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Dynamic Balancing in the Home Shop

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  • flathead4
    replied
    You're right, of course. L1 and L2 are physically connected to T1 and T2 in the RPC. I must have been having a minor brain cloud.

    Tom

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  • lakeside53
    replied
    In an RPC implementation, L1 is T1; L2 is T2. T3 is generated.

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  • flathead4
    replied
    L1 and L2 are passed though - utility "60hz". T3 is generated, and it can only be 60 hz at the rpc sycnchronous speed (say 1800 rpm) but in reality that will slip 3-5% (or more).
    Not the way I understand it but I'm not an electrical engineer. L1 and L2 are at 60hz but wouldn't both T1 and T2 be somewhat less than 60hz due to slip in the RPC induction motor/generator. I would also think T3 would be at the same frequency as T1 and T2 but not exactly in phase.

    Tom

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  • oldtiffie
    replied
    Fog and bog

    Originally posted by millwrong
    Oldtiffie!! Nice to hear from you! Hope you're back to enlighten us with pearls, and a boot for the lord!
    Thanks MW - appreciated.

    I only intend to appear on an occasionalonal basis and in between times I will descend/return to my domain in the primeval fog and bog.

    I see a lot here in many posts about people worrying themselves sick after having seen some others say that the machine or finish or what-ever has to be pretty well "Laboratory" standard and engaging in a needless, futile and quite unnecessary time-wasting expensive excercise to no avail or practical purpose.

    ("Laboratory" standard exceeds standards for "Inspection", and "Tool-Room" and is close to "Metrology" standard).

    Realistically, all a machine needs to be able to do is to get just that bit extra finish or size-keeping that you really need as perhaps opposed to what you want. "Need" and "want" are not necessarily the same though many cannot or will not tell the difference.

    Machinery's Hanbook is a great source of advice of finish vs. machine and as regards "working tolerances" for many applications.

    My T&C grinder hardly gets any use. An angle grinder and my pedestal grinder will do most of what I really need - a good wheel dressing as well as a hand diamond hone if used well do very well.

    My T&C and SG mainly get used for "(too) hard" stuff such as TC etc.

    I almost exclusively use HSS tools. If it is a lathe tool etc., I grind it on the face/front of my pedestal grinder wheel so that I get a nice "hollow grind" which can be "touched up" either on the lathe with a hand-held diamond hone or the pedestal grinder.

    If I am using a diamond or CBN wheel I use it on the T&C - but in most cases I hand-hold the tool and so use the T&C grinder as a pretty flash pedestal grinder.

    I never grind for "looks" - just all that is required for the surface to be functional for the purpose for which it is intended. Same on a mill or a lathe etc.

    A look at these pics might be quite useful and instructive as regards tolerances, limits and fits as well as surface finish and the machines best suited.

    It also applies to T&C and SG grinders.

    http://i200.photobucket.com/albums/a...and_Fits11.jpg

    http://i200.photobucket.com/albums/a...ness_card1.jpg

    http://i200.photobucket.com/albums/a...ess_table1.jpg

    http://i200.photobucket.com/albums/a...ess_table2.jpg

    http://i200.photobucket.com/albums/a...and_Fits10.jpg

    http://i200.photobucket.com/albums/a...and_Fits12.jpg

    http://i200.photobucket.com/albums/a...and_Fits13.jpg

    http://i200.photobucket.com/albums/a...and_Fits14.jpg
    Last edited by oldtiffie; 11-29-2011, 11:31 PM.

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  • Don Young
    replied
    Originally posted by lakeside53
    And.. if you are pulling frequency lower on two of three legs, then the phase relationships must also be altered. Basically a bunch of small pushing and pulling going on inside the load motor which means increased vibration.
    I certainly understand the logic of this but if one frequency is lower than another the phase relationship is continually changing at the frequency difference rate. If one frequency is 60Hz and another is 59Hz the phase relationship continually rotates all the way around thru 360 degrees once every second. A phase relationship which is continiously changing in the same direction does not seem consistent with observed operation and measurements of the RPC. I also do not think the counter EMF generated by a loaded single phase motor with its associated slip is at a frequency different from the voltage applied to the stator from the power source. I do not have a simple explanation of this seeming paradox.

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  • ces224
    replied
    Thansk JoeCB, Rich Carlstedt, and Macona for the info. I think I will start a new thread.

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  • lakeside53
    replied
    And.. if you are pulling frequency lower on two of three legs, then the phase relationships must also be altered. Basically a bunch of small pushing and pulling going on inside the load motor which means increased vibration.
    Last edited by lakeside53; 11-29-2011, 08:54 PM.

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  • lazlo
    replied
    Originally posted by lakeside53
    There's more to this problem than voltage balance; the frequency of the generated leg will vary by rpc load.

    L1 and L2 are passed though - utility "60hz". T3 is generated, and it can only be 60 hz at the rpc sycnchronous speed (say 1800 rpm) but in reality that will slip 3-5% (or more).
    Ah, that's interesting Andy -- good point.

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  • lakeside53
    replied
    Originally posted by lazlo
    Yes, absolutely. I was running my Harig with an unbalanced rotary, and the surface finish improved a lot when I switched to a VFD.

    That said, if you balance the rotary converter, you can get it within ~4% of phase perfect (IIRC), so maybe mine was a worst-case scenario.
    There's more to this problem than voltage balance; the frequency of the generated leg will vary by rpc load. Why?

    L1 and L2 are passed though - utility "60hz". T3 is generated, and it can only be 60 hz at the rpc sycnchronous speed (say 1800 rpm) but in reality that will slip 3-5% (or more). L1-L2 is 60hz; L2-L3 and L1-L3 will be less than 60hz; Not the best for critical surface finish applications.

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  • macona
    replied
    Originally posted by ces224
    I have been playing around with the idea of a dynamic balancer and found this thread. I have a set up similar to "A poor mans balancer" and have been using a coil to fire a timing light. What I would like to do is use a white LED as a strobe light so that it can be mounted in a holder and fire the light on the rotating object. Can some one tell me how to get the LED to fire as a strobe? I like the idea of using an accelerometer to fire the LED strobe circuit but I do not know enough about electronics to get it to work. Any help is appreciated.

    I would not use a white LED. White LEDs use a phosphor to convert Blue/UV to White with red, green, and blue phosphors. Due to this there is persistence in the phosphor.

    Just use a xenon strobe. The tech is old and works very well and would be easy to build or modify an existing strobe.

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  • millwrong
    replied
    Oldtiffie!! Nice to hear from you! Hope you're back to enlighten us with pearls, and a boot for the lord!

    Leave a comment:


  • oldtiffie
    replied
    Is it real(ly ?)

    Originally posted by metalmagpie
    And before you even balance anything, are you running your SG through a phase converter? If so, try it through a VFD. All shop-built phase converters by definition put out slightly unbalanced waveforms. This can be proven by running an idler motor on real 3-phase power and quantifying its level of noise, then hooking it up in the RPC and running it again. Inevitably it will be louder because the 3rd leg is always a bit out of phase (off of 120°, I mean) with the others.
    Very good advice mm.

    Even if the motor spindle - which incorporates the wheel mounting - is on a normal single phase supply, there is also the matter of "single phase cogging" which has been done to death in previous posts etc.

    http://www.google.com.au/#hl=en&cp=2...w=1280&bih=501

    http://en.wikipedia.org/w/index.php?...ofile=advanced

    Any or all of these faults which may or may not effect the dynamic balance may add to or subtract from each other and seemingly dynamic imbalance which may or may not be real or evident. Bearings may also be an issue.

    If it were my grinder, I'd put a glass of water on the motor and see how the rippled surface of the water looks. If it looks OK it should be OK but it is very subjective.

    My test on my T&C grinder is to "surface grind" a flat surface mounted on a magnetic chuck or vise. If it looks OK, it is OK.

    Some problems can be "over thought" and perhaps worried about unnecessarily - so I try to avoid them - as that can be frustrating and expensive for no real net tangible result/s or gain/s.

    Most of my "poor" T&C " problems have at least a component of bad "dressing" in them.

    Put a wheel on a 1 1/4" (31.75mm) shaft and put it all on a "trued up" balancing fixture. If the wheel is badly out of balance, the wheel assembly will soon "heavy spot" will soon "sink to the bottom". There is nothing "flash" about the balancing fixture so long as (round or flat) balancing surfaces are in the same horizontal plane.

    If by chance the grinding/wheel spindle is belt-driven by a remote/seperate motor than finding the fault (if any) may well be that bit more difficult.

    In reality in many small shops, a T&C grinder need only be as good as - or "just a bit" better than a "Baldor" bench grinder.

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  • 3jaw
    replied
    I used to balance rotors for centrifugal air compressors using a machine made by this company:

    http://www.schenck-usa.com/index.asp

    They have a great little book called "Fundamentals of Balancing" for $25. You have to register your info to be able to order it. I have the book and it isn't overly technical just the nuts and bolts of balancing and the calculations that are used.

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  • lazlo
    replied
    Originally posted by flathead4
    I don't like the sound of that...
    Geez! I just meant buy him a beer!

    Seriously, there are only a couple of dynamic balancers on the market, and most use the same software. You put the device to be balanced between centers, it rotates it, and tells you where to put the balancing weights. Basically, a very fancy tire balancer.

    So like a tire, you can get a quick and dirty balance, or you can futz with it to virtually any degree of accuracy. Its been awhile since I talked to the guy, but I seem to remember he said he could get 6 times stock NEMA balance pretty easily. When I asked about super precision balancing, as done on the Hardinge HLV_H, he said it was mostly a matter of time.

    I got the impression it was a lot like bringing a part to a grind shop: if you ask for a tenth across the whole part, they're going to charge you a lot. But if you chat them up, show interest in the shop, maybe tell them what you're working on,... and tell them to give it their best shot, a lot of times they'll get that good for a *lot* less.

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  • lazlo
    replied
    Originally posted by flathead4
    Does the unbalanced waveform result in measurable vibration in the driven motor and thus causes visible or measurable irregularity in the finish of the Surface Grinder?
    Yes, absolutely. I was running my Harig with an unbalanced rotary, and the surface finish improved a lot when I switched to a VFD.

    That said, if you balance the rotary converter, you can get it within ~4% of phase perfect (IIRC), so maybe mine was a worst-case scenario.

    Leave a comment:

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