Friday, September 27, 2013

turning ball screws

With all of the Tormach CNC lathe videos out lately, I've been inspired and have found myself back on the Harbor Freight CNC lathe project. Last time I posted about the lathe, I was in need of a longer ball screw for the carriage and I had messed up the leadscrew nut that I had (I was an idiot one day when I was dismantling the lathe and the nut slid right off the screw sending several bearings bouncing across my studio floor). I purchased some replacement parts, but I needed to turn down the end of the new longer ball screw to be able to adapt it to the bearing housing and the stepper motor coupler. I did some research on how to turn the screw down and to determine how tough a job this was going to be. 5 bears Research had a lot of info on the subject that I found very helpful. The screws are hard so I knew it was going to be a tough cutting job on the lathe. I chucked the screw up on my old South Bend and got things centered well before facing and center drilling. During these operations, I determined that outside of the screw is EXTREMELY hard and the center is very soft. Center drilling was a breeze and once I had the live center butted up against the divot, I began cutting. A lot of people anneal the ends, but warping sometimes occurs from this. Other people grind through the outside coating until they are down to softer steel beneath the surface. I decided to just suck it up and ruin a couple carbide bits while rough the shape. Once I got past the screw ridged, things started to progress more easily and then I switched out to a fresh bit and did all of the finish machining. I got the end turned and the bearing fits very well on the shaft. I still need to thread the end for the jam nut, but that should be straightforward enough. I'm going to size the screw to the lathe bed, shorten it,  and then decide if I need to to turn the other end for a end bearing block. A lot of lathe conversions just let the screw float at the other end, but supporting it makes more sense to me. I'll see if I can design a simple bearing block and drill and tap the bed casting. Stay tuned...

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