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CNC Machined Boat Propeller

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  • CNC Machined Boat Propeller

    I got one of these new CNC machined props in for repair last week. I know they have been around for a while, but this is the first one I've got in for repair.
    I would love to see one of these being made. I'm guessing they are done on a six axis mill or I guess what they call machining centers now.
    I wonder how long it takes to machine one of these from start to finish. I'm sure they are cast and then milled. Both sides are milled and also around the hub.
    I would imaging that they use a ball end mill and the prop rotates as the quill comes down cutting across the blade face. It must be moving pretty fast as you can see the skipped marks from the cutter, very consistent though and you can feel the wash board effect. I would think that would cause turbulence in the water.
    Two of the blades were ground a little on the back side for balancing, something milling can't accomplish. So there is some manual work done on these.
    I'm kind of wondering how cost effective this process is as compared to say an investment cast prop. I just don't see any point in machining a prop in this manner other than some innovative selling point. And when you have to get it repaired........ there goes all those pretty consistent mill marks.

    JL............

    I had to weld it up with Nibral TIG rod.







    Finished......


  • #2
    That is very strange. I have Nibral 24x26 CNC props on my boat and they they smooth as glass.

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    • #3
      They are cast blanks. Just thick so the different cups can be cut. The super high end offshore race boat props are CNC cut from a forged block then heat treated, x rayed and polished by hand as sharp as knife. Then X rayed again. One guy I know has over $20k into 3 props for his turbine poker run boat.

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      • #4
        I'm sure the CNC prop is cheaper to manufacture than a hand massaged one. They both start out as a casting, but the CNC machine doesnt get paid vacation, call in sick days, doesnt get paid time&half over 40hrs etc.
        The cuttings and chips from CNC can be recycled as clean metal and hand massaged swarf contains abrasives and grit.
        The CNC props can be duplicated with precision as a 'matched pair' or a duplicate size, whereas a hand finished prop is subjective in its measurements.
        I have run hand finished props that were measured within 1" of each other and yet they actually run sea trials as if 6" difference. CNC props are more accurate than hand finish.

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        • #5
          Originally posted by 3 Phase Lightbulb View Post
          That is very strange. I have Nibral 24x26 CNC props on my boat and they they smooth as glass.

          Some of the first generation CNC milled props had only the blade profiles CNC cut.

          In the early 90's the big thing was laser cut.

          JL.................

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          • #6
            Originally posted by Ringo View Post
            I'm sure the CNC prop is cheaper to manufacture than a hand massaged one. They both start out as a casting, but the CNC machine doesnt get paid vacation, call in sick days, doesnt get paid time&half over 40hrs etc.
            The cuttings and chips from CNC can be recycled as clean metal and hand massaged swarf contains abrasives and grit.
            The CNC props can be duplicated with precision as a 'matched pair' or a duplicate size, whereas a hand finished prop is subjective in its measurements.
            I have run hand finished props that were measured within 1" of each other and yet they actually run sea trials as if 6" difference. CNC props are more accurate than hand finish.
            Not necessarily, Most are investment cast. When they crack the mold there is very little clean up to do, especially in the aluminum ones. They don't even balance the aluminum props. A little cleanup of flash, squirt some paint on them and out they go. Stainless, bronze, brass and nibral are usually given a little more attention, especially when it comes to balancing probably because they are heavier and any out of balance is noticed more. Kind of like a flywheel effect.

            JL..................

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            • #7
              Amazingly enough this past weekend a friend was showing me one of these that he picked up out of the scrap bin at work. It also was clearly milled to finish size also.



              Here's a video of their intended purpose.

              https://www.youtube.com/watch?v=y71j-S1SvbE
              Location: Saskatoon, Saskatchewan, Canada

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              • #8
                Not all that impressive a finish, I’ve done better than that on my homebuilt machine in aluminum. Not 5 axis either just 3 cutting. Maybe a positioning 4th axis so the same toolpath on each blade by rotating the prop. The toolpaths would be blended differently on a 5 axis machine.

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                • #9
                  Originally posted by JoeLee View Post
                  Some of the first generation CNC milled props had only the blade profiles CNC cut.

                  In the early 90's the big thing was laser cut.

                  JL.................
                  My Nibral props are CNC finished. About $2500 each.

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                  • #10
                    It would seem to me that a prop cast into what is the final configuration without any machining required would be the most rugged. You don't interfere with the cast surfaces in any way except for polishing, and that allows for the 'as cast' surface hardening to remain uniform and effective. Cnc machining it seems like a way to get it closer to spec in sizing, volume, balance, etc, but at the cost of longevity. Am I off base here?
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                    • #11
                      I have seen only larger ones machined:

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                      • #12
                        I don´t think cast is "stronger", too much.
                        It has a hard skin, afaik, but this is not mentioned as imparting great strength in ultimate load or rigidity.

                        Forged is quite different and is both stronger and harder on the surface than cast.

                        CNC machined parts from solid like auto engine blocks are seen as 4x - 400% stronger than cast/machined normal blocks.
                        Some race/performance people use them.
                        I don´t know where the 400% comes from.

                        Fwiw, near-net casting, and cnc machining, a 50 cm d propeller should take 2-8 hours.
                        For a cheap as-machined part, no special metrology.
                        Prop vs turbine.

                        A 40 cm D part might take half the time, 1-4 hours.
                        Costs might be 50$ / hr for manufacturing quantities qty 1000+, down to 50$ / part cost, depending.

                        So the CNC machining might cost 50$ for 40 D qty 5k,
                        200-400$ for 40 D, qty 1k, depending.

                        And 200-800$ machining 50 cm D prop, qty 1k+.
                        Any 4-axis machine, or 3 axis with rotary table of 1 or 2 axis could do it, fairly efficiently.

                        My (guess)estimates are based on what our Haas customers might do and charge, according to them, when I sold the haas cnc machines, and visited 200 shops in 4 countries, and my 8 sales guys visited 2000 shops.

                        CNC machining is really cheap if you order lots of the same parts or families of parts.
                        So 4 sizes of prop with 2-5 variations each, counts as one part if the total order is 5000k +.

                        E.g. A horizontal might machine the prop in 15 minutes per side, plus finishing, and do 2-4-6 props per side of tombstone, x 4, x possible pallet pool.
                        So a 50$ per-part machining cost at 2 / hr might bring in 100$/hr x 24 = 2400$ / day, semi-unattended or near lights-out.
                        10 days to run 5k, 24.000$ charge, 1000$ or so in tooling, 2k in labour for 5k units.
                        The horizontal might need 0.5-1 hr labour per 8 hr shift to swap 8-16-32 parts, once.
                        Cost 3-50$ or under 6$ each maximum.

                        Originally posted by darryl View Post
                        It would seem to me that a prop cast into what is the final configuration without any machining required would be the most rugged. You don't interfere with the cast surfaces in any way except for polishing, and that allows for the 'as cast' surface hardening to remain uniform and effective. Cnc machining it seems like a way to get it closer to spec in sizing, volume, balance, etc, but at the cost of longevity.
                        Am I off base here?

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                        • #13
                          Originally posted by darryl View Post
                          It would seem to me that a prop cast into what is the final configuration without any machining required would be the most rugged. You don't interfere with the cast surfaces in any way except for polishing, and that allows for the 'as cast' surface hardening to remain uniform and effective. Cnc machining it seems like a way to get it closer to spec in sizing, volume, balance, etc, but at the cost of longevity. Am I off base here?
                          Your not way off base but surface hardness, ruggedness and stuff like that doesn't make a prop preform better or last longer. If you hit something it's going to bend, break off or chip. The thought here is the prop is an expendable part of the drive. If you hit something would you rather have the drive shaft or gear case be damaged or a bent prop?

                          JL.................

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                          • #14
                            Originally posted by garyhlucas View Post
                            Not all that impressive a finish,

                            I was thinking the finish might help shear/shed water tension. Like a golf ball deal.
                            Andy

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                            • #15
                              As a propellor user that’s had many repaired and tweaked can’t see CNC machining is useful . CNC duplicates many parts exact economically. Propellor in my experience has to be tweaked to get optimal performance. Even vessels that are supposed to be identical often “like” different pitches. Cast propellor on a new or re-powered vessel often comes off for adjustment after trials.

                              If the end result is tailored to the job, why CNC, cast it with a mid range pitch & Hammer on mandrels to suit. Only reason I can see is selling off the shelf wheels to owners that don’t know much, most of the pleasure boat market.

                              Boats
                              Last edited by boats; 04-13-2019, 08:23 AM.

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