I have just turned a couple of aluminum cylinders which are parts for a
machine I'm building. They are about 3.5" in diameter and 24" long.

The finished accuracy I achieved is about 0.010". That is, the
diameter at the "wide" end is about 0.010" greater than the diameter on
the "narrow" end.

This is accurate enough to work in my application, but it raised some
questions for me:

Is the level of accuracy I achieved "typical"? I set up the tailstock
as closely as I could, before machining the cylinders, aligning two
dead centers. Should I find a more accurate way of setting up the
tailstock? How would that be done? Would using a pre-made precision
cylinder of known accuracy, then measuring the clearances, be the most
accurate way?

Suggestions?

Thanks.

- Don

Is the level of accuracy I achieved "typical"? I set up the tailstock
as closely as I could, before machining the cylinders, aligning two
dead centers. Should I find a more accurate way of setting up the
tailstock? How would that be done?

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.

ducque wrote:

I set up the tailstock as closely as I could, before machining the
cylinders, aligning two dead centers. Should I find a more accurate way
of setting up the tailstock? How would that be done?

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
--
Motor Modelle // Engine Models
http://www.motor-manufaktur.de
DIY-DRO - YADRO - Eigenbau-Digitalanzeige

Hi Nick,

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

Cheers,

Zed

ducque wrote:

I have just turned a couple of aluminum cylinders which are parts for a
machine I'm building. They are about 3.5" in diameter and 24" long.

The finished accuracy I achieved is about 0.010". That is, the
diameter at the "wide" end is about 0.010" greater than the diameter on
the "narrow" end.

This is accurate enough to work in my application, but it raised some
questions for me:

Is the level of accuracy I achieved "typical"? I set up the tailstock
as closely as I could, before machining the cylinders, aligning two
dead centers. Should I find a more accurate way of setting up the
tailstock? How would that be done? Would using a pre-made precision
cylinder of known accuracy, then measuring the clearances, be the most
accurate way?


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

ducque wrote:

I have just turned a couple of aluminum cylinders which are parts for a
machine I'm building. They are about 3.5" in diameter and 24" long.

The finished accuracy I achieved is about 0.010". That is, the
diameter at the "wide" end is about 0.010" greater than the diameter on
the "narrow" end.

This is accurate enough to work in my application, but it raised some
questions for me:

Is the level of accuracy I achieved "typical"? I set up the tailstock
as closely as I could, before machining the cylinders, aligning two
dead centers. Should I find a more accurate way of setting up the
tailstock? How would that be done? Would using a pre-made precision
cylinder of known accuracy, then measuring the clearances, be the most
accurate way?

Suggestions?

Thanks.

- Don


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