From extrusion to injection molding

Charles is working on a tabletop plastic injection molder based on the Gingery design, but the machining needed for the heater chamber is a bit more than what his Sherline mill can handle, so I ran it on the big Keiyo Seiki (which also gave me an excellent excuse to finally get a big Rohm drill chuck for the tailstock). I figured he’d probably want to see the steps needed to do the machining, so I took some photos of the process (which simply reminded me of why I need a better camera, as the autofocus on mine in abysmal).

After taking off the 5C collet chuck and installing the standard 3-jaw chuck on the lathe, I chucked up the piece of 1.5″ steel rod, powered up the RPC and lathe, and got to making chips.

The bar was cut quite nicely as it was, so rather than face the end first (and have to fiddle with getting the tool bit exactly on center) I center drilled it.

Then I faced the end with a beater carbide tipped bit that had come with the lathe.

I started drilling with a screw machine length 1/4″ drill bit (I love the screw machine length bits as they’re short, strong, and don’t wander as much) and then switched to the much longer 1/4″ bit shown here.  One big advantage to drilling on the lathe rather than on a drill press or mill is that long holes are much straighter.  On a drill press, the workpiece is stationary, and the drill bit is allowed to wander off of its axis a bit.  However, on lathe, the workpiece spins, and the path of least resistance for the drill bit is to seek the rotational axis of the workpiece, ensuring that the hole stays straight.  From what I’ve read, the most accurate hole drilling is done by rotating both the workpiece and the drill bit simultaneously (in opposite directions, of course).

Once I had the 1/4″ hole drilled all the way through, I moved up to a 15/32″ bit to bring the hole nearly to size.

Then came the 0.501″ reamer.  I put the lathe into a very low gear, as high speed will kill a reamer quick.  Lots of Tap Magic and backing out frequently to clear chips made the procedure go without a hitch.

Finally, I flipped the part around, faced the other end and topped it off with a a slight chamfer where the piston will actually enter.

And then the moment of truth – the 0.500″ ground rod that will serve as the piston slid through cleanly with zero wobble.  I also machined the end of the piston where the linkage rod will connect.  Unfortunately, my camera refused to focus for any of those shots.  Fortunately, this means that no photographic record of my ‘oops’ exists (the endmill caught the workpiece and slapped it back through the 5C block I used for workholding).  It was a good reminder that steel is not as forgiving to machine as aluminum, so I removed the 4 flute endmill I had been using and switched to a flycutter and took nice small cuts to create the flats needed.

2 thoughts on “From extrusion to injection molding”

  1. Damn, you are just 2 steps ahead of me…

    I still have to machine an extrusion nozzle, and I see you’ve already done it and done the extrusion!

    Good work!

    Lawrence (pp3pd forums shanghailoz)

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