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Small RT vs. Dividing Head

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  • Small RT vs. Dividing Head

    I thought I would seek the wisdom of the folks here as I think through something.

    I have been keeping my eye out for a smaller dividing head on Ebay etc. The smaller ones are more popular and seem to go pretty high--especially if they have the tailstock and a full compliment of indexing plates. I want to stay with a smaller (depth) indexing solution so it will work not only on my Bridgeport, but also on a mid-size horizontal mill I have and perhaps even on my Sheldon 12" shaper.

    I have been toying with whether to just buy one of the B&S Universal dividing head knockoffs from China that are available pretty reasonable. I am also entertaining whether one of the import 6" rotary tables with a tailstock and dividing plates might be even more useful in some ways. I have a 12" horizontal Bridgeport rotary table, so the 6" jobbydoo would probably only be used in its vertical position (axis of rotation parallel to table). This makes it roughly equivalent to the dividing head, but it won't pivot on the horizontal axis like the dividing head would.

    On the other hand, the up-side is that if I wanted to use it for rotating work during a cut (as in milling a grove around the circumfrence of a part), rather than just for indexing, I would think it would be better suited to that. It would also be handier if work didn't lend itself to being held between centers or in a chuck.

    Anyone out there using one of these little RT's that way? Is there another factor that I am not considering that makes a true dividing head more useful? I don't have any specific project in mind at this time, but figured that the day I want to use one, trying to find a dividing head in a hurry would be annoying at best so I am trying to decide just what to do in advance.

    Paul Carpenter
    Mapleton, IL

  • #2
    one of the advantages of using an indexing head vs. a vertical rotary table is more tool clearance when using the former. the larger diameter of the rotary table makes it difficult, if not imposible, to position the cutting tool close to the face of the table. there are ways around that problem by extending the tool or by fixturing the workpiece to extend beyond the face of the table. also, if the part to be machined is large enough in diameter that will not be a concern. since you already have a horizontal rotary table, i would go for the indexing head. and, if need be, you have the option of using the rotary table mounted to an angle iron for use in the vertical.


    • #3
      That's a good point. I keep thinking of the work held in the jaws of a chuck where the chuck sticks out from either the rotary table or the front of the dividing head. In that case, the clearance from say a vertical mill spindle or horizontal milling cutter is about the same and is a function of the chuck jaw length, size of chuck etc...roughly the same either way.

      If, on the other hand, the work is held between centers then the extra "nose" that a dividing head has is a useful advantage. A dead center set in the center of a rotary table is closely surrounded by the 6" diameter table. In the nose of a dividing head, there a sort of spindle nose that is pretty small in you a clearance advantage there.

      Building on the original is a dividing head for use in actually feeding work into a cutter? I do know that some dividing heads are feed screw driven for use in say spiral milling, but that's a bit different than hand cranking a dividing head to mill a groove etc. I do also know that my horizontal rotary table has a 90:1 ratio where the dividing heads I am familiar with are 40:1. I don't know if that influnces whether its wise to use a dividing head this way or not.

      Paul Carpenter
      Mapleton, IL


      • #4
        Gear ratios and scales you can get used to. In my view the stuff you'll be trading off are

        - relative stiffness of rotab vs the dividing head- rotary table doesn't have the little spindle letting the chuck flop around. Be careful with dividing heads using a threaded spindle- cutting forces can start unscrewing the chuck pretty much guaranteeing a wreck. Solution is a bolt thru the spindle bore w/ nuts on each end. Bummer if your job needs the spindle bore clear...

        - do you need intermediate angles- I have used my universal dividing head at 45 degrees before, quite handy. The universal feature also lets you tram in the head, whereas if the rotary table/mill table aren't flat then you're stuck shimming. This last is theoretical on my part- just a guess I don't have a rotary table.

        - it can be difficult to set up a rotary table w/ axis parallel to mill table, that big base stuck up in the air can be hard to work around.

        - OTOH, a universal dividing head set vertical uses up a good deal of the z envelope of a Bridgeport- getting clearance for drills may require using collets.

        - You can do a lot with a dividing head w/o a tailstock- I have one which may fit mine but have never gotten around to using it. More important to have the plates- but don't sweat that too much, within reason plates w/ the desired #'s of holes can be adapted to fit- its a quick and straightforward job.

        Clearly the easy answer is "have both". I got one of the 6" universal B&S clones, haven't had a convincing reason to get a rotary table yet- though I won't hesitate if/when I find one cheap.




        • #5
          "rotary table doesn't have the little spindle letting the chuck flop around"

          You mean like in a lathe headstock?