Posts Tagged Sherline
The evacuation tip nearly touched the inside end of the base spigot!
I had to cut the shaft and half the body off the shell drill in order to fit it into the space above the tube base and below the chuck:
A slightly larger shell drill would still fit within the pin circle, but the maximum possible hole diameter in the base really isn’t all that much larger:
The getter flash covers the entire top of this tube, so I conjured a side light for a rectangular knockoff Neopixel:
There’s no orientation that doesn’t require support:
A little prying with a small screwdriver and some pulling with a needlenose pliers extracted those blobs. All the visible surfaces remained undamaged and I cleaned up the curved side with a big rat-tail file.
I wired the Arduino and Neopixels, masked a spot on the side of the tube (to improve both alignment and provide protection from slobbered epoxy), applied epoxy, and taped it in place until it cured:
The end result looks great:
It currently sends Morse code through the base LED, but it’s much too stately for that.
Now, with the 0D3 tube properly clamped and aligned in the Sherline mill:
I can slowly run an end mill down onto the spigot:
Eventually converting the whole post into black dust in the vacuum cleaner:
That was completely uneventful, which is pretty much the whole point of good fixturing, isn’t it?
Applying the vacuum cleaner while milling seems to have kept the dust out of the base, although I’m not sure I can pull that trick off every time.
In order to clamp the tube in a V-block, the clamp must position the tube’s centerline so the envelope will clear the V groove, thusly:
The clamp now extends into the V-block and surrounds the entire Bakelite tube base:
The little divot captures the clamp screw and the slot lets the whole affair compress just enough to firmly squeeze the entire tube base.
The tube data table now includes columns for the envelope OD and the base OD, although only the 0D3 (and similar) Octal tubes in my collection have a bulging envelope and a smaller base. You can build clamps for cylindrical glass tubes if you like; I don’t vouch for the accuracy of the table contents.
For whatever it’s worth, the 6SN7GTB tube I started with has a 32 mm Bakelite base and the 0D3 tube has a 29 mm base. That should probably justify two separate entries in the table, but I’m making this up as I go along.
The OpenSCAD source code as a GitHub Gist:
Rummaging in the Hollow State Electronics box produced the shapely 0D3 regulator tube with an intact spigot / key post in its base:
Because the glass envelope (1.5 inch = 38.1 mm OD) extends beyond the base (1.125 inch = 28.6 mm OD), the simple base clamp must let the tube extend over the workbench:
There’s no way to clamp that mess in the Sherline, so, rather than freehanding the shell drill, I misused a Dremel slitting wheel to grind away the end of the spigot, which normally extends a bit beyond the pins so you can’t possibly insert the tube into the socket the wrong way:
The missing end exposed the hole in the middle of the post and showed this tube’s evacuation tip didn’t extend into the spigot. Emboldened by that, I continued the mission until the wheel wouldn’t reach any further:
That didn’t work well, but at least I didn’t break anything and nobody will ever see those mauled pin tips.
Obviously, the only way to do this right is to clamp the tube properly and mill the spigot flush with the socket; it’s time for more 3D printing…
Clamping the octal tube into the Sherline let me set the XY=0 origin to the center of the base with the laser dot (visible near the front):
Find the edges, touch off the half the 32.2 mm diameter, then align the drill at XY=0 directly over the exposed evacuation tip:
Make a very shallow cut to verify the alignment:
Just inside the scuffed ring from the drill, you can see the fractured ring where the original one-piece Bakelite spigot / key / post broke off.
Then extract the drill from the chuck, file more relief behind the cutting edges so they actually cut, re-chuck, and continue the mission:
Pick a nice Bakelite ring out of the drill:
And eventually you can see all the way to the glass envelope:
The (knockoff) Neopixel LED sits directly below the evacuation tip and is about the same diameter, so much of that enlarged opening will be in shadow. Despite that, the tube does seem noticeably brighter:
Drilling that tube was so harrowing that I can’t imagine similar surgery on an intact octal base.
Perhaps just slicing off the tip of the Bakelite spigot and gluing a single very bright red/orange LED in place, rather using than a (knockoff) Neopixel a few millimeters away, will suffice.
Or just give up, top-light these tubes, and move on?
One of the octal tubes in my collection has a broken spigot / key post that lets some light in through the bottom of the normally opaque Bakelite base:
Perhaps drilling out the base would let more light pass around the evacuation tip, but that requires a shell drill to clear the tip. Some doodling suggested a drill with 12 mm OD and 8 mm ID, which was close enough to one of the smaller homebrew drills in my collection that I decided to see how it worked:
You (well, I) can’t freehand such a hole, particularly with a glass tip in the middle, so I needed a way to clamp the tube in either the drill press or the Sherline. A pad for the clamp screw in a V-block seemed appropriate:
The screw hole sits at the 1/3 point to put more pressure near the pin end of the base. Maybe that matters.
The setup looks like this, with a small red laser dot near the front of the base:
The tube rests on a random scrap of plastic, with the hope that the drill won’t apply enough pressure to break the glass envelope.
In normal use, the V-block would be oriented the other way to let you cross-drill the cylinder. In this end-on orientation, drilling torque can rotate the tube; compliant padding for more traction may be in order.
The OpenSCAD source code as a GitHub Gist now includes a module that spits out the clamp:
After drilling the platter for a Noval tube, I finally made a fixture to hold the platters firmly, but gently, in the proper position for drilling:
The platter sits more-or-less flush with the surface, where credit-card plastic pads work fine. Thinner platters may require compliant padding.
The solid model has locating pips at ±50 mm from the center and airspace below the platter for the drill bit:
The 1.16 inch hole spacing matches the Sherline’s tooling plate. The center hole seemed like a Good Idea, although it has no purpose right now.
The OpenSCAD source code is the same as before; just set
Layout = PlatterFixture; and it’ll produce the right thing.