Thoughts on milling curves with cylindrical tools

[pontiac428]

Two threads came up in the last month about creating noncircular arcs with conventional mills (in my case a 2" face mill). One option presented was to tilt the mill head to create an elliptical curve along the X axis. While this creates a climbing cut that can't be corrected (spiral motion into an interrupted path), it seemed like one way to get it done.

I've been working on a design for a shop-built press brake, and decided to make my own dies (of course). I thought I'd try this technique in Solidworks and see how it works out. I knew that it would form a true ellipse with the minor radius equal to the cutter radius and the major radius equal to the sum of the cutter radius and the sine of the head angle.

Turns out that even with a reduced depth of cut (machine z offset) the angle has a limit. I tried 15 and 20 degrees, but to get the depth I want for this case I would have to run this cut a lot closer to horizontal. That gets way outside the tool's intended work envelope. So my take home message is that this technique is okay for shallow cuts, but gets ugly quick as you try to go deeper.

Decided it's better to try to make my brake die clearances another way, but I thought I would share my result since it has a picture to go with it, and the technique may be useful at some point.

 

[verbotenwhisky]

I wonder if using the same strategy (tilting the head) you could use a smaller diameter mill to make 3 or 4 cuts then come back with the 2" face mill for the finish.... Just spit balling here...

 

[homebrewed]

I needed to machine a concave cylindrical section on part of a multi-part mold I was making. The section had to have a .5" radius, about 3 inches long. To make that I stood it on its end and used a home-made boring tool installed in my mill. It turned out pretty good.

 

[francist]

I remember using a similar technique but for wood. Well, similar but different, you know?

Anyway, first time was in high school to create the hollow tunnel in the centre of a guy’s waterski he was making. We set an auxiliary fence at about 30 degrees off the usual feed direction into the table saw and ran the ski against that. For each pass we’d bring the blade up a sixteenth or so and run through again. Worked great — made a nice smooth hollow.

Second time was making a large cove moulding for the plinth of a very large plaque. Again, set an auxiliary fence at a skew off the blade and ran it through multiple passes. The more the fence is skewed the shallower the curve, the less it’s skewed the deeper and more elliptical the result.

 

[homebrewed]

Here's a photo of the assembled mold:

 

[pontiac428]

Using the boring head would be great for a true circular cut going edgewise. It would probably be similar to using a fly cutter with the head tilted, just a different starting point and range of angles. I think either sounds feasible in softer materials.

The press dies will be made with 1", 1-1/4", and 1-1/2" thick alloy steel. Lots of metal to remove. I should stick to the heavier tooling and just make conventional cuts for this project.

Ellipse cutting may come in handy someday, not a bad thought experiment.

 

[Parlo]

I found this dimension for the two end points of the part arc, the chord length. ( A )
I then transposed this chord onto the cutter diameter.
This gives a short dimension perpendicular from the centre of the chord to the perimeter. ( B )
The chord dimension ( A ) on the cutter perimeter would cut the part chord length.
Use the distance ( B ) from the centre of the chord to the perimeter as the hypotenuse and the depth of groove required ( C ) as the opposite side to the tilt angle.
Sin tilt angle = C/B
This will cut an ellipse through both end points of the arc & also the midpoint at the required depth.