"SteveF" wrote in message
. com...

"Dave Hinz" wrote in message
...
On Thu, 20 Jan 2005 05:16:33 GMT, SteveF wrote:
Let's say I have a small lathe (length) and need to make a threaded
screw
longer than the max length of my lathe. Obviously this can be done in
theory by just threading one part, removing from the chuck, reinserting
further back in the headstock and continuing the thread. But in
practice
can this be done with enough accuracy for something like a replacement
lead
screw for a lathe? Yes, I am aware of parts on Ebay and long threaded
rods
from MSC, etc. - this is more of a "how to use my lathe" machining
question.

Can you use the lathe to turn the stock, and use a die to cut your
threads, or is this for a thread you don't have a die for? Novice
question
on my own part as well.


Well, that gives me two more questions for the group.

One - Assuming some experience on the part of the operator in cutting
threads and a good lathe, how much less accurate is using a die than
cutting with the lathe?

Die cutting would not be very accurate for a long thread. The die will want
to wander and the concentricity will be bad. I missed the part about cutting
a lead screw in the OP. Precision lead screws are usually ground. Although
some ball screws and lead screws today are cut out of hardened blanks on
thread whirling machines. http://tinyurl.com/5bn5d
Non precision lead screws (those not used in a machine tool) are for the
most part thread rolled. Long threads like those on all-thread rod are
through-feed rolled on a double roll type thread rolling machine.
You can easily make a long machine screw to the accuracy required by cutting
the threads in sections on a manual lathe. The problem with cutting a lead
screw on a lathe is that all of the accumulated error in the lathes drive
train, plus deflection, run out, slide wear, and thread form errors in your
tool all stack up and prevent you from making a lead screw that's as good as
the one in the lathe.


Two - How could one measure the thread to thread distance accurately to
see how much error was created when reindexing after moving? Looks like
thread micrometers only measure the diameter.


That is what the optical comparator was invented to do, check thread form
and thread lead.
http://www.qualitydigest.com/may02/html/optcomp.html
Modern manufacturers of ball screws and precision lead screws use special
electronic gaging machines that check the lead, run out, and accumulated
error.


Dan