Lathe Cross-slide Gib

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Sunday, Dec 5 2021

I noticed lots of tool deflection when cutting some materials. If I take too much if a cut I’d expect the lathe to stall, especially at low speends. Instead the tool deflects downwards. A lot.

Cross Slide Lift

Cross Slide Lift

Cross Slide Lift

I set up an indicator to measure deflection. First I put the dial indicator on the saddle and applied some up and down pressure to the compound slide with it set perpendicular to the saddle. Little to no deflection was seen. That didn’t surprise me because I’d changed the way the saddle mounts and is adjusted several years ago.

I moved the dial indicator to the top of the cross slide. It didn’t take much pressure in the direction tool cutting forces would take to show .005” to .006” of deflection on the cross slide. Why? Some internet research quickly led me to the source of the problem. The gib strip that came with the lathe is made to very loose tolerances. It is small enough that it can slightly rotate, allowing the side of the cross slide to lift. The suggested fix is to replace the gib strip.

I can buy new gibs from Little Machine Shop, but they are on back order. And what is the fun of that? A 6” strip of 1/4” by 3/8” brass and some time and I can make my own.

Deflects under cutter pressure

Deflects under cutter pressure

Deflects under cutter pressure
Added support for the end

Added support for the end

Added support for the end

I used a vice held within a vice to hold the material at the desired angle for milling. Because of extra stick out I added some end support. Still, cuts were light, about .010” cut per pass until I got close then .005” cuts for the final pass or two. I finished up sanding to fit without binding.

Once the gib fit I marked the location of the adjusting screws and used the tip of a center drill to make the dimples where the adjusting screws sit.

Much better. With the cross slide adjusted for slight pressure but no binding deflection is less than .002” and I need to apply a lot more pressure to generate that amount of deflection.

First use with the new set up was using a boring bar to make a hole larger. The material was delrin – not much of a test. I’m looking forward to see the difference when working with other materials.

 

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