For 1920s Epiphone
Banjo Nuts
© Frank Ford, 2008; Photos by FF

This weekend I did my first "production run" on my little Rambold turret lathe.  It's a swell old tool, bigger than a jeweler's lathe, but built along the same lines with the cantilever bed.  Despite its diminutive size, this machine takes regular 5/8" shank turret tools, and is very substantially built.  In its current configuration it has a variable drive with a top spindle speed of 7500 RPM.  An eBay purchase, it required a full rebuilding, but today it shines like silver and hums like a bee:

It's REALLY crowded in my small garage shop, so I can't stand back and get a good front view of the machine.  Here's a shot from the other oblique angle:

That "tower" on the left is the control unit for the variable frequency drive that runs the three phase motor under the table.

My first task was to grind a pair of "form tools" from high speed steel tool blanks to the negative of  the profile of the turned areas on the original nut I was to copy.  I simply ground them by hand until they fit the curvature:

For the broad one, I used a regular bench grinder:

To produce the more complex shape, I spent some time with my Foredom tool with a carbide cutoff wheel so I could pick out detailed curves:


Before too long I had a nice fit:


I set the tools into the holders on the quick acting cross slide - the rear one cutting "upside down" and the front one mounted conventionally.  Both tools have their cutting edges set directly on center.


Once I was all set up, I loaded the appropriate drill bits in the turret, adjusted everything and set about making brass shavings.

Here's Rambold in action:

1 - Move stock (9/32" hex brass) forward in the collet:

2 - Run the turret forward to press the stock with a stop rod so it goes precisely in place and lock the collet:

3 - Present second turret tool and spot drill:

The spot drill is just enough oversize that it leaves a nice little chamfer when the part is drilled.

4 - Run clearance drill halfway through the nut:

There's no need for dozens of threads in a banjo nut - by convention they are generally drilled for clearance at the bottom.  The turret has adjustable stops on all six positions, so each operation goes to just the preset depth.


5 - Drill for 8-26 tap:

This operation drills just deep enough to make a clear hole through the part, but not so deep as to start a new hole in the remaining brass. These nuts will be nickel and/or gold plated, and threaded after plating and polishing.


6 - Run in first form tool:

The tool is clamped close to the tool block, so you can't see it here.


7 - Back off  the slide and engage second, "upsided down" form tool:

These closeup shots were a bit difficult because of space problems, so I didn't notice the focus problems at the time.

8 - Plunge parting tool and cut off the piece:

9 - Repeat 200 times.

Here's the final product:

And it looks pretty much like a new version of the original:

The original on the right is  a deeper  yellow because it has most of its original gold plating.

Off goes the batch into the tumbler for polishing:

After polishing, I tapped the parts by holding the 8-26 tap vertically in the bench vise, and ran the nuts up and down using a chuck on my Makita variable speed hand drill.



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