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Question about gib

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  • Question about gib

    I was thinking about the rigidity of lathe carriage that rides on dovetailed way. Unlike the cross slide which has tapered gib, the carriage itself has a regular gib which is adjusted using four 1/4" setscrews. That means one side of the dovetail is supporting by 4 screws! The cross sectional area the 4 screws combined is about 0.2 square inch which makes for very weak support.

    Does this mean that you can put more pressure on one side without the gib than the other side?

    Tapered gib shouldn't have this problem since the entire surface of the gib is supported. So, why aren't all gibs tapered?

    On large milling machines are all gibs tapered?


  • #2
    Albert: those 4 screws presssure is distributed by the gib i guess. The gib stock is pretty thin but it musrt distribute pressure over a much larger area.

    I like tapered gibs. Few years ago iwould have said they were on only larger, high class machines.
    But i have seen them (i think) on cheap stuff too. Since the adjusting mechaninisms differ I am just guessing they are tapered gibs in some light duty cross fed vices. cant really see into the casting for sure.
    Seems to me the taper gib is (or could be) cheaper since you don't need such a close fit. First asain that i knew had taper was an enco knee type of some sort. I was impressed, friend said it was good mill but cheap. the gib looked like it was cut by a drunk ,day after pay day, with a dirty torch (on the none sliding side) but it worked well. The asian are getting better fast!


    • #3
      The side without the gib should be the side that is taking the cutting theory, at least.
      Try to make a living, not a killing. -- Utah Phillips
      Don't believe everything you know. -- Bumper sticker
      Everybody is ignorant, only on different subjects. -- Will Rogers
      There are lots of people who mistake their imagination for their memory. - Josh Billings
      Law of Logical Argument - Anything is possible if you don't know what you are talking about.
      Don't own anything you have to feed or paint. - Hood River Blackie


      • #4
        Steve, I understand that the pressure is distributed over the surface area of the gib, but it's still the 4 screws that are supporting the gib.

        I've found that tapered gibs are much more finicky when adjusting because the cross slide can feel tight one way but loose in the other direction. It depends whether the motion is wedging the gib or moving away from the taper.



        • #5

          The screws are there to prevent the carrage from lifting off the lathe bed and slapping down. With a heavy carrage on a static machine you do not need much more. My Maximat 7 has box ways (wraps around the flat bed) which secures the carriage from moving in any direction other than along the lathe axis. As a portable (500Lbs!) machine this makes sense.


          • #6
            Albert:you are right about the loose gibs binding. Most well designed taper gibs have a screw at both ends and no slop.
            I try to use one srew to trun the gib in until the unit moves nicely, then tighten the other end until it has a slight drag. I think- no way to see- that i once put so much pressure to stop the movement you speak of that i actualy "spung" the gib , made it ride on the side because it wss slightly curved. You are right about them being touchy, touchy .