Accuracy turning a cylinder

No, you're accuracy isn't too good.

What I do is turn a sample piece between centers. Then, measure both ends (without removing the piece). Set up an indicator on the part you just turned, and adjust the tailstock for 1/2 the difference in size. Repeat until you get it were you want it.

With a dial indicator and a rod, you can measure the center-, bed- and tailstock spindle alignment of a lathe.

Make a clamp/holder for the dial indicator at one end of the bar (you will find out how it has to look). Put that bar into the chuck and a dead center in the tailstock. Adjust the indicator to touch the center from top down. Then rotate the chuck 180 deg. With the difference, you see how much the tailstock is out of height. Turn 90 degres, note the reading and turn 180 degrees and you see how much the tailstock is front/back.

This assumes, that your spindle is aligned to the bed. There is a way (with that tool) to check that. But you didn't ask for.

Nick

Hi Nick,

I'd be interested how to check that the spindle is aligned to the bed, using a dial indicator.

Cheers,

Zed

Most likely your lathe bed has a twist in it. That's why it is so important to level the lathe with a master precision level.

Jon

A couple of thousandths is "typical". If the lathe is set up properly and you are skilled in lathe operation you can get errors in tenths. The lathe must be levels with a precision level or trued up using a ground bar and a dial indicator. Any bending in the ways will move the tool post to and from the work, causing the diameter to vary.

Another problem could be the way you chuck on the work. IF you are using a tail stock you should have centers in both ends and the piece driven by a lathe dog. IF that is not possible, you should grab the piece with the chuck by only a quarter inch or so, this is so that the piece will not be warped or bent by the chuck jaws.

another problem could be that you may not have all the backlash out of the compound or cross slide. You always want to make sure that you take up all the backlash by moving into the work. Don't back either slide off and then cut, any backlash will push the slide further back and cause an error.

There is plenty of info on how to set up a lathe in the archives of this group.

A simple way to get the piece straight is to set the compound at 5.7 degrees which will make the compound scale read 10 to 1. you can then slowly move the compound in and have good control of it. Mark the piece with a marker as to how many thousandths you have to move the compound to get the correction as you make the cut. a dial indicator on the compound makes life easy to read the travel. Remember that you are reading radius and the mikes are reading diameter so you want to move half the error.

John