I wanted a hollowing system.   At this writing it  remains  unaccomplished, but I'm working on it.

 I looked and looked and wasn't really happy with anything I saw.  Price was too much for what they were, bearings were crud, Depth problems with chattering.  On and on and on I kept running into issues.

Save one:  The KOBRA.

The guy who makes ( or made) them never had a website never had a retailer selling them and seems to have dropped off the earth.

There are old posts about it but none recently.

So I designed something I thought was maybe  better.  Maybe lots better.      It's been a journey.

I turned  four bearing knuckle assemblies from 2.5" 4140 steel.   It's tough steel.  Each knuckle had a component made to accept  Tapered Roller bearings  one on each end with a steel cap to engage the bearing and  one bearing in the muddle where the  two knuckle parts meet.

 It bears noting that the size of these knuckles was solely determined by the fact that I got the taper bearings dirt dirt dirt Cheap~!!  So the bearings were the controlling factor.   It'll kill you what these bearings were made for: A Ferris Lawn Mower Blade mount assembly. 

I rather suspect I'll never tax them to their engineered limits.

Forget the fact that the diameters were WAY too big for my little Elgin Jeweler's lathe.  Oh yah  this was tough sledding.

My original plan was to weld  a 3/8" bar in between the knuckle assemblies.  But after building them and thinking long and hard I decided that welding on parts with  bearing races was a really dicey thing .  So I went with mechanical attachment.

The knuckles are 2.490" OD  so I got some 3" A36 steel and made mating rings a dead exact match.  Cut the rings in thirds and  drilled and tapped the things into the knuckle assemblies  with 4  1/4-20  half inch long allen bolts per attachment.  Yes maybe it's overkill. 

  I made four bearing  pairs per the  drawings

The tolerances I was working to were on average within 0.002" TIR concentricity and coplanarity and the Diameters were about +/-0.002 because they are bearing race mounts.

 

 The mating surfaces are O ring sealed.

 

They look like this unassembled

Those for 1/4" holes you see are solely to facilitate me getting a little brass drift in to  tap  bearing races out.

But I had to drill  and tap the sides to put these on

 

 

 

 

Like my little spot-face job to seat the allen bolts?

 

 

 

These ( above) are  A-36 steel ( very weldable) they attach to the knuckle OD  a 3/8" thick A-36 steel bar 2" long by 1&1/4  wide will be welded to these attachments to connect one knuckle to another.

The knuckles get bearings

 

 

And a little cap

This one is all steel.  Others I made are composite so I could make them from smaller diameter bar stock and do less work

 

Here is a composite one with a Delrin annular ring that snap fits into place

 

here's a pic of the four  knuckle assemblies  more or less in the relative  positions that they'll be  in once I get the steel flat bar stock welded to the little attachments

 

They total height is sufficiently below the  Center height of my spindle.   The attachments for the tool post mount and control handle are not on the upper far right knuckle component as yet. 

I haven't made up my mind how bit the attachment will be whether a third of a ring or a half of a ring.  The thing I'm welding to it to  stick the tools in and the handle are 1.5" diameter.  so there'll be more welding going on than with the knuckle to knuckle connections.

 

 

So this will all be for nought if the welding  process produces failure.

Imagine that?  Weeks of work looking at a single point of failure in an operation that may produce catastrophic error by misalignment or warping  from heat.

And I'm no welder.

Who is the saint of metal smiths?

Ahh yes

St. Eligius

If you got nothing better to do and a moment to spare,  mention me to him will ya?

 

 

Oh yah there's a movie

 

 

 

 

Edited September 10, 201510 yr by Cliff

I had to Google the Kobra system to get an idea what you were doing. Looks like you have a fine start. Welding shouldn't be much of a problem with your detachable weld points. 

Watching the Kobra video on setup, it looked quite tedious. Because you are designing it around your own lathe, the setup should be a little easier. I wonder it there is a more precise way to dial-in the cutter height. Perhaps a cam lobed bolt that could be turned to vary the height and a separate locking bolt to keep it there.

Once it's set to a lathe's centerline, then  it's set.    There's no need to change the height unless one is going to a different lathe.  I should be able to take mine off and put it back with no attention paid to height  unless I start using goofy cutting tools that bend out  the plane on center.

Any tool with a 5/8 shaft will fit what I'm building and I can build adapters for half inch or metric assuming I ever end up with metric tools  ( shudder ).

 

Did you know that  a person is statistically more likely to be in an accident when working in metric?    It's true. The super vast majority of industrial accidents   arise when people are using metrics.

Won't catch me thinking in millimeters no sir.

 

 

 

 

True. I skin my knuckles all of the time when working with metric fasteners. My mentality- that wrench is a little loose, but it should work OK-- OW! DANG! (or similar terms)

I would have this all welded up by now but for the fact that I was unable to score 3/8" x 1.250 steel bar at the BORG or Lowes.

I had to order it.  A 4' bar is about the  same price, maybe cheaper, from Metals Depot with shipping than it would have been at the BORG if they had it.

 

Anyway All I have left to do is cut some 2" pieces jig 'em and weld 'em then  tap some 3/8 holes and cut a couple pieces of metal to mount it on the bed.

Paint of course. There's always paint.   I'll use Rattle Can Black.

 

IT LIVES  The Franken-Hollower Lives

 

One thing for sure you can't accuse me of using too little steel.

Here is is without paint.   The welds are all my crappy TIG welding . I have it mounted in my vice because the lathe mount isn't done yet.

 

In the top most knuckle There is a hunk  of 5/8" bar stock in the tool end and the other end is a socket for a handle

 

Gotta have a birds eye shot

 

 

Stretched out  I get slightly better than 28 inches I can bore with the thing and there's not a scosh of wobble

And it's nice and square.  That was  a bit of a jigging thing for welding.   I couldn't have those knuckles all cocked off at goofy angles.

 

 

Of course there's a movie.

Cecil Demille did it.   There's  some racy scenes with  Lana Turner

 

 

Edited September 15, 201510 yr by Cliff

You Da Man!

put it on the lathe today

Trammed it and  corrected a little run out.

 

 

This is sorta like the old Gene Autry serials before the movie started- leaves you hanging and wanting more!

 

Can't wait to see it in operation!

Can I get a WOW!!

And those racy scenes were there. 

Like Lew, now I can't wait to see it in full action.

Amazing!