Cutting a donut

[Keith Larrett]

I may have the opportunity to cut out a bunch of wooden "donut" shapes. Approx. 15" in diameter with a 2" cross section diameter. It would need to be a two-sided cut, which I think I can figure out. Probably four profile cuts with a round over bit. Two per side.

What I'm not sure of is hold down when the piece is flipped over. Using a round over bit I can't leave tabs. Any suggestions would be appreciated.

[scottp55]

Keith,
Maybe worth reading?
http://forum.vectric.com/viewtopic.php?f=2&t=26073
Moulding Toolpath is a Profile cut, so maybe leave tabs on first side?
scott

[knight_toolworks]

unless the roundover is special or you can't get a bit that size I would do them on a router table it is faster and easier. cute donuts and while they are cutting round them over on a router table.

[Ajcoholic]

I may have the opportunity to cut out a bunch of wooden "donut" shapes. Approx. 15" in diameter with a 2" cross section diameter. It would need to be a two-sided cut, which I think I can figure out. Probably four profile cuts with a round over bit. Two per side.

What I'm not sure of is hold down when the piece is flipped over. Using a round over bit I can't leave tabs. Any suggestions would be appreciated.

Thinking about this for a bit this morning... while I was out shoveling after our first real snowfall of the year, and playing outside with my kids.

I was at first thinking to use the roundover and cut both inner and outer profiles - then flip. However, the only way I could see holding down the half done ring, would be to 3D cut a negative of the ring into a piece of material, and gasket it and use vacuum to hold down the ring. Probably very difficult to get the tolerances necessaty spot on.

Then I was thinking, it would be far better to profile either the inner, or outer edges of BOTH sides. Then create a fixture that either grips the entire outer profile, or inner (think some sort of expanding circular ring) and then profile the opposite perimeter.

This would require more flips. But, it would be safe and positive IMO.

SOmething like this... cut out the exterior of the circular blank, and radius the outer edge. Flip, and radius the other edge.

Now, on a fixture that is designed to clamp around the now cut outer profile, cut the interior hole and profile, flip and profile again.

I think if you used some of those sideways acting toggle clamps, or made a 4 piece (in quarters) fixture that would clamps tight around the ring, with some sand paper 2 sided taped to the "jaws" to grip the ring firmly, it would do fine.

Did I make myself fairly clear?

[robtown]

I'm with Steve on this one.
Cut out the discs and do the roundover(s) by hand. Wayyyyy faster. Single bit operation, no flipping, and the roundovers will go much more quickly.

[scottp55]

+1 on Steve after playing with it! :)
(didn't realize moulding toolpath had so few options, and still takes a long time:( )

[Chuck Keysor]

Keith, maybe I am missing something, but why not just cut out a front half, and a back half, and then glue the two halfs together to make each donut? Chuck

[bobmoore]

you said a number of them so if there is enough to justify a little expense I would get a 1.0"r round over bit. Round over the 1st side to 1.0 " deep, flip into custom female vacuum fixtures, route side 2, sand and go.

[steve_g]

Keith…
A method I used recently involved a flip op with a radius bit. My radius cutter left a ¼” flat web 1/16” thick between the part and the scrap. I followed this up with a 1/4” endmill and left two ½” 3D tabs in the center of the part. Doing tabs in this manner left the radius cut smooth and unblemished by the tab process. This method is only feasible if it’s OK to have a narrow flat spot in the center of the two radii. In my case, I deliver the parts in the scrap matrix and the customer cleans up the tabs.
SG
http://www.talkshopbot.com/forum/attachment.php?attachmentid=29315&stc=1

[Keith Larrett]

Thanks very much for sharing your collective knowledge. I had thought of the custom female vacuum fixture, but not having much experience with vacuum fixtures I had been leaning towards Andrews idea of an external clamping fixture. What I had completely missed was the obvious, cutting them out with a straight bit and then rounding over on the router table, suggested by Steve.

When I first got the CNC I was thinking it would be the answer to everything. I learned in short order that there are often other tools in the workshop that are better for certain applications. It's funny how we often forget that :)

Chuck, I do like your idea of cutting a front and back and then gluing them together. If there is an issue getting 8/4 stock that would be a good alternative as it could be done from 4/4 stock.

Thanks again for all the suggestions!

[Keith Larrett]

[FONT=arial][SIZE=4]
Keith…
A method I used recently involved a flip op with a radius bit. My radius cutter left a ¼” flat web 1/16” thick between the part and the scrap. I followed this up with a 1/4” endmill and left two ½” 3D tabs in the center of the part. Doing tabs in this manner left the radius cut smooth and unblemished by the tab process. This method is only feasible if it’s OK to have a narrow flat spot in the center of the two radii. In my case, I deliver the parts in the scrap matrix and the customer cleans up the tabs.
SG


Thanks for the explanation and diagram Steve. I was writing my previous reply while you were posting.

I couldn't visualize how to do the tabs and leave a smooth profile on the radius cut, but see how leaving the 1/16" flat can achieve this.

I will ask if the flat would be acceptable as this is a good approach as well. Thanks!

[Burkhardt]

.... I had thought of the custom female vacuum fixture, but not having much experience with vacuum fixtures I had been leaning towards Andrews idea of an external clamping fixture. ....

If you have to make many of these donuts, I would not discount the vacuum fixture(s). A flat one to machine the first side and a female half-donut mold for the other side. For larger quantities (more than 5 or 10?) it might save you a lot of time and a little material. I use that method for bowls and dishes, below is a disposable fixture for a squarish bowl. The narrow donut would need a better gasket, though.

That said, I would probably turn the donuts on the lathe with a similar custom vacuum chuck. Even faster and you can sand and finish the surface on the lathe with minimal effort.

https://lh3.googleusercontent.com/-WfyHZiksVRo/VGgWZ9PNh6I/AAAAAAAAFcE/Sd_KnsgaCzg/w1462-h853-no/DSC02871.JPG

[Keith Larrett]

If you have to make many of these donuts, I would not discount the vacuum fixture(s). A flat one to machine the first side and a female half-donut mold for the other side. For larger quantities (more than 5 or 10?) it might save you a lot of time and a little material. I use that method for bowls and dishes, below is a disposable fixture for a squarish bowl. The narrow donut would need a better gasket, though.

That said, I would probably turn the donuts on the lathe with a similar custom vacuum chuck. Even faster and you can sand and finish the surface on the lathe with minimal effort.



Thanks. I will look further into the vacuum fixture. If this happens it will be more than 5 or 10.

Actually, the prototype was turned on the lathe. The same issue was encountered, namely, hold down. That's why we were exploring making them on the CNC.