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Preventing cross-slide lift when boring?

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  • Preventing cross-slide lift when boring?

    Previously, we discussed the mini-lathe's poorly-fitting gibs (http://bbs.homeshopmachinist.net/thr...work-like-this). Since then, I have obtained a mill and made deeper, flat-bottomed holes in the gib for the dog-point screws, like this:



    Now, this nicely solved the problem of the gib being "cocked" and it now nestles neatly into the fixed bottom of the saddle dovetail as shown above.

    Unfortunately, this did nothing to cure the cross-slide "lift" or "tip". When downwards tool pressure occurs left of the cross slide, as when boring, the right side of the cross-slide lifts up because the left side of the cross-slide acts as a lever.

    I could make new dog-point screws with fatter tips to fit the flat-bottomed holes more snugly, but there must be clearance for the dog points to fit into the holes in the gib, so whatever clearance I leave so the thing can still be assembled, will directly translate into lift.

    In my increasingly desperate hunt for a solution, I found this:

    http://madmodder.net/index.php?topic=5406.0

    which shows a beautiful picture of what looks like might be the correct setup using conical holes and pointed setscrews to make the gib "float". What do you think of this solution, and if it looks good, how do I accurately locate the new holes to center the gib vertically?

    Thanks in advance for any thoughts...

  • #2
    Looking at the amount of daylight in the photo that is the worst gib I've seen, it would appear that a new one that actually fits might be a good idea, with it as it is you could probably lift the crosslide with a plastic tea stirrer!
    Surface plate, file and scraper job should fix, boring hand making a gib but therapeutic!
    Mark

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    • #3
      I would just mill the screw side of the gib flat (vertical). I can't imagine why it wouldn't come that way.

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      • #4
        Which side of the slide is the gib on? If it's on the left side, the right side should tuck down as the dovetails come snug, but normal turning and facing would place a lot of pressure on the gib and the screws. If it's on the right side, that side would tend to tip up when boring, or when turning with a tool bit hanging out to the left, which is sometimes required.

        I suspect that design would dictate that most cutting forces should be taken by the fixed dovetail. The only way I can see to come close to this on both sides of the slide is to use a tapered, well-fitting gib. This means of course that at least one set of dovetails can't be parallel.
        I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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        • #5
          Invert your forces. Run the spindle backwards, so it tries to push down on the gib side. The other side can't lift so easily, as it is basically tightly fitting male/female dovetail.
          Amount of experience is in direct proportion to the value of broken equipment.

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          • #6
            It is, bar non, the very worst gib setup that I have ever seen in my entire life. My first reaction was that there is no way they seriously made it that way.

            I don't know if it CAN be fixed with the parts that are in place.

            The thickness of the gib makes it nearly thicker than it is tall, meaning it will be very unstable. That is pretty necessary to fill the huge gap between the slide assembly and the dovetail it supposedly slides on. But it cannot be stable.

            The gib is around a quarter inch tall (~6mm), but the gap looks to be around 0.375 (9 or 10 mm). pushing on that with the idea of making it more stable seems like it cannot possibly work very well, unless you nearly lock it up by tightening.

            The only solution that seems like it can really work well is to braze on a strip to the slide, to fill some of that huge gap with rigid material, and then drill and re-tap for gib screws. That would make the screws more secure by giving them a longer thread, and it would allow making the gib thinner, so the assembly would be far more rigid.

            The screw gib is a sloppy compromise anyway, compared to a taper gib, and this crazy "solution" only makes it a lot worse.

            Maybe an even thicker gib, with screws through it up into the body of the slide to snug it so it can only slide in and out, might be nearly as good. The existing screws would still snug it "in and out", and the added ones would keep it from cocking, which is what such a thick "over-square" gib wants to do.

            It looks like someone in china messed up the design of the unit, and then decided that they would "fix it" by a thicker gib, possibly assuming that "nobody will notice, the people who buy these aren't very mechanical anyway, it won't matter".

            I don't know that the assumption is good, and the "fix" of a thick wobbly gib is the stupidest ever.

            Typical chinese cheap goods fix.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              the cross slide (one on top, right?) gib on my ancient mini lathe is screwed to the top of the cross slide with the slack taken up by a captive screw in the gib that pushes against the cross slide to tighten. Seems to work really well and I can get some very nice finishes boring in alu, although parting off/ grooving isn't a whole lot of fun.

              I don't have a pic of the gib unfortunately. The ones in those pictures look like they'll just pivot around the dovetail if any rotational force is applied

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              • #8
                Darryl, the gib is on the right (tailstock) side. I agree that putting them on the left (headstock) side, the "lift" problem would be practically eliminated but may lose some rigidity when facing and turning.

                Forestgnome, I agree the angle of the screw side of the gib needs to be vertical where the screws hit it in order for them to keep the gib from cocking.

                Going back over the previous thread, the following things were suggested:

                "the gib should be close fitting between the green and blue parts" -- I read this as, I've now got the opposite of what I wanted -- I should have put the gib on the "ceiling" instead of on the "floor".

                "there should be some space between the bottom of the gib and the flat surface on the base" Dang, I wish I had read this closer last time. So, I screwed up and did it backwards!

                Ok, so, I agree the general design sucks, but I think we can fix it. In order to locate my dog point recesses, I think I should remove the gib screws and C-clamp the gib to the ceiling of the (movable portion of the) cross-slide female dovetail. Then use a close-fitting transfer punch (I will have to make one from O-1, but that's no problem) through the gib screw holes to centerpunch the locations for new dog-point recesses. Is this going to be acceptable or is there a better way?

                Once the locations are determined, plunge flat-bottomed holes with a 1/8" endmill and make them as deep as I dare to make the gibs effectively thinner. I'm thinking that ideally, the hole should be at least as deep as half the height of the gib (to make it "under"square instead of "over"square)?

                mattthemuppet, your post came in while I was typing the above. This idea, like
                J Tiers, sounds promising, but if the gib is locked to the ceiling, then the gib adjustment screws wouldn't be able to push it to take up horizontal slack? I like the concept of positively holding the gib, but want to make sure I understand it before I trash another gib.
                Last edited by tylernt; 01-30-2014, 11:55 AM.

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                • #9
                  I would make a new gib that fills up the space a lot more. Make vertical slots in the gib where the screws push in from the side. Put some bolts in from the top to draw the gib up tight against the slide. The holes would need to be a little slotted or oversize to allow for lateral adjustment with the side screws. The top screws can be snugged, then the side screws adjusted, then the top screws tightened.
                  Kansas City area

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                  • #10
                    what Toolguy said The top holes (ceiling of the gib I guess) are slightly oversized so that you adjust the gib first, then tighten the top bolts.

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                    • #11
                      Ok I see, yeah that would work. And be very rigid.

                      I'm not sure I'm sold on drilling holes in my cross slide just yet though. I think I will try the more traditional configuration first with deep flat-bottomed hole positioned to locate the gib as high as possible, and then consider drilling if that still allows too much lift.

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                      • #12
                        This is why Bessemer invented the Blast Furnace:



                        Just joking, well almost.

                        Phil

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                        • #13
                          Still dont like all that daylight!, Bessimer aparently patented the bottom blown converter as opposed to the blast furnace, the BF makes iron, you stick it in the converter to burn off the carbon from about 5% down to less than 1% or steel, and im not picking honest, its just that im sad enough to have spent 30 years playing with them (= 2 murders over here or 2 life sentances)
                          The interesting thing is that bessimer bought the process of a Texan farmer who was working on it according to the shorter history of technology book, Bessimer was a buisnessman just as ruthless as Edison.
                          The process didnt improve significantly untill the work of Gilchrist and Thomas who came up with the Basic Process (prior to that the process was Acidic, ie the slag was acidic, so eat the bricks amongst other features like inability to remove Phos)
                          Later it became the Basic Oxygen Steelmaking process or BOS process, my last residency was the Lab, called the BOSLAB!
                          Other than that id replace the gib with a better fitting one, theres plenty of room!

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                          • #14
                            Toolguy re-explained my idea of a larger gib with screws into the top of the crosslide casting.

                            That's exactly what I was suggesting. It has been done before, some machines were designed that way. And it will work.

                            You would only need those screws to be 'tight" enough to prevent the gib from cocking. You don't have access from below to tighten them after adjusting the real gib screws anyway, so the screws would go in from the top if you wanted to tighten them really tight. Heads on the bottom would be easier, though, looks like there could be room

                            One problem though, when you tighten them, you will also pull up the gib slightly, which will tighten it on the male dovetail. I think I would leave them just snugged.... You might consider using some Belleville spring washers (cone spring washers) to keep tension on the screws but allow movement.
                            Last edited by J Tiers; 01-30-2014, 06:49 PM.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Originally posted by phil burman View Post

                              I think Toolguy and Jerry have some valuable suggestions.
                              You cannot have daylight on two sides...period !

                              The most stable form is a triangle and you need to either drive the Gib down and in , and back it up on the third side (Gib Screws )
                              or lift it up to the top with top mounted screws as was suggested.
                              My first suggestion is to put a vertical slot in the Gib, where each screw is located.
                              The slot must not be wider than the screw, as you do not want the screw to move with the slide and leave the Gib in place !

                              Now lets talk about what you have .
                              If you lower the Gib as I mentioned, the screws will be exposed on the top of the Gib , Thus allowing the slide to lift !
                              I know, I just told you to drive it down....I did that so you would see what it should be.
                              Since the Gib is so short on height, you can't do that and must consider J's comments BUT
                              If the underside surface of the slide is not a finished/plumb surface, that does not work.

                              The best solution involves making a new Gib IMHO.
                              But if you feel un-qualfied, then drill the gib screw holes almost through and close to size (OD) of the screws , with the Gib pulled up tight to the top of the space. Put in long screws and the screws will support the GIB and prevent lifting of the slide.
                              Adding top screws is optional
                              Can't make a silk purse from a sows ear...or something like that

                              Rich
                              Last edited by Rich Carlstedt; 01-30-2014, 08:44 PM. Reason: spelling
                              Green Bay, WI

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