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  1. #1
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    Default Resonant frequencies and surface finish ?

    After reading some posts on the MILL DRILL group. I am prompted to ask If anyone can elaborate more. Some general thoughts : Cast iron vibrates - the frequency is dependant on various physical properties .When a machine is running , there are certain resonant points or Q points that , if avoided , will improve your finish on work pieces. Rigidity will help to aleviate any resonant oscillations . I guess each situation is different , how the machine is mounted ..floor type and rigidity . Mike

  2. #2
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    To techalogical for me.

  3. #3
    Dave J Guest

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    From me too
    Then throw in 240 volt motors, LOL
    They say you get patterns on 240 volt lathes, but I am yet to see it. I think it might get lost somewhere along the belts and gears etc.

    Dave

  4. #4
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    Default Info

    John,



    I think you are mixing up resonance, rigidity, and dampening effect. For example, if you are milling a part rigidly clamped to one side of a 36" milling bed with a 4 flute mill, you could possibly have a base resonance from the following: Iron transmits vibrations about 16000 ft/s with a base of 6 ft (from one end of the bed to the other side of the bed and back kind of like how you would measure VSWR for cable that has mismatched impedance). 16000/6 = 5333 cycles/sec. Then if you excite its resonance with a 4 flute mill you get 5333/4 = 666 Hz. If it was extremely rigid then you would basically have a tuning fork that would have increased vibration from rigidity. Now if you put something that absorbed the energy in the path, you would get a dampening effect that would decrease the amplitude of the vibration.


    Now what is actually happening when you tighten different parts of a mill is the flex that is being induced at that connection is changed so the frequency of how much it is flexing is also being changed so the base frequency (and harmonic frequencies) are also changing. At that point, the dampening effects of the structure can absorb the energy faster than it can build up because you are no longer at at the resonant frequencies. However, the same thing can sometimes happen if you loosen things a little depending on the structure. As far as rigidity goes, think of the difference between a 1 lb bell and a 1 lb bag of sand. The bell is a lot more rigid but the amplitude is much greater than the bag of sand. Also the frequencies of the less rigid sand is much higher than the bell because the path the energy has to travel is much shorter (assuming that the sand is made of the same stuff as the bell). Now if you really fortunate, you can generate a frequency that generates a reflected wave that is 180 degrees out of phase of the tool deflection. This will show up as a "sweet spot" that makes machining nice.



    Bob


    From: jgedde <jgedde@...>
    To: [email protected]
    Sent: Thursday, October 6, 2011 1:44 PM
    Subject: [mill_drill] Re: RF45 - Improving Performance

    Hi Rick,

    Thanks for the feedback. I did tighten up the X and Y leadscrew backlash with a noticable improvement. Thanks for the suggestion.

    I only partially agree with your assessment about the deflection however. While pushing on the head is by no means a quantitative test, it does have some relative merit as far as measuring rigidity.

    I agree with your statement that a constant force results in a constant deflection, however, milling operations are only a constant *average* force - as each flute passes the workpiece, force will increase dramatically. The trick is to avoid exciting any natural resonances in the workpiece or mill structure. As a fellow EE, I can explain it this way: increased rigidity (aka stiffness) decreases the amplitude of the resonances and raises their frequency. Making the mill stronger by some means would increase the Q of these resonances reducing the chance of exciting the "bad frequencies."

    What does rigidity offer? Increased accuracy to a degree, much better surface finishes, resistance to vibration caused by the motor or in the geared head), less chatter, and less chance of breakage of carbide tooling.

    That said, my mill has had time to settle into its new base (increase it's height by 7") which has been bolted to the concrete floor. This in combination with the leadscrew tightening has resulted in much better cuts all around.

    But, if there are modifications those of you have made, I'd be interested to hear them. I'm toying with the idea of gussetting the column or bracing it to the concrete wall behind the machine (I suspect the latter would affect tram with temperature though).

    John

    --- In [email protected], Rick Sparber <rgsparber@...> wrote:
    >
    > John,
    >
    > I don't own a RF45 so my comments will be general.
    >
    > In order to evaluate my mill, I started by removing as much play in the X and Y was as possible. They should be snug and not drag to the point where you have trouble turning the crank. I'm sure you have already done that.
    >
    > Then I would cut a test surface with the Z axis locked. Without disturbing the test block, attach a DTI to the spindle and run the X and Y feeds to map the surface. When I do this on my RF30 I see a maximum variation of about 0.0002". Say you get this kind of variation. Then with proper technique, you should be able to cut surfaces to this accuracy. If you see variations of, say, 0.002", then something is loose and shifts over time. I use a torque wrench and a DTI to provide a consistent stimulus and a means of consistently measuring the response.
    >
    > Pushing on the head and seeing deflection with respect to the table is not a good test. I can lean on my mill head and see a few thou of bending. Doesn't matter during milling because the force is constant. This causes a constant deflection which means it can be canceled.
    >
    > Just remember than constant errors can be canceled. Errors that vary over time cannot be canceled. At best you can minimize them.
    >
    > I hope this helps more than confuses.
    >
    > Rick
    > Rick.Sparber.org
    >
    > On Oct 4, 2011, at 7:07 PM, "jgedde" <jgedde@...> wrote:
    >
    > > While I'm reasonably happy with my RF45 clone (a PM-45 with power down feed), especially for what it cost, I'm wondering how to improve its performance. Mainly rigidity. While it's got gobs of power, I'm most often limited by rigidity issues.
    > >
    > > There is some slop in the quill requiring it to be tightened down tight before cutting anything. Also, an indicator on the spindle shows 0.002-0.004" deflection side to side and 0.002" deflection front to back when I push on the head.
    > >
    > > This is a big improvement over what I had before bolting the stand down to the floor.
    > >
    > > The head gib is a disgrace, but has since been scraped and sanded much better than it was. Nonetheless, I have to keep the head gib so tight to get any semblance of consistency out of it, that raising and lowering the head is an effort.
    > >
    > > There was a guy on another forum who dramataically improved his RF45. He promised a write-up but he has since dissapeared.
    > >
    > > While I realize it's not a Bridgeport, I can't help but wonder if performance can be improved upon. As far as it being Asian, well QC issued have cropped up here and there but nothing major. My Grizzly G4003 lathe is a delight. Shows what the Chinese can do...
    > >
    > > Any suggestions, mods, etc. would be hugely appreciated.
    > >
    > > John
    > >
    > >
    > >
    > > ------------------------------------
    > >
    > > Yahoo! Groups Links
    > >
    > >
    > >
    >













    Fri Oct 7, 2011 11:02 am

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  5. #5
    Dave J Guest

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    So thats why they leave the sand in the Chinese machinery,


    Dave

  6. #6
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    Quote Originally Posted by Dave J View Post

    They say you get patterns on 240 volt lathes, but I am yet to see it.
    Dave
    Dave,

    I have the 13x40 version of your lathe, a couple of years older. The follower wheel below was faced at three different speeds, starting at 350 RPM, then 250 RPM as the diameter increased, and finally 200 RPM for the final section.You can clearly see a vibration band at the 250 RPM speed. It doesn't always do this!

    Regards,

    Frank.

  7. #7
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    At 380rpm my AL335 visibly rocks back and forward, it's what I'd expect to see if I had something fairly out of balance on a 4 jaw, but it does it even with small bits of round in the 3 jaw. All the other speeds are fine, so I just don't use that speed any more!

    I think in my case its a combination of crappy belts and flex in my 'stand' (as they call those two wonky cabinets). Hopefully the link belts I've ordered will improve things, but I'll probably have to wait until I make my big solid stand to fully solve the issue (I think the channel I used when I raised the height is actually flexing when it starts rocking).

    Interestingly, I don't remember it doing this rocking behaviour at 380 when I got it (even with the risers), and I don't remember fiddling with the leveling feet between it not doing it, and when it started.

    So maybe the belts ARE the cause, and they've gotten more of a 'set' to them over the time I've had it - it's definitely always had a growling noise in the lowest speeds which I improved dramatically (but didn't stop) by loosening the belts off quite a bit.

    Or maybe I just never used 380rpm until the day I noticed it

  8. #8
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    Would seem that if the lathe is rocking that it cant be level/flat,or if it is level/flat then your support for the lathe definately needs looking into,before continueing with anything.

  9. #9
    Dave J Guest

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    Hi Frank, good to see you back around.
    I have done a lot of car fly wheels over the years and sometimes it takes a few different carbide tools before I get the right finish, because of the hard spots and the type of cast.
    I find I take the lowest speed needed for the outside of the fly wheel and continue through at that it seems to work fine on mine.

    I do understand about the 240v motors on paper, but think after going through the belts and getting geared down through the gear box, I think it is minimal. If it was one to one belt drive I would think it would show more.

    It's a bit hard to see your picture, but do you think it was the tool?

    I have a 3 phase motor here that Bryan gave me, and I am deciding on what to put it on. If it goes on the lathe it would be good to have the variable speed for jobs like this, but I really don't think I need it.

    Dave

  10. #10
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    Quote Originally Posted by Dave J View Post
    It's a bit hard to see your picture, but do you think it was the tool?
    Dave
    Dave,

    Doubt it. The material was mild steel, tool was HSS, and nothing besides the speed was changed between the three sections - same tool, same set-up, same piece of material, same depth of cut. I have noticed sometimes on surfacing cuts on some materials that the finish is slightly worse, though still acceptable, at 250 RPM than at other speeds - no obvious burrs or anything on the headstock gears though.

    Frank.

  11. #11
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    Frank, that's very interesting. Do you have a VFD on your lathe where you can change speeds on the fly? I couldn't get your picture beyond "weeny" size but I think I can see the band you're referring to and it's quite dramatic. Is there any vibration etc in the machine at this speed? If you back off the tool so it's not cutting is there any vibration?

    So thats why they leave the sand in the Chinese machinery,
    Ha ha, ah so THAT'S the reason!

    Pete

  12. #12
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    Pete,

    No VFD, just the standard gearbox and single phase 1 1/2 HP motor. I cannot feel any unusual vibration at this speed normally, and didn't at the time, but I could see that the finish was not good, so stopped the lathe and moved to the .next lower speed, then restarted without changing anything else. I could have got rid of the markings with a skim cut at another speed, but left it as a talking point - it has caused some interesting discussion over the several years since it occurred.

    I've blown up the photo a bit for you to show the vibration band a bit more clearly, but it is a poor definition image originallly enlarged from a low resolution photo, so there's not much detail.

    Another oddity when this lathe is making a reasonable cut at this speed (nominally 250 RPM) with only a fluorescent light on, is that you occasionally get a very strong stroboscopic effect which makes the chuck appear to be stationary. This has happened about three times in the last 12 years, but I doubt very much that it would be related to the vibration markings on the follower wheel - it looks odd when it happens though.

  13. #13
    Dave J Guest

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    Thats strange Frank, I cant say I have had a problem like that. I do find some times if the tool is not ground spot on it will leave a bad finish on mild steel, but I don't seem to have any problem with vibration.

    Dave

  14. #14
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    Quote Originally Posted by franco View Post
    Pete,

    No VFD, just the standard gearbox and single phase 1 1/2 HP motor. I cannot feel any unusual vibration at this speed normally, and didn't at the time, but I could see that the finish was not good, so stopped the lathe and moved to the .next lower speed, then restarted without changing anything else. I could have got rid of the markings with a skim cut at another speed, but left it as a talking point - it has caused some interesting discussion over the several years since it occurred.
    Yeah I bet it has! What about next time it happens you stop the lathe, don't change anything, and simply re-start it? If you did that a few times and it comes straight back then at least it eliminates the possibility that it was actually the change of speed that cured it and not simply stopping. Just at thought, frankly I have NFI. Excuse the pun

    Pete

  15. #15
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    Quote Originally Posted by pipeclay View Post
    Would seem that if the lathe is rocking that it cant be level/flat,or if it is level/flat then your support for the lathe definately needs looking into,before continueing with anything.
    It's certainly not level - I've mentioned in previous posts that I have a twist in my bed that I can't adjust out (the back leveling foot on the tailstock just hangs in midair when I try), probably due to the previous owner not bothering to set it up properly.

    That said, I do have all 6 feet down on the ground, and it's perfectly stable at any other speed, its only at 380 that everything seems to conspire to make it rock. It's certainly in no danger of falling over, but the movement is perceptible by eye. It will have to do until I happen across enough heavy duty steel at a good price to make the stand I want for it, which should hopefully improve stability and also allow me to untwist the bed.

    All of which is not particularly relevant to the discussion at hand, I simply wanted to share my observation that certain speeds can indeed create a resonant effect which is not as obvious at any other speed. This phenomenon can also often be observed in cars, sometimes a vibration will be observed at certain road speeds, and will go away if you speed up or slow down.

    I can certainly see the original posters point, being that resonance is a form of vibration, and vibration degrades surface finish. So if you can find a speed on your machine that has the least resonance, in theory you'll get the best surface finish (assuming you can use a suitable feed for your material at this speed). Whether that has a noticeable effect on surface finish or not would depend greatly on the design and mass of the machine itself, the mounting of it (including the floor it sits on), the drive system, the material being turned, the speeds and feeds involved etc etc. And in theory you could have two of the same lathe in the same condition mounted identically doing the exact same job in the same way, and still get a slightly different finish, as I would guess the castings are unlikely to be 100% identical (unless you're talking about extremely high quality toolroom lathes).

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