Where to get knee gib? - Page 2
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  1. #21
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    I agree that there's not much cast iron to drill into and its brittle on top of that.

    I just checked the tensile strength of bronze brazing rod and also silver. It is tens of thousands of psi tensile.

    If my joint is 1/4' square, and the tensile strength is 50,000, then we're talking 3100lbs tensile strength. IF thats anywhere near accurate, I cant imagine that much lift pulling up on the gib.

    Most of the replies seem to point to "braze it and it should be fine"

    Anyone sees if I'm way off here please speak up!

    What I still am not sure of is how to adjust the gib. I guess of I put an indicator and cannot move it by rocking the table left and right, it is good?

  2. #22
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    See post #9 for general gib adjustment.

    Adjustment depends on areas of dovetail wear. The table ways usually wear in the center as that is where most work is done. A lot of wear is due to operators leaving the table, saddle, or knee gib clamped.
    A table gib adjusted for center of travel will certainly be too tight when the table is moved to extreme right or left. This accelerates wear of the nut and the screw.
    When the knee gib is too tight the knee will 'hang' when moving downward and then drop suddenly. The knee should lower smoothly when the gib is properly set. If the knee hangs moving up then maximum force will be applied to the repaired section of the gib.
    John
    Last edited by jhruska; 12-11-2019 at 10:13 PM. Reason: definition

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    thanks john I'll try to tighten and feel (somehow) the travel until it seems tight...

    I dont get how a hang up moving downward would push up on that gib tab??? do you mean when cranking upward (where the gib screw is lifting the gib?)?

  4. #24
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    If brazing with aluminum-bronze filler material,
    it is usually 32,000 psi tensile strength.
    Cast iron can be between 20 to 40,000 psi depending
    on the grade. Brazing with aluminum bronze filler
    is a very good match for repairing cast iron
    and will have original or better strength and functional
    properties. Brazing also has less variables to go wrong
    than electric welding.

    --Doozer

  5. #25
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    Quote Originally Posted by FlyinChip View Post
    thanks john I'll try to tighten and feel (somehow) the travel until it seems tight...

    I dont get how a hang up moving downward would push up on that gib tab??? do you mean when cranking upward (where the gib screw is lifting the gib?)?
    The gib will "stick" to the column dovetail as the knee is lowered because it has been tightened beyond the optimum point of engagement or due to lack of lube. The friction force will try to slide the gib up when the knee drops down.That same friction can drive the gib into a deeper engagement when the knee is raised. The gib screw will load the repaired head of the gib to prevent the gib from sliding any deeper.
    As Doozer said, the braze should be a good repair.
    John

  6. #26
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    Quote Originally Posted by jhruska View Post
    The gib will "stick" to the column dovetail as the knee is lowered because it has been tightened beyond the optimum point of engagement or due to lack of lube. The friction force will try to slide the gib up when the knee drops down.That same friction can drive the gib into a deeper engagement when the knee is raised. The gib screw will load the repaired head of the gib to prevent the gib from sliding any deeper.
    As Doozer said, the braze should be a good repair.
    John
    according to the backlash on the adjusting screw and the slot in the gib and flange on the adjusting screw, which should add up to as little as practically possible. but if its >.025" then i agree you will have a problem.

    anyhow, i disagree with doozer on the paper thin section of the gib. its .455" wide on the fat end, which is why i suggested simply bolting on a plate rather than dealing with the stress riser of brazing and then cleaning up the slot.

    from FlyinChip: "If my joint is 1/4' square, and the tensile strength is 50,000, then we're talking 3100lbs tensile strength. IF thats anywhere near accurate, I cant imagine that much lift pulling up on the gib."

    but its not tensile its sheer and bending, and only half of the cross section is in tension the other half in compression, and the (presumably) sharp corner at the bottom of the milled slot is a significant stress riser. you may find that it only takes 1/6th of the cross sectional area in tension to snap the tab off, or about 500 pounds for a 1/4" thick cross section loaded 1/8" away for your supposedly 50Kpsi cast iron. (which might only be half of that to begin with)

    anyhow we know this is an issue because they do break, and milling the slot back and brazing it should significantly increase the strength simply because the braze material is ductile and can handle the stress riser inherent to the design. a generous radius at the bottom of the slot would probably double the strength of the stock design.

  7. #27
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    Quote Originally Posted by johansen View Post
    ...anyhow, i disagree with doozer on the paper thin section of the gib. its .455" wide on the fat end,...
    Of course you do.
    If the gib is .455" thick
    and you center a 1/4" screw thread within that width
    you get (.455-.25)/2 = .102" per side of meat.
    100 thousandths of cast iron is really thin
    and is just asking to crack.
    But this is just my opinion.
    Yours is obviously different.

    -D

  8. #28
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    The OP is probably satisfied with his solution, but if an extra measure of safety was desired, there seems to be a decent amount of room between the wiper cover and the top of the gib (at least on my BP). Enough room it looks like a small piece of metal could be attached on top of the repaired gib for added reinforcement.

    Would using a #8 machine screw (0.16" dia.) to attach an additional metal plate add strength w/o compromising the wall integrity of the gib? It would leave about .147 wall thickness.

    Even a small #8 screw has a surprising amount of cross sectional tensile strength, especially if grade 5. I've no way of calculating the pull out strength of cast iron used in the gib, but if the threads were sufficiently deep I'd think the force required for pull out would be significant.


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