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  • dovetail angles

    This is probably pretty straightforward, but it's worth discussing anyway. If you were making a slide of some kind, and were able to choose what angle to put on the dovetails, what factors would you consider in making that choice?

    Here's a short list of scenarios-
    the slide would experience mainly side forces
    the slide would experience mainly downward forces
    the slide would see upward forces
    the slide would see both side and tilt forces

    The questions arise when you consider how to keep play to a minimum in the various possible relative motions between parts, while at the same time keeping friction to a minimum. Another part is the ability to make the dovetail accurately in the first place, this depending on the available cutters and the ability of the machine to handle that without damaging it. How much this affects the choice of dovetail angle- I'm not experienced enough to know.

    In general, I see two angles in use, one is 45 degrees, the other is 30 degrees. From my perspective, I see the shallower angle better able to keep a slide positioned when the force is sideways against it, and the 45 degree angle better to keep a slide from lifting off its base with vertical forces being applied. I also see the 45 degree angle making the sliding action more friction prone.

    When you add in gibs, it gets a bit more complicated. Another factor to consider is whether the application is to adjust the slide then secure it, or whether it should be free to move at all times by the turn of a dial.

    Anyone care to elaborate on this topic, or point to a suitable read on the subject? I'm sure many of us would find that interesting.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  • #2
    45 degree, keep it simple.

    I used to fix screw machines that made parts in seconds, no need to reinvent the wheel.

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    • #3
      I will offer nothing more than observation. One the most common angles for machine slides appears to be 55*. It is used on my Sheldon lathe and Rockwell mill and is what Machinery's Handbook presents in their dimensioned drawings for dovetailed machine slides.
      Jim H.

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      • #4
        90

        Why not include 90 degree dove-tails aka "square slides"?

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        • #5
          Which is better may depend on the exact details of the application, but 60 degree seems to be the overall winner for general use.

          According to machinery's handbook (as quoted by sherline), the theoretical ideal dovetail angle is 55 degrees (for metal, for wood 80-83 degrees). I haven't found that section in my edition of the handbook. The weaker material, the shallower the taper. The standard (out of 45 or 60) angle which is closest to this is 60 degrees (probably what you are thinking of as 30 degrees). Standard angles are 45/50/55/60 but 50/55 degree cutters are rare. 60 degree cutters can also use standard triangular carbide inserts.

          A lot of older machines had 55 degree dovetails and british whitworth threads were also 55 degree. 55 also seems to have been more common for production machines (if you need a lot of cutters you can get any angle you want or lean the head over) but for small volume using a standard cutter helps. Sometimes odd angles may be used to make it more difficult to get parts or accessories elsewhere.

          60 degrees is a lot nicer than 55 to work with. For example, you can make a 60 degree reference by hand scraping 3 blocks at a time out of 4 to make up 180 degrees total against a surface plate. You can use a threading gage or 30-60-90 triangle.

          An easy mistake to make with dovetails is to do some back of the envelope calculations on one side of the dovetail and forget that in some directions, both sides come into play.

          Against side forces, only a single side of the dovetail opposes the force. Against straight upward lifting, both sides support the force. Tilt, however, puts the lifting load on one side though that side then has some leverage against the other. For downward force on the tops of dovetails, a 45 would be weaker. At 45 degrees, the throat is narrower for a given tail width; 60 degrees usually lets you keep more metal where you need it. 60 degrees will exert more spreading (shear) force when lifted but the sides won't be as badly undercut.

          Here is an older discussion:
          http://69.223.24.78/archive/index.php/t-33579.html
          I think there are some errors there, though. And another:
          http://www.practicalmachinist.com/vb...l-t157899.html
          http://www.practicalmachinist.com/vb....html?p=687024

          There is also the issue of play, though. A 45 degree may allow less motion in the lifting direction for a given amount of play. However, for a straight lift, the play is taken up on both sides so the motion may be less than the side to side play. For tilt, the geometry gets a little complex. But 60 degrees gives you the same amount of play in both the horizontal and vertical directions. If you need 1mil of play in both horizontal and vertical to slide freely, you can get that with a 60 degree dovetail but for a 45 degree dovetail, you end up with 1.4mils of side play to get your 1mil of vertical play.

          If you are going to use adjustable gibs, it is easy to lock a gib for a 60 degree dovetail in a 45 degree dovetail.

          60 is easier to scrape.

          45 might have an advantage if your part is very shallow. A 45 degree cutter has the advantage that you can cut 45/50/55/60 by tilting the head so it is good to have at least one around for odd parts but a 60 degree cutter may be stronger and can use carbide inserts.

          Bear in mind that the inside corners on dovetails are stress risers. A round relief there (like a hole drilled perpendicular or a ball end mill at an angle) reduces the stress, makes it easier to get a precision straight edge in there for scraping/measuring, and reduces the stresses on the corner (weakest part, where chipping tends to occur) of your dovetail cutter but can involve another setup. It also provides a chip grove for debris when in use.

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          • #6
            Well, 45 degrees has more of an acute angle than a 60 degree (30 deg off vertical). probably less upward bending force on the 60 degree, but more "wedging' horizontally.

            Square dovetail like on Atlas shapers are easy to make, and can be made about as strong as you want by adjusting thickness, etc.
            1601

            Keep eye on ball.
            Hashim Khan

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            • #7
              Thanks for the discussion. I wasn't aware that 55deg was an optimum, but it kind of makes sense. Best of both worlds essentially, and a shorter length of cut than with 60deg. Either way that's a special cutter.

              I just checked my crosslide- my plastic angle finder says 53deg. I think that would be an error, so it's probably 55.

              It's possible that my next project requiring dovetails will be the custom carriage/crosslide I started building last year. Part of that needs to be built-up, another can be milled from solid like normal crosslides. I will probably go with 60deg for ease of fabrication, but I haven't settled on that yet.
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #8
                "I just checked my crosslide- my plastic angle finder says 53deg. I think that would be an error, so it's probably 55."

                Don't be too sure. BeBep told me the other day he was working on a Bridgeport that had 52 degree dovetails. My Rockford planer has about 50 to 51 depending so there may be some uncertainty. It stands to reason that every manufacturer has gages and standards that may or may not conform to some theoretical ideal. Then there is such a thng as compensating manufacturing error.

                If your dovetail manufactured and specific use, it only makes sense to use a commonly available dovetail cutter that's close - 60 degrees. That is, if you're using a vertical mill. If you're using a shaper or a planer where the dovetail angle is set by the slide swivel you can make them any angle you wish.
                Last edited by Forrest Addy; 11-28-2009, 10:05 AM.

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                • #9
                  Dove tail

                  What's the angle on this dove tail?
                  http://en.wikipedia.org/wiki/De_Havilland_Dove

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