Spiral cut bowls

[bob_c]

Hi - All
This is my first posting on the Shopbot forum. I would like to share a project that I just completed a few weeks ago. A set of three nesting bowls made from walnut. The unique part is they were “spiral cut”, one continuous tool path from top to bottom inside and outside. Here are a few pictures that show the set-up and the finished product.


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I plan to present a “show and tell” at the March 21 “Camp Shopbot” held in Ontario California hosted by Gabe Pari. If you are located in the southern California area, attend and learn more.

Bob Chubb

PS If you do plan to attend, please contact Bill Palumbo at
> http://www.shopbottools.com/campshopbot.htm <

[scottcox]

Bob,

Beautiful work!

Can I ask what software you used to create the spiral toolpath?

[bob_c]

Thank you Scott.

The program is one that I wrote a few years back for machining surfaces (Bezier surfaces). Basically it cut along the surface parametric flow lines. Just recently added the spiral cut option.

I am presently up-dating the documentation. When that’s complete, I will provide a link for down loading both the software as well as the documentation. This won’t happen until late March.

Bob C.

[mikeacg]

Bob,

Your bowls are very nice! You've set the bar pretty high with your first post. I can't wait to see what else you have up your sleeve...

How does your spiral cut compare timewise with the standard raster path?

Thanks for sharing!

Mike

[mmccue29]

Very nice. cant wait to see how you did it

[woodworx]

robert,
Were you at last years cali camp?

[myxpykalix]

Robert,
That is very cool and I would be interested in finding out your method. It looks like you also have to have a very good registration method to cut both the inside and outside precisely which i'm sure is key here. Will the documentation cover that also?
I've often thought about cutting bowls but never have done any of that. I'll be anxiously awaiting your next post. Good Job!

[bill1]

I'll say "Excellent Work" very well done.

[woodworx]

Jack,
Looks like they are pocketed down into the plywood that is screwed to the spoilboard. This provides great registration when flipping over to machine the back side of a board.
I am sure this is a distinctive machining strategy, but does this have any advantages over a standard 3D raster or offset machining? Looks like there were some pretty heavy machining marks in the sides that were left from the mill as well. Did you do any roughing with an end mill or did you just use a ball nose?

[tkovacs]

Very nice Bob. Having done some spiral cutting it is definitely the way to go. Looks like you did a rough cut first. What pitch and tool did you use for the final cut?

Terry

[bob_c]

Mike,
I have no numbers to compare the time difference between other cutting methods and the spiral cutting I did. I pulled out some numbers for one operation. As an example, the outside finish cut on the larger bowl took 11 minute and 24 seconds. Using a1/2 D ball end, 50 revolution, 80 steps per revolution and a feed rate of 1” per second. This produced a peak height of .0025 max. _ .0006 min. does this help?

Justin,
Yes I did attend the last two southern Cal camps.

What you are looking at in the first two pictures are the cutting of the first outside finish cut using a ½”ball. The remainder of the bowls are shown in a rough condition following the roughing operation, which used a ½” flat end cutter, leaving .125 for clean up.
Another point, I don’t think other 3d cutting methods (raster, V-carving, etc.) allow under cutting or back draft. If you will note, in the illustration below, the inside of the lip has approximately .1” overhang. The entire inside was roughed and finished using a .75 d spherical shaped cutter.


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Jack,
Yes, the registration between the two positions is critical. This was achieved using 1/8” wooden dowel pins (4 per part). The other important item is holding the part when it is separated from the parent stock in second position. This was accomplished using paper and glue place on the base of the bowl and the tooling board.
The attached illustration should give you some idea of the strategy used.

The finish I used was relatively easy. After a minimal amount of sanding to remove any machine marks, two coats of wipe on poly (water based) were applied. Being water base the first coat raised the grain a bit. So a second sanding was required before applying the second coat.

Terry,
I think I E-mailed you about a year ago about your spiral cutting and told you that where I was heading with my next project.

Bob C.

[myxpykalix]

Thanks Robert, I will wait for your full documentation in March. This looks like a neat project to look forward to. thanks

[bob_c]

Hi - All
As promised I have made the necessary updates to the documentation and a few software tweaks. Also included is a DXF translator I use for moving point data into surface files (text format).

I will welcome any and all feed back regarding the success, problems or failures using the programs.

The writing and testing of the programs were done totally in a Mac environment. The only PC testing was done using “VMware Fusion” virtual PC software. At present I know of only two PC issues. 0ne is that the “capture screen” button does not work, as it was written using Unix code. Also some misaligned text in the DXF translation program.

For the PC users: The BezierEdit and SurfMill programs are in folders, which contain items that must not be removed. To install, simply drag their respective folders to the desktop.

Here is the link to the package containing the programs for both Mac & PC, compressed in a zip format.

> http://homepage.mac.com/rchubb/program_package.zip <

Bob C.

[myxpykalix]

I downloaded your files and for me it looks a little daunting so I hope we get some good questions and answers to help some of us who sit in the back of the class to get a good handle on this. This looks like an extreme amount of work to put this together so thanks for taking the time to do this and share.