Midnight Special

A [metal] lathe center height tool

lcht.jpg      lcht-parts.jpg

This tool is used to quickly set a lathe bit at the center line. I actually prefer the “pinch the ruler between the bit and part and and adjust until the ruler is vertical” method (I learned this technique from a Keith Fenner video, it is quick, easy, accurate and only uses a 6″ rule and eyeballs) but sometimes it is really awkward. I saw this tool in one of Tom Lipton’s (oxtoolco) videos (see the video (left link) for the gory details on use, etc).

The long story – I’ve been watching a lot metal working videos lately, sort of a continuing education thing (or, in my case, learn how to use the tools) and learning lots. After watching Tom’s video, I was “wow, a useful tool I think I can make”.  So out to the shop I go, it is only 10pm so lots of time (oh the irony; oh the bad puns). No plans, no numbers, just gonna twiddle some knobs, flip some levers and paint it green. And I almost pulled it off; when I called it quits at 2:30, it was to wait for the LocTite to cure. Did the finish machining the next morning.

The nut is a 32 TPI thread inside a chunk of 1″x0.0625″ [seamless] bicycle fork tube. The thread is about 0.75″ long, which was about half way along the tube.  The screw is 1″ rod  necked down to 7/8″. The flat in the nut rides on the flat of the screw and keeps things nice and straight. This was all fly by eye and lever turning until things fit. I used the same boring bar to cut both threads.

The end caps were cut from 2″ rod, the top is 1.5″ diameter, the bottom 1.75″. They press fit on the screw and nut (well, the other reason I quit for the night was I over bored the top cap (#$#$!!!), but was able to fix it with thin shim stock and red (272) LocTite (I don’t have the green); if you look at the full size image, by the dark blob at the bottom you can see gap at the ends of the shim).

After the ends were pressed on, I turned the reference surfaces: both sides of the top cap and the bottom. Now I know the top cap moves up and down parallel to the surface and is perpendicular to the shaft (verified on a surface plate).

The one thing I spaced out was the lock screw. After about the third on the fly change while making the screw and nut, I said screw it. If I need one, I’ll do a retrofit or rebuild. But the tool does work nicely.

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