More than 50 years but ... ISTR there is an illustration in that book right at that point that shows a line tilting up at a few degrees, so no, it isn't tool top-rake they mean. Many other variables in that.
More likely the intent is to compensate for the work to want to climb, the tool to be more subject to being forced downward as the radius, therefore the lever-arm, increases its force on both.
I've always taken it as a conceptual guideline only, and one most appropriate to lightly-built lathes, lantern toolposts, and overhung Armstrong tool-holders.
Heavier machines with stiffer toolup overall, one adapts to and sets up as required to make a given combination work best............
Bill
Not to be contradictory Bill but if I understood you (or the book) correctly......
....in my experience and given ideal tool geometry and sharpness, (don't we always try?), with "lightly-built lathes" on a long reach, it is crucial to set the contact point
no higher than center and a touch below for safety.
When set even a
tick above center, the trajectory of a tool point that has the potential to flex under the downward thrust of turning, is
into the work, all bad, no good.
There is nothing about the tool contact point being "wrapped down around" the part, (slightly below center) that hurts an ideal penetration into the work and lift of the chip, that can't be compensated for with a little additional top rake re-establishing the included angle.
Conversely, bore work
demands a slightly high contact point to direct any potential flexure
away from the part, the bending moment of the cutting tool support amplified by the additional reach of the bar.
The
theory of setting the cutting point even slightly high in a pre-emptive attempt to "take out the lash" or "buttress" against
anticipated flexure, is analogous to the tendency of amateur shooters to push forward and down
before squeezing one off,
gouging into the dirt, rather than cutting straight into the target.
When a tool's support assembly flexes under the pressure of the cut, that signals just one thing, the pressure generated by excessive depth of cut under the circumstances, overcame the equipments ability to resist. With that in mind, the obvious thing to do is to reduce the depth of cut, because lashing out in frustration over the lathes lack of rigidity wont fix it and like the shooter,
pre-setting a down-and-in trajectory, won't improve the accuracy.
When you've adjusted the depth of cut to
not overcome the stiffness of that particular set up, it's nice to just have to skim off the slight smooth bulges created when the tool flexed
away, rather than scrap the part because you gouged
into the stuff you wanted to keep.
Now, if I had one of them super lathes, I'd just jam in most any ol' kind of tool, wherever I wanted and join the lathes laughter at the expence of the parts pain..... hell, I'd purposely pick way over-sized material, giving me enough to make smokin' blue 6's and 9's with, just 'cuz.
A feller don't even need to walk clear across the shop and
look at the lathe, to tell that it's super, after he's dug a 2 ounce, smokin' blue chip from between his collar and neck!
Bob
Now I didn't get that stuff from a book and maybe I've just gotten away with it while doing the wrong thing and then making up stuff that isn't true to explain it....cause and effect not always revealed accurately, to all... I'm serious about that and willing to listen.
UPDATE: OK Bill, just checked to see if anything had been added since I got called away while typing this and found an absolute jewel:
"There are fewer things that are 'absolute' than we think there are," oh yeah, that was you!
