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Collet Questions

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  • Collet Questions

    I want to drill and ream a series of 1/4" indexing holes in the cast iron back plate for a 6" Bison 4-jaw independent chuck.

    1) Can I safely hold a C drill (0.008" under a 1/4") in a 1/4" R-8 collet so I can center drill, drill and ream with the same tool holder?

    2) I couldn't find the answer in either a 1976 edition of Machinery's Handbook or the new Handbook for the Metalworking Industries by Hanser Gardner. The only acknowledgement of the existence of R-8 collets I could find was on page 767 of the latter book. Are the limits of R-8 collets in there somewhere?


  • #2
    It should work. Give it a try.
    Why not just use a drill chuck in an R-8 holder?
    Has anyone noticed that genuine Bridgeport collets have a step machined in the relief slot to prevent over stress by closing too much? I have never seen them on other collets.

    [This message has been edited by JCHannum (edited 12-06-2003).]
    Jim H.


    • #3
      Another Collet question..

      I am wanting a collet set to use the Morse #3 drills in the chuck, add a vise to the table on my lathe.

      What kind of collet do I need? It has to be chucked.. Is there a chuck specially for this?



      • #4
        There are MT sleeves with straight OD's. Use them all the time in my turret lathes.


        • #5
          Collis makes the sleeves you are looking for. They are a little expensive and are available in diameters from 1 1/4" to 2-1/2" for MT#1 through MT#6


          • #6

            My Japanese MT3 set of collets have a 5/32 hole machined at the narrow end of the three slots to allow for closing to the limit of the slots without exceeding the elastic limit of the metal. Is that what you mean? I imagine the hole is there to prevent cracking as a round hole in a straight line cut will distribute the stress over a larger area and reduce the chance of cracks.

            A properly designed collet should be able to close to the design limit without deformation.
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            • #7
              It looks like this:


              It is interesting, I have only seen it on Bridgeport, I have Royal, Lyndex, Hardinge and others and they don't have it. I assume that it is to keep the collet from closing too far.
              Once a certain point of over closing has been passed, the gripping power and accuracy of the collet will be lost, especially with three segmented collets. This most likely will happen before the elastic limit has been reached. The more segments, the wider the gripping range TG, ER and DA collets have 8, 16 or more as size increases.
              Just curious, since Bridgeport developed the R-8 Collet, it might be a patent thing.
              Incidentally, the size of available R-8 collets seems to be by 1/32"s, 5-C is 1/64"s so that is probably limit to which they can be closed and retain acceptable accuracy.
              Jim H.


              • #8
                JC. I don't think thats the reason. Since their spring tempered you'd really have to pull it up into the spindle to over stress it. I think that what your seing is the web that is left before the collet is heat treated. It is then is removed after it is HT and then ground to size. If the collets were slit completely through , after HT they'd look like a big three petaled flower. Depending on the hardness the web can be sawn or slit with an abrasive saw after grinding.


                • #9
                  I got the bid on some MTaper sleeves extensions that are "Flat" on the female end. 1 1/8th through hole in my spindle.

                  Perhaps, Thou I have lost about everything I have bid on lately. Seems like a wildfire the flagrant bidding on "own" items by sellers.



                  • #10
                    "1) Can I safely hold a C drill (0.008" under a 1/4") in a 1/4" R-8 collet so I can center drill, drill and ream with the same tool holder? "


                    Yes you probably can "safely" hold the drill. Whether it will run concentrictly is something else.

                    .008" undersize plus the shank of most drills are slightly under the drill's size so you could be .009" or more undersize.

                    The R8's are tool holding collets, not so tolerant of holding under/oversize. On the other hand 5C collets are generally workholding and seem to a little better with undersize, but .008" under on a small diameter 5C and you lose a major portion of your grip.

                    In my shop we almost exclusively use 5C's up through 2" with power closers. I know in 1" and under bar work if the stock was .008" undersize we would be limited to very light cuts and the concentricity would suffer.

                    We recently ran an undersize job. It was nominally 1/4" stock, but the customer purchased the raw material (1030? steel) sheared to length form a wire company (5000 pieces). Wire gages are not fractional, IIRC the material was about .243". We had to die cut a 1/4-20 thread on one end and found with collet closer turned up as high as possible a 1/4" collet wouldn't hold the material well enough. We had to order a wire size collet from Hardinge ($$$$) to complete the job.


                    • #11
                      Rustybolt, that is a possibility, but I doubt it. It would be easier to slit completely through rather than re-index and stop at the same point each time. The step appears to be uniform in size and location for many years worth of B'port collets which are of different manufacture, but I have never seen on one by another manufacturer.
                      Another alternative would be to use a metric R-8 collet. 6mm=0.23622".
                      Emergency collets are also available, they are soft, can be machined to size and hardened if desired.
                      Jim H.


                      • #12
                        Three thoughts:

                        1. If it's a 4 jaw independent, why is such great attention to accuracy needed in it's mount. All work will be indicated to center anyway.

                        2. Use a drill chuck.

                        3. On the Bridgeport collets, the slit looks like at least 0.02" at it's narrowest. AT LEAST that much. That's 3 x 0.02" = 0.06" total distance off the circumference if you tighten it up until they actually close. Since the diameter is the circunference / pie, that's about a 0.019" difference in the diameter. I would never tighten any collet of less than 2 or 3 inch size by that much.

                        Those slits must have some other reason for being. Perhaps a rough cut to 75% of the depth to save wear on the final saw or wheel.

                        Paul A.
                        Paul A.
                        SE Texas

                        Make it fit.
                        You can't win and there is a penalty for trying!


                        • #13
                          A collet should hold work +/- a couple thou of its intended diameter. I think a "C" drill in a 1/4" collet is probably pushing your luck.

                          You might try making a simple split bushing, "C" size i.d., 5/16" o.d., and holding the drill in a 5/16" collet. It won't be the same collet for all your steps, but changing only collets may be slightly less of a pain than putting in a drill chuck. Of course, if the machinist gods are smilling on you, you could try making a 0.004" wall split bushing to hold the "C" drill in a 1/4" collet.
                          Try to make a living, not a killing. -- Utah Phillips
                          Don't believe everything you know. -- Bumper sticker
                          Everybody is ignorant, only on different subjects. -- Will Rogers
                          There are lots of people who mistake their imagination for their memory. - Josh Billings
                          Law of Logical Argument - Anything is possible if you don't know what you are talking about.
                          Don't own anything you have to feed or paint. - Hood River Blackie


                          • #14
                            The MT sleeves that have a flat in the socket are "usem-up" sleeves. When the taper on the drill has become galled from spinning you gring a flat along the bad taper and put it into the socket with the flat. The flat keeps it from turning. These adapters are usually #2 to #2, #3 to #3, etc. They really extend the unusable length of the drill.


                            • #15

                              If the elastic limit of the material has not been exceeded then there is no change in the shape or properties of the collet (NB). Under the elastic limit the metal acts as a spring and returns completely to the "as made" shape. There is stress that acts upon the metal's crystalline structure and the elastic limit will change with time as the metal is repeatedly stressed. The usual failure mode in that case is cracking when the elastic limit has not been exceeded, not bending, bending being elastic deformation until the yield strength is reached.
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