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Permanent thumbscrew dump mold

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  • Permanent thumbscrew dump mold

    Here's a fun little project I recently completed. After every little casting session I do with my benchtop foundry, there is usually a bit left in the crucible that I dump off into an ingot mold, only to end up in the crucible again for the next pour. Small volumes, but it's always bugged me as being a waste. I got thinking of some simple small volume shapes I could cast and make permanent dump molds of and maybe shift away from just dumping off ingots all the time, to creating useful parts instead. A dump mold is usually a simple shape to dump the waste in. It doesn't have the complex sprue, runners, risers, etc of a permanent mold, it's simple a cavity to "dump" whats left into. This is the first of a few different ideas I had.

    The threaded inserts are 0.75" od x 1" long and tapped 1/4", 5/16, and 3/8" unc. They ride in straight 0.751" reamed bore and can be set at different height for different shoulder lengths. I'll make some metric ones too as needed. Simply wind in a bolt of the size you want and it get's captured by the pour. The natural draft of the cavity is the rads of the lobes connecting at the parting line, and I'm relying on shrinkage to release from the straight sided shoulder bore.

    First test didn't go as planned.

    Seems there are some venting issues that didn't allow the lobes to be filled. The rest of the mold worked as I imagined it, so it was time to do some modifications to the sprue and venting. The excess on top is easily cold chiseled off allowing the part to come free of the mold.

    The sprue is easily faced off leaving a flat surface​. Tiny bit of porosity in the center.....

    Back to the drawing board to modify the venting and open up the sprue, this is what I came up with.

    and these are the results. I opened the sprue dia up to the max, and vented the entire flat top out. I wanted to keep the top opening round to aid in the chiseling off excess or else I would have just cut the lobed shape straight up.

    I also added some 0.003x0.337" parting line vents out the side of the lobes

    First pour in 1/4" this time, and all lobes filled perfect. Success......

    When it came time to face it off however.....

    I'd consider it a 90% success for the intended goal which is to better utilize that small volume left at the end of a pour instead of dumping off ingots to remelt again. NOT make 100% perfect thumbscrews trying to put carrlane out of business....... I don't think I'll ever 100% get away from the porosity in the center because I don't have a feeder to feed that core as it cools. For most of my regular casting sessions with my little benchtop foundry there isn't enough volume left at the end of a pour. I might not ever get 100% fills of this mold, but it'll be worth giving it a shot instead of just dumping off ingots all the time. It won't take long to build up a little inventory of various sizes that are handy around the shop for tooling builds, or for use with my lure/jig molds.

    I have a few other ideas for simple small volume items that I can also make dump molds for and will share them too when I get there. I won't do anymore dedicated pours to test this one though and play around with it more, it'll have to wait until the next casting session. Not sure there's anything I could really do with this to mitigate that, but I might turn a pouring tube to act as a bigger riser on top to feed that cavity as it cools better. Not sure I'll have enough volume to fill it all the way, but it would probably help. I also thought about machining down the top of the mold to almost get rid of the sprue entirely and force the shrink bubble deeper below the surface. If that doesn't work I have to re machine the upper mold half so.... I'll pour a few more during the next casting sessions and go from there. I can also just simply plunge a ball endmill in the top to turn it into a design feature too.....

    Cheers, Dan.

  • #2
    Good plan!

    Nice looking mold cavity also, is that CNC? I think I see the typical CNC machining marks around it.
    CNC machines only go through the motions.

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    • #3
      Yes, It's all cut with my Tormach. I modeled it, and the CNC just went through the motions......


      • #4
        Awesome Dan! Nice looking mold!

        Thanks for sharing....
        Ontario, Canada


        • #5
          Originally posted by RSG View Post
          Awesome Dan! Nice looking mold!

          Thanks for sharing....
          Thanks. This was sort of a test of workflow for me for some future ideas, as much as it was about the end result.


          • #6
            That's a very sweet idea!

            I wonder if part of the issue is the mold freezing the metal too fast. Did you pre-heat the mold before lifting the crucible for the other part and then the knob?

            I know from my bullet casting that even round ball does not form cleanly until the mold is up to the proper casting temperature. Even after letting the mold sit with a corner in the pot of melted lead for a minute it's still pretty common to get incomplete fills for the first couple of casts. But once heated correctly it's all good as long as I cast fast enough to keep the temperature up.

            Chilliwack BC, Canada


            • #7
              Ya, I preheated it both times. Perhaps not enough the second time. The first time I had a propane torch on it for about 10 minutes. The second pour, not that long. I'll try and find my infrared thermometer to quantify the preheat temp next time.

              I've been picking away at it today like a scab, and have exposed the top. I'm about 98% sure this second one is simply an air bubble due to the way I pour, and perhaps helped by a slightly lower preheat, and not a shrink related issue like I initially thought. Looks like a bubble formed around the top of the bolt head, and the sprue froze up before it could escape. I can't photograph it very well. Thinking maybe I should have a poking rod handy to immediately give it a poke before it freezes up to try and release that bubble if there is one. I know I said I wasn't going do anymore dedicated pours for this, but now I'm curious......

              I have a silicone lure mold for resin casting that almost always catches a bubble in the sprue that I have to poke free. When I cast lead I always run a few pours through the mold to preheat it without hooks. pour and let them soak, the dump them out. Rinse repeat a few times and it'll pour just fine. Not something I can do with this one keeping the original purpose in mind, but I'll try in the future to get more heat into it.


              • #8
                Could the black oxide or zinc, depending on which bolt, be causing the airbubble? As in maybe it's not air but some gas produced by the oxide or plating burning off?

                If you're hitting it with a torch for even 5 minutes I would think that it should be hot enough.

                Something I've seen for some other casting process was to soot up the mold. I assumed that the soot was to act as a release agent. But perhaps it acts as an insulator as well? A candle does a nice job when held close to metal. Get it so the tip of the flame is touching the metal and the black builds quick.
                Last edited by BCRider; 03-21-2023, 02:45 PM.
                Chilliwack BC, Canada


                • #9
                  Maybe, good thought. I'm not sure. Black oxide I'd doubt, but zinc maybe. It's vapor temp is higher than the pouring temp of the aluminum, but further analysis is beyond my knowledge. It could be a possibility. I did pour that one pretty pourly though, so there's no doubt I introduced some turbulance. It wasn't so much a steady pour as it was a "why isn't this coming o...oh **** it's already over"


                  • #10
                    Maybe try grinding some flats on the insert in the bottom to help vent the center? I only worked with injection molds, but more vents was always the first move when troubleshooting.
                    Traverse City, MI


                    • #11
                      Wow.. This really appeals to me.
                      I have a knob collection and also
                      a crank and handle collection.
                      Knobs are like man jewlery for
                      my machines. Your knob is lovely.
                      Kind of like mickey mouse fingers.
                      Very devine I must say. I would
                      totally buy one from you if you were
                      selling them. 3/8-16 would be perfect
                      for the power feed on my Gorton 9-J
                      mill. It has a really small bat lever
                      knob, but you have to loosen it and
                      also slide it sideways. The bat lever
                      is not big enough for your hand to
                      grab and get that sideways pressure
                      exerted to the mechanism. So it
                      needs to be big enough to turn and
                      slide. That is why I find your knob
                      so attractive. It fills the mechanical
                      need better than the Gorton knob.
                      Let me know.



                      • #12
                        Doozer, I'm flattered you find my knob so attractive. How long of a shaft stickout do you require for my knob to meet your needs? Do you prefer Zinc, or would you like it black oxide. I have not used the 3/8"-16 insert yet, so the next time I do, It's yours if you want it.


                        • #13
                          Originally posted by Dan Dubeau View Post
                          Doozer, I'm flattered you find my knob so attractive. .....
                          Um.... you might want to re-phrase that?

                          Getting back to a possible effect from the surface on the screws. I assume you de-greased the black oxide surfaces? But since the function of the oxide is to hold the protective oil or other anti rust product could it be that it's set in there pretty well?

                          As an experiment I wonder about grinding or machining off the finish to bare metal for the part in the cast portion just to check.

                          Chilliwack BC, Canada


                          • #14
                            I'm secure enough in my manhood I can appreciate a good compliment .

                            Neither one I've tried yet was black oxide. I'm not sure what the coating on the first one was. Grade 5 5/16-18 bolt from tsc bolt bins, somewhat greenish coating, burns off easily. The 2nd was a galvanized 1/4-20 from a scrap bins. Neither was degreased, but both were torched.


                            • #15
                              Like doozer ( I do) I also have a knob collection, sounds a bit kinky but there we go, mostly cracked ones looking recently, making little jigs for wood butchering, ( I do like wood, at my age a bit of wood is rare, hard wood anyway, plenty of soft wood over here .
                              the hand screw is reminiscent of really high end stuff like German saws ( there’s a few on a kaltenbach upcut extrusion saw I have) great when your hand is all greasy from lube, unlike the round form .
                              plus they don’t get brittle with age like plastic.
                              id like to see a brass one polished myself
                              nice work, very competent bit of machining, well done
                              sorry for all the workshop innuendos, has to be done , blame doozer, he started it!