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Thread: Knife Edges for Crank Balancing

  1. #31
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    I did not disagree with the 'stationary' statement, Folks...

    My 'hold and release' technique achieves the same thing.

    What bugs me is the drawing. Showing it level would make too much sense.

    And the countermass must have bubbles.

    ....Cotten
    PS: Tried to watch the U-tube, but it just sits and spins for me.
    Is that George Yarocki?
    Last edited by T. Cotten; 12-20-2017 at 02:43 PM.
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  2. #32
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    Hello Folks,

    A pair of "straight" edges turned up yesterday but I need some advice. They are 750mm, say 2' 6", long so I would need to shorten them to my intended 2' long knife edges but before I use them I want to make sure that they are straight. I don't have a surface plate (it is something that I want/need to get but my small workshop is already at greater than 100% capacity so I have been holding off on getting one) so the obvious thing to do was to compare them against each other. Resting the two beveled edges together (see picture) there is a gap in the middle. Using feeler gauges as a guide I can get a 0.002" gauge between them for about 6" to 8" but not a 0.003". The gap then tapers out over the next 6" or so at both sides.

    Now I am aware that this is not definitive, they could be much worse than this if one is convex and the other concave but I don't have a known straight edge to hand to compare at the moment.

    So my question is, assuming I can verify how straight these actually are, what is an acceptable level of straightness?

    Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway? I can return these to the vendor and swap them for higher quality/more expensive items but before I do I want to gauge what level of straightness I am aiming for?

    Thanks






    John

  3. #33
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    Quote Originally Posted by TechNoir View Post
    I don't have a surface plate (it is something that I want/need to get but my small workshop is already at greater than 100% capacity so I have been holding off on getting one)
    The problem with surface plates is they weigh so much that you can't just put them out of the way when they're not being used. Since Nature abhors vacuums and uncluttered spaces they will fill up with items "temporarily" placed on them. But, I digress. With a surface plate determining how straight your straight edges are is, ahem, straightforward. Without it the least expensive alternative I can think of is a precision straight edge, and those aren't all that cheap.

    Quote Originally Posted by TechNoir View Post
    what is an acceptable level of straightness?
    That can be calculated. To arrive at a good estimate, assume you have a straight edge that tilt downward by 0.002" over a distance of 1 ft. This makes it an "inclined plane," for which equations aren't needed because we can cheat using an on-line calculator such as the one here:

    https://www.engineeringtoolbox.com/i...es-d_1305.html

    To see where this leads, assuming there is no friction, and plugging 10 kg (22 lbs) into the calculator along with an angle of arctan(0.002"/12") = 0.01 deg., a force of 0.0017 Newtons is required to pull that weight up the plane. The same 0.0017 N is trying to get it to slide/roll down that plane. Plugging in the acceleration of gravity, this is 0.17 grams. That is, an imbalance of only 0.17 g will cause your flywheel to roll down that slope. But, put another way, if you have 0.18 g on the other side of the flywheel, it will roll up the slope. What this means is if your parallels are bent or tilted by this amount you wouldn't be able to balance your flywheels better than a negligible 0.2 g because you wouldn't know if they were rolling because of weight imbalance or because of tilt of the straight edges. Assuming I haven't made a mistake in my calculation...

    Quote Originally Posted by TechNoir View Post
    Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway?
    This also can be calculated. Or, we can again cheat by using an on-line beam calculator. However, I'll leave that as an exercise for the reader because you'll have to enter the dimensions of your straight edge and look up Young's modulus for steel to get a result. Once such calculator is at:

    http://www.engineeringcalculator.net...alculator.html
    Last edited by BoschZEV; 12-23-2017 at 09:44 AM.

  4. #34
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    Hi Bosch, thanks for taking the time to reply, I know that you are very busy at the moment so your words are very much appreciated.

    Since posting my last post I have had a think and a chat to a friend. My lathe bed is a vee way and so whilst it is an old lathe with relatively worn ways the portion of the front way just behind the vee is relatively unworn as the saddle does not touch it. Using this as a comparator I think most of the problem is with one of the straight edges rather than a combination of both.. My friend has a lathe which has had relatively little use since its beds were professionally reground so I am going to compare the straight edges against his lathe bed also. I shall see how they all compare and if the results are consistently the same then I should be able to determine exactly where the high spots on the straight edges are. Then I will decide if I am going to return them or address the high spots myself.

    I still might invest in a precision straight edge as per your suggestion, I am sure I will get some use from it more than just checking these knife edges.

    Thanks very much for the calculation, Your conclusion:

    Quote Originally Posted by BoschZEV View Post
    What this means is if your parallels are bent or tilted by this amount you wouldn't be able to balance your flywheels better than a negligible 0.2 g because you wouldn't know if they were rolling because of weight imbalance or because of tilt of the straight edges. Assuming I haven't made a mistake in my calculation
    Seems to say that 0.2g is an acceptable tolerance when balancing these old engines. The balance I have ordered is accurate to within plus or minus 0.1g so if I could reduce the error in the straight edges to, say, 0.001"" then I think we are good to go.

    You were also kind enough to point me in the direction of calculating the sag in the "beams"

    Quote Originally Posted by BoschZEV View Post
    This also can be calculated. Or, we can again cheat by using an on-line beam calculator. However, I'll leave that as an exercise for the reader because you'll have to enter the dimensions of your straight edge and look up Young's modulus for steel to get a result. Once such calculator is at:

    http://www.engineeringcalculator.net...alculator.html
    This I had thought about a bit more (maybe because I work in Civil Engineering and there is someone talking about this sort of stuff everyday in the office even if I am not a Civil Engineer myself). I already had plans to beef up the straight edges by sandwiching them between some larger section steel bars. However your first calculation, giving an indication of the effect of slope on the balancing operation, will enable me to determine just how much sag is acceptable and how beefy the sandwich needs to be or maybe just shorten the beam a bit.

    I will post an update once I have advanced things a bit further although it might be a week or two due to some seasonal distractions.

    Thanks very much once again your thoughts and comments are very much appreciated.

    John
    Last edited by TechNoir; 12-23-2017 at 10:41 AM.

  5. #35
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    Quote Originally Posted by TechNoir View Post
    ...So my question is, assuming I can verify how straight these actually are, what is an acceptable level of straightness?

    Also, when placing a, say, 20 or 30 pound crankshaft assembly on knife edges then how much flex do you get in them anyway? I can return these to the vendor and swap them for higher quality/more expensive items but before I do I want to gauge what level of straightness I am aiming for?
    John!

    I had the edge of my planer blades ground flat as a pair to assure they were matched and "straight"
    And yes, a flywheel assembly definitely can make rails sag. I have to place the assembly quite close to supports, even with 1 1/16" rails, and would suggest that the supports be no more than six inches apart.

    ....Cotten
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  6. #36
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    Quote Originally Posted by T. Cotten View Post
    and would suggest that the supports be no more than six inches apart.
    ....Cotten
    Hi Cotten,

    Thanks for your input. I had planned supports further apart but given the weight of these old crankshaft assemblies I can see why you recommend 6". I will weigh my 20F and my Matchless and see what level of sag my proposed apparatus calculates as with these weights and adjust the length to suit.

    I have 2 straight edges, each 2' 6" long. If my "good" one turns out to be good then if I were to half it I would still have two good edges each 15" long. This would seem more than adequate given your 6" recommendation.

    Many thanks to you too for your help with this, I am learning a lot.

    John

  7. #37
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    I learn too, John!

    My apparatus taught me that planer blades were not a great idea, although I use smaller ones on a wheelhub, rim, and tire poise.
    Somewhere I posted that if I were to do it all over, I would use ground round rod, and now I must add shorter length.

    There's complications in holding the blades, as even setscrew tension warped them in the fixture, and can only hold them 'finger tight'.
    (Please note the mirror behind my edges in my previous attachment. It is valuable especially when placing the flywheel assembly squarely on the rails, as it is as if you stood back a few feet for perspective.)

    An ideal fixture would be monolithic,.. cut, ground, and polished from granite, with perhaps agate 'edges'.

    (*sigh*)

    Cotten
    Last edited by T. Cotten; 12-23-2017 at 05:27 PM.
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  8. #38
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    Quote Originally Posted by TechNoir View Post
    Seems to say that 0.2g is an acceptable tolerance when balancing these old engines.
    I didn't mean to imply that. I actually think you can get by with less sensitivity than that, i.e. 1 g or even worse. More serious issues might be sag and/or the tendency of the relatively wide and thin bars to twist under the weight.

  9. #39
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    Quote Originally Posted by T. Cotten View Post
    I would use ground round rod, and now I must add shorter length.
    Cotten

    I did take note of thhose comments and my planned setup would not be difficult to adapt to rods or to shortening the length

    Quote Originally Posted by T. Cotten View Post

    (Please note the mirror behind my edges in my previous attachment. It is valuable especially when placing the flywheel assembly squarely on the rails, as it is as if you stood back a few feet for perspective.)

    Cotten
    I saw the mirror and guessed that was its purpose.


    Quote Originally Posted by T. Cotten View Post

    An ideal fixture would be monolithic,.. cut, ground, and polished from granite, with perhaps agate 'edges'.

    (*sigh*)

    Cotten
    Oh for an ideal world, but thats a whole other conversation!



    Quote Originally Posted by BoschZEV View Post
    I didn't mean to imply that. I actually think you can get by with less sensitivity than that, i.e. 1 g or even worse. More serious issues might be sag and/or the tendency of the relatively wide and thin bars to twist under the weight.
    I have pondered the possible distortion of the knife edges and have some ideas for variations on the basic principles. At the moment I am planning on sandwiching the straight edges between flat bar however the outside of the sandwich could be substituted for angle or tee section to increase rigidity.



    John

  10. #40
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    Quote Originally Posted by TechNoir View Post
    At the moment I am planning on sandwiching the straight edges between flat bar....
    What about milling a slot in a length of ground round bar and clamping your straight edges into the slot. Then, if you wanted, you could rotate 180 degrees and use the ground round bar edge instead. You get extra rigidity and 2 sets of edges to choose from.

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