tips on how to machine a large arc

Cabinet makers often use a similar technique to cut coves. The work is fed diagonally across a table saw blade.
 
Illinoyance said:
Cabinet makers often use a similar technique to cut coves. The work is fed diagonally across a table saw blade.
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Yes, I've done that a few times although not for many years. Works well. Start with a low exposure to the blade, run a pass, nudge the blade up a sixteenth or so, take another pass, etc, etc. If the heighth increments are kept small the side loading to the blade is manageable. For the fence, a board is clamped to the table at the desired angle and the workpiece run against that.

-frank
 
So a 10" cutter mounted in the spindle could do the job in a series of light plunge cuts.
or
might be simplest to rough out the curve with a band saw and use a sanding drum to finish to the scribed line.
 
What is missing from this thread is what machining options are available to the OP. From the original post, I assumed that no mill was available.

If a mill is available, I would make up a jig consistying of a base plate with a pivot pin and a clamping system for the part. The part would be fastened to the jig and the jig rotated into the cutter to cut the arc. The plate could be a piece of plywood. Given the lack of precise control, I would use conventional milling to make the cut.

I used this technique many years ago to cut curved slots. For better control, I attached a lead screw to move the plate through the arc but for the production of a few parts, it wouldn't be necessary. Just take light cuts to control the forces.
 
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Janderso said:
Mr. Whoopee, you introduced an idea that sounds revolutionary.
Could you please explain it a bit more for us hobby guys?
I appreciate it.
Thank you.
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I'm afraid I can't explain the math, it's one of those things that I was taught without an explanation of the underlying calculations. Try to envision the concept. With the spindle perpendicular to the table, the cutter produces a flat surface, which can be considered an arc of infinite radius. With the head rotated to 90 deg. (spindle parallel to table) it will cut an arc equal to that of the cutter. In between, going from parallel to perp, the radius gets progressively larger. I don't believe it generates a true radius, but it's close enough for the girls I date.

I'm trying to get a clarification of the underlying math, will post if I find it.
 
I could hold the parts in the mill vise. Mount the right angle head to my BP and use a flycutter with a 5" radius.

By now re have probably exhaustedall the ways to make the radius





'
 
I have used the tilted head method of making an arc. You are actually making an ellipse. and not a true arc. As long as you are less than 90° of arc needed this will usually be close enough for non precision fits.

I generally lay it out in CAD so that I can draw the actual arc that I need and then play with the ellipse until I find the best (or at least acceptable) fit to the arc. I can then use those numbers to set it up. The drawing will also let me visualize just what the error will look like before I commit to making chips.
 
MrWhoopee said:
I'm afraid I can't explain the math, it's one of those things that I was taught without an explanation of the underlying calculations. Try to envision the concept. With the spindle perpendicular to the table, the cutter produces a flat surface, which can be considered an arc of infinite radius. With the head rotated to 90 deg. (spindle parallel to table) it will cut an arc equal to that of the cutter. In between, going from parallel to perp, the radius gets progressively larger. I don't believe it generates a true radius, but it's close enough for the girls I date.

I'm trying to get a clarification of the underlying math, will post if I find it.
Click to expand...
I believe that the surface cut will be an elliptical cylinder rather than circular and only then if the cutter has zero width in the direction of the axis of rotation. For a non zero width, there will be a small flat at the bottom of the cut and anomalies at either extreme.
 
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