DIYbanter - View Single Post - Mathematical analysis of Rollie's Dad's Method

Joseph Gwinn

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