On Jun 10, 12:17 pm, "Lynn" wrote:

After reading those responses, and doing some actual thinking (!), I'll
probably
just ignore my alignment imperfection.

But then you put this really large *BUT* in there...

However, putting some thoughts together, I think that what actually
caused the
head and tail stock centers misalignment is worth considering.

But if misalignment is apparent, and the axis of both the spindle and
tailstock
are exactly parallel, that would be one case. (a difference in the height
from the bed)
But suppose the misalignment was due to either the spindle or tailstock not
being
perfectly parallel to the bed. Were the headstock (spindle) not exactly
parallel to the
bed, then it seems like the further the tail is from the head, the worse the
problem
could be. Does that make sense?

Absolutely NOT. Totally unfounded. This is a false trigonometric
argument. PERIOD.

This is not a compounding, cumulative error. This error could only be
of a multiplicative nature IF one end of the extrapolated triangle was
WAS set, and the other was NOT set.

You have nattered this math problem down to the point you are chewing
on the bones.

Let me try it this way:

Imagine yourself standing in front of your lathe with the headstock on
the left hand side. We are not looking at bowls, winged shapes,
warped and punky wood, offset turnings, etc. For purposes of
illustration, think of a round piece of hard maple spindle stock.

Say your headstock/tailstock (now known as hs/ts) is out of alignment
of a perfect center by 1/100 in one inch. That would mean that if
one side was perfectly stationary, then in two inches it would be
2/100 (I know, 1/50th). At 3 inches, 3/100ths, and so on and so on.
Then you get to 14 inches, which is the max working length between
centers on your lathe.

WITHOUT the end ts being secured (remember, working left to right) you
would have a cumulative effect of being 14/100ths ( a little more than
1/16th of an inch ) off center.

BUT BUT BUT.... your ts holds the piece in place, at near perfect
coeccentricity at the ts. In your case, the difference at the hs/ts
detail is about 1/100. This means that at the other side of the two
FIXED revolving axises of the lathe, it is still only 1/100.

Don't believe me on how little this matters?

Think about it this way. When you turn a spindle, you are moving your
piece into a concentric shape with every pass. So try this:

Make a temp gauge that is 1" in diameter (a one inch wrench will do
fine), or use a good set of calipers. Take a piece of wood about a
foot long or so, and secure it between centers. This long piece will
help you understand why the effect doesn't multiply.

Round this piece of wood carefully, until you get it a perfect one
inch diameter from hs to ts.

Wasn't that hard, right?

Where did the difference or cumulative error go? It went nowhere
because there isn't any!

(In case anyone is actually reading this... the error would show up on
the ends as the surfaces perpendicular to the error.)

The important thing to remember is that YOU will establish a round,
coeccentric shape every time you turn. By the nature of the lathe,
you are spinning the wood on an axis determined by you. You
establish a new line of coeccentricity.
The lathe spins the wood on that axis, and that is all there is. With
both sides of your line secured, there is simply no way for the error
to accumulate or compound.

Besides, since you are looking at this with your machinist's
micrometers and calipers, how do you secure your wood to the lathe?
Do you use a spur and tailstock revolving center? When you attach or
suspend your piece to the lathe, is your technique such that you can
gauge both sides (hs/ts) to less than 1/64 of an inch difference in
penetration to hold that piece perfectly.

I doubt it.

And once again, it doesn't matter if you did. The sides are the
bearing/holding points for a piece of spindle work. When you turn it
round, you have brought it into round, the level of perfection being
your limitations as a turner.

But while you are nattering this problem into tiny pieces, think about
this: NONE of this matters (even for a hypothetical argument) if you
get a piece of wood that had dried unevenly, has hard spots or voids,
is kind of green, it is reacts quickly to exposure to air, has a few
knots, is left on the lathe overnight, has hidden bug tunnels in it,
and on an on. All of those things will throw your piece out of round
by affecting the density of the wood in different areas of the piece.
THAT is what you watch for.

You are over thinking this. Go outside, put some wood in the lathe
and turn a spindle every way you can to see if you can get it to screw
up.
It won't happen.

Enjoy your lathe. These are nice machines. I have two, and do almost
all my turning on them and have for years. Don't ruin this for
yourself by dwelling on pointless details that in the end don't
matter.

Robert