Some Offset Design Experiments

[Dave Delo]

Finally decided to get off my duff and make an offset fixture to hold a sphere and put some design into it. While I've done some between centers offsetting before, that just allows me to put lines in 1 direction or axis. This jig allows to rotate the sphere and crisscross the line. This is no David Reed Smith level jig but it did work well and will hold a 2.75" to 4" sphere without any adjustments.

Took a couple rubber chuckies that I wasn't using and attached them to a 3/4 x 10 bolts that were epoxied into a couple pieces of scrap wood I had laying around. Offset for this style is 3/4". Took another scrap piece and tapped the 3/4 x 10 hole for the tailstock and then attached that piece to the bolt piece.

Tried it out on a cherry piece that I had previous done and put 3 splits into the piece that where perpendicular to the existing splits. I was pretty satisfied with (no picture) the results so I tried to put a few more cuts into the piece at a 45 degree angle.........well that didn't work out so well. Not sure if it was because of grain direction or maybe trying to feed a little to fast but I lost the point of the vortex tool and got a little section chip out.

Dressed up the tip of the tool a bit (still need a little better point) and tried it out on an oak sphere. Nice clean cuts so I'm satisfied with the 1st trial runs. Have to say I was a little worried and had a little pucker factor spinning a 4" ball at 1,000 rpm's but after it was done, I'd say if the piece wasn't as secure as it was the cut quality wouldn't be there.

Thinking about masking off the sphere and then cutting and then maybe spraying a color into the splits. Here's a couple pics of today's adventure.

 

[Bill Boehme]

Wonderful. Thank you for showing this.

Would you consider writing a tutorial for the How To section showing this and along with some details on making the fixture?

 

[Dave Delo]

Bill,

Thanks for the kind words but this edition is just a prototype/1st attempt at making something to fit a specific application. Seems to work okay but is pretty clunky and needs some refinement.

The inspiration/idea for something like this comes from David Reed Smith who has instructions for making a variable eccentric cup chuck at
http://www.davidreedsmith.com/Articles/VariableEccentricCupChucks/VariableEccentricCupChucks.htm

If anyone was going to make something of this nature, I'd follow his type of plans. I just did it this way because I had all materials on hand and didn't need to buy anything.

All I've done is drill & counter-bore holes in the correct position and epoxy in place a bolt that protrudes 4 threads to accept 3/4" x 10 tpi threads inside the rubber chucky. Here's a couple pics.

 

[John Torchick]

Good pictures of the setup. Thanks for the link to D. R. Smith.

 

[Dave Delo]

Had some time today to experiment further with the jig so I made an almost 3 3/4" cherry sphere. Just some random splits at 90 degrees to each other in sets of 3 with one split sitting in-between. Experimented with rpm's because there can be a fair amount of vibration. At 400rpm's the machine runs smooth and tried a couple splits with not good results so needed to get up a little over 1,000 to get it to smooth out and gave great results (nice crisp cuts). Think I have this process set up to the point I'm comfortable with it and just need to make a bunch of spheres to have on hand to experiment with some more thought out designs.

 

[Raul McCai]

Oh that is way cool

 

[olaf Vogel]

Thinking about masking off the sphere and then cutting and then maybe spraying a color into the splits. Here's a couple pics of today's adventure.
View attachment 21097

Dave,

Very nice work. I've seen such designs before, but never drilled down to see how its done. Thanks for showing it!
One question in the pic above, both the head and tailstock end are off the centre axis. And I'm sure you align them when first clamping the sphere.

How do you entire those two ends stay aligned?
Is it just the pressure on the sphere?

Great work!

Olaf

 

[Dave Delo]

Olaf,

The headstock side cup and the tailstock side cups are both offset equal or are mirror images of each other. The tailstock side does swing freely but it's easy to hold the headstock side + sphere with your thumb & index finger and hold the tailstock cup with your pinky finger and apply pressure with the tailstock wheel with your right hand. Pretty much just a visual line-up and the pressure applied secures the sphere without any movement while machining.

Dave

 

[Tudor Joseph]

Hello Dave, have watched this thread with interest. I first saw some pieces using this technique some 20+ years ago but there was no info. on how it was done so I developed my own solution. The pics should be self explanatory, the main difference is that I used a "Trumpet" like clamping device using the tailstock pressure to create the friction drive from the offset drive cone. The trumpet, which has an internal cone at the front, simply follows the eccentric displacement of the sphere without any creep or scuffing of the ball. This eliminates the need to have a mirror-image tailstock support. I made the 60 degree grooving tool using a triangular carbide insert. This needs a smooth "rocking" action (similar to a parting tool) in order to produce a good finish without a "catch"! The available offset is from Zero to 1/4" which is adequate for balls up to about 4" dia.
Best wishes from across the pond.
Tudor

 

[Dave Delo]

Tudor,
Thanks for posting this. I'd agree there isn't much online help for performing this specific task. As stated previously David Reed Smith's site is about the only place I've seen that shows some different ideas for secure holding.

Thanks for your picture/idea for the tailstock side. I had tried something along those lines with a flat surface on the tailstock side but it just didn't hold secure enough to satisfy me. Going to try out your method and see what happens. I'd imagine this might help in reducing vibration. Here's a pic of the cutter I'm using.

 

[Tudor Joseph]

Hello Dave,
Personally I would use square material for the cutter. This would provide a flat base against the toolrest keeping the cutting face horizontal and reducing any tendency to snatch or twist.
Tudor