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Trouble with deep facing groove in brass tube

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  • Trouble with deep facing groove in brass tube

    Note: I posted this also in the "other" machining forum. I'm trying to get as much input as possible. Thanks in advance.

    I am having trouble creating this feature in the end of a 2" brass tube.



    The large entry groove is about 1/4" wide x 1" deep with a square shoulder at the bottom (it can have a small radius). In the middle of that groove is a narrow groove about .180" wide x 1/2" deep with a full radius at its bottom.

    Altering the design is not an option.

    I had some success with this trepanning tool (I had to modify the radius and weakened it) until the insert caught and broke the tool. There is an unmodified one next to the broken one.



    I then, with with this style of trepanning tool, had some limited success, but the ID and OD dimensions weren't true enough.



    I plunged the intial slot about .210" wide and then attempted to open up the slot. The slot has some taper in it, and I thought the tool must be rubbing and deflecting, so I attempted to grind the side radius a little to give greater clearance. So far, I am unable to determine if the OD is narrowing down, the ID wall is expanding, or both. I have a gage ring that binds as I try to insert it.

    I'm very frustrated with the large groove and haven't even begun to tackle the smaller groove at the bottom.

    I'd like any and all suggestions on how to machine this feature(s). I am willing to take the parts to another shop, if I knew what process would be best. I am only showing this one aspect of the part, as it is proprietary.

    I am also willing to invest in other tooling. I bought carbide tipped hole saws to try and create the initial slots (didn't work at all). I bought the Valcut tool but didn't try it as the bit that is narrow enough, will not go deep enough.

    I thought about plunging with the pictured tool and then using an endmill on a dividing head to try and open up the slot and give greater "trueness", but there is no way I can see to use a small endmill to machine a 1/2" deep slot that begins in a 1" deep pocket.

    I'm whipped!
    Last edited by Jim Caudill; 09-13-2011, 01:02 AM.

  • #2
    Use a tube shaped tool

    For the depth and width of the features, you probably want to make a tool similar to one on the market called a rota-broach. It looks like a very stout hole saw with a triple chip cutter pattern and relief areas that allow the chips to exit. You will need a series of four, a roughing tool for the large groove, a finishing tool for the large groove, a roughing tool for the narrow groove and a finishing tool for the narrow groove.

    A parting type tool with a curved body will likely deflect and roll up in that deep a groove.

    Comment


    • #3
      Your drawing is broken.

      Once that is fixed make a blank that would fit that space then remove parts until you have a holesaw. You're just cutting brass, so go slow, use coolant, and clear chips often.

      Comment


      • #4
        Originally posted by 2ManyHobbies
        Your drawing is broken.
        Your comment lacks insight. How is it broken? You may think it's obvious but it could be "broken" in more than one way. I haven't stared at it, but in how many ways is it broken?

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        • #5
          I did buy carbide-tipped holesaws made by Morse in 1-1/2", 1-5/8", 1-11/16", and 1-3/4". For some reason the holesaws required way too much pressure on the downfeed of my Bridgeport. I would have thought they would have cut thru that brass like butter, but it was not so. The brass was supplied to me, so I can't be sure of the alloy. The drawing calls out "brass" with no specific alloy being specified.

          I have several annular cutters (forget the manufacturer - could be rotabroach) and they were my first thought. Their "wall thickness" is too great. Thinning the annular cutter didn't look like an option due to the "tooth design" which places angled facets both at the ID and OD of the tools.

          This is not the actual part, but a "test slug" that I am using to try and develop the grooving technique. The actual parts are already made, and are just lacking this final grooving operation. There is too much time and money invested in the parts to do any experimenting on them. When I can produce the desired grooves in a "slug", then I'll run the actual parts. This 2" brass is costing about $10 per inch!



          jpfalt, your comments show great wisdom. You were able to predict what I am learning thru experience. The issues you are describing are probably what led the Valcut folks to use the dimensions they chose for their cutting tools. To get "stick-out" you have to increase cross-section, and that makes for a tool that is too wide to reach the 1-1/2" depth. Their 5mm cutting tool is only reccommended for 3/8" depth! Making my own "Rota-Broach" type cutter is beyond my abilities, although I did consider trying to thin the wall of my exisiting annular cutters (teeth design issues). As already mentioned, I have considered sectioning an existing hole saw or eliminating any number of teeth and creating greater chip clearance.
          Last edited by Jim Caudill; 09-13-2011, 12:39 AM.

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          • #6
            Do you know for a fact that these parts have been made correctly before?

            Just because someone can draw it on paper doesn't automatically mean that it is possible to do.

            A special ground trepanning tool seems the only way to me, but something less than .180" wide sticking out over 1.5" with a full radius on the end is going to be a challenge.

            Dave

            Comment


            • #7
              Thanks Dave. No, these parts have never been made. My niche in machining is doing prototype, tooling, and experimental support. Almost nothing I make is ever a production part. If it becomes a production part, it goes to someone else. I typically make onesy-twosy type parts or assemblies. I once made about 150 coil cores, but that was a long time ago.

              I used to have a Hurco CNC mill, but sold it because I never was able to economically make it work for me. If you are only making 2 parts, the setup/programming time takes as long as to make it on a manual machine. The CNC did allow me to make some parts that I just couldn't do on my Bridgeport and lathes.

              Comment


              • #8
                CNC lathe with a live Y axis. Farm it out.

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                • #9
                  A CNC mill and 4th axis or for that matter a manual mill and a RT.
                  I just need one more tool,just one!

                  Comment


                  • #10
                    Thought about that but can't tell how long the part it. If it is reasonably short than it can be done on a cnc mill, no 4th needed.

                    Comment


                    • #11
                      The parts are less than 6" overall length. I know a couple of good CNC shops with vertical machining centers (one is the shop where I bought my Hurco's). I'll take the parts to them tomorrow and see what the owner's say. I think they'll balk at the depth and diameter required for the smaller slot. With their high flow coolant systems, they maybe able to flush the chips out sufficiently to allow the use of a 1/4" shank with 1/8" cutting dimension. A reground 3/16" ball end mill (reduced to .180) could be used as a final tool to generate the radius at the bottom.

                      Comment


                      • #12
                        Generating the form is not the issue, there is more than one way to do that. Tool rigidity is the problem.

                        I suggested a trepanning style tool as that would be the most robust (ie rigid) tool that I could think of. Any type of rotating or conturing tool would have to be even smaller for clearance purposes, not to mention chip evacuation. Or am I missing something?

                        Dave

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                        • #13
                          "Altering the design is not an option."

                          Does the part have to be one piece? Maybe you could turn it from two pieces and press them together. Functionally the same, but easy to machine and inspect.

                          Even if you can figure out a way to make it in one piece, inspecting it is going to be a pain.
                          Any products mentioned in my posts have been endorsed by their manufacturer.

                          Comment


                          • #14
                            Originally posted by winchman
                            "Altering the design is not an option."

                            Does the part have to be one piece? Maybe you could turn it from two pieces and press them together. Functionally the same, but easy to machine and inspect.

                            Even if you can figure out a way to make it in one piece, inspecting it is going to be a pain.
                            Can it be don on the mill w/rotarytable .
                            Reg. slotmill and a ball-endmill ?

                            CS

                            Comment


                            • #15
                              Chances are you will have to make your own custom tool, so what I'd suggest is turning some steel to a tube shape, with ID and OD to match what you're doing, then add some cutting edges onto it. I would use three cutters, equally spaced.

                              One way to do it would be to turn the tube to sizes, then drill three dimples equally spaced in the end. Silver solder three ball bearings into those, then grind on each to leave a half-round cutting edge behind. If you grind just over half the thickness of the ball away, you'll have a round form with some relief angle behind it.

                              Seems the initial slot is about 1/4 inch wide- seems you're good to machine that, otherwise make a tool with 1/4 inch ball bearings to do that part. The next one is 3/16 wide so use 3/16 balls for that one. The tools are self-centering by virtue of having three cutters, and it wouldn't be difficult to hold them in the tailstock. All you need besides some pressure behind it, is to keep it from turning.

                              It might make more sense to mount this tool on the mill table, centered under the spindle. Mount the workpiece in the spindle and lower it onto the tool. You will have to keep raising and lowering the spindle to allow the chips to fall out, but at least they will fall out. That might be a large part of the battle- avoiding jamming because of the chips.
                              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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