Replacement Focusing Knob

I have an old Bushnell spotting scope that I bought a while back.  Overall, it is in good condition.  One of the problems is that the focusing knob has been cracked from tightening down the set screw that holds it on.  As a result, it slips on the shaft and you cannot adjust the focus reliably.  Time to fire up the lathe.

FN1

I took some measurements and sketched out a rough drawing to work from.  Basically, it is a cylinder with a stepped hole in the middle and a threaded hole normal to the center axis where the set screw goes.

FN1a

I decided a steel 1″ knob would be a little overkill.  Instead, I chose to use some aluminum I’d recently ordered.  This is the first aluminum project I’ve done on my lathe.

FN2

The original knob was a little over 1″ but 1″ is the largest piece of aluminum I have.  The outer diameter isn’t a critical dimension though.  I took a light pass to clean up the surface. I was very happy with the surface finish I got from the old girl.

FN3

Next, I put a slight bevel on the bottom of the knob.  I did the same to the top after removing a little material.

FN4

To increase grip on the knob I wanted to put a knurl on it.  I only have one size knurl with my quick change tool post which made selecting the size of the knurl easy.  I slowed the lathe down, fed the knurling tool in, and engaged the feed to move it to the top of the knob.  The knurl didn’t come out perfect though as the final spot I knurled is slightly different than the rest.  I’m not sure why.  Knurling takes a bit of practice.

FN5

Moving on, I removed the live center from the tail stock and used the chuck to drill the small hole that the focusing shaft will go into.

FN6

Next, I removed the small drill bit and drilled the larger hole to fit the spotting scope.

FN7

To drill the perpendicular hole for the set screw, I removed the bar of aluminum and headed over to the drill press.  I then drilled and tapped the 8-32 hole for the set screw.

FN8

Before I’d removed the part from the lathe, I turned my tool post to bevel the edge of the large hole I’d drilled in the bottom.  Before I can part off the knob I need to align the tool post to be perpendicular to the work.  This is important for the cutoff operation.  If the cutoff tool isn’t perpendicular to the part then the sides of the tool will rub the work which can result in a poor finish and possibly break the tool.  (Technically, the cutoff tool has to be aligned with the direction it is being fed in.  Since I’m feeding in with the crossfeed it needs to be perpendicular.)

FN9

I parted the knob off, filed down the post left on the top of the part, and sanded the top down to 600 grit to leave a nice brushed finish look on the knob.  I thought this was a good alternative since I couldn’t think of a way to hold the knob to clean up the top of it.  After that, I put the knob back on the scope and can now focus without issue.

FN10

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