.

Best of luck with it.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[GregLocock]

I suspect that you are really interested in the dynamic stiffness measured between the workpiece and the tool. You aren't going to get very far on that using a balance measurement.

In other words, you are probably interested in a plot of Ftool/(Displacement of tool wrt workpiece) against frequency.

This is a tricky thing to calculate, and is also tricky to measure.

If it were a simple structure we would do an experimental modal analysis of the lathe, from which we could derive the dynamic stiffness. In practice the resemblance between values measured on a static lathe, and a running one, would be quite different.

Your static load cell can be used to some extent for dynamic work, look at its frequency response. But bear in mind that in practice we use special purpose low mass load cells.



Cheers

Greg Locock

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

 

[EdDanzer]

What are you trying to do that requires knowing dynamic stiffness?
Having operated several different makes of CNC turning centers and machining centers, knowing the static stiffness can determine the metal removal capabilities of the machine. The dynamic stiffness will need to include the work piece, tooling, and cutting parameters. Kennametal has a system that listens to the cutting tool and adjusts the speeds and feeds to optimize the productivity of you machine and tooling.
This is what we did to test VMC stiffness. We have also done spindle torque tests, and a test of stiffness of KM, Capto, ABS and stick boring bars in a CNC turning center.

http://www.danzcoinc.com/html/machining_center_stiffness_tes.html

has some information.

 

[BobM3]

I've seen mutiaxis load cells designed specifically for mounting tools in lathes and measuring dynamic loads. I believe they were piezo electric. You have to make sure the compliance of the load measuring device doesn't contribute to the springiness of the system.

 

[kingmeister]

I was going to put a DTI on the spindle head and another on the tool. The force is applied with a pneumatic piston and the load cell mounted on the end of that. Am I right in saying it does not matter about the stiffness of the components in the middle of the stiffness loop (i.e load cell, bracket and piston) beacause I am measuring the displacement outside of that loop. And all the force indicated by the load cell will be transmitted through those components.

 

[GregLocock]

Yes, that's fine, so far as it goes.

Cheers

Greg Locock

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