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Thread: Tapered Gibs !

  1. #1
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    Question Tapered Gibs !

    Hi Guys,

    I have a question, how do I go about making a tapered gib strip ?

    As some of you will know I'm in the process of making a modified Books tool and cutter grinder, (see projects). I've discovered that the table gib strip is banana shaped. It only touches the table dovetail at the ends !

    I found that I was unable to consistently get the same gauge readings at either end of the 240 mm long piece of metal placed in the vice. No problem over the 75 mm width of the vice and I probably wouldn't have noticed the difference until I wanted to ensure that the long length measured the same at both ends.

    Anyway I've stripped it out and removed the burrs that were on it and put it back. The machine is a lot better and better adjusted, but I would like to replace the bent gib with something that is at least straight.

    10-06-2019-001.JPG
    The measuring setup.
    Best Regards:
    BaronJ.

  2. #2
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    Hi John
    I will struggle to explain this seeing as I'm terrible at explaining but here goes.
    Start with a decent bit of stock larger than you actually need, and longer. This way you can support the bit in the middle (by either end) which you will machine into the tapered key.
    The ends are clamped to angle plates leaving the centre section free for machining.
    Machine the straight side then offset the angle plates for the tapered side. Top and bottom are just clearance and could probably just be linished though machining would be better.
    Hope this helps.

    Phil

  3. #3
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    The simple way is a surface grinder equipped with a sine table. But, alas, I don't have one (yet).

    I used a tilting angle plate swivelled to the angle of the taper and a big slitting saw. See attached picture. Followed by a lot of scraping so the closer you can machine the angle the less scraping you'll do.

    PDW
    Attached Images Attached Images

  4. #4
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    Baron if your material is not stress relieved you will make more bananas.

  5. #5
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    Hi Guys,

    Thanks for the information and PDW for the picture. Its starting to make sense !

    I have a fairly large chunk of brass plate, about 12 mm thick that originally came from the bottom of a bank door. Heavens knows how old it is, I've had it for a good many years waiting for something to use it on. How likely is it to be suitable for use as a gib strip ?

    Thanks:
    Best Regards:
    BaronJ.

  6. #6
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    Before trying to make a new gib I would try straightening it by peening with a cross peen hammer or even a welding slag hammer.
    Work on the convex side of the gib by putting straight line bruises that you can barely see at right angles to the length.
    You will be surprised by how easy it is to change things.

    Robert

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    Brass would work I guess, but the usual material is CI.

    Michael

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    There is some videos on Youtube on making them. I'm actually in the same boat in that I have a DM45 Mill Drill that needs a new Gib for the column, because the original was never thick enough. I going to use a to use a piece of round cast iron bar and get the gib out of that. You can usually buy cast iron bar off eBay.
    All The Best steran50 Stewart

    The shortest way to do many things is to do only one thing at once.

  9. #9
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    G'day John,

    i would second Robert's suggestion of straightening the existing tapered gib rather than manufacture another one. If the only problem is that the gib is bowed, then selective use of heat on one side followed by relatively quick selective cooling can usually straighten the bowing up. If the material is cast iron it should be no problem to straighten in this way - the only caveat being that we are taking gentle localised heat and gentle localised cooling, not heat to red hot and drop in a bucket of water. I am presuming that apart from the bowing, the tapered gib is correctly proportioned for the slide it is mounted in.

    Regards

    Quentin

  10. #10
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    Another thought John,

    What caused the gib to bend? I presume that you have checked where the gib fits to make sure that there is no unsupported section or a damaged section which is inducing a bend into the gib when it is tightened up. Just a thought, as generally I didn't think that tapered gibs bowed or bent when correctly fitted, being supported on the two opposing faces when adjusted up.

    Another thought, is one of the faces of the tapered gib or one of the faces against which one of the faces of the tapered gib rougher than the other, so that there is a different frictional resistance occurring on each of the the faces in contact, resulting in one side being "stretched" whilst the other is being "compressed" when the gib is adjusted into position?

    Regards

    Quentin

  11. #11
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    Hi Quentin, thankyou for your comments.

    The gib in the machine is original and made from steel. One side looks plain whilst the other side appears to have been scraped, little swirls on the surface that bears on the table dovetail. If you lay it down on a flat surface with the scraped side up it rocks from end to end.

    I've read since that the gib should be bowed with the ends of the bow towards the table. Though I don't know if there is any truth in this because I cannot find any reference on the net to this effect.

    I must admit that since I filed off the burrs and gave the gib a rub on some 600 grit emery paper the table movement seems a whole lot more smooth and there is definitely far less side play in its travel.
    Best Regards:
    BaronJ.

  12. #12
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    Quote Originally Posted by BaronJ View Post
    ...I've read since that the gib should be bowed with the ends of the bow towards the table. Though I don't know if there is any truth in this because I cannot find any reference on the net to this effect.
    I doubt that very much as authors like Connelly devote pages to scraping of gibs and making sure they are flat. The perfect gib would be flat and in full contact with the mating parts so that effort was uniform and there was no slop anywhere along the travel.

    One of the things I've done with my current lathe and the last is to scrape the compound gib flat for much improved performance. One day I'll get around to replacing the cross slide gib too and fix that bit up.

    Michael

  13. #13
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    Hi Michael,

    The place where I found that reference is in the links below the posts

    Tapered Gib Adjustment
    Best Regards:
    BaronJ.

  14. #14
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    I sort of understand what is being said there, and I think it applies more to the fixed side of a gib installation. On the side that slides you still want flat to flat contact. On the side that does not, contact on the two ends are preferred to contact just in the middle.
    I know at the scrapefest that I went to, we also spoke about things like re-scraping saddles, where you would relieve the centre of the saddle slide, just so that it would wear in before it then wore out.

    If it were mine, I would be scraping the contact side to be flat and scraping (or otherwise removing metal) from the stationary side so it were flat (or possibly slightly relieved in the centre).

    Michael

  15. #15
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    Hi Michael,

    Thankyou for your comments, they make sense to me. The gib on my machine is bent the other way and only the ends touch the table. However since I cleaned it up, the difference is quite noticeable. I cut the first slot in my Brooks grinder table yesterday and the work and slot are more parallel than they would have been if I'd not taken the time to measure it.
    Best Regards:
    BaronJ.

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