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  • bar-bed lathe

    The temperature in my garage is likely to be 110F until mid-October, here in Phoenix. Since I can't work in the shop, I sometimes day-dream about lathe design.

    For a home-built lathe a bar bed design, using rods like a Wabeco http://www.lathes.co.uk/wabeco/ seems appropriate, eliminating gibs as a source of chatter.

    Although rigidity is good, weight (just for the sake of it) is bad, at least on lifting day. The rods could be hollow, saving weight but sacrificing almost nothing in stiffness. Are there commercial bushings mated to matching hollow rods?
    Last edited by aostling; 08-22-2007, 07:23 PM.
    Allan Ostling

    Phoenix, Arizona

  • #2
    If it were 110 degrees in the shop I wouldn't be daydreaming about lathe design but rather on getting air conditioning in the shop.
    Last edited by J.Ramsey; 08-22-2007, 02:06 PM.

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    • #3
      They are called "linear bearings", I used to have to make shafts that used them.

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      • #4
        I would not use hollow bars. Even though there is no gain in stiffness, mass is important even for a small lathe.

        You could fill a hollow bar with cement however.

        If you Google "linear motion" you can find all kinds of parts for such an idea.
        Gene

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        • #5
          Check out the "Baker" lathe, made in the 1950's I think, it was about as good a bar bed lathe could get. Model Engineer had an article on it if you can find it.
          The shortest distance between two points is a circle of infinite diameter.

          Bluewater Model Engineering Society at https://sites.google.com/site/bluewatermes/

          Southwestern Ontario. Canada

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          • #6
            Solid bar...no gain in stiffness? Calling all Mechanical Engineers....you want to address this?

            This is about as common as the mis-understanding about fluted rifle barrels being stiffer than a solid barrel.

            I don't know how you remove chatter by having round ways either? I assume you were referring to stick-slip which is a function of way friction. While linear bearings may not have this problem, neither would flat roller ways...which is the more parallel comparison. The latter are used on some surface grinders where table lift is not a concern.

            Paul
            Paul Carpenter
            Mapleton, IL

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            • #7
              Solid is the stiffest, but the gain is not great.

              The formula for resistance against bending is:

              1/10 * ((D^4 - d^4) / D)

              So a little increase in outer diameter (D) gives a big gain, a little decrease in inner diameter (d) a little gain. It's a design parameter thing.

              But in essence, solid has the biggest resistance against bending.

              Because d^4 becomes: 0^4 which is 0.

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              • #8
                Originally posted by MCS
                Solid is the stiffest, but the gain is not great.
                Yes, that's how I should have stated it.

                I'm renting a condo, so can't really air condition the attached garage. Although I have no immediate plans to do so, I may move in a few years. The mass of my machine tools (and everything else I own) is something I would like to minimize. Hence my desire for a weight-conscious lathe design.
                Allan Ostling

                Phoenix, Arizona

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                • #9
                  aostling. I got some hydraulic cylinders that were used for a car carrier. Some of the rods were more than six feet long. They were 1.25 in diameter and I was suprised to find that they were hollow. The hole is about .750.

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                  • #10
                    Originally posted by Rustybolt
                    aostling. I got some hydraulic cylinders that were used for a car carrier. Some of the rods were more than six feet long. They were 1.25 in diameter and I was suprised to find that they were hollow. The hole is about .750.
                    Rustybolt,

                    We can compare this hollow rod with a solid rod of the same OD.

                    As MCS said in the post above, weight of a hollow rod is proportional to the difference of the squares of the OD and ID. Stiffness of a hollow rod is proportional to the difference of the fourth powers of the OD and ID. We'll compare the 1.25 OD/0.75 ID rod with a solid rod of 1.25 OD. Doing the sums ...

                    The hollow rod has 64% of the weight of the solid rod.

                    The hollow rod has 87% of the stiffness of the solid rod.

                    So the reduction in weight is significantly larger than the reduction in stiffness.
                    Last edited by aostling; 08-22-2007, 10:50 PM.
                    Allan Ostling

                    Phoenix, Arizona

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                    • #11
                      There was a fairly long discussion on making a bar bed lathe some time ago, but I can't find it. I have some ideas on how it could be done, using a surface plate, shop made spacer pairs, and epoxy with glass fiber and fillers, such as sand. There was one fella who seemed quite interested in making his own bar bed lathe, though I don't recall his name. I wonder if he ever got going on it-

                      At any rate, my procedure would have called for careful alignment of the bars on the surface plate, then some casting with epoxy to create spacer bars that would maintain the spacing of the bars and keep them in the same precise plane. The headstock, tailstock, and carriage are all build-ups that are mated to the bars, again using epoxy to create the fit to the bars. Some of these 'fits' will become sliding bearigs (carriage), and others will be permanent mountings (headstock and mounting bases fitments). Once some spacer bars are made, some are used to locate a third bar that would define the spindle axis. All three bars would then be in some state of perfect alignment, and the epoxy is cast into the build-ups to solidify this alignment. Obviously, the third bar is removed and spindle and tailstock components are fitted. I'm still intrigued with the idea of doing this as a project.

                      If I"m not mistaken, part of that discussion revolved around the damping characteristics of sand filled epoxy, and part of it concerned ways of making the bed more rigid and stable. Concrete filling was also one of the topics.
                      I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                      • #12
                        deleted, as this post got repeated in #14 below.
                        Last edited by aostling; 08-23-2007, 01:09 AM.
                        Allan Ostling

                        Phoenix, Arizona

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                        • #13
                          I would use induction hardend shaft for the rods , that stuff is tough.
                          Last edited by tattoomike68; 08-22-2007, 11:27 PM.

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                          • #14
                            Originally posted by deltaenterprizes
                            They are called "linear bearings", I used to have to make shafts that used them.
                            They used to be made by Thomson Industries. There were recirculating-ball bushings, and roundway bushings. The bushings were intended to run on a mating hardened shaft. But I don't think these shafts were available hollow.

                            I'm wondering if there is a current product line from some manufacturer, featuring hollow shafts.

                            But perhaps I'm making too much of this hollow shaft notion, and the saving of weight in the bed. The CNC lathe which S_J_H showed us two days ago, using HiWin bearing blocks, looks hard to beat.
                            Allan Ostling

                            Phoenix, Arizona

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                            • #15
                              Even with solid shafting, I think you will find your lathe lighter than the typical cast iron lathe bed. Last I knew, the linear bearing blocks were quite pricey in larger sizes. I rescued a smaller one from the trash and I keep debating just what little mechanism to use it for due to the fact that I won't be buying another one if I come up with a better application in the future.

                              I am not saying that the linear shafting way idea is inherently bad, but I do recall reading that some of the wood lathes made this way were sort of second rate due to flex as you approach the center of the shafting...and a wood lathe is not nearly as critical as a metal cutting machine. A conventional lathe bed allows for about the same deflection (little or none) everywher while a long round shaft supported only on the ends can deflect much more easily. Mass is your friend with machine tools. You only have to deal with moving it once in a great while, but you live with a less than solid machine every time you use it.

                              Once you cross the threshold from something you cannot just bend over and dead lift, you end up using tools to handle the moving. At that point, whether it weighs 400 or a thousand pounds, you will have to employ something to lift and then roll it so depending on just where you end up weight wise, you may not need to fret a few pounds...or even a few hundred.

                              Anyone know what one of the 9x20 lathes weighs? 400# or so sticks in my head. I know I can carry my 7x14 mini lathe without too much difficulty and its really disproportionately rigid due to its short length so there is less of a sacrifice due to its light weight.

                              Paul
                              Paul Carpenter
                              Mapleton, IL

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