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Rotary Broach Version 2 - Shanksize question

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  • Rotary Broach Version 2 - Shanksize question

    Ok so i've revised the broach designs thanks to the help provided from robert and peter in one of the other threads on the subject, but one think remains, Do i want to use a 12 or 16mm spindle bore?

    i figured 16mm would be nice and would give rigidity but leaves the spindle wall a little thin (see below) so perhaps 12mm is enough after all most of the force is axial not radial. oh yeah and i chose 16mm as its likley ill be wanting to broach some larger forms in plastics and possibly aluminium. ill be using smaller forms for steel/brass/bronze etc.

    I suppose i could make two spindles but rather not... 12mm limits me to a maximum hex of 10.3mm accross flats as i would rather not be using a form larger than the shank with 12mm, 16mm limits me to 13.8mm accross flats within the 16mm shanksize, but it would also allow more rigidity if i were to have a form larger than 16mm..... sooo ideas/thoughts?



  • #2
    1/2" is common 3/8" is also but not as much.

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    • #3
      I don't think I'd worry much about a 16mm tool on a 12mm shank, particularly in softer materials. You've got good lateral support on the tool holder.
      .
      "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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      • #4
        Originally posted by TGTool
        I don't think I'd worry much about a 16mm tool on a 12mm shank, particularly in softer materials. You've got good lateral support on the tool holder.
        Fair point, i think im probably just over estimating lateral movment, cause the theory being if all is well it should all be pretty much axial thrust, so ill give 1/2" a try, thanks

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        • #5
          What angle did you decide on for the centreline of the broach to the shank?

          Mark.

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          • #6
            well, i did a fair bit of thought on that, all of the commercial examples i've seen are 1 degree, with a 1degree 30second back clearance on the broach.

            I was tempted to try a 1 degree 30 second angle from shank to broach centreline to increase shearing action and put 2 degrees clearance on the broaches but from what i have read the more clearance on the broach its self the more spiralling that occurs. anything less than 1 degree and the force and contact area is going to increase a load so im not sure, i'll probably make up a 1 degree shank and then shim it out a bit to gain a larger angle to experiment a bit, but as the big boys seem to all use 1 degree and not hide it ill probably stick with that, i'm hoping that ill get a chance to build the new design sometime next week, but am snagged up a bit with sorting out a shop move at the moment (hopfully *crosses fingers* )

            Intresting thing about it is im starting to enjoy designing things almost as much as actually making the stuff, I guess i just really like knowing exactly how things work.
            Last edited by sconisbee; 05-15-2008, 05:37 PM.

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            • #7
              I don't believe increasing the angle will change the shearing action. The angle only affects the extent to which one corner of the broach receives all the tailstock pressure as distinct from its opposite corner. If you can manage come concavity on the face of the cutting tool it would be to your advantage.

              If you think about it, a 1 degree shank-to-broach angle with a flat face toolbit means you also get a positive 1 degree rake. So you'd preferably have a lower centered tool the way endmills are ground fishtailed in order to free the cutting action.
              .
              "People will occasionally stumble over the truth, but most of the time they will pick themselves up and carry on" : Winston Churchill

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              • #8
                Thats pretty much the conclusion i came too also about the dishing of the face and have planned to dish the front of the broach a little, not sure of specifics yet.

                Your probably right about the spiralling however having never handled one of these units in the real world i have to go by what i've read, so thats why i plan on experimenting a bit but ill use 1 degree as a starting point, the good thing on this project is, if i mess things up and my plan doesnt work im only down on material and time and the material was free this time around.

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                • #9
                  re: holder

                  I agree about keeping close to 1 degree. This also keeps your form closer to the actual shape of the broach. Too much angle, and then you start getting close to having a hole a different size than the broach. 1 degree will keep very good accuracy.
                  1/2" bore is really the industry standard, and you may be able to use broaches with forms larger than 1/2" shank size. See the few larger broaches in the image below. The broach farthest to the right is larger than its 1/2" shank.

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