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A New Screw for Old Narex
© Frank Ford, 2008; Photos by FF
I bought this "hardly
used, perfect condition" Narex boring set off eBay from one of those dropoff
resellers who obviously knows nothing about nothing, or at least claims
that kind of ignorance:
It cost me a good deal less than one of those imported "Criterion" style boring heads, so I don't suppose I have too much to squawk about. But, one thing was amiss. The adjusting screw had no threads at all:
And, the brass insert in the slider had been nicely stripped, for good measure:
From the misalignment of the set screw, it's clear it had been messed with.
I chose the brass insert for my first line of attack, and whacked it out of place:
Clearly, it's time for new threads, and they are left handed:
My first thought when I encounter left-hand threads is "Jeez, why me?" Then, moments later I remember that lefty threads are actuallly easier on the lathe because the travel is away from the headstock. I bored out the insert to remove the old threads, and proceeded to add a nice new set of threads with the same pitch - 20 TPI:
Having just concocted my own unique thread diameter, I measured the inside diameter using my handy new plug gage set:
When I got a nice slip fit, I made a note of the diameter, and applied this little formula:
I have this note posted on a couple of machines for easy recall.
Working backward then, I used the inside diameter measurement as the tap drill size approximation, and added the reciprocal of the thread pitch (1/20th of an inch) to come up with the major diameter of my external threads:
Turning a 1/2" diameter piece of 303 stainless steel to .413" diameter, I was ready to thread it up:
Threads mostly done:
I turned out that my starting diameter was a bit too big, so I simply continued cutting the threads deeper until I could screw the brass piece on. The fit was a bit tight when I finished:
After I touched the new external threads with a file and some fine emery cloth, I coated them with fine lapping compound:
And I ran the brass back and forth under slow power to smooth out the fit:
I parted off the new threaded piece:
And, finding it was a tight but acceptable fit in a 10 mm collet, I set up to drill and bore:
Boring the length for a nice slip fit on the adjusting screw:
Sorry about the focus on this one.
I turned the shoulder of the original adjusting screw so my threaded piece would run all the way up:
Here they are, ready for assembly:
A bit of that green Loctited bearing retaining compound, and I had a nice solid assembly:
Back to the body, I reinstalled the brass sleeve, tapping it for a new set screw:
Locking it in place:
Installing the adjusting screw:
And the little protective cap over the back end of the block:
NOW I have a really fine Narex boring head, solid and ready for work:
And, I learned some new tricks in the bargain. . .
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