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switching between a 3 and 4 jaw chuck on a Rotary table?

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  • switching between a 3 and 4 jaw chuck on a Rotary table?

    I started a thread on mounting a 4 jaw to my 8" Vertex RoTab that has 3 slots. Got some wise advise and just bought an 8" 4 jaw Sanyou/CME for $111 on Epay along with a 3MT arbor with a machinable head to center the table to mill spindle. I'll turn the blank head to 7/8 to fit my largest R8 collet. I'll also turn some 3" alum rod I have on the shelf down into a crown for the 7/8 head for snugging the 4 jaw chuck.

    So I got to thinking might as well have a self centering 3 jaw for quick work too. Whats a good way to work with two chucks on a table? For $55 I can buy a little 70mm 3 jaw already mounted on a MT3 arbor /draw bar. that seems like the best way probably, covers most of the round stock I use most often.

    Just thought I'd throw it out there for any system you might like using yourself- thanks

  • #2
    Athree jaw scroll chuck will probably be easy to mount on a three slot table. I have some Pratt bernerd three jaw scroll chucks which as standard came with both front and rear mountings. This prompted me to see if there was space for making them totally front mounting. I checked the threaded rear holes and found they could be drilled right through and have counterbores on the front for SHCS. The picture shows a 100mm chuck I bought which had three rear mount threaded holes for 6mm screws. I was able to convert them to front mount and also add another three, so the chuck now has 6 front mount holes. This is for added security as I like to fit this type of chuck to a backplate with a reduced register. The register is about 0.020" loose to allow fine adjustment if necessary.

    Click image for larger version

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    • #3
      Every scroll chuck that I use has either 3 or 6 front mounting holes for bolting to an adapter.
      In this case an 8" 3 jaw mounted on an adapter for the common American standard A2-5 spindle taper.

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      • #4
        OK so how many of those Pratt Bernered chucks you sitting on? I mean if they're stacked in the corner....
        PM ;-)



        Originally posted by old mart View Post
        Athree jaw scroll chuck will probably be easy to mount on a three slot table. I have some Pratt bernerd three jaw scroll chucks which as standard came with both front and rear mountings. This prompted me to see if there was space for making them totally front mounting. I checked the threaded rear holes and found they could be drilled right through and have counterbores on the front for SHCS. The picture shows a 100mm chuck I bought which had three rear mount threaded holes for 6mm screws. I was able to convert them to front mount and also add another three, so the chuck now has 6 front mount holes. This is for added security as I like to fit this type of chuck to a backplate with a reduced register. The register is about 0.020" loose to allow fine adjustment if necessary.

        Click image for larger version

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Views:	160
Size:	416.1 KB
ID:	1941200

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        • #5
          I have my grandfathers 5" Cushman 3 jaw from about 1900, the scroll nuts are proud of the body- a real knuckle killer (old old old). Funny thing is he was a trained MIT engineer and this OLD lathe was his first machine tool, he ended up with 50 employees making springs for like 75 years. I think thats why he set it aside for 65 years before I ever saw it. It's perfect, like it never did a days work.

          Perfect for me anyway! I dont do much of a days work either!


          Originally posted by Bented View Post
          Every scroll chuck that I use has either 3 or 6 front mounting holes for bolting to an adapter.
          In this case an 8" 3 jaw mounted on an adapter for the common American standard A2-5 spindle taper.

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          • #6
            oops meant to post my 9" Seneca Falls lathe, it's a good one. less than .001 runout. Wish I had learned more from all the parts I've made on it- but instead- I seem to have found an endless horizon of mistakes.. I learn from every third one ;-)Click image for larger version

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            • #7
              So let me get this straight...
              You want to mount a 8" lathe chuck to your rotary table with a 3 morse taper arbor??
              Something does not compute.

              -D
              DZER

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              • #8
                well I want to ALLIGN the chuck with the arbor. My Rotab is 3MT so I'm going to make a 3MT allignment pin to center the rotab to the mill spindle quickly. The 4 jaw itself has a 2.5" ID so I will turn a cap or crown to fit over the arbor so the same pin will also register the chuck. Bolt down and I hope to be within a couple thou. So now I'm thinking about adding a 3 jaw and wondering what a good approach. I guess making another cap/crown to fit the ID of a flatback 3 jaw. With a 3 jaw (as Bented suggests) I can prob use T slots bc my Rotab is 3 slot.

                The confusion is that I'm also looking at these little India built 3 jaw chucks for about $55 on Ebay that come on a 3MT. Just seems awful easy. 70mm chuck size a bit small but when I think about it will handle 90% of what I do.

                Edit: these come on an M12 threaded drawbar type arbor.

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                • #9
                  Old Mart that is just what I'm fixing on if I go with a larger chuck instead of the "chuck on a stick" India 70mm type. How did you figure out if you were safe to blow out the threaded sockets? Did you take the thing apart? Never dug into a scroll chuck. Independent chucks look foolproof enough.

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                  • #10
                    Bill, three jaw scroll chucks are made to be dismantled and cleaned on a regular basis. There is no way to get into trouble working on one. Take yours to bits and assess the consequences of drilling through the rear mounts with regard to how close the holes get to the tee slots, and the bores for the key bevel gears and the little holding screws. Use a Sharpie and a trysquare to mark the line of the holes on the outside of the chuck body. Also check the space for the counterbore for the screw heads. If you can get the holes missing by 3/16" you are good to go. If you wanted to keep the rear mount holes, then look at drilling a set of three spaced halfway between them.
                    Last edited by old mart; 05-04-2021, 12:14 PM.

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                    • #11
                      Old Mart, that's a good marching plan. I prob should buy a tri square, looks like a great thing for layout and finding centers. Looks invaluable for shooting a line off of circle. So I guess my 100 year old Cushman is overdue for a bath!

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                      • #12
                        Just my opinion-I wouldn’t bother with the three jaw. Or maybe would be better to say I wouldn’t put much effort into a quick change system, because swapping chucks would negate efficiency gain, and the “wrong” one would always be on the rotab.

                        If you usually do fussy work you need the four jaw and for a short run quantity just mark two jaws and leave the other two alone. Three jaw for bigger quantity or wide size range/non-fussy work.

                        That said, I COULD really go for a D1-6 spindle nose screwed to the table to share things with my lathes.

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                        • #13
                          What ever lathe spindle mount you have, cam, threaded, whatever.....make an exact duplicate of that that catches the centre register of the RT and bolts to the RT. Then any tooling you use on the lathe can be used on the RT, without even removing the work if you like.
                          in Toronto Ontario - where are you?

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                          • #14
                            What works for me is an 8" 4 jaw scroll chuck on an 8" Vertex H/V rotary table. The 4 jaw holds round and square very securely. The rotab has 4 T slots and the chuck has 4, 1/2" grade 8 bolts, 1 next to each jaw, holding T nuts. The rotab was set up on the mill, dialed in for center, the chuck put on, T nuts lightly tightened, chuck dialed in, and cinched down. I've been using this setup for years and never needed to change it. I have used it in horizontal and vertical mode many times each.
                            Kansas City area

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                            • #15
                              If you have a 3 jaw on the table, it needs aligned carefully. If you have a 4 jaw, it only needs a rough centering... you are going to check alignment anyhow, and can set it each time to be "perfect".
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