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Jan 09

Making a replacement lathe leadscrew nut

My milling machine has a #3 Morse Taper spindle. I’ve spent a few days making #3 Morse Taper arbors from steel, which I’ll use to make various tools for my mill.

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Unfortunately this took a toll on my lathe’s topslide screw nut. That little bronze coil spring is actually the threads from inside the nut.

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After doing a little research I found that acetal plastic is just as strong as bronze for this kind of application, and much easier to make into a nut. The simplest way to do this is through compression molding.

Here I’m drilling a hole through a piece of steel shaft to make the main body of the mold.

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Progressively boring out the hole to 20mm

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The original bronze nut fits inside, so it’s looking good.

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Next I drilled a 12mm hole through the side of the mold.

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This allows the leadscrew to be fed through the mold and plastic blank.

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Next, a section of acetal plastic is machined to 20mm diameter and inserted into the mold. The mold is used as a template to drill the initial hole through the plastic.

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The plastic makes some impressive swarf.

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The mold and the drilled plastic blank.

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The plastic blank is sawed to length, and two 20mm diameter spacer plugs are machined from steel. These will apply even pressure to both the top and bottom of the mold, to prevent any possible bending of the leadscrew.

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The leadscrew is dusted with graphite powder to make it easier to unscrew when the pressing is finished.

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All set up, ready to apply pressure. The 6-tonne press is overkill for this task, but I’ve heard that if you don’t use your tools your wife may be inclined to ask questions about why you need them.

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Heating the mold and leadscrew with a heat gun. Since we’re applying a couple of tonnes of pressure it isn’t necessary to heat the mold to the melting point of the plastic, we just need to soften it enough to retain a good impression of the leadscrew threads.

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Cooling…

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The thread impressions look pretty good for a first try.

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Pressing the completed nut out of the mold.

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New nut fitted to the compound slide.

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Threading the compound slide leadscrew back into place.

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The nut was a little tight, but unlike the original bronze nut has no detectable backlash. It smoothed out nicely after being run to and fro a few times. I made sure to make two nuts while I had the equipment set up, so I’ll be ready if it fails again. :-)

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