As I mentioned in " Millrite MVI spindle bearing repair - Second report"
posted on 5 September 2010, I'm looking for a way to measure total runout
without use of a $250 precision test bar, and Rollie's Dad's Method of Lathe
Alignment http://www.neme-s.org/Rollie%27s_Dad%27s_Method.pdf was suggested as
an approach that could be adapted to the task. We will call this the RDM
method, or just RDM.

Reading various postings about attempts to use RDM, people seemed to be having
some problems getting it to work. As did I. I may now know why. A simple but
fundamental error may have crept in over the years.

Consider a circle rotating about an axis displaced from the center of the
circle. (This is all in 2D, and the various axes are perpendicular to the plane
of the circle.) Using RDM's nomenclature, the radius of the circle is R, and
the distance between rotation axis and circle center is X. In other words, X is
the runout.

Using a dial indicator or a dial test indicator, we will rotate the circle about
the rotation axis and measure the maximum and minimum values. We have adjusted
the indicator so that measurements are all positive (or all negative), with
greater absolute values signifying greater distances from the axis of rotation.
We will assume positive measurements in the following paragraphs.

Now, by geometry, the maximum reading will be (R+X), and the minimum reading
will be (R-X).

By RDM, we compute 0.5*[(R+X)+(R-X)]= 0.5*[2R]= R, which is the radius of the
circle, regardless of the runout X. If we measure the diameter D with a
micrometer and compute R-D/2 as suggested, what we get is a measure of the
departure from roundness of the circle. We do not get the runout, which has
already cancelled out.

If we instead compute the difference, subtracting the smaller measurement from
the larger measurement, it's the circle radius that cancels out instead, and we
now get the runout 0.5*[(R+X)-(R-X)]= 0.5*[2X]= X that we seek, uncontaminated
by the radius R of the circle. (Assuming that the "circle" is in fact round
enough.)

Wherever we compute this difference, the radius R of the circle at that location
will cancel, yielding the total runout X at that location.

Now, I bet that Rollie's Dad knew this and was solving for X and not for R, so a
small error crept in as the method was passed along.


If one measures X at different locations along a round rod, it is possible to
fit the data to a linear equation, and this equation can be used to predict
total runout as a function of position along the rod.


I made the needed measurements on my Millrite, so will use the data in the
following example:

Close to collet: Max=0.0020", Min=0.0015", so X=0.00025".

Away from collet by 4.135" (from the DRO):Max=0.0040", Min=0.00145", X=0.001275".

Now, fit these data points to the equation y=a*x+b (X and x are not the same),
with all runouts multiplied by 1000 for convenience:


0.25=a*0+b, so b=0.25 mils.

1.275= a*4.135+0.25, so a=0.2479 mils per inch.

The full equation is thus y= 0.2479*x+0.25, yielding mils of total runout as a
functionm of distance in inches from where the "close to collet" measurement was
taken.

In other words, the total runout is a quarter mil plus a quarter mil per inch
along the rod.

As mentioned earlier, the Millrite MVI specs are 0.0005" total runout near the
spindle nose, and 0.001" at 8" from the spindle nose, using a test bar. This
yields the equation y=0.5+0.0625*x.

Near the spindle nose, we are seeing only 0.00025" total runout, half the
allowed 0.0005" runout. This is probably due solely to the lateral runout of
the bearing closest to the nose, and cannot be much improved.

At 8", we would see about 0.25+8* 0.2479= 2.2332 mils, or 0.002233" total
runout, which exceeds the 8" total limit by a factor of 2.233.

The key problem is the angular error, 0.2479 versus 0.0625 mils per inch.

A machine made in 1965 need not apologize for having only twice the runout it
had in its youth. That said, properly orienting the outer races may help a
great deal.



I should also list the fundamental assumptions underlying the above methods:

First, while the rod need not be straight, it must be quite round at all places
measured, so a piece of raw stock will not work. What will likely work the best
is precision ground shafting, which is quite round but may have a few
thousandths of curve per foot.

Second, in the above linear fit, we implicitly assumed that the line between the
two measurements does not cross the axis of rotation. While this is usually
true, it is not guaranteed. A quick test is to measure in at least three places
along the rod, and plot the value of X as a function of position along the rod.
If they fall in a line, no significant crossover. If they form a V, there is
crossover. If necessary, one can keep track of runout directions and fit to
the actuals.

Third, we implicitly assume that the measurements all fall on a common line (are
colinear), and that this common line and the axis of rotation together define a
common plane (in other words, the lines are not skewed with respect to one
another). This is never quite true, although is is usually true enough. To
detect skew, one measures both runout and clock angle at a minimum of three
places along the bar and does some fancy math.

I hasten to add that the machine accuracy specs make the same assumptions.


Joe Gwinn

In article ,
Ade V wrote:

did gone and wrote:

[snippage]

Consider a circle rotating about an axis displaced from the center of the
circle. (This is all in 2D, and the various axes are perpendicular to the
plane of the circle.) Using RDM's nomenclature, the radius of the circle is R,
and the distance between rotation axis and circle center is X. In other words,
X is the runout.


[snippage]


By RDM, we compute 0.5*[(R+X)+(R-X)]= 0.5*[2R]= R, which is the radius of the
circle, regardless of the runout X. If we measure the diameter D with a
micrometer and compute R-D/2 as suggested, what we get is a measure of the
departure from roundness of the circle. We do not get the runout, which
has already cancelled out.

Unless I'm mistaken, RDM is _trying_ to cancel the runout? The runout is
getting in the way of determining how accurately the spindle is aimed
relative to the bed. Whilst runout is its own issue, it's not what RDM
is trying to fix...

Although the article doesn't say that, it may be that RDM is ultimately trying
to measure bed twist, but the method as published cannot achieve that. The
published method measures rod diameter despite runout, and there is no way to
deduce bed twist from rod diameter.

What the article claims to be measuring is the deviation of the spindle rotation
axis from parallelism with the ways, in two dimensions, horizontal and vertical.
The published method cannot do this, but changing only the math from summing the
runout max and min to taking the difference allows this deviation to be measured.

What was done with the information was to cleverly shim the headstock where it
rests on the bed, to achieve parallelism.

Actually, with the sum, we don't get the full radius, we get the change from
some unknown constant, because we never zero the dial indicator at the unknown
center of rotation, we set the dial indicator up at some convenient offset, and
go from there.

In article ,
Joseph Gwinn wrote:

In article ,
Ade V wrote: (on 9 September 2010)

did gone and wrote:

[snippage]

Consider a circle rotating about an axis displaced from the center of the
circle. (This is all in 2D, and the various axes are perpendicular to the
plane of the circle.) Using RDM's nomenclature, the radius of the circle
is R, and the distance between rotation axis and circle center is X. In other
words, X is the runout.


[snippage]


By RDM, we compute 0.5*[(R+X)+(R-X)]= 0.5*[2R]= R, which is the radius of
the circle, regardless of the runout X. If we measure the diameter D with a
micrometer and compute R-D/2 as suggested, what we get is a measure of the
departure from roundness of the circle. We do not get the runout, which
has already cancelled out.

Unless I'm mistaken, RDM is _trying_ to cancel the runout? The runout is
getting in the way of determining how accurately the spindle is aimed
relative to the bed. Whilst runout is its own issue, it's not what RDM
is trying to fix...

Although the article doesn't say that, it may be that RDM is ultimately trying
to measure bed twist, but the method as published cannot achieve that. The
published method measures rod diameter despite runout, and there is no way to
deduce bed twist from rod diameter.

What the article claims to be measuring is the deviation of the spindle rotation
axis from parallelism with the ways, in two dimensions, horizontal and vertical.
The published method cannot do this, but changing only the math from summing
the runout max and min to taking the difference allows this deviation to be
measured.

What was done with the information was to cleverly shim the headstock where
it rests on the bed, to achieve parallelism.

Actually, with the sum, we don't get the full radius, we get the change from
some unknown constant, because we never zero the dial indicator at the unknown
center of rotation, we set the dial indicator up at some convenient offset,
and go from there.

On reflection, I think Ade V has put his finger on the answer.

Rollie's Dad (RD) was measuring using a dial indicator on the lathe carriage
sensing the spinning rod, the intent being to see how well aligned the headstock
was, using a random bit of round rod held in a chuck with random crookedness.
So, RD wanted to cancel the runout and crookedness, yielding the diameter of the
rod plus some constant.

The easiest way to visualize the runout is to imagine the spindle axis tracing
out a cone in space. To align the headstock, the axis of that cone is made
parallel to the bedway. The raw measurement is a combination of runout, actual
rod diameter, and deviation of cone axis from parallel. Cancelling the runout
yields the local apparent rod radius, which is the combination of actual rod
diameter and cone axis deviation. If the rod is a perfect cylinder, with
constant radius everywhere, then a constant dial indicator reading as the
carriage moves implies that the cone axis is parallel to the bedway. If the rod
radius varies with location, one must measure the actual rod radius and subtract
it to get the distance to the cone axis.

Now, by contrast, I'm currently interested in the runout that RD ignores, and
want to ignore the rod radius that RD uses.

So, to summarize (in the context of a vertical mill):

One half the *sum* of the the indicator measurements (corrected for rod radius)
yields the deviation from perfect parallelism between Z-Axis ways and/or quill
motion, ignoring runout and chuck crookedness. This is RD's Method (RDM).

One half the *difference* of the the indicator measurements yields the runout,
ignoring crookedness and imprecise parallelism.

One set of measurements can be used to compute both non-parallelism and runout.
In practice, the only part of non-parallelism that is adjustable in most
vertical mills is tramming.

One can also deduce crookedness of the chuck by running the test with the same
bar rotated into different positions, so bar crookedness can be separated
mathematically from chuck crookedness.


A practical note: I've found that R8 spring collets don't hold the rod quite
rigidly enough, probably because of a very slight mismatch between actual rod
diameter and actual hole diameter of the collet, so the rod is clamped in a ring
versus over an extended area. If one tugs on the rod, it will permanently shift
by a few tenths, and won't usually return to zero if the rod is plucked and
allowed to vibrate down to zero. A Jacobs Superchuck is somewhat more secure in
that while it also moves, the bar returns to ~zero when it is plucked. I will
next try an Albrecht keyless chuck, which is likely far more precisely made than
the superchuck.

What should work far better is a R8 to ER arbor, as ER collets have a far better
grip on a rod than a R8 spring collet. And ER collets have many more uses than
a test bar.

We may also be seeing the R8 arbor shifting in the spindle, but given the taper
it should return to zero when the bar is plucked. A test bar would have the
same problem with shifting in the spindle.


Joe Gwinn

On Mon, 13 Sep 2010 09:12:17 -0400, Joseph Gwinn
wrote:

In article ,
Joseph Gwinn wrote:

In article ,
Ade V wrote: (on 9 September 2010)

did gone and wrote:

[snippage]

Consider a circle rotating about an axis displaced from the center of the
circle. (This is all in 2D, and the various axes are perpendicular to the
plane of the circle.) Using RDM's nomenclature, the radius of the circle
is R, and the distance between rotation axis and circle center is X. In other
words, X is the runout.


[snippage]


By RDM, we compute 0.5*[(R+X)+(R-X)]= 0.5*[2R]= R, which is the radius of
the circle, regardless of the runout X. If we measure the diameter D with a
micrometer and compute R-D/2 as suggested, what we get is a measure of the
departure from roundness of the circle. We do not get the runout, which
has already cancelled out.

Unless I'm mistaken, RDM is _trying_ to cancel the runout? The runout is
getting in the way of determining how accurately the spindle is aimed
relative to the bed. Whilst runout is its own issue, it's not what RDM
is trying to fix...

Although the article doesn't say that, it may be that RDM is ultimately trying
to measure bed twist, but the method as published cannot achieve that. The
published method measures rod diameter despite runout, and there is no way to
deduce bed twist from rod diameter.

What the article claims to be measuring is the deviation of the spindle rotation
axis from parallelism with the ways, in two dimensions, horizontal and vertical.
The published method cannot do this, but changing only the math from summing
the runout max and min to taking the difference allows this deviation to be
measured.

What was done with the information was to cleverly shim the headstock where
it rests on the bed, to achieve parallelism.

Actually, with the sum, we don't get the full radius, we get the change from
some unknown constant, because we never zero the dial indicator at the unknown
center of rotation, we set the dial indicator up at some convenient offset,
and go from there.

On reflection, I think Ade V has put his finger on the answer.

Rollie's Dad (RD) was measuring using a dial indicator on the lathe carriage
sensing the spinning rod, the intent being to see how well aligned the headstock
was, using a random bit of round rod held in a chuck with random crookedness.
So, RD wanted to cancel the runout and crookedness, yielding the diameter of the
rod plus some constant.

The easiest way to visualize the runout is to imagine the spindle axis tracing
out a cone in space. To align the headstock, the axis of that cone is made
parallel to the bedway. The raw measurement is a combination of runout, actual
rod diameter, and deviation of cone axis from parallel. Cancelling the runout
yields the local apparent rod radius, which is the combination of actual rod
diameter and cone axis deviation. If the rod is a perfect cylinder, with
constant radius everywhere, then a constant dial indicator reading as the
carriage moves implies that the cone axis is parallel to the bedway. If the rod
radius varies with location, one must measure the actual rod radius and subtract
it to get the distance to the cone axis.

Now, by contrast, I'm currently interested in the runout that RD ignores, and
want to ignore the rod radius that RD uses.

So, to summarize (in the context of a vertical mill):

One half the *sum* of the the indicator measurements (corrected for rod radius)
yields the deviation from perfect parallelism between Z-Axis ways and/or quill
motion, ignoring runout and chuck crookedness. This is RD's Method (RDM).

One half the *difference* of the the indicator measurements yields the runout,
ignoring crookedness and imprecise parallelism.

One set of measurements can be used to compute both non-parallelism and runout.
In practice, the only part of non-parallelism that is adjustable in most
vertical mills is tramming.

One can also deduce crookedness of the chuck by running the test with the same
bar rotated into different positions, so bar crookedness can be separated
mathematically from chuck crookedness.


A practical note: I've found that R8 spring collets don't hold the rod quite
rigidly enough, probably because of a very slight mismatch between actual rod
diameter and actual hole diameter of the collet, so the rod is clamped in a ring
versus over an extended area. If one tugs on the rod, it will permanently shift
by a few tenths, and won't usually return to zero if the rod is plucked and
allowed to vibrate down to zero. A Jacobs Superchuck is somewhat more secure in
that while it also moves, the bar returns to ~zero when it is plucked. I will
next try an Albrecht keyless chuck, which is likely far more precisely made than
the superchuck.

What should work far better is a R8 to ER arbor, as ER collets have a far better
grip on a rod than a R8 spring collet. And ER collets have many more uses than
a test bar.

We may also be seeing the R8 arbor shifting in the spindle, but given the taper
it should return to zero when the bar is plucked. A test bar would have the
same problem with shifting in the spindle.


Joe Gwinn

From reading your original post.

Rollie Dads Method of measuring bed twist is useful and well
established.

For the commonplace case of a reasonable straight and round
test bar it gives the correct answer.

As you point out, Rollie's "correction" for diameter is in
error. At both the near and the far measurement points, the
rotational axis is truly defined by the difference between the
highest and lowest clock readings and is independent of diameter
and diameter change.

However the detail content of Rollie's description is very
useful. I think it might be helpful to post to the drop box an
agreed RTM Mk2. Changes needed could be pretty small :-

In each paras 5 and 8 delete the last sentence

"why this method works" - delete or modify

Jim

In article ,
wrote:

On Mon, 13 Sep 2010 09:12:17 -0400, Joseph Gwinn
wrote:

In article ,
Joseph Gwinn wrote:

In article ,
Ade V wrote: (on 9 September 2010)

did gone and wrote:

[snippage]

Consider a circle rotating about an axis displaced from the center of
the
circle. (This is all in 2D, and the various axes are perpendicular to
the
plane of the circle.) Using RDM's nomenclature, the radius of the
circle
is R, and the distance between rotation axis and circle center is X.
In other
words, X is the runout.


[snippage]


By RDM, we compute 0.5*[(R+X)+(R-X)]= 0.5*[2R]= R, which is the radius
of
the circle, regardless of the runout X. If we measure the diameter D
with a
micrometer and compute R-D/2 as suggested, what we get is a measure of
the
departure from roundness of the circle. We do not get the runout,
which
has already cancelled out.

Unless I'm mistaken, RDM is _trying_ to cancel the runout? The runout is
getting in the way of determining how accurately the spindle is aimed
relative to the bed. Whilst runout is its own issue, it's not what RDM
is trying to fix...

Although the article doesn't say that, it may be that RDM is ultimately
trying
to measure bed twist, but the method as published cannot achieve that.
The
published method measures rod diameter despite runout, and there is no way
to
deduce bed twist from rod diameter.

What the article claims to be measuring is the deviation of the spindle
rotation
axis from parallelism with the ways, in two dimensions, horizontal and
vertical.
The published method cannot do this, but changing only the math from
summing
the runout max and min to taking the difference allows this deviation to
be
measured.

What was done with the information was to cleverly shim the headstock
where
it rests on the bed, to achieve parallelism.

Actually, with the sum, we don't get the full radius, we get the change
from
some unknown constant, because we never zero the dial indicator at the
unknown
center of rotation, we set the dial indicator up at some convenient
offset,
and go from there.

On reflection, I think Ade V has put his finger on the answer.

Rollie's Dad (RD) was measuring using a dial indicator on the lathe carriage
sensing the spinning rod, the intent being to see how well aligned the
headstock
was, using a random bit of round rod held in a chuck with random
crookedness.
So, RD wanted to cancel the runout and crookedness, yielding the diameter of
the
rod plus some constant.

The easiest way to visualize the runout is to imagine the spindle axis
tracing
out a cone in space. To align the headstock, the axis of that cone is made
parallel to the bedway. The raw measurement is a combination of runout,
actual
rod diameter, and deviation of cone axis from parallel. Cancelling the
runout
yields the local apparent rod radius, which is the combination of actual rod
diameter and cone axis deviation. If the rod is a perfect cylinder, with
constant radius everywhere, then a constant dial indicator reading as the
carriage moves implies that the cone axis is parallel to the bedway. If the
rod
radius varies with location, one must measure the actual rod radius and
subtract
it to get the distance to the cone axis.

Now, by contrast, I'm currently interested in the runout that RD ignores,
and
want to ignore the rod radius that RD uses.

So, to summarize (in the context of a vertical mill):

One half the *sum* of the the indicator measurements (corrected for rod
radius)
yields the deviation from perfect parallelism between Z-Axis ways and/or
quill
motion, ignoring runout and chuck crookedness. This is RD's Method (RDM).

One half the *difference* of the the indicator measurements yields the
runout,
ignoring crookedness and imprecise parallelism.

One set of measurements can be used to compute both non-parallelism and
runout.
In practice, the only part of non-parallelism that is adjustable in most
vertical mills is tramming.

One can also deduce crookedness of the chuck by running the test with the
same
bar rotated into different positions, so bar crookedness can be separated
mathematically from chuck crookedness.


A practical note: I've found that R8 spring collets don't hold the rod
quite
rigidly enough, probably because of a very slight mismatch between actual
rod
diameter and actual hole diameter of the collet, so the rod is clamped in a
ring
versus over an extended area. If one tugs on the rod, it will permanently
shift
by a few tenths, and won't usually return to zero if the rod is plucked and
allowed to vibrate down to zero. A Jacobs Superchuck is somewhat more
secure in
that while it also moves, the bar returns to ~zero when it is plucked. I
will
next try an Albrecht keyless chuck, which is likely far more precisely made
than
the superchuck.

What should work far better is a R8 to ER arbor, as ER collets have a far better
grip on a rod than a R8 spring collet. And ER collets have many more uses
than a test bar.

We may also be seeing the R8 arbor shifting in the spindle, but given the taper
it should return to zero when the bar is plucked. A test bar would have the
same problem with shifting in the spindle.


Joe Gwinn

From reading your original post.

Rollie Dads Method of measuring bed twist is useful and well
established.

I think it measures far more than just bed twist. Any misalignment between
spindle axis and bedway should show up.


For the commonplace case of a reasonable straight and round
test bar it gives the correct answer.

But lots of people seem to have had problems. Now, I was never sure if it was
due to inadequate explanation, or some error in the method itself. Or maybe
both.

I'll have to try RDM on my lathe, for the experience. I think I recall having
tried it, and having gotten nowhere, but don't know why. It may have been that
I didn't really understand what I was doing mathematically, and so was doing
random things.

I ordered a R8 to ER25 collet chuck today, so I will soon be able to get better
runout data.


As you point out, Rollie's "correction" for diameter is in
error. At both the near and the far measurement points, the
rotational axis is truly defined by the difference between the
highest and lowest clock readings and is independent of diameter
and diameter change.

I'll have to think about this, as it seems to me that the difference yields the
pure runout regardless of where the cone axis might be.


However the detail content of Rollie's description is very
useful. I think it might be helpful to post to the drop box an
agreed RTM Mk2. Changes needed could be pretty small :-

I had been thinking along these lines as well. At the very least there is an
extension to RDM, and there may be a small correction as well.


In each paras 5 and 8 delete the last sentence

I'm not sure I understand. Please quote the sentences to be deleted.


"why this method works" - delete or modify

Yes, modify.


Joe Gwinn

On Wed, 15 Sep 2010 23:46:28 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

On Mon, 13 Sep 2010 09:12:17 -0400, Joseph Gwinn
wrote:

SNIP

From reading your original post.

Rollie Dads Method of measuring bed twist is useful and well
established.

I think it measures far more than just bed twist. Any misalignment between
spindle axis and bedway should show up.


For the commonplace case of a reasonable straight and round
test bar it gives the correct answer.

But lots of people seem to have had problems. Now, I was never sure if it was
due to inadequate explanation, or some error in the method itself. Or maybe
both.

I'll have to try RDM on my lathe, for the experience. I think I recall having
tried it, and having gotten nowhere, but don't know why. It may have been that
I didn't really understand what I was doing mathematically, and so was doing
random things.

I ordered a R8 to ER25 collet chuck today, so I will soon be able to get better
runout data.


As you point out, Rollie's "correction" for diameter is in
error. At both the near and the far measurement points, the
rotational axis is truly defined by the difference between the
highest and lowest clock readings and is independent of diameter
and diameter change.

I'll have to think about this, as it seems to me that the difference yields the
pure runout regardless of where the cone axis might be.


However the detail content of Rollie's description is very
useful. I think it might be helpful to post to the drop box an
agreed RTM Mk2. Changes needed could be pretty small :-

I had been thinking along these lines as well. At the very least there is an
extension to RDM, and there may be a small correction as well.


In each paras 5 and 8 delete the last sentence

I'm not sure I understand. Please quote the sentences to be deleted.


"why this method works" - delete or modify

Yes, modify.

Joe Gwinn


It's true that the method also shows up spindle alignment and
possible carriage alignment change but in practical terms,
adjustment of bed twist is normally the only available method of
correction.

RDM is often proposed as an alternative to the use of a precision
level which only detects bed twist . It may be worth discussing
this method in an RDM revision because the method is often
described as a series of level measurements of the bed surface
with the lathe as a whole needing to be precisely level.

If you have precision level it, is very much simpler to mount
the level on the crossslide and observe the change in reading as
the carriage is traversed.

It is unnecessary for the lathe to be precisely level because you
are now measuring directly the effect of bed twist or distortion
on the cutting tool location. Even if the lathe were large
enough and flimsy enough for gravity induced deflections to be
significant this would be indicated directly by this method



Both para 5 and 8 amended to only read "Average the high an low
readings (add together and divide by two) to get the "near end
average distance".
Delete " if you suspect .........."

An alternative wording could be " Note as the reference
distance,the mid point between the two readings"



The centre line of the bar mounted in the chuck describes a cone
whose centre line is coincident with the lathe rotational axis.
The reference distances describe a line truly parallel to that
axis.
The reference distances assume that the lathe bed is straight.
Bends or bumps would introduce their own errors.

Jim

In article ,
wrote:

On Wed, 15 Sep 2010 23:46:28 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

On Mon, 13 Sep 2010 09:12:17 -0400, Joseph Gwinn
wrote:

SNIP

From reading your original post.

Rollie Dads Method of measuring bed twist is useful and well
established.

I think it measures far more than just bed twist. Any misalignment between
spindle axis and bedway should show up.


For the commonplace case of a reasonable straight and round
test bar it gives the correct answer.

But lots of people seem to have had problems. Now, I was never sure if it
was
due to inadequate explanation, or some error in the method itself. Or maybe
both.

I'll have to try RDM on my lathe, for the experience. I think I recall
having
tried it, and having gotten nowhere, but don't know why. It may have been
that
I didn't really understand what I was doing mathematically, and so was doing
random things.

I ordered a R8 to ER25 collet chuck today, so I will soon be able to get
better
runout data.


As you point out, Rollie's "correction" for diameter is in
error. At both the near and the far measurement points, the
rotational axis is truly defined by the difference between the
highest and lowest clock readings and is independent of diameter
and diameter change.

I'll have to think about this, as it seems to me that the difference yields
the
pure runout regardless of where the cone axis might be.


However the detail content of Rollie's description is very
useful. I think it might be helpful to post to the drop box an
agreed RTM Mk2. Changes needed could be pretty small :-

I had been thinking along these lines as well. At the very least there is
an
extension to RDM, and there may be a small correction as well.


In each paras 5 and 8 delete the last sentence

I'm not sure I understand. Please quote the sentences to be deleted.


"why this method works" - delete or modify

Yes, modify.

Joe Gwinn


It's true that the method also shows up spindle alignment and
possible carriage alignment change but in practical terms,
adjustment of bed twist is normally the only available method of
correction.

RD also mentioned shimming the headstock where it rests upon the bedway,
but you are right that the adjustment options are limited. In practice
one might do both in alternation, so the lathe converges to as perfect
alignment as can be obtained given only those two "knobs".


RDM is often proposed as an alternative to the use of a precision
level which only detects bed twist. It may be worth discussing
this method in an RDM revision because the method is often
described as a series of level measurements of the bed surface
with the lathe as a whole needing to be precisely level.

This could work, but it would be necessary to separate the effects of
headstock misalignment and bed twist, or one could end up turning the
wrong knob, and making things progressively worse.


If you have precision level, it is very much simpler to mount
the level on the crossslide and observe the change in reading as
the carriage is traversed.

I do have a 6" Starrett model 98-6 precision level (0.005" per foot per
division), although I usually slide it around on the tops of the bedway
V rails, the method recommended in the Clausing manual. This will not
work for all bedway designs, but the ride-the-carriage method should
work universally.

The tops of the V rails do not wear in normal use, as the carriage rests
on and wears away the flanks of the V rails, so for older machines using
the level on the tops of the rails should be more accurate than riding
the carriage. But I'll have to think about this - I don't know how
important it will be in practice.


It is unnecessary for the lathe to be precisely level because you
are now measuring directly the effect of bed twist or distortion
on the cutting tool location. Even if the lathe were large
enough and flimsy enough for gravity induced deflections to be
significant this would be indicated directly by this method

Yes. Maybe this also explains how people adjusted lathes in ships at
sea.


Both para 5 and 8 amended to only read "Average the high and low
readings (add together and divide by two) to get the "near end
average distance".
Delete " if you suspect .........."

An alternative wording could be " Note as the reference
distance,the mid point between the two readings"

Ahh. These reference the original RDM description, not my postings.

The original description could be clearer for sure. I would start with
the why-this-works explanation of the math, then move on to some
specific applications, on the theory that the detailed method is easier
to understand if one knows how and why it all fits together.


The centre line of the bar mounted in the chuck describes a cone
whose centre line is coincident with the lathe rotational axis.
The reference distances describe a line truly parallel to that
axis.
The reference distances assume that the lathe bed is straight.
Bends or bumps would introduce their own errors.

Not to mention uneven bed wear near the headstock, often an issue with
HSM iron. But again, I wonder how important this is in practice.


Joe Gwinn

On Thu, 16 Sep 2010 09:06:46 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

On Wed, 15 Sep 2010 23:46:28 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

On Mon, 13 Sep 2010 09:12:17 -0400, Joseph Gwinn
wrote:

SNIP

From reading your original post.

Rollie Dads Method of measuring bed twist is useful and well
established.

I think it measures far more than just bed twist. Any misalignment between
spindle axis and bedway should show up.


For the commonplace case of a reasonable straight and round
test bar it gives the correct answer.

But lots of people seem to have had problems. Now, I was never sure if it
was
due to inadequate explanation, or some error in the method itself. Or maybe
both.

I'll have to try RDM on my lathe, for the experience. I think I recall
having
tried it, and having gotten nowhere, but don't know why. It may have been
that
I didn't really understand what I was doing mathematically, and so was doing
random things.

I ordered a R8 to ER25 collet chuck today, so I will soon be able to get
better
runout data.


As you point out, Rollie's "correction" for diameter is in
error. At both the near and the far measurement points, the
rotational axis is truly defined by the difference between the
highest and lowest clock readings and is independent of diameter
and diameter change.

I'll have to think about this, as it seems to me that the difference yields
the
pure runout regardless of where the cone axis might be.


However the detail content of Rollie's description is very
useful. I think it might be helpful to post to the drop box an
agreed RTM Mk2. Changes needed could be pretty small :-

I had been thinking along these lines as well. At the very least there is
an
extension to RDM, and there may be a small correction as well.


In each paras 5 and 8 delete the last sentence

I'm not sure I understand. Please quote the sentences to be deleted.


"why this method works" - delete or modify

Yes, modify.

Joe Gwinn


It's true that the method also shows up spindle alignment and
possible carriage alignment change but in practical terms,
adjustment of bed twist is normally the only available method of
correction.

RD also mentioned shimming the headstock where it rests upon the bedway,
but you are right that the adjustment options are limited. In practice
one might do both in alternation, so the lathe converges to as perfect
alignment as can be obtained given only those two "knobs".


RDM is often proposed as an alternative to the use of a precision
level which only detects bed twist. It may be worth discussing
this method in an RDM revision because the method is often
described as a series of level measurements of the bed surface
with the lathe as a whole needing to be precisely level.

This could work, but it would be necessary to separate the effects of
headstock misalignment and bed twist, or one could end up turning the
wrong knob, and making things progressively worse.


If you have precision level, it is very much simpler to mount
the level on the crossslide and observe the change in reading as
the carriage is traversed.

I do have a 6" Starrett model 98-6 precision level (0.005" per foot per
division), although I usually slide it around on the tops of the bedway
V rails, the method recommended in the Clausing manual. This will not
work for all bedway designs, but the ride-the-carriage method should
work universally.

The tops of the V rails do not wear in normal use, as the carriage rests
on and wears away the flanks of the V rails, so for older machines using
the level on the tops of the rails should be more accurate than riding
the carriage. But I'll have to think about this - I don't know how
important it will be in practice.


It is unnecessary for the lathe to be precisely level because you
are now measuring directly the effect of bed twist or distortion
on the cutting tool location. Even if the lathe were large
enough and flimsy enough for gravity induced deflections to be
significant this would be indicated directly by this method

Yes. Maybe this also explains how people adjusted lathes in ships at
sea.


Both para 5 and 8 amended to only read "Average the high and low
readings (add together and divide by two) to get the "near end
average distance".
Delete " if you suspect .........."

An alternative wording could be " Note as the reference
distance,the mid point between the two readings"

Ahh. These reference the original RDM description, not my postings.

The original description could be clearer for sure. I would start with
the why-this-works explanation of the math, then move on to some
specific applications, on the theory that the detailed method is easier
to understand if one knows how and why it all fits together.


The centre line of the bar mounted in the chuck describes a cone
whose centre line is coincident with the lathe rotational axis.
The reference distances describe a line truly parallel to that
axis.
The reference distances assume that the lathe bed is straight.
Bends or bumps would introduce their own errors.

Not to mention uneven bed wear near the headstock, often an issue with
HSM iron. But again, I wonder how important this is in practice.


Joe Gwinn


While it is true that there are many secondary errors that can
contribute to the total alignment error I believe that bed twist
is both the commonest major error and is also the easiest to
correct.


With level measurement, if the sides of the V ways are worn, it
is even more important to use the carriagre mount method as this
then shows the total effect of both wear and misalignment on the
cutting tool location.

Jim

In article ,
wrote:

On Thu, 16 Sep 2010 09:06:46 -0400, Joseph Gwinn
wrote:

In article ,
wrote:

[snip]


It's true that the method also shows up spindle alignment and
possible carriage alignment change but in practical terms,
adjustment of bed twist is normally the only available method of
correction.

RD also mentioned shimming the headstock where it rests upon the bedway,
but you are right that the adjustment options are limited. In practice
one might do both in alternation, so the lathe converges to as perfect
alignment as can be obtained given only those two "knobs".


RDM is often proposed as an alternative to the use of a precision
level which only detects bed twist. It may be worth discussing
this method in an RDM revision because the method is often
described as a series of level measurements of the bed surface
with the lathe as a whole needing to be precisely level.

This could work, but it would be necessary to separate the effects of
headstock misalignment and bed twist, or one could end up turning the
wrong knob, and making things progressively worse.


If you have precision level, it is very much simpler to mount
the level on the crossslide and observe the change in reading as
the carriage is traversed.

I do have a 6" Starrett model 98-6 precision level (0.005" per foot per
division), although I usually slide it around on the tops of the bedway
V rails, the method recommended in the Clausing manual. This will not
work for all bedway designs, but the ride-the-carriage method should
work universally.

The tops of the V rails do not wear in normal use, as the carriage rests
on and wears away the flanks of the V rails, so for older machines using
the level on the tops of the rails should be more accurate than riding
the carriage. But I'll have to think about this - I don't know how
important it will be in practice.


It is unnecessary for the lathe to be precisely level because you
are now measuring directly the effect of bed twist or distortion
on the cutting tool location. Even if the lathe were large
enough and flimsy enough for gravity induced deflections to be
significant this would be indicated directly by this method

Yes. Maybe this also explains how people adjusted lathes in ships at
sea.


[snip]

The centre line of the bar mounted in the chuck describes a cone
whose centre line is coincident with the lathe rotational axis.
The reference distances describe a line truly parallel to that
axis.
The reference distances assume that the lathe bed is straight.
Bends or bumps would introduce their own errors.

Not to mention uneven bed wear near the headstock, often an issue with
HSM iron. But again, I wonder how important this is in practice.


Joe Gwinn


While it is true that there are many secondary errors that can
contribute to the total alignment error I believe that bed twist
is both the commonest major error and is also the easiest to
correct.

Bed twist is certainly easy to remedy, but in an old lathe, many things
are worn a bit, and there is a distinct limit to how close one can get
to the original factory performance. So, by "important in practice",
I'm assuming an old lathe used by a HSMer.

People also shim headstocks, and I've always wondered if they were
unknowingly fixing a bed twist the hard way.


With level measurement, if the sides of the V ways are worn, it
is even more important to use the carriage mount method as this
then shows the total effect of both wear and misalignment on the
cutting tool location.

It's true that the carriage mounted indicator will show the combined
effect of misalignment and wear of bedway (and carriage), but if one
adjusts the headstock so the spindle axis parallels the effective bedway
near the headstock, it will be misaligned away from the headstock. It's
a tradeoff to be sure, and most work is close to the headstock. But
shorter workpieces are less sensitive to misalignment in the first
place.

I guess my instinct is that it's best to align the spindle axis with the
unworn bedway. But this will be a matter of personal preference, at
least partly determined by how worn one's lathe really is.


Joe Gwinn