A narrow bed is better for linear guidance.
What I mean is, narrow and long are best for guidance.
Think of the single inverted vee and flat type bed...
Only the vee way provides the guidance.
And the vee is narrow and long.
If wear develops, crab (angular guidance deviation)
is minimal with a vee way. This is because the hypotenuse
of the triangle has a very small (shallow) angle. Wear can
occur in the vee, but since the length is long compared to
the width of the vee, not much crab happens.
With the 5" bed (or worse the 7" bed), if wear occurs, the
hypotenuse is diagonally all the way across the bed, and
quite a bit of crab can occur. This is why the inverted vee
and and flat way bed became the standard for lathe makers.
Just to clarify, wear that results in crab happens because
of the forces that originate from the location of the drive
pinion and the half nuts. They are in the apron, and the
front of the saddle. So with a inverted vee and flat type
lathe, the driving elements are near the inverted vee way.
With the wide dovetail bed of the Hardinge type lathes,
this is also the same, the saddle being driven by the
apron in the front, but because the diagonal points of
contact are completely across the bed, the whole saddle
is going to crab, as this wear space is 'taken up' before
linear movement happens.
Consider if the saddle were not driven by the pinion gear
in the apron, but in the middle of the saddle. The Porter-
Cable Production lathes were like this. The rack and the
drive pinion were in the middle of the bed.
This equalizes the force and
puts a central drive for the saddle. No wear from crabbing
can occur. Also think of the very old 1800s lathes, with the
lead screw in the center of the bed, and double inverted
vee ways. Again, the drive from the bed to the saddle
was central and balanced. Not a good screw location for
dirt and swarf, so the lathe evolved to having the leadscrew
to the front side. The engineering compromise is unbalanced
drive forces, and wear developed, causing crab when the
saddle feed is reversed.
So in reality, the Hardinge type dovetail bed is great when
it is new, but bad when it wears. The wings of an inverted
vee lathe would have to wear a whole lot before you would
see the saddle crab. The Hardinge saddle will exhibit crab
with no where near as much wear.
When I talk about this lathe design and how much of an
engineering compromise the Hardinge bed design is,
guys come apart like I am calling their girlfriend ugly.
But this is ground level engineering. It is interesting to
study these things and see what time and wear have
proven out with the various designs that have been created
over the years. Maybe some others can comment and
discuss their favorite lathe designs.
-Doozer
