Hi Ben,
Lead screws are not directly copied in the sense of replication. Instead,
they offer an accurate reference for geometrically based movement of the
lathe tool's movement.
The significance of the lead screw (as recognised by Maudslay in the 1700s)
is that when turned it provides linear movement to a nut in exact proportion
to angular motion. I.e.. If you put a nut on a screw and turn it a certain
amount then the nut moves along by an exactly proportional amount. Such a
nut typically covers several pitches of thread and hence tends to average
small linear errors.
The lead screw drives horizontal movement of the lathe tool cutting a new
thread. The key insight of Maudslay was that by providing adjustable
gearing between the drive to the lathe chuck which turns the piece being cut
threaded and the lead screw which drives the horizontal motion of the
cutting tool, then threads can be cut at different pitches (and obviously
different sizes) to the lead screw depending on the gearing and the work
piece size..
The gearing between chuck and lead screw can be seen as a rotational version
of an asymmetric lever between the linear movement of the nut on the lead
screw and the linear movement of the lathe tool. Consequently, this
leverage and the ability to produce screws at different sizes can be used to
help minimise errors on the thread being produced - in effect producing a
more accurate lead screw.
By the way, my apologies, it takes of course three surface plates compared
together to be sure one has a flat surface. Two can result in a
convex-concave combination.
Best wishes,
Terry
===
Dr. Terence Love
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Tel: +61 (0)8 9305 7629
Fax: +61 (0)8 9305 7629
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-----Original Message-----
From: PhD-Design - This list is for discussion of PhD studies and related
research in Design [mailto:[log in to unmask]] On Behalf Of Ben Shaw
Sent: Wednesday, 16 May 2007 1:17 AM
To: [log in to unmask]
Subject: Re: milling and flatness
I recall reading (I apologize that I can't remember where) that the milling
machine rested upon two distinct achievements: flat and straight ways, and
the precision lead screw. Terry gave us a description of how the former
could be achieved through ink and painstaking iteration, and how relatively
imprecise lathes can be used to produce very round products. The lead screw
couples translation with rotation. I don't recall how the first precision
lead screw was fabricated, but I do remember a comparison to viral
replication: once such a screw was in hand, it could readily be used to
fabricate numerous copies (at different scales), which could then be used to
generate additional copies. It's not clear how precision was increased over
time (paintaking iteration? or perhaps some sort of mechanical averaging?)
but no doubt there's an interesting story there.
Regards,
-Ben Shaw
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