Prewar Workshop: Line boring in the lathe

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This project requires a little out-of-the-box thinking: using the machine for something it isn’t built for. The project is line boring a brass veteran gearbox. The 4 bores are quite oval and tapered because of years of wear and tear and it’s leaking much oil through the bores. To fit new bearings, it’s best to re-bore them and because they are as worn as they are, I can’t indicate the old holes in the mill to get them straight. This means the best way is line-boring. I will show you in the article how I went about doing this.

I wanted to build a jig to do so, as I don’t own a mill large enough to handle a boring bar this long. I must admit I got very lucky everything fit in my small toolmaker lathe. I will show you the setup and how tight it fits. But I the end, everything worked out.

The jig was built in CAD to cut all the plates with my CNC plasma cutter. This makes it very easy and quick to build jigs like this for a single project. I started with fixing the main supports to the gearbox. They won’t come loose anymore until all is finished, so in the end, the bushings remain concentric with the bores.  I welded up the whole frame and in the end, I tacked in the 2 bushings. I didn’t fully weld these in because I’m afraid they will shift a little bit from the heat. The cuts I take are very light and the small welds held perfectly.

The boring bar is a homemade unit made of 1mtr long 25mm diameter drill rod. I drilled a hole and made a cutter to fit. It’s just long enough to clear both bores in 1 setup.

I think the job is best explained in the photo album so you can view that here: >click<

I don’t have pictures yet of the end result, because the work is still in progress. But I’ve finished 1 of 2 bores now and the result looks beautiful. Next step is making 4 new bearings and putting everything back together. If you have any questions, please ask them in the comments or per mail via [email protected].

Article by Jos van Genugten

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