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Stan Schaefer
 
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Default Inner threading turning tool out of HSS...Need for astronomy project

(Vincent Coppola) wrote in message . com...
Hello,

I am rehousing a webcam for use in astronomy and I turned down on my
minilathe a simple adapter tube out of aluminum to fit the standard
1.25" diam eyepiece holder.
I now have to turn an inside thread to hold
a standard filter. The filter requires a 1.125" x 40tpi thread. I am
trying to grind my own highspeed steel tool and having some difficulty
getting the threads to look right. They are there but look flattened.
Does someone have a dimension or procedure for grinding such a
tool to work with aluminum? Would the same tool work for delrin? I
have searched the web many times and I can turn up very little on this
subject.

Please advise,

Vince Coppola



For threading holes over an inch, I use a boring bar and make a
threading tip for it. When I say boring bar, I'm not talking about
one of those one-piece jobbies, it's a chunk of round bar with a
square hole in the end at 90 degrees and has a set screw to hold the
bit. I've made one up in about 20 minutes with some spirited filing
to square the hole out. I like to use the Tantung G I've got for
those threading tools, once you get them shaped and stoned, they hold
an edge for a long time. I like to use a bar with just enough
clearance for chips, stiff is good when it comes to threading. For
internal threading you have to watch the top-to-bottom clearance, if
you don't grind enough of an angle, the heel is going to be dragging
in your fresh cut and will mess things up. As another poster
mentioned, the compound has to be kicked opposite the direction it
would be used in when external threading. It also take a little
practice to get to thinking in reverse. Threading stops help a lot,
too. On aluminum, make sure you use some decent cutting fluid made
for the job. Tap-Magic works, Alumi-Cut is good, too. For 40 TPI,
you're going to want to use a thread gauge for both bit grinding and
setup, a magnifying glass will help with stoning to final contour.
You'll have to have a pretty good finish on the tool flanks to get a
good result on your threads. I usually use a black Arkansas stone for
final polish on something that fine. I didn't use to do that when I
first started out, it really does make a difference in final finish on
the threads.

Stan