# Try this again..  Undercutting experience?



## echesak (Oct 4, 2014)

Seems that my first post on this topic got lost in the aether...

I have a 1/2" plate of aluminum that has 2 opposing 1" bored holes, that are at a 2 degree angle.  I've machined a few of these by angling the mill head.  But with the CNC, I was thinking I should be able to bore these angled holes directly, while the plate is sitting flat.  I'd start first by machining out a straight bore at about 0.975 and then using an undercut (lollipop) mill bit to clean-up the edges and to angle the hole.  It seems that it would, in theory, be pretty simple to do, since it's basically a circle path, step-over, step down, then repeat.  But my CAM software doesn't appear to be able to do this directly (SprutCAM).  I've been working with Tormach support on this, but they indicated that SprutCAM can't do this operation.

So I was thinking of trying to code this up myself.  I'm very new to CNC and still learning.  So I was wondering if anyone here, with G-code expererience, wanted to help.  Alternatively, I'd be interested in seeing if anyone else's CAM software might be able to do this operation.  This would allow me to machine the part profile, internal structures and the angled bores, all in one set-up.  Sure I could make a 2 degree fixture, but I wanted to see if there was any experience doing this kind of operation here.

Here's a section view that shows that this undercut angled hole is possible with a 3/8" lollipop mill bit.



Any thoughts?

Thanks,

Eric


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## JimDawson (Oct 4, 2014)

The one thing that comes to mind is to create a series of circle profiles, where each one has the proper step down and step over relative to the previous one.  Then when run sequentially they would create the hole at the proper angle.  The step down might require a bit of experimenting to find the correct amount for a smooth surface.

I ran a part a few days ago using a 1/4 inch ball end mill, with a 0.012 step over and the surface looked like it had been ground.  That might give you a starting point.


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## echesak (Oct 4, 2014)

Hi Jim, 

I'll keep that number in mind.  Sure seems like it would be doable, in theory.  I just need to get up to speed with G-code, so I can give it a try.  Oh yea, I also need that $75 lollipop mill :-(.  But I might try it with a regular ball-end mill first, and just let it machine away the side that would be undercut.  This would be a good test and could be done with tooling I have on-hand.

Thanks for the look-see.

Eric


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## John Hasler (Oct 4, 2014)

JimDawson said:


> The one thing that comes to mind is to create a series of circle profiles, where each one has the proper step down and step over relative to the previous one.  Then when run sequentially they would create the hole at the proper angle.



Don't those have to be ellipses?  It seems to me that if you do circles as described with the plate flat the the hole will not be round in a plane perpendicular to its (tilted) axis.


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## JimDawson (Oct 4, 2014)

John Hasler said:


> Don't those have to be ellipses?  It seems to me that if you do circles as described with the plate flat the the hole will not be round in a plane perpendicular to its (tilted) axis.



John, you raise a good point.  I think you're right.  Assuming a 0.012 step down and a 1 inch hole, the offset per step is 0.0005.  The real question is, would the difference even be measurable in the home shop environment, or would the difference just be in the noise?   Now I'm going to  have to try it to find out.


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## John Hasler (Oct 4, 2014)

By my math the ratio of the major diameter of the ellipse to the minor is 1.00061.  Too small to worry about for a 1" hole in aluminum, I suppose.


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## echesak (Oct 4, 2014)

That's what I had calculated, John, 1.0006.  I guess to be technically accurate, I need to plot an ellipse in short segments and use this at each step.  However, I don't think it would matter either way, in this case.  I'm probably unlikely to hold a half thou on my machine, at this point.

Thanks for bringing that up.  I'm embarrased to say that I overlooked this point
Eric


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