Setting the tailstock offset

I needed to do a between-centres job on the lathes at TAFE a while back and found that the tail stock was so far off that the taper was over 0.05mm over only an inch or so of travel, so I needed to teach myself how to set it right. Here’s a how-to.

Here’s the axle that I was turning. I didn’t want to clamp up on the first bearing surface to machine the second one, so I decided to turn it between centres.

There’s the tail stock. If you look down the bottom, there’s an allen head bolt down the bottom near the bed. There’s one of these on each side of the tail stock and they tighten against each other. Random aside: These tail stocks kick ass… there’s a little flat are on top to put drill bits and a gearbox near the back for super fine feeds.

Here’s a close-up. In order to move the tail stock from side to side, you loosen one side, crank the other side a little bit and then tighten them both. I think the screw underneath it is for preloading the slide that it’s on.

There’s a vernier scale on the back to tell the position that it’s in, but frankly, it’s useless. Put a dial gauge on the cross slide to see how it’s moving.

Here’s a photo of that. You can’t clock off one side of the workpiece to get it right- It’s conical, so if you get one side right, the other will be wrong. What you want to do is take a cut, measure the diameter at two points. The difference is double the actual error, since you measured diameter and the offset is radial. So halve the error, divide it by the length over which you measured and multiply it by the distance between centres. That’s how much you need to move the needle of the dial gauge.

After a bit of frigging around, here’s the finish cut on the axle near the end…

…and here it is up against the shoulder. It’s off by about 0.005, which is close enough. If it was my own machine at home, it’d try to get it to the point where I couldn’t measure the error anymore, but someone will probably just fuck it up when I’m gone, anyway.

 

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