# DIY Hollowing System



## holmqer (Apr 8, 2012)

I recently completed this home made articulated arm hollowing system. It is what I used to make this

http://www.penturners.org/forum/f45/first-hollow-form-95717/

The piece I am turning has a 1/8" wall thickness, the laser is just starting to fall off the edge.

The design is a combination of the Elbo tool and the Kobra tool. I used 1" x 1 1/4" bar stock to make it. 

The 1" square design of the Elbo does not allow for adequate thread engagement of the set screws. I offset the hole for the boring bar to allow for M8 set screws (same hex driver as the set screws on Oneway chucks.

Rather then the more popular roller thrust bearings, I chose Super-Oilite thrust bearings. Super-Oilite is sintered iron impregnated with oil. They have no moving parts, and are much stronger than roller bearings, they can handle over 100x the shock load of the roller bearings that most folks use (P max of 4000 vs P max of 31), and since the joints do not need to move quickly, the superior V max of roller or needle bearings is irrelevant. You can get very high performance roller bearings that have comparable P max but at $0.60 for Super-Oilite vs over $20 for the rollers with washers with 6 needed, I went with Super-Oilite. Oilite thrust bearings are sintered bronze, to avoid electrolytic corrosion I went with the sintered iron type.

Made my own boring bars out of 4140 steel which I will have heat treated to harden them a bit. One end holds 3/16" square HSS tool bits, the other holds a 3/8" shank swivel mount.

The secondary banjo is made from 3" diameter steel rod with a 4" square plate on top of the ways and 3" for the clamp block. The 3" rod is counter bored to allow the 3/8" socket head cap screws to be below the surface.

The laser system is cobbled together from a laser diode and battery pack from Digi-Key. This lets me use a very low mass laser at the end of the laser arm. and not have to fool with a fixture to hold down the switch of a laser pointer. The low mass keeps the laser very stable in use.

With 7" of the boring bar extended over the tool rest there is no chatter.

When not in use, the hollower folds up and fits an a drawer. The arrangement of the bolts allows it to articulate like the Kobra system.

Total material cost around $75, $50 for the hollower and $25 for the laser system.

Having made my own, I appreciate why they cost so much. They are a lot of work to make.


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## bluwolf (Apr 9, 2012)

That is a fine piece of work. Well done!

Mike


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## holmqer (Apr 9, 2012)

Thanks, it is fun to use, and makes hollow forms easy, at least moderate sized ones.


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## Padre (Apr 9, 2012)

That is really cool Eric.


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## BRobbins629 (Apr 9, 2012)

Very nice. On my to do list.


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## leehljp (Apr 9, 2012)

That is a nice setup. Your experience in turning and tool making is evident! Some day, for me - maybe!


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