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  • Question about friction

    Hi Everyone,

    When you have two flat plates that slide against each other, is it better to have a smaller or larger area of contact?

    With a larger area, the force is more spread out, with a smaller area, there's less opportunity for friction. I know clutch disks and brakes like to have a large area so I'm thinking smaller is better, but those surfaces aren't lubricated.

    This is for the slide on my milling attachment. The slide plate is about 2 inches wide and 4 inches long. I'm thinking about filing out the middle of the plate so it just makes contact on the outer 1/2". That would also make it easier to scrape should I choose to do so.

    What do you think?
    Lee

  • #2
    I've only seen a few slides with a hollowed out center, and that's when they were 24" long and the center section was about 6". I've never seen a cross slide or compound on a lathe deliberately hollowed out in the middle. That's not to say that there aren't any, just that I haven't seen one.

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    • #3
      More trouble than that it's worth, to hollow them out.
      The "gain" will be negligible.
      Furthermore, you create pockets that swarf gets in, resulting in all kinds of mischief!

      Danny
      ---------------------------
      Wer anderen etwas vorgedacht, ....... When you propose a solution for someone's problem,
      wird jahrelang nur ausgelacht. ....... you will be ridiculed for years.
      Begreift man die Entdeckung endlich, ....... When the discovery is finally understood,
      so nennt sie jeder selbstverstÙ†ndlich. ....... everyone will say it is obvious.
      -- Wilhelm Busch --

      Comment


      • #4
        The more square inches of surface are in contact the higher the friction will be. If you machine the two plates flat and grease them they should move fairly easy.

        As to lathe cross slides and compounds the contact area is very small relative to the size of them because the contact area is on the dovetails on each side and the drag is adjusted with the gibs that take up the slop. They do not touch together in the center area between the dovetails.

        How are going to guide them? what will keep them running straight? How will you adjust the side movement?
        It's only ink and paper

        Comment


        • #5
          Theoretically, it makes no difference. The force of friction is equal to the normal force times the coefficient of friction.

          In practice, it becomes a little bit more complicated. In the case of bearing slides, I always opt for more surface area. This is because you want the surface to slide on a film of oil, not on the other surface. If the surface area is too small, the pressure increases (same weight but distributed over a smaller area). This pressure will cause the part to rupture the oil film and now you will be sliding on the steel surface.

          Comment


          • #6
            Originally posted by DannyW
            More trouble than that it's worth, to hollow them out.
            The "gain" will be negligible.
            Furthermore, you create pockets that swarf gets in, resulting in all kinds of mischief!

            Danny
            Thanks, I hadn't thought about that.

            I'm still curious about the original question in the abstract sense though.
            Lee

            Comment


            • #7
              Hmm, I guess you didn't read my explanation in my post #4 did you.
              It's only ink and paper

              Comment


              • #8
                Originally posted by Carld
                How are going to guide them? what will keep them running straight? How will you adjust the side movement?
                I am building it from a plan I found online. See this link and scroll all the way to the bottom for an exploded diagram:

                http://i.imgur.com/OkGaO.jpg

                It runs between two rails with a brass gib to take up the sideways slack. It is held in place by two wider rails that contain it -- and there is no provision for adjusting wiggle in that dimension. I'm going to have to just shim it by trial and error.

                It actually looks like a pretty poor design and I hadn't realized that at the outset. However it does have the advantage of being able to be constructed out of flat stock without any fancy cutting.

                Gib Material

                While I'm asking questions, I also wanted to ask what is a good type of brass to use for the gib. I ordered 385 brass, but ended up with 360 brass because the 385 was out of stock.

                The instructions specified "hard brass".

                Thanks.
                Lee

                Comment


                • #9
                  Originally posted by Carld
                  Hmm, I guess you didn't read my explanation in my post #4 did you.
                  Carl, before you posted I clicked to reply. Then I got a phone call, came back and wrote my message. In the meantime you had posted yours and I hadn't seen it yet.

                  darn, another call, be back in a while with an edit.
                  Lee

                  Comment


                  • #10
                    Id think a smaller area would wear much faster too.. Less metal to wear before it wears down. Course, that only matters if this is a sliding deal

                    Of course, if your milling attachment is designed to clamp down and not move, you still want a larger surface area way.. Better grip, less chance of damageing ways under more pressure (At least, changeing the rather small, poorly cast clamp on my tailstock for a huge one (So big it actualy sticks out behind the tailstock 3", Enough space for ANOTHER clamp to be mounted ontop if I ever find the need) made outta CRS sure did improve the holding force!.. but could also be because I perposefuly made the slide way with a 'poor' finish with lots of machine marks for extra grab/place for oil to go)
                    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                    • #11
                      I don't think you will have much trouble with it as long as you can devise a way to remove any wiggle or side play. Having looked at the plans there is a gib to take up side movement but nothing to adjust upward movement so that may be the issue to solve.
                      Last edited by Carld; 08-17-2010, 02:16 PM.
                      It's only ink and paper

                      Comment


                      • #12
                        Carld, in the abstract sense your statement:

                        "The more square inches of surface are in contact the higher the friction will be."

                        Is not correct. In it's simplist form:

                        F = mu x N

                        Where F = friction force or drag. mu = coefficient of friction for the two contact materials and N is the applied force at 90 degrees to the sliding surfaces. No area involved.

                        I have to support Fastrack's view with the qualification that if the surface area becomes to large then oil viscosity may come into play. So the real answer is that there is no standard answer as the friction in any system is dependent on many parameters that are inter-related.

                        Phil

                        Originally posted by Carld
                        Hmm, I guess you didn't read my explanation in my post #4 did you.

                        Comment


                        • #13
                          Originally posted by Carld
                          I don't think you will have much trouble with it as long as you can devise a way to remove any wiggle or side play. Having looked at the plans there is a gib to take up side movement but nothing to adjust upward movement so that may be the issue to solve.
                          OK, back from the phone call. As I said, our posts crossed in the mail and I did carefully read yours afterwards. At first blush it seems to conflict with Fasttrack's post but I think that the "film of oil" thing is the key difference.

                          Edit: I've got a plan to deal with the unaccounted-for play by using shims between the two stacked side rails.
                          Last edited by ldn; 08-17-2010, 02:27 PM.
                          Lee

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                          • #14
                            Originally posted by philbur
                            F = mu x N

                            Where F = friction force or drag. mu = coefficient of friction for the two contact materials and N is the applied force at 90 degrees to the sliding surfaces. No area involved.

                            OK, I'm not going to argue, but I have to ask, why is more area better when talking about a brake pad or clutch?

                            Heat distribution? Wear resistance?
                            Lee

                            Comment


                            • #15
                              because the contact area is on the dovetails on each side and the drag is adjusted with the gibs that take up the slop. They do not touch together in the center area between the dovetails.
                              That certainly is not true.
                              Hollowing out the contact area can only be done, if the short guide always covers the long guide, like the saddle is shorter than the bed.
                              This is not true for many cross slides and most of the compounds.

                              Edit:
                              And the reason why this is done is not to reduce friction, but to compensate for wear.


                              Nick
                              Last edited by MuellerNick; 08-17-2010, 02:33 PM.

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