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Old Feb 28, 2011, 10:56 PM
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How to CNC a Negative Draft Angle??

I'm hoping there are some more skilled CNC pro's on this forum to help out. I have a club build ongoing of a molded DLG. The wing will sit in a molded wing saddle in the fuse and as you can see in the attachments the saddle has a 6 degree angle. There will be a standard left and right handed mold milled out of corian. The saddle having the 6 degree angle is creating a negative draft angle which the CNC Z axis will not mill. I'm thinking maybe milling a insert for the saddle. Does anybody have any ideas on how this may be done or any other tricks of the CNC trade??
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Old Feb 28, 2011, 11:07 PM
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Ward Hagaman's Avatar
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I think you are on the right track with the insert. It might be hard to pull the part out with an undercut without pulling an insert first, but you might be able to rotate the fuse and get it out.

In an injection mold you would have a "slide" that would be pulled away from the part. This slide would run in a pocket the width of the wing chord.

Would it be blasphemy to suggest that you do the insert the "old fashioned way" and use an actual wing root and make it out of fiberglass? You could rationalize that this way you can adjust your mold for future cutting edge airfoils.
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Old Mar 01, 2011, 04:17 AM
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You can mill it if your spindle rotates to an angle, or with an angled table.
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Old Mar 01, 2011, 03:41 PM
G_T
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Or change the parting line to be horizontal rather than vertical. Your parting plane is no longer flat but that shouldn't be a big deal.

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Old Mar 01, 2011, 05:47 PM
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There are cutters to cut undercuts, and there is probably a way to do it depending on your CAM system. However, there could be an issue with pulling the part out of the mold unless the undercut is on in insert that could be pulled separate from the rest of the mold.
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Old Mar 02, 2011, 12:14 AM
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I use CamBam for my 2.D drawings and generating gcode for those drawings as well as imported 3D drawings. In CamBam there is a method to define a sloped vertical cut where the cutwidth is outside the desired path and gradually moves inward as the cut deepens. This intended to be used with a straight mill, but I have 'cheated' and used a ball end mill to create undercuts as the cut deepens. The thing you have to be careful with is retracting the bit after cutting because the cutting edge of the mill is actually under the top edge of the cut.

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Old Mar 02, 2011, 12:48 AM
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Don, that type of cutter is called a "lollypop" cutter. Never used one, but I've seen them in tool tables.
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Old Mar 02, 2011, 09:53 AM
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Quote:
Originally Posted by Ward Hagaman View Post
Don, that type of cutter is called a "lollypop" cutter. Never used one, but I've seen them in tool tables.
Or maybe a 'tootsie roll' cutter? ;-)

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