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Keyway problems

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  • Keyway problems

    At the museum, we have almost finished the Atlas 12 x 24 bench top lathe. When it was switched on the motor was being bounced up and down on its pivot by a belt/pulley problem. The double pulley next to the motor was wobbling so it was checked out. The 3/4" bore in the Zamak was not a great fit on the spindle and when the grubscrew was tightened the wobble got worse. fortunately a bush could be fitted and bored to be a good fit in the shaft. The bore completed, attention was turned to a keyway 3/16". We do not have any means of broaching so I decided to cut a slot by axially milling with a new hss slot drill. There was enough bush wall thickness to not worry much about a round rather than a square slot bottom. The 3/16" slot drill could manage about 1.2" depth in the 1.3" long bush, near enough for government work. The slot drill was advanced 0.010" each cut and plunged the whole depth each time in the gunmetal bush (leaded bronze) until there was enough straight sides for the key. The key fitted very well in the slot, but the shaft would only fit 1/3 of the key length. We eventually found that the keyway was not in line axially, like a very slow turn rifling groove. The fixture was checked, the mill was checked, everything was square, so the cutter must have deflected, even though the cuts were small and the feed rate was not fierce.

    I removed the bush from the pulley and turned up another to loctite in. This time I drilled the bush 5mm using a solid carbide drill where the bottom of the slot was to be and then used a solid carbide 5mm mill to start at the hole and move inwards to cut into the centre hole. I went down 1mm at a time to the halfway mark in the bush length, and then lined up with the drilled hole to do the other end of the slot. 5mm cutters do not usually manage 32mm deep. The slot looks good despite being milled in two stages without a DRO. I will now have to mill up a 5mm/3/16" stepped key for the pulley. The bore will have to be finished bored with an interrupted cut, but gunmetal is very easy to machine.
    The checking for squareness will have to wait till next Wednesday, I won't make the key until the slot proves to be straight.

  • #2
    What is gunmetal?

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    • #3
      Originally posted by Bented View Post
      What is gunmetal?
      It's a type of bronze that was used in making cannons and such, hence the name.
      Lynn (Huntsville, AL)

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      • #4
        I thought the Gunmetal and Leaded bronze were different things. No lead in Gunmetal.
        Nev.

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        • #5
          There are a couple variations of gunmetal bronze that are leaded. The original gunmetal bronze was not leaded.

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          • #6
            This is the type of gunmetal I used, Ialso use it for leadscrew nuts.
            Premium quality alloy of Leaded Gunmetal LG2 Bronze with excellent machining properties are available for sale at Kormax. Suitable for light & medium duty applications.

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